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GS750 running on 2 cylinders?

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pasteurized

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Well, I realize that my carbs need cleaning BADLY... bike won't start under its own power or with a kick unless i spray starter fluid in the airbox, so I am assuming the choke passages inside the float bowls are probably clogged. Gives me something to do this winter. Anyway once started the bike runs very weakly. The sound the engine makes and out the exhaust is very weak sounding and it won't run off choke, it will just die. Giving it throttle SLOWLY revs the bike. So I decided to pull off a plug wire and see what happened. Well, the bike runs absolutely no differently with the 1 or 4 cylinder wires disconnected. No difference whatsoever. Removing either the 2 or 3 wire kills it immediately. It is obvious that its running on 2 cylinders. I seem to be getting spark to 1 and 4, I can see it when the plug is removed and grounded, but I'm not sure if it is weak or not. My question is: is it possible that clogged carbs are the cause of the 2 cylinders not firing? The outer plugs are oily and wet looking, obviously because the cylinders are not firing. What do you think is the problem?
 
Sounds like you have a bad coil. If I'm not mistaken, the left coil (from the riders point of view sitting on the seat) fires cylinders #1 and #4, and the right coil fires cylinders #2 and #3.

The simple way to test, is to switch the coils or plug wires around, so that each coil now serves the opposite plugs. If the no-spark condition moves to the other two plugs, you know it's the coil.
 
is there any chance that the problem is related to the carbs? im just trying to hope that i only have to deal with one issue (carb cleaning) than 2 ;-)
 
If your idle jets are clogged up, you won't have a running engine. Do yourself a favor and take the carbs off, clean em up, and put them back on. If you don't want to do a full rebuild, just take them off and clean out the jets. It won't take long to do, just set aside a couple hours if it's your first time and go to it.

1 and 4, 2 and 3 run off the same coils too, so be sure to check your spark. If after you cleaned the carbs and verified the spark it still doesn't run right, check your timing, points gap and all that.
 
OK, next test:

Remove all four plug wires from the plugs, remove all four plugs out of the head, and re-install the plug wires on the plugs. Ground each plug to the block (use gravity, or jumper wires or lightly clamp with vice grips, etc.).

Turn the lights off in the garage, and thumb and hold the starter button, while sequentially looking at each plug tip.

If your coils are OK (and wires also) you should see a spark jump across the plug electrodes. If all four plugs spark, your coils are OK, and you probably have a carb problem. If only two plugs spark, you have a coil problem and you can trace the wires to figure out which coil.

Also, if your plugs are already fouled silly, go buy four new plugs (NGK B8ES) and do this test with clean new plugs.
 
Another way to check for spark, that doesn't envolve cranking the engine, is to just turn on the bike, and then turn it off again. When you turn it off the coils will spark once.
 
The real problem here is most likely the coil that drives the #1 and #4 cylinder, the left coil as mentioned earlier. Do the carbs if you wish, they probably need it anyway, but I don't think that is your immediate problem and you may end up chasing down a bad carb rebuild as well as trying to fix the coils at the same time. I recommend checking the coils first, it's easy enough to do without causing extra work for yourself. Either way you're going to have to remove the fuel tank. Before you remove the tank make sure you prime the carbs, you'll see why in a minute. Next switch the trigger wires to the coils, not the orange/white wires, but the other ones. I think one is white and the other is black/yellow. Next switch the plug wires on the #1 and #2 cylinders and #3 and #4 cylinders. Next, because there is still gas in the carbs, try and start the bike and try to keep it running for about 10 - 15 seconds to see which cylinders are firing. If it won't run on it's own that's OK. Just feel the pipes to see which ones are warm and which ones are cold. If the #1 and #4 are now warm and #2 and #3 are cold, the left coil is your problem. If not you wasted about as much time as took to read this because you had to pull the tank to do the carbs anyway. Hope this helps.
 
You have an ignition problem on your #1,#4 cylinders. Check input voltage to the left coil. Then check voltage at the positive lead on the left pointset.
Then check ohms (resistance) through the 2 and 3 plug wires with the caps in place. Resistance should be between 30 and 50 K. Check reistance between the terminals on your coil. resistance should be about 3 ohms, but anything between 2 and 5 is considered servicable. Next, set the max point gap opening on the 1,4 pointset (the one on the left side of the breaker plate) to 14 thousandths. Next, set the static timing on the 1,4 side to the |F1 that you will see when looking through the timing window. The continuity light should just go out when you come up to the mark when turning the crankshaft by hand. Use a 19 mm wrench to turn the crank. Have the spark plugs removed (it is much easier to be precise) Do NOT turn the crank with a wrench on the small hold down bolt in the middle of the crank end, you will wring it off.. 19 MM wrenchonly.

It should then fire on the 1,4 cylinders. You probably need to clean those plugs too if they have been soaked and carboned from not firing.

This will not solve your starting/choke/clogged carb problem, but it should help.

Cleaning your carbs is pointless if the ignition system isnt working. :-)

Earl
 
i'm having a similar problem with 2 and 4 (from the left, facing the front wheel)

1 and 3 get VERY hot VERY fast, but 2 and 4 are sluggish--- 4 gets hot after i let the bike idle for a bit, but 2 just stays warm, not really hot. i've a sneaking suspicion that it's either a bad coil or carbs that need some lovin'.


at any rate-- what carb kits would we be recommending for the 79 GS750E carbs? :)
 
your problem is different. :-) Yours really is a carb problem. A good soaking and replacing the "O" rings will do wonders for you. I soaked and rebuilt the carbs on my 79 750 a couple years ago and bought the carb rebuild kits from cycle recycle for $25 per carb. This was before Robert Barr had "O" ring kits for the VM carbs. If I had it to do now, I would just buy the "O" rings from Robert and clean soak/clean the jets I had. I dont know how much he's charging for "O" rings for 4 carbs, but I am pretty sure it isnt anywhere near $100. :-) :-)

You will have to vacuum synch the carbs after dismantling and assembly/re-installing them on the bike. Its not optional. :-)
A mercury carb synch/tool is going to cost you another $40.

Earl


m0unds said:
i'm having a similar problem with 2 and 4 (from the left, facing the front wheel)

1 and 3 get VERY hot VERY fast, but 2 and 4 are sluggish--- 4 gets hot after i let the bike idle for a bit, but 2 just stays warm, not really hot. i've a sneaking suspicion that it's either a bad coil or carbs that need some lovin'.


at any rate-- what carb kits would we be recommending for the 79 GS750E carbs? :)
Click to expand...
 
Would you suggest using berryman carb dip, or is there an alternative method that works better?

I'll have to look into the o-rings for the carbs, but it's really not important that i finish it quick quick, since it's not my only means of xportation :)

That carb series is definitely handy... clymer or haynes...hmmmmm
 
Berryman is fine, as is any solvent/dip intended for cleaning carbs.
I have both, Clymer and Haynes and I dont notice enough difference to prefer one over the other.

Earl

m0unds said:
Would you suggest using berryman carb dip, or is there an alternative method that works better?

I'll have to look into the o-rings for the carbs, but it's really not important that i finish it quick quick, since it's not my only means of xportation :)

That carb series is definitely hand
y... clymer or haynes...hmmmmm
Click to expand...
 
where could i go to obtain those o-rings, earl?
*EDIT* i found a site in a previous post-- cycleorings or something.

2nd question- is there anywhere I can get the float bowl gaskets by themselves? or is that just begging for problems?


When my manual gets here, i'm gonna get myself a mercury synch tool, and i'll hop tocleaning those puppies out.


next stop is wiring and ignition :)
 
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