ESE's works engine tuner

[Frits Overmars]

F5Daves suggestion of using a ball bearing ball and punching it to form the valve set seems to work. I have a few Mikuni float valves so it could be worth a try.

Shaping the valve seat with a bearing ball and a hammer definitely works.

[F5 Dave]

Drilling brass remember. Must try modified drill bits next time I do it.

[TZ350]

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Previous best ever dyno run with the F81M on Methanol.

Hopefully after I get the fuel flow issues sorted I will be able to pull these numbers or better consistently.



Took a look at the F81M's 1970's transfer port layout. Good news, it looks like there is a bit of room for improvement.

[TZ350]

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The making of a Methanol 4,2mm ID float valve for a Mikuni VM34.

Relative differences in length between a Dellorto and Mikuni float valve assembly.

Thought I would go the hard way and insert the Dellorto into the Mikuni.

Finished assembly. Hard to see in the picture but there is a reasonable relief around the Mikuni float valve body for the Dellorto fuel holes to open into.

Mikuni Methanol carburetor with enlarged fuel passages, Dellorto float valve and Wobbly adjustable power jet.

[oldjohnno]

Holley bowls are way better than anything else I've tried, if a bit bulky. Big flow rates, able to withstand fairly high pump pressure and not upset by vibration. Also externally adjustable float height and a variety of seat sizes and needle materials available.

[husaberg]

Holley bowls are way better than anything else I've tried, if a bit bulky. Big flow rates, able to withstand fairly high pump pressure and not upset by vibration. Also externally adjustable float height and a variety of seat sizes and needle materials available.

Very Nice
Cool adjustment procedure for these carbs also.

[oldjohnno]

I might just add a few thoughts on methanol with 2T engines, based on what I found running it in my LSR bike both on the salt and on the dyno. I had issues with consistency in both setting; usually what would happen is it would start a run fine then after maybe 5 or 10 seconds it would lean out, despite having a more-than-adequate supply.

It took me a long time to work out what was going on but eventually I found that fuel was collecting in the crankcase. Methanol tends to drop out at idle and low speeds, so it needs additional richness in the low speed circuits to compensate. Like most people I usually blip the throttle for a bit before a dyno run as a quick warmup, or if I'm sitting at the start line waiting to go I'll idle and maybe blip it a bit. While I did this, fuel was collecting in the crankcase, though I was completely unaware of it. It occurs quite quickly, and because the engine never really got hot it's persistent. The puddled fuel is consumed when the throttle is opened and the revs increase, this fuel being added to what the main circuit is supplying.

The problem is when you jet - if the duration of the run isn't long enough (and this is especially likely with an inertia dyno) a jet will be selected that will provide the right amount of fuel so long as it is being (unknowingly) supplemented by the puddle downstairs. If the dyno runs are short everything will seem fine and it'll make good power. But in a long run on the dyno or on the salt it would lean out every time. Some people suggested it was temperature related but that turned out not to be the case.

I added a handlebar mounted control that let me manually enrich the main circuit when it began to lean out, and this worked but it was a kludge. Later, I added individual needle valves between both the idle and intermediate discharge holes and the float bowl, to give better control of part-throttle mixture. This worked well, but I found that if the engine had been idling I could shut these valves right off - completely isolating the fuel supply - and the engine would continue to run for quite some time afterwards. That's when it all started to make sense.

Eventually I added a very small tank under the carb that supplies petrol to the idle/transfer circuits (directly with no float bowl). This completely eliminated the problem and jetting is now consistent no matter how long or short the run is. The main circuit runs methanol and is inactive at small throttle openings. As a happy side-effect the petrol helps put a little heat into the engine (which the methanol loves) and it automatically purges the alky after a run.

It's probably over-complex and impractical for most applications but for me it meant the jetting could be set accurately with confidence and the engine ran consistently every time.

[TZ350]

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Fuel dropout in the crankcase at low rpm, makes sense. I had that issue when I tried running a plenum. Fuel would trop out in the plenum. Surprising amount too. Fuel dropout killed off the plenum idea. A small petrol supply to the idle/transfer circuits sounds like a great way to get a clean running Methanol two stroke. I will look into it. Thanks.

Attachment 259989 Attachment 259990

This is the arrangement that I tried ages ago to get around the restriction of a 24mm carb. The idea was that the motor sucked from the plenum through a big unrestricted inlet and the 24mm carb had plenty of time to refill the plenum between suction events.

[TZ350]

Eventually I added a very small tank under the carb that supplies petrol to the idle/transfer circuits (directly with no float bowl). This completely eliminated the problem and jetting is now consistent no matter how long or short the run is. The main circuit runs methanol and is inactive at small throttle openings. As a happy side-effect the petrol helps put a little heat into the engine (which the methanol loves) and it automatically purges the alky after a run. It's probably over-complex and impractical for most applications but for me it meant the jetting could be set accurately with confidence and the engine ran consistently every time.

Methanol for the main jet and petrol for the idle/transfer circuit. It might prove to be overly complicated and impractical but looks like a very good idea worth exploring.



For road racing I probably need to have a bit of petrol handy for the longer races. So, I found this small diaphragm pumper carburetor from a four stroke weed eater. Plenty of places to hide it on the 250. After modifying the carb by cutting the un necessary bits off. I could mount a small tank and gravity feed the pumper with a small line running from it to the Mikuni's idle/transfer circuit.

[Grumph]

You're starting to get to the point where it may be less complicated to go back to the injection...

As a matter of slight interest, the speedway TQ's I tuned - all 4T fours - I ran the carbs slightly lean at idle.
Under a caution light circulating slowly, they stayed warm and didn't load up.
The guys running Hilborn injection were invariably slow off a caution as they couldn't lean them out.

[F5 Dave]

Might run fouls of pre72 rules there. Lean on idle can only mean lean on pilot on a slide carb. Seize city on 2 stroke roadrace. Closed throttle high revs for several seconds.

Well, at least on petrol. Maybe enough meth to not?

[TZ350]

You're starting to get to the point where it may be less complicated to go back to the injection...

Electronic Fuel Injection would absolutely be my choice for an alcohol two stroke engine. But sadly EFI is specifically forbidden for pre 72.

[Grumph]

Eventually I added a very small tank under the carb that supplies petrol to the idle/transfer circuits (directly with no float bowl). This completely eliminated the problem and jetting is now consistent no matter how long or short the run is. The main circuit runs methanol and is inactive at small throttle openings. As a happy side-effect the petrol helps put a little heat into the engine (which the methanol loves) and it automatically purges the alky after a run.

It's probably over-complex and impractical for most applications but for me it meant the jetting could be set accurately with confidence and the engine ran consistently every time.

Don't need the diaphragm pump. For a roadrace application I'd use a small float chamber alongside the carb supplying petrol/oil mix to the idle circuit only.
So metering is still through the one carb and though two fuels are used, no rules are offended.

Only the KISS principle.

[F5 Dave]

So. . If you made a float drain plug that had a pipe that sealed with a rubber tube to the pilot jet. . . Pause for Thinking. . And that lead to a secondary Float bowl because you don't want to flood it or starve it. . . You could have another carb bottom fed petrol. If that was mounted on a slide mount you could adjust the level to something that works ok.

And if you are really careful won't be a fire risk when crashing.

[Strokerhaus]

You're starting to get to the point where it may be less complicated to go back to the injection...

As a matter of slight interest, the speedway TQ's I tuned - all 4T fours - I ran the carbs slightly lean at idle.
Under a caution light circulating slowly, they stayed warm and didn't load up.
The guys running Hilborn injection were invariably slow off a caution as they couldn't lean them out.

Ive had similar experiences with 4T speedway bikes, whilst tuning carbs for gating after less than 10 stop starts without any runs in between we had washed the bores. Take care with very short dyno runs.