Left hand cylinder misfire from cold.

Hello, I had a problem with a misfire at tickover which with the help of this forum I thought I had sorted but I still seem to have a problem at tickover. When I start from cold the left hand cylinder runs intermittently unless I rev to over 4k when it clears. if I ride the bike staight away from cold it runs fine and after it warms up(after around 5mins) it will tickover OK. But if I start and run it on the stand it will initially backfire on LHS at around 3k and appears to go on to one cylinder(RHS) at tickover(1k). I have checked out everything, Boyer, earth,carbs etc all seem OK, I stripped and cleaned out the slow running on LH carb and the pilot now seems fine otherwise I assume it would not tickover OK when the engine is warm. Any ideas would be much appreciated. :? :?

HI Jaymac, I had a rough running and backfiring issue a couple years ago too. The Boyer wiring from the pickup on the RH side of the engine had work hardened and failed inside the jacket. As things warmed up the wiring plastic became softer and allowed the wire to separate inside the wire. It failed within an inch of the pickup plate. By the time I found it I had changed coils, leads, plugs, did a valve adjustment and carb rebuild.
How fresh is your cylinder head? Guide or sticky valve maybe? I seem to recall the Boyer ignition fires both cylinders together. Only matters in the one at TDC on compression though. Try swapping your HT leads to the other spark plug. Ie. rh coil to lh plug and visa versa. If the issue goes to the other side you'll know it's not carbs or cylinder related.
Best of luck to you.

Assuming everything else checks out as per the previous reply, including making sure the idle jet and pilot circuitry are totally clear, you could try what I did with a backfiring cylinder from cold. After letting it warm up a little, crack the throttle to the point where the offending pot is backfiring or runs roughly, hold it like that and screw the pilot air screw in or out and see if it responds and begins to run smoothly. If it does, you know it's a mixture problem.

I say this assuming that electrics, valves, head joint, etc. check out. Also, check for air leaks at the carb/inlet stub and stub/cylinder head, and the balance pipe connecting the two stubs.

When you get it sorted, let us know what it was.

Daveh has it. If the idle air circuit is working right it will be sensitive to small adjustments and if it's plugged up it will not. Always have some good known plugs on hand because this kind of situation is a plug killer always see if a new plugs clears the condition.

DucsandNortons said:

HI Jaymac, I had a rough running and backfiring issue a couple years ago too. The Boyer wiring from the pickup on the RH side of the engine had work hardened and failed inside the jacket. As things warmed up the wiring plastic became softer and allowed the wire to separate inside the wire. It failed within an inch of the pickup plate. By the time I found it I had changed coils, leads, plugs, did a valve adjustment and carb rebuild.

Click to expand...

This miss is a common problem with the Boyer. Many, including myself, have been fooled into troubleshooting the carburetor. The problem is actually built into the Boyer. Comes from soldering the leads to the pickup coils. The solder wicks up the wire, turning it into a solid wire. The vibration causes the solid portion to fracture. The rpm range from idle to ~3000 rpm will usually have a misfire from the broken wire. After 3000 rpm the vibrations are dampened by the isolastics and the misfire disappears. This lead one to think it's in a carburetor idle circuit. The permanent cure is to attach the wires with a crimped ring lug and small (4-40) bolt and nut. I've re-soldered them before and they still fracture. Ya', I know about heat sinks when soldering. I'm NASA certified for soldering techniques, but it ain't gonna' work.

Thanks for all the suggestions, I will try them in the next few days. I did take the bike on the road today to confirm fault after replacing the spark plugs, I had the same fault(not firing on LHS at tickover and below 2,000RPM) but once underway underload it picked up fine on both cylinders. However after about 5 miles it started to miss and spit back at low/medium revs, so returned to base and it missed intermittently most of way back. Once parked I restarted and adjusted tickover to just under 1000RPM and both cylinders were running correctly, I will now have to check the leads/boyer agian for loose connections or broken wires at pick ups.

How old are the plug wires? The old wires on my bike did the same. The right plug wire had continuity but the left did not. The bike ran but popped on the left side. Replaced the wires and problem gone.

I am still having problems with the LH cylinder misfire and backfire when the engine is first started and until it warms up(5 mins) If i ride off as soon as I start it runs OK and will tickover when I stop after a run but the LH seems to run uneven and the gases on that side are cooler then RH. Most replies suggested the problem was with the Amal pilot jet but I have cleaned that out with a drill as recommended, I would also expect that if it was the pilot I would have a problem when the engine is hot and not only when first started and cold. One thing I did notice at the weekend on first start up and the LH misfiring and popping if I pressed the tickler on LH carb briefly the problem would clear for a few seconds and it would run more even. Any suggestions for a cure would be appreciated.

Jaymac — you say you depressed the tickler and it ran more evenly. This is because you richened the mixture briefly. Sometimes you have to take the carb off to unblock the pilot jet and pilot circuit thoroughly, by spraying solvent through the passages and ideally using an air line, if you have one. If you look at the first pic and the drawing below on this link, it shows the pilot circuitry:

http://www.jba.bc.ca/Bushmans%20Carb%20Tuning.html

It also shows you how to check and adjust the float level, which you should do when the carbs are off.

I wonder if you have an air leak somewhere? E.g. carb flange to inlet stub, stub to head, balance pipe. You've checked these?

Do you have one petrol tap feeding one carb only, or do you have a link pipe that connects both carbs?

I would still not discount electrics entirely unless you know they are in good nick but it does sound as if it's a mixture problem.

Thanks daveh, I will do as you suggest and remove the carb and clean the pilot again, also check float hights. The carbs are quite new only fitted a few hundred miles ago and I understand the new build units can be a problem with pilot passage, I have also noticed that the RH float may be low as it is almost impossible to flood it with the tickler for starting where the LH will flood easily. I have completely rewired the Boyer to rule out problems with bad joints etc and although this has not affected the misfire on start up but the bike now runs well and cleanly once underway and warm.
As I mentioned when the bike has been run for around five mins it will tickover OK and does not back fire or misfire when revved on the stand like it does when cold and first started, however if I hold my hands over the end of both silencers the gases on the LH are cooler than RH and the firing is more even on RH where the LH tends to miss occasionally but again if I depress the tickler briefly it will become similar to RH (more even). I will also check for leaks on the inlet stub but as it runs well under load and at higher throttle opening I assumed this should be OK. The balance pipe on the MK3 is also fed from the engine breather so i am not sure if this could cause a problem, i will disconnect it and fit a standard balance pipe.

BTW The petrol feed has a link connecting both carbs and I have also tried with both RH and LH taps on.

It sounds like a low float to me off hand. Have you got a small graduated container? Drain the good carb and note the level in your container. Do the same for the other, are they exactly the same? A bravery test can be done by using one of your wife's measuring containers for this. Warning, this could cause gelding.

Thanks Cookie, I will check the floats and capacity, ( I have already nicked the wifes measuring container a couple of years back! she thought she had lost it and got another.)

Cheers

John