A65 Thunderbolt intermittent misfire mistery

[Dave1955]

Hi,

I am new to the forum but not classic bikes.  I currently own a 1953 BSA Star Twin (plunger), a 1968 BSA A65 Thunderbolt and (dare I say it) a 1978 Kawasaki Z750 twin.

I bought the Thunderbolt in August this year with an intermittent misfire problem.  I though it would be an easy fix, although experience should have told me that that is rarely the case.

As mentioned, the Thunderbolt has an intermittent miss-fire and I think it is only in the RH cylinder.  Therefore, I have completed the following work to-date:
Fitted Pazon electronic ignition.
New Ignition coils, HT leads and spark plugs.
Checked and adjusted the valve timing.
Checked and adjusted the ignition timing.
Refurbished the carb.
Complete re-wire as the wiring insulation was brittle in parts and therefore, I though a short circuit could be an issue.

After completing the above, the misfire problem remains the same.  My current thinking is it might be a sticking valve, but to be honest, I am clutching at straws.  Has anyone had a similar problem, or, suggestions towards a remedy?

Any assistance would be gratefully received.

Thanks
Dave

[DerekAnderson8]

Not good, i think after all you have done the only route now is as you say top end off and check valves.




derek

[Mike Farmer]

 

When it misses is it a single miss or several.

Mike

[Dave1955]

Hi Mark & Derek,
Thanks for your response.

Derek; That was my thought, but wondered if anyone else had any other ideas first.

Mike: It is a single miss but happens frequently.



Dave

[A10 JWO]

Have you got another battery, possible dead cell ? Just trying to help. Some ignitions need full voltage ?

[DerekAnderson8]

Compression test?

What colour are the plugs?

Have you had the head off on this bike, i had issues with head gasket on mine.

In fact i still have i think, as its oiling up plugs on one side only but i think that's down to possibly valve guides passing some oil!



derek

[Dave1955]

Hi,

Checked battery; ok.

Not done a compression test, but it feels ok.

Plugs look like it is running rich; same both sides.  More than likely because I am struggling to set the carbs up due to missing/poor running.

Doesn't appear to be passing oil.  Although I think I need to take the head off and change guides and valves.  The bike had been laid up for a long time (or so I was told) prior to me buying it and I wondered if sticking valves could be due to that. Especially as I had previously bought a bike that had been laid up with a similar misfire problem.

Dave

[DerekAnderson8]

pm sent

[EDDIE SIMPSON]

from my limited experience with an a65 spitfire. i found the highly tuned motor was very fussy about carburation. the new amals have removeable pilot jets and thats the direction i would go in.
check your spark with a strobe. if you have points are they insulated properly or sticking?

[finn]

HI CHECK YOUR SPARK PLUGS IF BLACK AND SOOTY RUNING RICH /IS YOUR CHOCK LEVER COMPLETLY OFF/CABLE TAUNT/ SOME PEOPLE RECOMEND HIGH OCTAIN FUEL MY THUNDERBOLT DOES NOT LIKE IT TO RICH PREFERS THE CHEEP PETROL THE GUY I BAUGHT MY THUNDERBOLT OFF SAID IT MISSED NOW AND AGAIN HEED BEEN RUNNING IT WITH CHOKE ON PLUGS GET FOULD THEN MISS FIRE PULL CHOKE LEVER RIGHT ROUND /CABLE PULLS CHOKES UP  / WHEN STARTED ADJUST PILOT SCREW TO HIGHEST TICK OVER THED RESET IDLE