Ignition problems - I'm stumped!

[Anonymous]

I'm having problems with my 79 GS550. Only two cylinders appear to be firing. When I warm up the engine, the header temp on cylinder 1 and 3 is around 400 degrees F and 2 and 4 are around 100 degrees F. Also, the cylinders that are suspected to be not firing appear to be dumping raw fuel into the headers and is dripping out of the joints where the header connects to the exhaust can. I put a brand new spark plug in each cylinder that is suspected not to be firing and it comes out wet with no signs of it ever having fired. If I undo each plug and rest it on the engine while cranking, each plug sparks. I wouldn't say it's the strongest spark I've ever seen, but each sparks.

OK, now for the weird bit. One coil fires cyl 1 and 4 and the other fires 2 and 3! Based on the wiring diagram in the service manual, it looks like the hv side of the coil has one pole connected to one spark plug and the other pole connected to the remaining plug. It seems highly unlikely to me that both coils would have one side fail at the same time. Does this make any sense? Is there something I'm missing here? Maybe a failing coil only has enough juice to fire one plug when both are connected? I'm really at a loss here.

I verified that the plug wires are connected to the right cylinders and all the low voltage connections seem fine. The plug wires are also numbered and they are plugged into the cylinders as numbered. I also adjusted the points to the proper gap. .3mm, and I gapped each spark plug to .6mm as indicated in the service manual.

The manual suggests switching the plug wires around to test for a bad coil, but it assumes that only one cyl isn't firing. I'd rather not risk bent valves, so the whole idea of switching plug wires around doesn't sound like a good idea to me.

I'm inclined to replace both coils since both appear to be bad, but I figured I'd put the question to those with more experience than me. Any assistance is greatly appreciated.

[focus frenzy]

that is not a ignition problem, but a carburetor problem.
you are getting way to much gas in the two offending cylinders.
the key item is that the dead cylinders are on separate coils.
what you can do is switch the wires around, 1 goes to 4, and vice versa, and 2 goes to3 and 3 back to 2.
the firing order will remain the same, you will be just changing the polarity of the fire across the plugs.
I suspect the problem will remain with the same cylinders.

[Anonymous]

I agree it is the carbs but they might also be delivering NO fuel.

[daveo]

If it's sparking laying on the head, it's most likely also sparking when it's installed, so, seeing as you know it's getting gas, make sure it's not too much, ie. leaking past the float valve, if it's not, put a compression tester on it, see what they read with the throttle held open or slides lifted while testing,(120# +/-) if you don't have a compression tester, check your valve adjustments look for suspected tight valve hanging open

[SqDancerLynn1]

If it is pouring fuel out of the exh I would suspect that you may also have fuel in the oil. From what you have checked it does sound like carb problem. Try changing the plug wires. Also check the petcock vacuum for gas

[KEITH KRAUSE]

Do you also notice a little fuel coming out of the floatbowl over flow tubes?Has to be either the petcock, float needle valves, dirt interfering with the float needle valves, or incorrect float level adjustment.
You also should check the coil/leads/plug caps for clean/good connections.
A weak spark won't properly burn the fuel.

[Anonymous]

Thanks for the feedback. I considered that it could be a fuel problem of some sort due to the large amount of fuel that drips out of the exhaust. I also noticed that when I tested the spark on one of the offending cylinders, a nice spray of gas came out of the spark plug hole where this did not happen on either of the operating cylinders. It seems odd to me that I would have two cylinders out, but I don't know what was done to the bike prior to me buying it. I wanted to remove the stock airbox and install some high flow air filters and rejet the carbs anyway, so I guess this is as good an excuse as any to take the carbs out and rebuild all of them. I'll probably just try leaning out the two cylinders that aren't running just to see if that helps, then do the rebuilds. Thanks for the advice! It is greatly appreciated.

[AOD]

If it is pouring fuel out of the exh I would suspect that you may also have fuel in the oil.

that can't happen on a 79 with VM carbs

check out the rebuild kits from Sirius (out of Canada) - i used one for my carbs and they're pretty nice. you probably have a clogged pilot circuit so fuel isn't getting in there right.

before you rebuild the carbs, carefully tighten each adjustmetn screw (2 per carb) down until it seats...and count the turns. most are between 1 and 2 turns. CAREFUL YOU DONT OVERTIGHTEN OR THEY'LL BREAK IN THE CARB BODY. please dont do what i did.

~Adam

[KEITH KRAUSE]

What will you do to try to lean out those 2 cylinders? Adjust the mixture screws? It's possible the previous owner made some unusual adjustments but it's not likely they would adjust only 2 that excessively rich. If you adjust the screws in, I think that will only be to try to compensate for the real problem. Compensating NEVER works. Just my experience.