Engine misfire

[bosco659]

I had been having lots of issues with split or cracked carb mounts for my PWK carbs on my ‘76. I bought and installed a set of aluminum adaptors with reinforced hoses (from Mike’s XS). Took it out for a ride and according to what I see in reading the plugs it’s now running leaner. Before the mount swap it would rev to redline without issue. Now on full throttle acceleration, when it hits 6,000 to 6,500 rpm it starts to misfire severely and I need to back off the throttle. Just checking that this is most likely a lean condition. I have main jets to swap but wanted to hear some opinions first.
I’ve posted pics of plugs before the mounts were swapped and after. Please note the before pictures were taken when it was hotter out, maybe in the high ‘80’s. After pics were in mid ‘70’s temperatures.

 

[bosco659]

Yet to be tested but I raised the needles one notch. Thinking it is more the main jets than the needles though.

The carbs with these mounts sit about 1/2” - 3/4” closer to the head. Does it make sense that this could have caused it to go leaner?

 

[Bushyeyed1]

You may be experiencing a rich stumble at the high rpm wide open throttle and that would mean you need to lean the main.

 

[bosco659]

You may be experiencing a rich stumble at the high rpm wide open throttle and that would mean you need to lean the main.

Thx - could be that but much more than a stumble. Originally was running a 125 main, then swapped to a 130. Ran pretty good with good plug color. New carb mounts then the misfire (pretty severe, once it starts the rpm’s will not go higher). The very clean plugs led me to believe I was going lean. Mains are pretty easy to change on the PWK’s so I can try going back down to the 125’s or up to 135’s.

 

[Jim]

Mains are pretty easy to change on the PWK’s so I can try going back down to the 125’s or up to 135’s.

Breaking up at WOT (wide open throttle) and looking at your plugs... looks lean to me. I think I'd try the larger main first

 

[Bushyeyed1]

The thing about reading plugs is that you basically need a clean plug and/or you have to run it at that throttle position for a while and cut it off before you go into another range. You spend most of your time on the needle jet and some of the pilot jet so that’s probably what your reading. The safe bet since your problem is WOT is adjusting the main one way or the other. So if you went lean earlier and it didn’t get better then richen it up. Also I always like to say go back to what you just changed. New carb mounts? Maybe the problem is something there.

 

[bosco659]

Breaking up at WOT (wide open throttle) and looking at your plugs... looks lean to me. I think I'd try the larger main first

I raised the needle a notch and will try taking it for a run but agree at WOT most likely is the main. Could carb mounts cause this? The mounts bring the carbs about 1/2” closer to the head but also hold the carbs perpendicular to the flange face on the head. The rubber mounts were angled slightly and kept the carbs more perpendicular to the ground.

 

[bosco659]

The thing about reading plugs is that you basically need a clean plug and/or you have to run it at that throttle position for a while and cut it off before you go into another range. You spend most of your time on the needle jet and some of the pilot jet so that’s probably what your reading. The safe bet since your problem is WOT is adjusting the main one way or the other. So if you went lean earlier and it didn’t get better then richen it up. Also I always like to say go back to what you just changed. New carb mounts? Maybe the problem is something there.

Agree re: reading the plugs. I tried a highway run and returned directly home to pull the plugs (I’m close to the highway). I should check for vacuum leaks around the new carb mount flanges. Thx

 

[Jim]

but also hold the carbs perpendicular to the flange face on the head. The rubber mounts were angled slightly and kept the carbs more perpendicular to the ground.

If the float levels were on the low side of acceptable before, changing the angle might have put 'em past what's acceptable. Might wanna check that. Maybe even set them to the high side of their tolerance.

I should check for vacuum leaks around the new carb mount flanges. Thx

A vacuum leak bad enough to affect WOT would mean idling would be damn near impossible.

 

[bosco659]

Good point about the vacuum leak. Looked at the angle of the carbs and they aren't tilted much at all. I’ll swap the mains which are easy on the PWK’s and if this doesn’t address the issue I’ll pull the carbs and check float level.. thx