Backfire at idle?

[OzJavelin]

Got my '78 GS750 running again (painted) and first time it was fired up it ran great, just started idled and ran fine. Next time I started it it just backfires all the time at idle, but above about 1500 it seems fine? The bike did this before I painted it (which I forgot). I changed the plugs and no difference. I noticed that if I remove plug lead 1 or 4 the idle engine will almost stall at idle. If I remove 2 it has a very small affect and removing 3 seems to have no affect at all. I'm going to swap the coils and see if it has any affect.

If it doesn't would I be better to follow electrical or carbs as my first course of action?

[OzJavelin]

Forgot .. All the exhaust pipes are warm ..

[tom203]

Forgot .. All the exhaust pipes are warm ..

Do pipes 2 and 3 still feel warm when you pull plug wires? wait 3 minutes and then decide;
Anyhow, you had a problem before its downtime, so it's likely that problem is only worse,especially if it's carb related. Sort out the electrical first ( yeah swap coils ) and then move on.
"above 1500 it seems fine" ? in neutral or riding it?

[tkent02]

Have you adjusted the valves, disassembled and cleaned the carbs, and synchronized them? Have you checked the points and ignition timing? Is it charging correctly? Are you running stock intake/exhaust or something else, and is it jetted correctly for that?
Is it backfiring as in popping through the carbs or backfiring through the exhaust pipe? There are way too many variables to start a guessing game.

[spyug]

As Tkent mentions , you need to pinpoint where the backfire is occuring, carbside or exhaust. If it is exhaust it is very likely an airleak on that side. A header pipe may have loosened up or more likely a hole in a pipe or at a connector or crossover tube. Usually, these don't show up until the bike is being ridden and deaccelerating, however so it more likely is through the carb.

Can you clarrify for us please.

Cheers,
spyug

[OzJavelin]

OK .. I thought I might get a divine revelation, but sounds like it will just be a hard slog. I'll pull the plug leads on 2-and-3 and see if the exhaust cool donw. Then I'll swap the coils and see if the behaviour moved to 1-and-4. And I'll check the points, and carbs, etc, etc ..

When I first tried to fire up my '79 GS550 it wouldn't run .. changed the plugs and it's been great for two years! Wish they were all that easy ..

[OzJavelin]

As a matter of interest I'm almost 100% sure the problem is with cyl#3 .. If I pull this plug lead there is no negative impact to idle and the misfire stops. Ive swapped leads 3 and 4 and no change (I haven't swapped coils). I've cleaned carbs and no change still. I've checked compression and all cyls are very close to each other.

I'm just hoping it's not a burnt valve ..

I'll do valve adjust and carb sync and see if that helps ..

[tom203]

As a matter of interest I'm almost 100% sure the problem is with cyl#3 .. If I pull this plug lead there is no negative impact to idle and the misfire stops. Ive swapped leads 3 and 4 and no change (I haven't swapped coils). I've cleaned carbs and no change still. I've checked compression and all cyls are very close to each other.

I'm just hoping it's not a burnt valve ..

I'll do valve adjust and carb sync and see if that helps ..

Doesn't the vaccuum for the fuel petcock come off the #3 carb on the VM carbs?
You swapped leads 3 and 4 ? why?

[OzJavelin]

Doesn't the vaccuum for the fuel petcock come off the #3 carb on the VM carbs?
You swapped leads 3 and 4 ? why?

Sorry .. typo .. leads 2 and 3 (same coil)

[OzJavelin]

Set petcock to prime and fired up. Started popping etc. Pulled the vacuum to petcock from carb 3 and popping stopped with vacuum line unplugged - sucking air. Plugged vacuum line and started popping again. Screwed out the adjustment on that carb half a turn and a LOT better! Returned the vacuum line to the petcock and still running ok. I think the carb needs more adjustment but looks like that is all it is .. Sweet!!!

Thanks TOM203!