Winter Layup - 1995 Ducati 900 Supersport

[Ocean1]

but wont a header tank remove the fuel supply pressure variations ? A header tank will always have say 200 mm of "head"...wont vary (much)

No, the extra volume in a tank adds nothing to the pressure compared to the existing tube. What's causing the problem is the flow restriction downstream from that, and at the very low pressure involved it doesn't take much to restrict the flow. I once organised a clear pvc tube return line up through the steering head and into the fuel cap, just like a normal vent tube. It meant I could see immediately if the pressure dropped at all, you could see the petrol level drop back down the tube.

There are systems that contradict almost every sensible design improvement you can think of, but they mostly involve weird fluids at much higher pressures. I recall one project where doubling the pressure produced almost twice the flow, doubling it again stopped it literally stone cold dead. General rule of thumb, though is doubling the line diameter gives 4 times the flow, at the same pressure.

Which reminds me of a Smokey Unick story. His response to the new, rigorously enforced fuel tank capacity limits was to increase the size of the fuel line to the engine, to about 2".

[eldog]

Tell me if I am wrong, because this is a long shot

apart from general wear and tear, your main problem that you believe you have now is running lean.
from your description this doesn’t occur when you go for a test ride apart from going up/down hills or spirited riding.

your calculation for amount of fuel required, is this for steady state? I ask this as I know someone who has a fuel rate meter on their dash, it’s acceleration which causes this to go min 3-5x more than highway (100kph) running fuel supply rate.

I am suspicious about the return line not being below the fuel level at all times. I wondered if the pump is unable to supply the instantaneous amount of fuel, fuel is then sucked down the line along with air. If this goes for too long air will enter the carb via the return line.

I would be keen to run a transparent hose from the Y to the tank to watch the fuel level.

Everything system I have worked with, always uses the path of least resistance, air, oil, heat etc.

i would also apply soapy water in case of vacuum/airleaks etc.

like those off-road bikes with the fuel hose through the cap.

you have been very thorough and the explanations/photos are excellent so even a dummy like me can understand

Ocean1 - ya beat me to it with a much simpler explanation

[OddDuck]

Tell me if I am wrong, because this is a long shot

apart from general wear and tear, your main problem that you believe you have now is running lean.
from your description this doesn’t occur when you go for a test ride apart from going up/down hills or spirited riding.

OK, maybe I should have been more clear... yep, it happens all the time. Even cruising. Open that throttle up and the bike leans out, no matter what I tune the carburettors to. Then it riches up at idle. About all I've ever managed to do is shift the point where it runs properly, at about 14:1. This is via measurement from the AFR gauge (which I acknowledge is grain of salt measurement) and examination of heads, spark plugs and header pipes. It's just that it gets way more obvious if I go up a hill (higher throttle) or start riding the bike the way it's meant to be used.

your calculation for amount of fuel required, is this for steady state? I ask this as I know someone who has a fuel rate meter on their dash, it’s acceleration which causes this to go min 3-5x more than highway (100kph) running fuel supply rate.

Well, I'd like a fuel rate meter. The calculation is for a theoretically perfect 900cc engine. 100% volumetric efficiency, perfect air-fuel ratio, etc. It's full of guesses. I took the 300 mL / minute figure from 100% throttle and thus 100% volumetric efficiency at 6000 RPM, the nominal peak output of the 900SS engine. It's a useful benchmark guesstimate number, that's all.

I am suspicious about the return line not being below the fuel level at all times. I wondered if the pump is unable to supply the instantaneous amount of fuel, fuel is then sucked down the line along with air. If this goes for too long air will enter the carb via the return line.

Yep, but the fuel pump is putting 5x the maximum expected demand along the feed / return line circuit. It'd have to be something spectacular happening for the carbs to suck the return line dry.

I would be keen to run a transparent hose from the Y to the tank to watch the fuel level.

Well, if it's working properly then that line will never show air. I'm confident that's the case btw, I'm just getting pressure variations due to velocity variations. What'd be more useful for direct experimental confirmation of what I'm guessing at would be some kind of ultra-sensitive fuel pressure meter, something with a decent scale from 0 to 2 psi or so. I'd connect that the carb supply line and see what I get. For the record, this is possible via a very tall length of clear piping and a T-joint, it's possible to work out the supply pressure via the density x height x gravity formula.

Everything system I have worked with, always uses the path of least resistance, air, oil, heat etc.

i would also apply soapy water in case of vacuum/airleaks etc.

like those off-road bikes with the fuel hose through the cap.

Vacuum: that thought had occurred to me - what if the tank breather valve has jammed? - but that's a brand new breather. The leaning out behaviour didn't change when I installed it. Tank fuel level or evaporation doesn't seem to affect it either.

As to vacuum leaks at the carb bodies, I'm confident that the seals are OK, I've had them apart enough times to be sure of the rubber gaskets top and bottom.

you have been very thorough and the explanations/photos are excellent so even a dummy like me can understand

Thank you, I just hope that this is useful to someone else out there.

[pete376403]

Is the fuel entry to the carbs above the lowest point of the tank (or where the petcock is)? Why does it need a pump at all?

[OddDuck]

Is the fuel entry to the carbs above the lowest point of the tank (or where the petcock is)? Why does it need a pump at all?

Yes, but only just - the height difference between base of tank and carburettor fuel rail is tiny, only 100 mm or so. The carb fuel rail is also quite a bit forward of the petcock, which is at the base rear of the tank. It wouldn't take much of a hill climb for this to go negative. Gravity feed just won't work with this bike unless a pumped weir is used, as you propose.

I can see what Ducati were trying to do with this pumped, recirculating system. I just don't think they did their detail diligence on it properly. I think the basic concept is workable - it's just that a very low flow resistance Y joint is needed, and probably much larger tank piping and fuel hoses on the fast circuit. The 6mm from the Y to the carburettors is OK, as discussed previously.

I'll have a go at some numbers and see what turns up.

[OddDuck]

Right, some numbers.

I got curious and ran some more numbers for the OEM Ducati setup, this time including the Y joint. I had to take a guess at the K coefficient for a narrow angle sharp pipe mitre joint, so the following is very much pinch of salt stuff, but this time the pressure variation comes in at 21% between idle and full throttle. The Y joint makes things worse. Of more interest is the fact that having all this resistance on the return line serves to increase supply pressure to the carburettors - these pressures are 0.84 psi (idle) and 0.66 psi (full throttle).

I tried calculating losses through the petcock and found something surprising, fittings aren't really that important compared to long lengths of narrow diameter tubing or piping. There is a term but it's about a quarter of what the 5mm pipe and 6mm tube loss adds up to. If losses are to be minimised then as Ocean1 said, increase pipe diameter.

If I go to a gravity weir inside the fuel tank to regulate supply head and run a single 1/4" fuel hose down through the petcock to the carburettor fuel rail, variation in supply pressure between static to nominal full throttle supply falls to 7%.

If I manage to use both sets of internal tank pipes, with a second parallel supply through the pipe previously used as the return line, the variation falls to just 1.5%. If I go this way, I might have to find and install some kind of inline petcock though, otherwise I'll have to completely drain the tank every time I need to take the carburettors off.

In both cases the supply pressure is just 0.32 psi.

This low pressure might just be a problem, it's probably worth running some tests on carburettor float level variation with demand against fuel supply pressure.

[OddDuck]

I'm keen to take a measurement of what's happening inside the float chambers while the engine is drawing fuel. I'd like to confirm what's happening with the fuel supply lines and also have a play with the needle valve seats.

Unfortunately that's easier said than done. Riding and trying to take a look - forget it. Dyno - don't have one, therefore can't take the engine to full load while the bike is stationary. About the best idea I've had is to simulate the engine draw by draining fuel from the bowls at around the same rate as would be encountered if the engine was running.

Exactly matching fuel flowrates to throttle setting isn't important for this measurement. What I want to see is if the fuel level in the bowls changes with fuel demand, in short, how consistent the behavior of the floats and needle valve is under various different fuel flowrates. Ideally the fuel level in the bowl should be consistent.

I've chosen to use three small peristaltic pumps to provide the fuel draw. For those not familiar with this type of pump, it's a flexible tube run around the inside of a cylinder, in a U-shape. A rotor carrying several rollers pinches the tube shut at equal spacings around the enclosed perimeter. If the rotor is turned, the pinched zones move along the tube, giving a positive displacement pumping action.

A peristaltic pump can also function as a valve, if the pinch is good enough. If the rotor is stopped, there won't be any leakage flow through the pump.

I've set the three pumps up to run in parallel electrically. They'll have parallel fuel lines in and out, too, meaning that any one can function independently of the other two. I can draw fuel in steps of zero, one pump, two pumps, all three, even on a simple power supply like direct connection to the bike battery. The enclosure was used to shield the sparking of the open commutator on the pump motor; this work is going to end up with petrol fumes getting generated throughout the garage and I want to minimise the chances of ignition.

The drain fittings for the carburettors took some machining. It's not possible to just purchase these, as far as I found. There are certainly aftermarket drain plugs, but all of the ones I saw function the same way as the OEM Keihin ones - unscrew the entire thing and presto, the carb drains. There wasn't anything on offer which carried a tube fitting and a screw valve.

The drain fittings also aren't threaded. The OEM plugs are threaded with an M16 x 1 thread; this is a very unusual thread and would require purchasing tooling to duplicate. It's a bit pricey to buy an exotic die or carbide insert lathe tooling in order to cut two threads. In the end I realised that I could use the heater element support brackets from earlier to press the drain plugs into place and therefore didn't need a thread at all.

This has to hold for the duration of a set of measurements. I'm not going on the road with this, so that makes life a bit easier. There's no need for hose clips everywhere and I can use cheap transparent vinyl tube.

The plan at this stage is to set the carburettors up beside the bike on some kind of a stand, run the fuel lines from the tank to the carbs, set up the drain pump module somewhere above the floor (this'll keep it above the worst of the fumes), and drain the carb bowls back into the fuel tank via the peristaltics. Power supply for the pumps can come straight off the bike battery. The stand pipes that I've cable tied to the fronts of the carburettor bowls will let me see what's happening to the fuel level and I can mark off increments with a Sharpie as I go.

I haven't metered the flow out of the peristaltics yet, that'll have to wait until I have the rig as a whole in place. It'll be good to do that, I'd like to confirm that I'm getting a maximum of around 300 mL / minute, as calculated. The pumps are cheap aquarium dosing pumps off TradeMe and were quoted as having a flowrate between 19 to 100 mL / minute, presumably dependent on fluid handled and pressure gradients.

[pete376403]

Keep a fire extinguisher somewhere nearby. Move the other bike out of the garage. :-)

[AllanB]

So you now have two Ducatis in parts ?

[OddDuck]

No no, the ST2 is still intact and running. The whole point of all those measurements was to work out what was there without taking the clutch hydraulic system to bits.

Tempting though...

Thanks Pete - I've gotten away with petrol so far but yes there is always a first time!!

[OddDuck]

I took the first measurements today. Some interesting results...

First things first: I purchased and installed a fire extinguisher. Dry powder (messy but effective), bracketed for easy access just inside the garage door - it's where I can get at it if I've bailed out from a fire in a hurry. There's only about eight seconds of spray in the thing but it's way better than crossed fingers and hope. For the sum of $34 there's really no excuse for not having one of these.

The measurements themselves didn't exactly go to plan. I got the setup in place as envisaged: carburettors off the bike and on a stand, extraction pumps feeding back to the tank, stand pipes accessible and ready for marking. It all went fine until one of the cheapo peristaltic pumps started leaking petrol everywhere.

Stripping the pump down established that the silicone rubber tubing had cracked. Maybe the petrol had got to it, but I'm more inclined to think that the cheap pump had simply been sitting unused for too long and the rubber had finally let go. It's possible to spend hundreds on a single pump, I spent about $22. You do get what you pay for.

And then there were two... I kept going and found a few results.

1) Clutter on the fuel line really does affect float bowl fuel level. In-line fuel quick connect couplings, fuel filters etc will introduce supply pressure drops and therefore drops in fuel level with increasing fuel demand.

2) Needle jet seat size will also affect float bowl fuel level. This seat size is very important, if the seat is undersized then it'll be the dominant effect, stronger than having clutter installed.

3) Perfect float valve behaviour appears to be impossible. No matter how large a seat is installed, there will always be a drop in level with demand, it's just the way that the mechanism works. Ideally this valve would go from sealed to 100% flow instantly but it doesn't, there's always a proportional band between closed and full flow, and this affects changes in fuel level.

4) Float needle valves actually like vibration and intermittent flow, this helps them to operate closer to ideal.

5) As far as I could tell, float fall vs demand isn't linear: it rapidly falls to a given level and then holds there. There's a curve which levels off. The trick for tuning will be to get static level and full demand level as close as possible.

6) Misalignment of the needle valve seat broadens the proportional part of the response, the initial fall on the level curve.

The last one was a surprise. More about this later.

And numbers...

I didn't really take a proper measurement of what the pumps could do, but it looks like they can deliver 100 mL / minute, as advertised. That means that for full throttle, as calculated (not measured unfortunately), I need to simulate 300 mL / minute fuel demand. With one of the peristaltics down, the best I can do with this setup is 0, 100, 200 mL / minute.

Setup 1: 6mm hose directly connected to the tank and Y joint, an inline fuel filter and quick connect fitting between Y and carburettors, 2.0 mm needle valve seats. Fuel level fall between idle and 2 pumps was nearly 6 mm. I saw very clear progression in the stand pipes at each step.

Setup 2: 6mm hose to Y joint, 6mm hose to the carburettors, 2.0 mm needle valve seats. Improved behaviour compared to above but I still saw 5 mm fall.

Setup 3: 6mm hose to Y joint, 6 mm hose to the carburettors, 2.6 mm needle valve seats. Very interesting behaviour here: one carburettor saw nearly 6 mm fall, the other just 2 mm. The problematic carburettor had a 4mm fall between no demand and the first peristaltic, unlike the other, which moved by 1 mm or less.

At this point pump 1 managed to chew up its hose and fail. It was repairable but by this point I'd had enough of hanging around in the fumes and set about clearing stuff away for the day.

[OddDuck]

Something I noticed during the flow / level measurements: the needle valve seat is very slightly shorter than its housing. The retaining screw clamps down on the housing, not on the seat, as shown by the marks where the dished screw head has carved into the aluminium but not the brass. There's around 0.10 mm of float.

I've made up a pair of washers from brass shim stock and fitted these: 0.25 mm thick, 7.5 mm ID, 10.8 mm OD or thereabouts. The idea is both to allow the seats to be nipped up properly, but also to be held flat and square in their bores. The OD of the washer has to be smaller than the OD of the seat's locating face (in this case, on the other side of the seat). Pressing anywhere inside the circle will naturally press it flat.

IMG_2031 hopefully shows the two marks that the dished securing screw have left on the housing (6 and 9 o'clock), it wasn't easy to photograph this.

IMG_2035 shows the seats pressed fully home in their bores, no washer, and a visible gap between securing screw and the seat.

IMG_2044 shows the O-ring seal for the inlet snorkel. Apparently this is not necessary in non ram applications, my experience so far has been that without it, fine dirt slowly moves into the air jets and then goes through the carburettor. I've just fitted these seals tonight, haven't tried running with them yet.

IMG_2047 shows the seat reassembled with the washer fitted.

IMG_2051 shows the seat, washer, float and needle fully assembled - the seat should be square but due to the off-center screw, the washer has lifted a bit on the open side.

As to why I think this is important... three reasons, really.

1) consistency between carburettors when setting static float height.

2) seat height and thus float height remain fixed during carburettor operation.

3) the seat face is always presented square to the needle, this greatly helps the needle to seal properly and go from sealed to full flow in the shortest motion possible.

I'll have to make further measurements to be sure that this improves things. The last measurement of float height did seem to make a case for it - quite a large variation was seen between two carburettors running apparently identical settings.

The very fine washers were made by clamping shim stock between two plates, drilling the center hole, and then clamping again between two thicker washers on a mandrel to turn the outer diameter down. It was fussy but it was the only way I could think of to get these made without bending the edges of the metal.

[OddDuck]

I set the fuel measurement rig up again today to check the valve seat work.

A couple of changes:

1) the fuel drain pumps were now run off a regulated power supply

2) a stand tube was set up on the fuel line, at the carburettors inlet.

I'd rebuilt pump 1 and 3 earlier in the week with replacement silicone rubber tubing, slightly thicker walled than the original. This was better but still gave reliability problems: the rubber both swelled with fuel and also slipped under pumping action in the pump housing, ultimately bunching up on one side and jamming the flow. I had to keep pulling the tubes outward to get the pumps running again. The photo shows the pump assembly at the end of the day's testing, with the tubes just about pulled right through the pump housings.

Before starting work with the bike I checked output of the pumps, using a volumetric measuring cylinder:
Pump 1: 75 mL / minute
Pump 2: 92 mL / minute (still running the original tubing)
Pump 3: 75 mL / minute

I then checked flow with all three running simultaneously: 242 mL / minute, in short, they're adding up nicely and there aren't issues with non-linear behaviour on the drain side of things. All of this was done through the experimental setup so that pump behaviour wasn't altered.

The carburettor bowl stand tubes had a problem, last time around: the peristaltic pumps set up quite a bit of vibration in the fuel level and it became difficult to read them. Some damping of fuel motion was needed. I provided this by putting a double-ended cotton bud down the 6mm tubing. It let fuel through but stopped rapid motion very well.

Then to the bike... I started the fuel pump, watched the level in the inlet stand tube rise, tried running the pumps, and was very pleased with the result. Fuel level in the bowls dropped by 3 mm and was consistent over both bowls, with identical initial fuel heights. Later runs showed even tighter behaviour, approx 2.4mm between zero demand and full load. It looks like the 2.6 mm seats and the nip-up washers are working. I'd also guess that the needle valve's rubber tips bed in to the seats, if they're left standing for a while.

The inlet stand pipe showed me something interesting: I'd been correct about pressure variation on carburettor inlet with fuel demand. The pressure on the inlet really does drop with increasing fuel supply. I hadn't quite been right about how much it was, it was roughly 13% instead of 21%, but it was there.

The photo of the inlet stand pipe with four bits of masking tape on it shows this result. The two upper pieces of tape show fuel height (and therefore pressure via p = density x gravity x height) when the fuel pump is running. The highest piece of tape is for no fuel demand. The lower of the pair is for all three peristaltics running. This shows supply pressure variation of 13%, as measured by change of height above the carburettor inlet. This is for 242 mL / minute, of course. I couldn't measure for 300 mL / minute but if scaling is applied and the effect is linear, the variation will be around 16%.

The two lower pieces of tape show the same thing, with the fuel pump off and the carburettors now fed purely by gravity and siphon from the tank. In this case the supply pressure variation is much worse, roughly 45% or so. This is quite different to the 7% I'd calculated earlier, probably due to flow losses through the fuel filter. I'd calculated the 7% for the tank's internal piping and fuel lines only.

In both cases this didn't affect the zero to peak variation of the bowl fuel level much. It still stayed at a neat, tidy 3mm or so. What did change was base level. Between pump running or not running, the base height shifted by around 1 mm.

The same thing happened when I sealed the tank (so the tank breather valve came into play) and drained the peristaltics into a bowl: the inlet pressure to the carbs changed (it increased slightly), supply pressure varied by around 13%, carb bowl level varied by 3mm or less, but base height very slightly increased. It looks like fuel line supply pressure affects the point where the needle valve will seal off.

So as far as I can see, this does validate the Ducati design. It's not perfect, but it's as good as it needs to be. The Ducati fuel delivery system also gives supply pressure higher than gravity could manage, which probably helps, and it does it without using valves or diaphragms (as a pressure regulator would need) so long term reliability is good. Ultimately the only modifications needed to improve the system were a bigger needle valve seat and making sure that seat was properly mounted.

Modifications to actively avoid: doing anything to the fuel supply lines. The achille's heel of the system is that it seems to be horribly vulnerable to anything introducing a pressure gradient.

Anyway, it now looks like I have a stable foundation for proper carburettor tuning.

[OddDuck]

I made a modification to the way the carburettor breather line is arranged, after running a couple of fairly simple calculations. Previously I'd simply run a tube from the breather to a point where a fuel overflow wouldn't go all over the bike, as per every bike I've owned previously. That means a line run down the side of the engine, similar to the battery vent tube. This time around I've gone vertically upward and applied a crankcase breather filter to the end.

This is a violation of the normal experimental procedure, by the way. If you've got a multi-variable system and you want it to go better, change only one variable at a time. If you change two or more, and it goes better or worse, there's no clear explanation why.

If I get curious enough later, it won't be that big a deal to run a hose down as previous and see what, if any, change there is compared to the changes I've made in the float bowl already. For now, I think this is the most promising way to go.

The reason for this change is worth going through. It turns out that there's a bit more engineering in a carburettor vent tube than I'd thought.

Carburettors work, fundamentally, by a pressure differential pushing fuel into the intake airflow. There's got to be positive pressure (relative to the carburettor throat) in the float bowl chamber to do this. The usual story is that high air velocity through the carburettor's throat - the venturi - reduces the local air pressure, thus sucking the fuel in. The unspoken assumption is that the fuel bowl is (of course) at normal atmospheric pressure, easily arranged via a vent. There's a pressure push between the two, the fuel continually refills, no problem so no worries.

If there's a significant air leak between the float bowl and the throat, that's it, the carburettor won't draw fuel. If the carburettor vent is blocked, pressure in the carb bowl will drop to match the intake and again, the carburettor won't draw fuel. If there's a mispressure in the float bowl, it'll directly affect the air-fuel ratio, forcing it either rich or lean. If there's a variable mispressure in the float bowl, the carburettor will be impossible to tune.

None of this is from my own direct experience. These are all pretty common discussions on the adventure bike forums, since dirt getting into carburettor breathers is a fairly common problem.

I think it's possible that I've been seeing a speed-dependent mispressure of this type, as well as the fuel bowl draining to a low level. The old vent tube was arranged with a square-cut end, at roughly 90 degrees to slipstream airflow, at the front left corner of the engine. It's a very rough suction tube, in short, drawing air pressure out of the carburettor bowls and thus making the mixture leaner with speed. It shouldn't be a strong effect, but... how strong?

I don't have a wind tunnel. That'd be the way to really test this: set the bike into a wind tunnel, sitting on a dynamometer, and rig manometers at all areas of interest. Get local pressures at air intake, carburettor throat, carburettor bowl, vent tube, and anywhere else of interest with the motor running and under load. In the absence of this sort of facility, it's get the pen, paper and calculator out. The calculations below are riddled with assumptions and are best treated as a guide only, but still, after the experience with the fuel lines, I'm confident that they will show a trend.

First: Bernoulli's flow continuity equation. Along a flow streamline:

Pressure / (density x gravity) + velocity squared / (2 x gravity) + height = a constant

It's possible to relate flow in two different locations, different speeds, heights, pressures etc by this relation, as long as they're in the same streamline. In this case I looked at free flow in the slipstream, then a momentarily captured pocket just at or inside the hose opening.

It turned out that the pocket just inside the hose will see a pressure drop of roughly 0.46% of an atmosphere, at 100 kph. Absolute pressure in this situation is 100,862 Pa, compared to normal atmospheric pressure of 101,325 Pa.

That's not much of a difference, shouldn't be a problem... except that it's not relative to the atmosphere. It's relative to whatever pressure is there in the carburettor throat. So, using the same equation, assuming 100 % volumetric efficiency at 6000 RPM, full throttle, and flow through the carburettor only 1/4 of the time, I estimated carburettor throat pressure (absolute) to be around 98,535 Pa. That's against normal atmospheric pressure of 101,325 Pa.

I.e. 98% of the normal atmospheric pressure is still there, the carburettor operates to draw fuel into air on just a 2% pressure differential.

Right. Maybe this would explain why carburettors can be a bit fussy.

Looking at the 0.45% variation in light of this, between zero slipstream and 100 kph, there's suddenly a 17% variation in pressure between fuel bowl and carburettor throat, purely due to the fact that the drain tube ends out in the slipstream. If I placed it somewhere on the bike that was tucked out of the breeze, this effect goes away. Of course if the new location's ambient pressure doesn't match the intake ambient pressure, there's a whole new set of problems to sort out.

The next thing I twigged to was the reason that Ducati used a shrouded box to cover the vent tubes in the OEM Mikuni carburettors: it's not a cover at all. It's a resonator. The OEM Ducati design, shown in exploded view in the attachments (thanks Stein Dinse) clearly shows the use of Helmholtz resonation on the air pressure side of the carburettor. They've done the same thing they did with the airbox, though, and used a shared resonant space for two unevenly timed sets of pulses. The pulses will tend to mess each other up. The design shows two tubes going into one resonator, with the outer resonator apparently unused. It's been a while but from very vague memory I think the two resonators were connected via a hole, this might have been done to broaden the frequency response.

This would be done if there was a standing pressure wave set up in the float chamber by the action of the fuel being drawn, and if achieving resonance on the breather lines would help. This does, to my mind, make sense. Fuel isn't drawn continually. Instead there's a brief, fast draw during induction, then the bowl slowly refills during compression, expansion, and exhaust periods. It's cyclic and would look like a sawtooth wave pattern. Physical motion might be quite small but what's important here is the pressure front on the top of the fuel during the induction; if this pressure can't be maintained at some engine RPM band then there's going to be a lean spot in the carburetion. If this pressure is inconsistent due to RPM / throttle interactions then the bike's going to be very difficult to tune.

If there's a standing wave in the carburettor vent, installing things like in-line fuel filters would tend to dampen this out and thus wreck any resonance tuning that the manufacturer applied. The same would go for changing the length or diameter of the vent lines.

It's not really possible to set this resonator up on the Keihin FCR 41's. The two bowls share a vent line via a T-joint and so they talk to each other first and the air line second, plus the fact that there's a hard 90 degree angle between the single air line and either of the vent chambers means that waves won't travel through the system properly. Waves will travel back and forth between carburettors instead, assuming of course that they can channel out of the float bowls (there's another 90 degree angle here). It's entirely possible that this is deliberate design. The FCR's were always intended to go onto a huge range of bikes, fitted by owners as aftermarket upgrades, and so fussy things like resonators would probably be best avoided. For now, about the best I can do is to get the breather vent tube out of the slipstream, and into a zone of the same pressure as the intakes.

I've put the line up into the same under-tank area that the main pod filters are housed in. There's a crankcase vent filter pushed onto the end to keep the usual bugs / road grit out of the carburettor bowls but not affect the supply pressure too much (I chose the absolute lowest flow resistance I could find). Maybe it's too close to the inlets and will be affected by them, but there were limited options for placement. I might have better luck taking it forward and tucking it somewhere behind the front fairing. At this stage it's just try it and see. It isn't that big a deal to revert or change.

As a side note, the line's as close as possible to straight up. I'd read that any sort of loop in the line can trap fuel, and once that happens (under heavy braking or acceleration, say) it will eventually choke the carburettor bowl of atmospheric pressure and thus the engine will stop.

[OddDuck]

A very quick note - no photos sorry - filled tank with fresh petrol, made sure sump was full and tried the first start on the engine since rebuild.

No issues. Gave it a few squirts of the accelerator and then a few turns on the starting motor. It caught and ran. The engine made oil pressure after a few seconds (go moly-based assembly lube!) and after that it was good to give it some revs just to hear the sound again.

Very pleased. Miss my bike.