missfire ??

Help, has anyone on here ever had a proplem with a missfire when hot. The bike runs sweet from cold but when its being raced, after about two laps it starts to miss. We have checked all the usual things and swapped coils etc. The tappets have been checked and re checked, but it still develops a miss when its being ragged hard. Any ideas would be helpfull.

any idea what sort of revs and speed this is kicking in at?

Curvy or pointy? If pointy, pre or post twin spark head?

C

My mate Allan had identical fault on his carby at the end of the season-tried all the same stuff as you have-replaced ecu and it ran fine!

TC

Mr Wayne has disappeared!

I had the same misfire when I had too large a main jet in the carbs. I also had the same thing when the tilt switch came loose on the pointy. The heat part was a red herring.

C

Cheers guys for your ideas. Its a carby, mini twin spec and it happens when you are woking it hard. At anglesey we did 2 laps and it ran fine, then for the rest of the race it started to miss and as the race went on it got worse and in the end we had to retire. We begged borrowed and stole from around the paddock and replaced plugs,coils leads ecu and it still missed. Then the carbs were taken off and swapped with another set and the thing still missed. When we got home the engine builder removed it and checked every thing, rebuild it and all was good. Then we took it to thruxton for the next meeting and it all happened all over again.
Next stop a gallon of fuel and a match

[Biker Biggles]

Fuel starvation?Is any of the fuel/venting pipework kinked or broken allowing a vacuum to build up in the tank over a few laps?Or just a faulty fuel pump.

[andrewsmith]

Originally Posted by Biker Biggles

Fuel starvation?Is any of the fuel/venting pipework kinked or broken allowing a vacuum to build up in the tank over a few laps?Or just a faulty fuel pump.

This
I think you need to start at the top and work down.

- Fuel Tank: I can put money on the inside looking like a old oil drum (mine does and its being sealed for the cost of £40 for POR-15)
- Pump: if the above is bad it'll be fouled, just as quick to swap out (My bikes was varnished as it had hardly ran in 7 years)
- Lines: how old are they?
- Fuel Filter: put one in (or replace)
- Carb bowls: Will be dirty inside with deposits (do the bowl screws while you there, £2 for 25 alan head machine screws isn't a bank breaker)
- Rubbers (carb and air box): can they be tightened enough to make a good seal.


Its possible for starvation to appear as a tight valve, massive back popping, missing, no idle (or a combination of the above)

What exhaust system and jetting are you running?

My money is on a leaking CV seal in the carbs.

Whip the airbox off and with the engine hot, check the sliders both return to closed at the same rate (by lifting them up with your finger., engine off.). if one is faster than the other, that one is leaking.

C