Heat soak misfire

[Guest]

Hello All,

I'm hoping you can help me with a problem which has been going on for over a year now and it's got me well puzzled.

Bike is a 79 GS750L which is almost completely stock apart from some shorty silencers on 4-2 pipes.

It was running fine until last summer when it went into 'limp home mode' on the way back from the pub. It ran fine on the way there, on the way back it was misfiring badly, maybe only on two cylinders. I thought maybe a main jet had come loose. It would tick over and rev in neutral but wouldn't accept more than say quarter throttle when riding without going onto two (or maybe four barely firing).

Next day it started up fine. I can ride 12 miles into town to work with no issue and back again in the evening - it just seems to happen mainly when its been stood for a while and run again while still warm / hot. Heat soaking into something? I cant seem to make it happen when I want it to though.

Plugs look fine, right colour. Carbs in balance and mixtures set using Colourtune. Apart from slight hesitiation just off idle , goes like stink when not misfiring.

I replaced the condensers last year - thought that'd fixed it but it's back again.

I got a couple of spare coils and I've so far swapped one out and cleaned all the terminations, no change - might still be other coil though?

I was convinced it'd got to be ignition related rather than fuel as it runs so well most the time, but I'm open to suggestions. I've tried removing the tank cap, running on prime. Can floats stick intermittently? could the carb manifold o-rings cause this much trouble?

Charging system seems OK, running on superdream regulator but maybe putting lights on makes it slightly worse - might coil relay mod help? I'd also suspected the cheap ignition switch but don't know how this would tie in with the heat connection.

If anything obvious springs to mind I'd be most grateful

Regards

Andy

[Cipher]

cracks in coil bodies leaking electricity

[TxGSrider]

After looking at the pic you're using, I suspect it may be fire damage. lol

[Brendan W]

Have a close look at the coil supply plugs. They can look ok but hide a lot of corrosion where the wires attach to the terminations.

[Cipher]

you can spritz heavily salted water on teh coils and wires in the dark. If you get a cross talk lightshow then things atre leaking.

rinse copiously with fresh water.

[zuluwiz]

I had a problem with my old GS1000 years ago. It would run all right for a while then start to lose power, and finally quit. Finally tracked it down to the boots between the carbs and head, they had cracks that would let air in. Slathered them with silicone seal and they were fine after that. Have you looked at that?

[Nessism]

How long since your last valve adjust?

[Andyjwatson]

Thanks for replies.

Nessism I must confess I never had the cam cover off tight valves losing compression when hot?

I was convinced it was something ignition / coil related - I've now swapped out both coils (albeit with other old ones) and it's the same I'm not so sure

I've had another look at the points / condensers -there's almost no sparking at the points when ticking over hot. I've reset the timing with a strobe. There's a good strong spark off each plug lead when running on the other three.

It still won't run under load though.

The inner two headers seem to be running cooler than the outer two at tickover by about 40-60 degrees C. I tried the colourtune plug in number 2 and the combustion colour was all over the place, yellow blue and missing.

I've checked the compression warm and got something like 135-125-125-135 although the tester wasn't screwing in all the way on the middle two pots for some reason.

When I took the tank off, some fuel came out of the vaccum pipe connection at the tap - could a faulty diaphragm let fuel be sucked through?

Regards

Andy

[Cipher]

oh my fuel on the vacuum side of the diaphragm
tres mal mon frere
You can check it by attaching a hose and using your mouth to draw vacuum on it

[Nessism]

A small amount of fluid in the vacuum line is normal. This is apparent when using sync gauges where some fluid tends to build up in the lines. Using a clear tube for the vacuum line will tell the story though. Or suck on the tube like mentioned, just be careful.

Oh, and adjust the valves before you burn them. It's just plain idiotic to ignore this piece of critical maintenance. And yes, the valves will hang open when the engine is hot if the clearance is too tight.

[Brendan W]

This sounds like condensors. What happened when you replaced them or how long was the honeymoon? Are they the right size and what is the insulation on the posts like?

[Andyjwatson]

My daughter got a house (if you can call it that) on 1st September and I've spent every evening and weekend working on that ever since!

I've just acquired a tank off another GS (not sure if 550 or 750) and during a bit of down time earlier today I had a go at swapping the tanks. The tank didn't fit but while I had the 750l tank off I rebuilt the tap with a new diaphragm kit. The old diaphragm washer in the centre (where the spring seats) was broken in four pieces and sitting in the bottom of the tap.

I couldn't resist having a run up and down the village and it goes like a train - no misfire

Can it be this simple? I had thought bad vacuum at high revs = fuel starvation but wasn't expecting this to be the problem

Happy new year to all

Andy