Misfire

[Shedman]

Has anybody got any ideas on this because i am running out of straws to grasp at. My 71 lightning has a severe misfire at 4000 revs changed plugs several times replaced electronic ignitions twice for ones i know are good because they came of my 71 firebird changed the coil and caps changed the carbs again off the firebird all made no difference. The misfire is so severe that it wont ride through it and the banging and coughing makes the whole bike shake and shudder. Checked the compression and it was a bit low so i took the top end off de glazed the bores and fitted new pitons and rings genuine bsa reground the valves checked for broken springs but found nothing untoward the sprins are srm and have only done 500 miles but still severe misfire. Ive even dis connected the alternator because i thought the solid state regulator might be interfering with the ignition module. Ive wired the ignition direct to battery to try that but still misfiring badly. Ive been at this for the last month and am getting nowhere. If anybody has had this sort of problem before i would appreciate any advice or ideas to try because ive just about run out.

[ChrisG]

Does the misfire happen on both or just one cylinder? If its the one then it could be the carb on that side. I've got a pair of Lightning carbs you are welcome to try if it helps. I don't know the condition of them as they were handed down to me from someone else, but maybe worth trying each one in turn to see if it makes any difference.
Send me an email on editor@bsaownersclub.co.uk if that could be of help
Chris

[Shedman]

Ive tried a nother pair which i know to be good as they are off my 71 firebird and they made no difference and as for which side or both cylinders mifiring i cant really tell but i think its both. Ive tried both richening and weakening, running without air filters but couldnt try playing with air lever because once it goes into misfire i dont want to take one hand off the handlebars, the misfire is that bad

[DerekAnderson8]

fuel flow? are the carbs getting enough fuel? what about breather on tank.

Sorry if its too obvious.

cheers

derek

[Shedman]

Tried lifting and lowering float height. Fuel flow is good and have also been running on both taps and one tap but still the same

[DerekAnderson8]

and up to 4000 all is well?

Does it not want to rev out at all? i had issue with needle position but i guess you have tried that.

What colour are the plugs?

derek

[ROYC]

Try it with the petrol filler cap loosened off.

[Shedman]

Up to 4000 it pulls like a train, the plug colour anything from light grey to black dependant on the different setting ive tried filler cap loosened makes no difference. It usually starts first kick but just wont go over 4000

[DerekAnderson8]

The springs are definitely ok and correct for the job? seems strange after all the changes it wont go past 4000 and still has same symptoms.

Sounds like a real pain, it can put you off a bike for good until you get it sorted or sold! 

good luck

derek

[ROYC]

Is it advancing enough at 4000 ?

[ducati2242]

Is it advancing enough at 4000 ?

That was my guess too whilst reading the earlier posts .

[Shedman]

With the strobe you can see it advancing as you rev it to the correct timing mark. I have played with the timing setting more advance and more retard to see if i could break the 4000 barrier but again made no difference. What i cant get my head round is its 4000 give or take revs no matter what settings its on

[ducati2242]

have you tried changing the condensor and points . Thats the only things left .

[Shedman]

Its on electronic ignition and i have tried 3 different ones which i know to be good as they came off my other bikes and 3 coils. At the moment im torn between the canal or a box of swan vestas

[Admin]

What petrol are you using, supermarket or a branded one? I have a Spitfire that had a similar misfire, it disappeared when I started using Shell V power. BP do a similar type of fuel, but empty your tank first. Also have you tried changing the plugs?
Rob