Valve clearances and lunched motor

[Max Preload]

Valve clearances tend to tighten, not loosen, due to creep in the valve stem and seat recession so it's important to check them for being too tight because they'll burn valves much more readily with that situation. Cam wear doesn't come into that because the valve should never be touching the non-lift section of the cam on a conventional spring return valve - that's what the clearance is designed to avoid.

[Max Preload]

For instance, from the factory mine is set to start reducing petrol @ 13,500rpm (it doesn't cut the ignition and is a much better way of doing things if you ask me) and completely cut it at 13,900rpm.

I doubt that very much because a lean mixture at those sort of revs for nearly any duration is fatal. Cylinder temperature rises rapidly.

It's much better to cut the ignition too, for mechanical reasons.

[Mental Trousers]

I doubt that very much because a lean mixture at those sort of revs for nearly any duration is fatal. Cylinder temperature rises rapidly.

It's much better to cut the ignition too, for mechanical reasons.

It runs lean for a very short period of time (like 10th's of a second) and quickly ends up too lean to fire at all but is still squirting some petrol in there, which then takes some heat away.

Cutting fuel gradually means it approaches the cut off point slower, ie it's losing impetus by the time it hits cut off. Cutting the ignition means it's still going hard up to the cut off point so it's easier to over-rev, ie momentum takes it well over the cut off point.

[Max Preload]

It runs lean for a very short period of time (like 10th's of a second) and quickly ends up too lean to fire at all but is still squirting some petrol in there, which then takes some heat away.

I'd say, if anything, it's immediately cut to well below the minimum explosive limit if not completely stopped.

Cutting fuel gradually means it approaches the cut off point slower, ie it's losing impetus by the time it hits cut off. Cutting the ignition means it's still going hard up to the cut off point so it's easier to over-rev, ie momentum takes it well over the cut off point.

Momentum isn't going to result in over revving. As soon as the ignition is cut, acceleration stops and indeed reverses due to drag. Newton's first law.

[Mental Trousers]

I'd say, if anything, it's immediately cut to well below the minimum explosive limit if not completely stopped.

From what I've seen the sampling period for engine revs could be improved quite a bit and the air/fuel maps are quite coarse so it's likely to do exactly that.

[Flip]

Shit there are a lot of experts here.

Check your valves because the valve clearances do change for 2 reasons, 1, valve seat recession which closes the tappet gap and 2, wear in the cam-follower-rocker etc which increases the tappet gap.

If the tappet gap closes too much the valves are held open and the valve burns out. If the tappet gap is too large it reduces the motors efficiency, makes noise and causes extra wear to the cam-follower-rocker mechanism.

There is not a motor maker that would back the mixture off at full revs-power, why because exhaust valves run red hot, if the mixture leaned there would be free oxygen in the combustion gasses and these would reduce the valves to their component metal oxides very quickly. What the ECU does is retard the ignition, which reduces the power very effectively and backs the revs down very quickly.

Just imagine what the oxygen sensor would see and do if the exhaust gasses suddenly went lean, it would dump a whole pile more fuel into the system to keep the combustion within safe limits.

[Owl]

There is not a motor maker that would back the mixture off at full revs-power, why because exhaust valves run red hot, if the mixture leaned there would be free oxygen in the combustion gasses and these would reduce the valves to their component metal oxides very quickly. What the ECU does is retard the ignition, which reduces the power very effectively and backs the revs down very quickly.

Well I'm certainly no expert, but just curious.

My S3 is supposed to start cutting fuel at 9450rpm and ignition at 9700rpm. Redline is 10,000rpm, so is this not normal? I've never found the limit, so I can't say what the fuck happens.

[Brian d marge]

Im no expert either ,,, just been doing it for donkeys years and have lots of brightly colored paper ,,,

if people actually sat down and looked at what they do , rather than what they think they do , the difference might be interesting

Brakes , oils , tyres you be surprised how good they actually are..., and how little of the potential we use

Some countries do not have the purchasing power or spares , and you would be shocked and amazed ( I was ) at what the ie engine can take try kero for two stroke fuel...

and think about this ,,, IF the bike was bought for say 1500 ....you can buy a second hand engine for 600 dollars and all things being equal it probably quicker to change the engine ... ( thinking cbr here as them is what I know )

is it even worth doing major maintenance ???


Me ... I bought an enfield both old and new ,, They have/had a different design philosophy cheap labour cheap parts , increased wear parts in the engine ( the oil pump worm drive is almost a change with every filter ,,,joking but you get the point)

Stephen

[Max Preload]

What the ECU does is retard the ignition, which reduces the power very effectively and backs the revs down very quickly.

Good point. Just as it does with detonation when detected by a signal from a knock sensor.

Just imagine what the oxygen sensor would see and do if the exhaust gasses suddenly went lean, it would dump a whole pile more fuel into the system to keep the combustion within safe limits.

Well, that's down to the ECU and the fuel map. The O2 is only a feedback so may be ignored in that part of the map in favour of a default duty cycle on the injectors. I've never heard of it being done though except for cold start enrichment when the O2 sensor is useless.

[Mental Trousers]

All of the Triumph maps I've looked at (mostly factory maps for Daytonas and Speed Fours using TuneBoy) all reduce fuel until it's cut completely. It's a separate section for inputting when to start cutting and when to completely kill the fuel.

[pzkpfw]

...and think about this ,,, IF the bike was bought for say 1500 ....you can buy a second hand engine for 600 dollars and all things being equal it probably quicker to change the engine ... ( thinking cbr here as them is what I know )...

So basically you are hoping the previous owner of the second hand engine did the maintenance that was required?


(Having said that, my bike is getting lower and lower in value, so it's hardly worth selling to "upgrade". So I'm almost considering buying one like it as a "spare". The idea would be an engine out could easily have stuff like valve clearances done, in leasure, on a nice bench. Then an engine swap when ready.

Just dreaming, really. I'm far too cheap/lazy to do that...)

[Brian d marge]

So basically you are hoping the previous owner of the second hand engine did the maintenance that was required?

(Having said that, my bike is getting lower and lower in value, so it's hardly worth selling to "upgrade". So I'm almost considering buying one like it as a "spare". The idea would be an engine out could easily have stuff like valve clearances done, in leasure, on a nice bench. Then an engine swap when ready.

Just dreaming, really. I'm far too cheap/lazy to do that...)

well its that ole how long is a bit of string thing really , assume average use ...500 for cash

hoping it aint knicked

Stephen

its frames I need for my bike , have 3 engines ,,,one frame

[george formby]

Interestin.

It seems most of this thread is about fours. How important are regular checks on a 5 valve parallel twin & er, how difficult is it?

[pete376403]

About half as busy as a 5 valve inline four. I did the clearances on an FZR1000 with the cylinder head on the workbench. It was straightforward, but fiddly (the shims are tiny, about 6mm diameter) Pays to be methodical if you dont want to be taking the cams out more than once.
I don't think I'd like to do it with the motor in the frame.

OTOH old Suzuki fours are a piece of piss. With the right tools you don't even have to take the cams out.

[george formby]

About half as busy as a 5 valve inline four. I did the clearances on an FZR1000 with the cylinder head on the workbench. It was straightforward, but fiddly (the shims are tiny, about 6mm diameter) Pays to be methodical if you dont want to be taking the cams out more than once.
I don't think I'd like to do it with the motor in the frame.

OTOH old Suzuki fours are a piece of piss. With the right tools you don't even have to take the cams out.

Thank you. I will start saving up then. I'm a mechanical numpty & the last time I looked at a shim it belonged to a CB 750 integra, about the size of a $1.50 coin.