78E Lugging at low RPM -- what am I missin?

[buggrin]

I don't like doing it, but since I can't physically wrench on the bike anymore I am forced to take it to a local shop. (Good guy & only one I trust). The first time the carbs ever needed cleaned (since I bought it new) was 3 years ago and this guy did a good job. Now after experiencing similar symptoms -- low RPM lugging, back firing while decel, and rough idling-- he has been thru the carbs multiple times and it's still lugging.

We've been thru all the steps in the help section (this guy works on carbs almost exclusively and he was impressed with web site) and while the decel backfiring is gone, when going from 2 to 2.5K RPM with a healthy amount of throttle the missing is STILL there. Also it idles fine at times, and misses at other times while idling. If the choke is pulled out one click the idle rpm goes way up but the missing at low RPM problem is gone. Also, if I stay out of the throttle and accellerate slowly the missing doesn't happen.

The engine/carbs/air intake is stock except the 4 into 2 Jardines. Rejetting was not needed for the exhaust change.

The choke "fix" tells me it's a lean problem but after many hours and way too much expense it's still a mystery. After 33 years of happy marriage, divorce is looking better and better. I spend a lot of time between 2 & 4K RPM and know after all these years how good it can pull. this flat spot is making me crazy.

[WMarshy]

Bob,
What all has changed since the last time it ran good...just a carb cleaning? Sounds to me like you might have a vac leak. Check/track all your vac lines to ign timing and petcock vac lines. I had my left side petcock vac line come off and was running like crapola at low end. I suggest using starting fluid or propane to see if you have a leaking carb holder. What shape are the rubber holders in, are they cracked at all or brand new, if they are new do you have a gasket between the head and them..? Also, check the rubber caps on sync ports...Ive also read about the brass sync tube working lose from the rubber holder... Any increase in RPM with the propane or starting fluid will verify a leak...GL

[buggrin]

Air leaks?

Bob,
What all has changed since the last time it ran good...just a carb cleaning? Sounds to me like you might have a vac leak. Check/track all your vac lines to ign timing and petcock vac lines. I had my left side petcock vac line come off and was running like crapola at low end. I suggest using starting fluid or propane to see if you have a leaking carb holder. What shape are the rubber holders in, are they cracked at all or brand new, if they are new do you have a gasket between the head and them..? Also, check the rubber caps on sync ports...Ive also read about the brass sync tube working lose from the rubber holder... Any increase in RPM with the propane or starting fluid will verify a leak...GL

It was running great until I ran it hard one day. I never do that, but was showing off............ After that the symptoms began. Symptoms were the same as when the carbs were completely gunked up. This time the carbs were clean. Your point is valid. Maybe wringing it out thru the gears a couple times created a leak somewhere. The carb man looked at the boots and declared them good -- just superficial cracks.

Thanks for the thoughts. I will check for leaks as you suggest. Will report back.

[TopCatGr58]

Devil's advocate...alternative problem!?

Hey Bob,

I don't want to add more to your possible diagnostics budget, but a simple test would be to take off and cap the vac. adv. hose, and then see if you get the miss and studder. If no, then this can point to the Pu Coil wires and not the carbs. After 30 years, those PU coil wires have flexed quite a bit, and they are known to wear and get cracks and such, and the flexing of them under varying vac. adv inputs....ie. strong vs. weak throttle input can cause the timing plate to rotate flexing and pulling the broken wires apart enough to prevent the signal to the TCI=miss, one the timing plate come back from rotation....like very slow throttle input, then the wires reconnect, and little or no miss. With the choke on, changes in vac. adv. input and rpms, so not necessarily indicative. JAT!

This is also covered in the tech tips, nice photos and such!

T.C.

[petejw]

just something else to throw
out there, what have you got your float heights
set at, an incorrect setting will result in a lean
or rich mixture.

[buggrin]

PU coil wire

Hey Bob,

I don't want to add more to your possible diagnostics budget, but a simple test would be to take off and cap the vac. adv. hose, and then see if you get the miss and studder. If no, then this can point to the Pu Coil wires and not the carbs. After 30 years, those PU coil wires have flexed quite a bit, and they are known to wear and get cracks and such, and the flexing of them under varying vac. adv inputs....ie. strong vs. weak throttle input can cause the timing plate to rotate flexing and pulling the broken wires apart enough to prevent the signal to the TCI=miss, one the timing plate come back from rotation....like very slow throttle input, then the wires reconnect, and little or no miss. With the choke on, changes in vac. adv. input and rpms, so not necessarily indicative. JAT!

This is also covered in the tech tips, nice photos and such!

T.C.

I've checked this in the past & it was always OK. Don't know if he checked or not. Maybe it's finally given up the ghost. will add to the list. thanks

[buggrin]

float height

just something else to throw
out there, what have you got your float heights
set at, an incorrect setting will result in a lean
or rich mixture.

the missing on decel is gone which tells me the lean condition is at least better. can't tell you where the float height is set or how it was measured. Also, the plug color was good -tan. (altho I don't know if he checked the color exactly according to the hints he did run it then check)

I'll add your question to the list. thanks