idle missfire

Status
Not open for further replies.

bigstu

Joined
Jan 2, 2009
Messages
280
Just got back from a long run from North UK to Switzerland n back.
Got some duff petrol out there, bike woun't idle or pull...eventuly dumped the gas out of the tank/carbs and bike immediately felt better, but had developed a persistant missfire on the left pot (timing side).
Plug looks brown, tried twiddling idle screw but to no avail.
Checked all the Boyer / coil connections but again still there...

Any Ideas?
Stu.
 
I have proposed this before and I'll do it again.
When you don't know if you have an ignition or carb problem the easiest way (to figure out what it is) is to hook up the strobe light and see that it flashes without interuption. If it looks OK you know you have a decent spark and it's likely to be a carb problem.

/Per
 
I would look at the last thing that happened. Would suspect the bad gas may have had impurities that may have clogged up the tiny idle passage in that carb. See http://www.jba.bc.ca/Bushmans%20Carb%20 ... html#PILOT. I know this page is often quoted but it happened to my bike too. Also, did you dump the float bowls and clean the gauze filters in the fuel line connections to the bowls?
 
Thanks Batrider
I will look at the site you suggest.
I dumped the float bowls but didn't look at the gauzes.
I have poked at the pilot by removing the air screw and prodding with wire...
Thats not a bad idea with the strobe PerG, I will try this. I should get a difference between the left and right pots.
Stu
 
Yes, if the plug doesn't give a spark the strobe light will not flash, easy to see if ignition is OK. Actually very easy at idle.

/Per
 
Have done the strobe thing and get a good flash pattern....have also changed the plug....
May have a look at the valve clearances, as I havn't rechecked them since I rebuilt 2000 miles ago now. Maybe one is tight?
I will also investigate the carb further. The bike dosn't run bad on the road, it is just annoying.
Stu.
 
Stu,
Now you know that ignition is working properly so it is fuel and compression left to troubleshoot.
I don't belive you have a valve clearance problem but that would be an easy check for you to do to rule that out.

As you mentionthe bike runs fine but how is the engine pulling when taking off from standstill? If weak, I would bet my money on Batriders proposed clogged idle circuit. Maybe more cleaning is required?

/Per
 
Per
The bike seems pretty good on the road.
On idle one pot goes thud thud thud, the other goes fluff, thud fluff....
when you increase the throttle it goes on to a more persistent missfire, which definately pops ans misses untill the throttle is on maybe 1/3 revs, then it clears and run fine.
Stu
 
Still sounds like a clogged pilot circuit. I'm chasing something similar on my bike and I swear I'm ready to ditch the Amals if I can't get this sorted out. Been inside the left carb 4 times now and can't find anything wrong. I feel your pain.
 
bigstu said:
Per
The bike seems pretty good on the road.
On idle one pot goes thud thud thud, the other goes fluff, thud fluff....
when you increase the throttle it goes on to a more persistent missfire, which definately pops ans misses untill the throttle is on maybe 1/3 revs, then it clears and run fine.
Stu
Click to expand...


Are you running a Boyer? If so, the miss is probably caused by it.
 
Jim
I am running on a boyer. I have swapped the wires from the stator to the box over and it still does it on the same pot.
I have also put a strobe on at idle and you cannot see a missing flash....and I have checked all the electrical connections....
Stu
 
bigstu said:
Jim
I am running on a boyer. I have swapped the wires from the stator to the box over and it still does it on the same pot.
I have also put a strobe on at idle and you cannot see a missing flash....and I have checked all the electrical connections....
Stu
Click to expand...

Maybe try swapping the plug leads over, then try swapping the plugs over.
 
Triton Thrasher said:
bigstu said:
Jim
I am running on a boyer. I have swapped the wires from the stator to the box over and it still does it on the same pot.
I have also put a strobe on at idle and you cannot see a missing flash....and I have checked all the electrical connections....
Stu
Click to expand...

Maybe try swapping the plug leads over, then try swapping the plugs over.
Click to expand...

Swapping stator leads really won't tell you anything because the plugs fire together on a Boyer, right? If you're using the stock type coils, I've found it's easiest to swap coil and lead together. My leads are too short to reach over. I had a resistor cap go to infinite resistance once that acted a lot like what you describe. Happened fairly quickly. I read a long time ago that on the analog Boyer one disables the advance by getting the stator leads reversed. BTW
 
btw
I heard the wires from the stator to the box fracture....heat and vibration....I swopped both ends of the wires....not just at the stator....all I was checking for is whether the wire itself was fractured. Does this make sense?
If I am swoping coils thats a different matter, I would have to study the diagram to figure that out. That maybe the next check.
Stuart.
 
bigstu said:
btw
I heard the wires from the stator to the box fracture....heat and vibration....I swopped both ends of the wires....not just at the stator....all I was checking for is whether the wire itself was fractured. Does this make sense?
If I am swoping coils thats a different matter, I would have to study the diagram to figure that out. That maybe the next check.
Stuart.
Click to expand...

Hi Stuart,

It's the short wires that run from the stator plate to the connectors inside the points cover that fracture. The lead wires that run the distance from the motor to the box seem to have no problem.
 
Status
Not open for further replies.
Back
Top