Engine miss firing after warm-up

[Peter in Aus]

Hi, I have a 1949 BSA A7 500cc Twin long stroke with cast iron head motorbike that has just been rebuilt it starts and runs very good idles good and sounds sharp, the magneto has been replaced with a Tri-Spark electronic ignition kit which test ok with a very good spark the timing has been rechecked and is ok, the electrical system has been upgraded to 12 volts using Alton AC generator with Neg. earth, It has all new wiring and switches. The carburettor has been rebuilt with all the right jets and needle etc the float level is correct and there is no fuel blocks. (using 92 octan stranded fuel)
The bike runs and sounds good with plenty of power til it warms up (about 5 to 10 Km) then the note of the engine changers and starts to miss fire, if I let it cool down for minute or two it runs ok for a bit and starts miss firing again, sounds like it is running out of fuel but I have checked and rechecked this and there is always plenty of fuel there, I also rebuilt another carburettor and tried that but made no difference.
 I have tried a few different spark plugs NGK B6HS & B8HS and a Champion L82C (all new) but doesn't make much different.
I would like to know if any one have come across this problem before and what the fix is please.
I feel it is a spark plug problem and would like to know if any one have any suggestions as to what spark plugs I could try or do you think I'm barking up the wrong tree?

[Brian]

Peter usually when a fault shows up when the engine gets hot its almost certainly a ignition problem.

One thing it would pay to check is valve clearances. If by chance one has closed up that will also show up as the fault you have described.

I cant help with the electronic ignition as I only use magneto's but others on the forum will as I am sure someone will have the same system fitted as you have.

[muskrat]

G'day Peter  .
Give us an intro about yourself and bike in "bikes, pictures and members.
Sounds like a coil or black box problem. Where on the bike is the coil and black box? Try moving them to a cooler place.
Brian's suggestion of tappet clearance is also worth checking. Give them another two thou and see if it improves.
Cheers

[chaterlea25]

Hi Peter,
Welcome to the Forum 

A common cause of similar problems can be a blocked tank cap breather hole,
Next time when the misfire starts, loosen the petrol cap
I had an issue with some petrol taps in which the seals reacted with the petrol and partially blocked causing similar issues

When running electronic ignition it is advisable to add earth wire connections from all points to one central location
usually where tha battery is earthed
Another  possibility is a bad joint in the wiring somewhere,
hotwire the ignition box directly from the battery through an inline fuse

HTH
John

[trevinoz]

Hi Peter,
                  Did you fit new valve guides?
Possibly one of the exhaust valves are nipping up when hot and freeing when the engine cools enough.

Trev.

[Triton Thrasher]

When you increase revs from idle, does the ammeter needle move from "discharge" up to zero and maybe a little bit into "charge."

[Peter in Aus]

Hi All Thanks for all the tips I will work through them all but I think Trev is on to it I did have new valve guides fitted and the valves may be sticking a bit will keep you posted
Peter

[bsa-bill]

Anyone going to mention Ex or inlet air leak

[Peter in Aus]

Hi, Checked the tappets, were OK but backed the Ex. off a few thou and added some upper cylinder lube to the fuel (Flash Lube 20 Ml to tank full) went for a spin about 40 Ks didn't miss a beat so that seems to have fixed the problem. Just goes to show you can bark up the wrong tree Easily!
Thanks again for all the help
Peter    

[trevinoz]

Pete,
       Are you setting the clearances to 0.015"?

Trev.

[Peter in Aus]

Oooops I said I backed the Ex. tappets off a bit I meant the inlets (I've got exhaust on the brain)
Yes Trev I had them set at .015 Ex, and In.
Regards
Peter 

[flatdeck]

Hi Peter, Sounds like a fine job. Mine is is dire need of the same treatment generally I think (same model, same year). I would appreciate any info you would be prepared to share on specs you have used for timing, carb parts and settings and details on the ignition system. Regards, Dave in NZ

[Peter in Aus]

Hi Dave, I don't know if I can be of much use to you but as far as the Tri- Spark Electronic goes it is easy to fit though you have to replace the auto advance gear with a fixed gear I had no trouble getting one of these from Draganfly  England. I sold my magneto on line which payed about half the cost of the Tri-Spark kit. Timing is a breeze compared to the magneto, as you know you can't use a timing light so it is just static timing, the hardest part is getting the piston 5/16 BTDC then all you have to do is rotate the stator till the red LED just comes on lock it up and job is done, just be carful with the bolts that hold the stator in place I managed to strip one which caused me a few hassles, the instructions with the kit is very good and easy to follow.
One thing though you have to have 12V system for me this was not a problem as the bike (basket case) didn't have a generator or regulator so it was just as cheap if not cheaper to buy Alton AC generator Kit about $650AU. you can just fit a 12V bat. and do away with the 6V generator and just keep the battery charged up, this was not a option for me, could not stand that! 
With the Electronic Ign. you can open up the spark plug gap a bit, I set mine at .035 and it starts first kick every time.
Hope this is of some help,
Peter 

[flatdeck]

Thanks for the info Peter. I hope to be able to devote some time and pocket money to the bike over the next couple of years and electronic ignition sounds like a good plan. Regards, David

[Peter in Aus]

Hi all, just a up date on the engine misfiring.
I was still having trouble with it misfiring, backfiring and spitting back through the carby when worm! Finley got sick of it and ripped the head off.
Took the head back to the repairer that did the job, told him the storey, he said he would have a look, he found that the Ex valves had been sticking he could see scuff marks on Ex valves and guides, he eased out the guides .001 thu
"Presto" no more problems, so Trev you were wright it was the valve guides.
Cheers
Peter