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Misfiring. Badly. Suddenly.

  • Thread starter Thread starter lemonshindig
  • Start date Start date
L

lemonshindig

Guest
As some of you may recall, I soaked my carbs overnight, sprayed out the passages, and reassembled them all about a week ago because she wouldn't run for ****. That was the second time I rebuilt the carbs. At that time, I also rinsed out my fuel tank, installed an in-line fuel filter, and sprayed out my airbox.

As some of you may also recall, I recently broke down twice on the highway, right at three hours into the ride. I believed that I had a faulty ignition coil that would cut out once it got hot. Once the bike cooled for an hour, it fired up and ran great, both times.

Fast forward to yesterday, it broke down about two and a half hours into the ride. It was running but missing badly. I assumed it was the mysterious heat-related issue again. So I stopped, waited, and no change. This morning, no change. I tore the carbs apart once more, found nothing, sprayed out every single passage again and confirmed there were no blockages, reassembled, and still missing like crazy.

The points are opening and closing. I measured the primary coil resistance on both coils to be good. I tried to measure the secondary coil resistance and wasn't able to get a reading on either coil.

I replaced the plugs.

This thing is missing bad. I've put about 800 miles on it in the last 9 days, and I have about 400 left to go. The change in running was brought on suddenly, which lead me to believe it was a carb issue. But the fact that I had an inline filter, found nothing, and all the passages were spraying clear suggests otherwise.

I want to check compression next. Compression was great three months ago. Brace yourselves: I never checked the valves. I might have burned one.

Throw yourselves at me. Err, your knowledge anyway.
 
lemonshindig said:
I never checked the valves. I might have burned one.

Throw yourselves at me. Err, your knowledge anyway.
Click to expand...

How tight are they? Too tight to have good compression? Too tight to tap a little?

No worries, top end rebuilds are fun. Ask Cliff.
 
tkent02 said:
How tight are they? Too tight to have good compression? Too tight to tap a little?

No worries, top end rebuilds are fun. Ask Cliff.
Click to expand...

I was running about 110 psi cold (roughly) when i did the compression test a few months ago. I believe the bike has 12k miles on it.
 
Probably weren't any burnt valves when you checked the compression, they wouldn't all be even. Definitly time to check them, unless you have a record of it being done. Even then it was probably done wrong.


But it's probably not the misfiring problem, do what Rustybronco says. Then adjust the valves.
 
The points and condensers are new. They've got maybe 1000 miles and three months on them. I did not see any arcing on the points.

Once I verify compression is good, I am suspecting a vacuum leak. Perhaps a vacuum leak at the airbox to carb boots. I am thinking Carb #2 is the culprit, because i noticed that with all of the fuel screws 1 turn out, carb #2's air mixture screw sweet spot is only 1/2 turn out. Which means to me that it's getting a lot of extra air. Also, the bike appears to have idling problems. Normally it idles fine, I set it at 1100, but sometimes I come to a stop, choke off, fully warm, and it goes down to 400 or so. I adjust the idle up to 1100, drive away, and at the next stop light it's idling 2k. Is that the "hanging idle"? It is a new symptom.

I suspect the boots between the carbs and the box because I have to put a good deal of pressure on the airbox to get the bolt holes to line up. Previous to this trip, I didn't have the airbox bolted in place.
 
Last edited:
Did replace the coil(s) mentioned in the 1st post? Other heat related things to check would be plug caps, and maybe pull them off and trim the wires back. Maybe that was covered in the other thread for your coils, just throwing ideas out there.
 
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