misfire
 

Hi all

Following a rebore, and a new BSM exhaust, which works on standard jetting, I am getting the occasional misfire when the throttle is open approx 3/4. Do I need to adjust the mix, and if so, do I adjust the pilot screw on the bottom of the carb?
The spark plug colour looks OK. I have also just started using unleaded petrol, rather than LRP, but I am using Pro Boost, on the advice of the person who rebored the barrel.
The misfire was there when using LRP however, so I don't think this is relevant.
Any advice would be appreciated.
Cheers Dave

Can be anything according to the symptom, but the idle mixture screw is only for the idle mixture. Check air filter first. Also please specify what exactly you mean when you say 'misfire'. Sounds like the spark plug skips once!?

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Fred, XTZ660, Holland.

Hi Fred
The filter seems OK, and I've changed the plug and cap. Also the CDI unit is only about 1 1/2 years old. I,m pretty sure the coil is OK.
It feels like it is missing a beat, and happens between 3 1/2 to 4000 revs. I guess it feels as if it might be choking a bit. It may be a blockage in a jet I suppose.

Dave

Leave the pilot mixture screw alone, it is only relevant from closed to 1/4 throttle max.

Does the misfire occur at a steady throttle or when you're accelerating as well?

Steve

Hi Steve

It has been occuring at both. However, today I put a cooler running spark plug in it, and also thoroughly cleaned the air filter and that seems (touch wood) to have solved the problem.
I think maybe it had been running a bit hot after the top end rebuild. I'm still running it in.

Dave

So it's not a 'misfire' as in 'ignition not firing' [you wouldn't notice it missing a beat] but more like a mixture problem. Can be both too lean or too rich. Try to listen to the sound from the exhaust. If it sounds like blurry it's probably too rich, and if there's no sound but no reaction to the throttle it's probably too rich. But it is best to go to a test bench and have it analyzed. As we say here : meten is weten! [to measure is to be sure]

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Fred, XTZ660, Holland.

Hi

Thanks for all your help. Its now running smoothly with an 8 plug back in, so it was the filter. It looked clean, and had only been serviced (allegedly) abou 400 miles ago, but when I washed it out, it was filthy.

Cheers Dave