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Not firing on #4 cylinder

  • Thread starter Thread starter Anonymous
  • Start date Start date
A

Anonymous

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Hi
I've had my '83 gs650l down for a while. During that time I installed a new MAC exhaust system (4 into 2). Cleaned the carbs and installed brand new float valve assemblies. Then I couldn't get it to start. So I bought Accel super coils and brand new NGK spark plugs. Now it runs, but I noticed that it was shooting gas out of the right side exhaust. So I checked and the fourth cylinder is not firing. So I did a compression test and I have 170 on all four. Any Ideas? I would apreciate any help.

Josh
 
Re: Not firing on #4 cylinder

Do you have spark on the #4 plug?

Did you set the float heights? (new assemblies do not come set for height)

Earl


kingoftheroad said:
Hi
I've had my '83 gs650l down for a while. During that time I installed a new MAC exhaust system (4 into 2). Cleaned the carbs and installed brand new float valve assemblies. Then I couldn't get it to start. So I bought Accel super coils and brand new NGK spark plugs. Now it runs, but I noticed that it was shooting gas out of the right side exhaust. So I checked and the fourth cylinder is not firing. So I did a compression test and I have 170 on all four. Any Ideas? I would apreciate any help.

Josh
Click to expand...
 
Yep, I have a nice fat blue spark, and I the float height is set correctly.
 
Does #4 not fire at idle only and fires OK at higher rpm, or does it not fire at any rpm?

Have you vacuum synched the carbs? Are you sure #4 is drawing the same vacuum as the other three cylinders?

Have you removed the float bowl drain plug on the #4 bowl, and drained the bowl into a cup to be certain there actually is fuel in that bowl and in the correct amount?

Earl


kingoftheroad said:
Yep, I have a nice fat blue spark, and I the float height is set correctly.
Click to expand...
 
Having raw fuel coming from the right side exhaust does not guarantee it is coming from the #4 cylinder.

Is the spark plug wet on the #4 cylinder?

Earl
 
I did a bench synch before I put them back on the bike. It isn't firing at all on any RPM that is why all the fuel is coming out of the exhaust.
 
Sounds like the slide in the #4 carb is stuck and unable to raise.

Earl

kingoftheroad said:
I did a bench synch before I put them back on the bike. It isn't firing at all on any RPM that is why all the fuel is coming out of the exhaust.
Click to expand...
 
Well I guess that is possible, but when we were trouble shooting, we swaped the #1 and #4 slide, along with the float assembly and all jets. The #1 continued to run correctly.
 
Well, you have a blue spark on #4 plug and #1 runs normally, so it is not an ignition timing problem. Youre blowing fuel out the exhaust, so not a fuel availability problem. Fuel is passing from the carb, through the cylinder and out the exhaust, so not a clogged jets problem. It appears to be a mixture problem. In this instance, all fuel and no air. That would likely be a synch problem with the #4 carb. Swapping slides and jets with the #1 carb would have no effect on that condition. Try raising the slide height setting for the throttle off position. Also, I would change the spark plug as the #4 plug is probably fouled out now.

Either the #4 slide setting is way off, or the #4 slide is not raising. I dont see any other possibility

Earl

kingoftheroad said:
Well I guess that is possible, but when we were trouble shooting, we swaped the #1 and #4 slide, along with the float assembly and all jets. The #1 continued to run correctly.
Click to expand...
 
You DO have the vacuum tube connected on the #4 carb?

Earl
 
#4

#4

So the pipe on #4 is cold after start up and you have the spark plug wires in the right order, and not crossed
 
Between the 1 and 2 carb there is a vacuum balance Tee, and between the 3 and 4 carb, there is another vacuum balance Tee. The 1,2 Tee should be connected to the 3,4 Tee. If you run a vacuum operated petcock, there needs to be a middle Tee between those two to provide a "draw off" to operate the petcock.

Also as Gee-s-is says, you do have the left coil spark plug leads going to the 1,4 cylinders and the right coil powering the 2,3 cylinders?

Earl

kingoftheroad said:
what vacum tube?
Click to expand...
 
Are you talking about the vent tubes? These are not cailifornia style, the only vacum connection is on top of the #2 which runs to the petcock. Yes, the coils are running to the correct cylinder. This is what is really getting me, all three of the other cylinders are running fine. In fact if it were'nt for the gas coming out of the exhaust I wouldn't have even known it wasn't firing. It idles fine and the throttle is crisp like it should be.
 
fire

fire

Ok you have good spark, you have compression and fuel, what you need to do is do whats called a running compression check, you need a gauge with a relaese pressure valve,you pull #4 plug, put the gauge in after the bike is warmed up, start the bike and let it idle, push your release button and hold it open, then watch to see what the gauge will climb to, now take the revs to about 2,500 rpm, doing the same holding the pressure release valve open the whole time, if the pressure is lower while reving the engine than at idle, you have a flat cam or it spit out the shim, but more than likely a bad valve spring, that is allowing the valve to bounce once your off idle, the only other thing I can think of is if the cylinder is firing and the pipe getting hot, then you have water vapor coming out of the exhaust and not fuel, when the weather is cold here and you shut the bike down for several hours it will steam like an old locomotive for a few minutes and in the mouring for about 10 mins.
 
Well, I can try the comp. check, but it is definitely fuel and the pipe is not getting hot. I will pull the valve cover tommorrow, if it is not the shim, is there anything that I should look for on the cam or valve spring? By the way the bike only has about 6,500 miles on it. Which valve, exhaust or intake?
 
Sounds like the previous carb work resulted in the #4 problem.
Any gas overflowing from the bowl overflow line? Some aftermarket valve needles are poor quality and fit. I've seen new ones leaking at the very first start up.
Did you make any mixture screw adjustments or take the jet needle assembly apart?
From my experience, you can't substitute a bench synch for a vacuum tool synch.
 
The vent tubes are not leaking and the float valves I believe are functioning properly. I planned on synching them with merc sticks, but the bike should still run good with only a bench synch. Also, they are keyster float assemblies.
 
#4

#4

we still never got an answer as to weather the plug is coming up wet or not, also a plug that has gotten saturated with gas and then cleaned up will fire in open atmosphere and not under compression, so I hope you have replaced the plug? you may want to first before you pull the v/c is unplug the ign. box so there is no spark to the plugs and no power to the coils, pull all four plugs with the choke off crank the engine over, with your face away from the spark plug holes look to see it #4 hole is pumping out a lot more gas than the others, all 4 will pump out a small amount, if #4 is like way more you need to go back into #4 float bowl, please do this out side with the power to the coils DICONNECTED and NO available ignition source around you, if you find that you go in to the bowls and every thing is ok, it may be in the choke assembly in that carb, but do this before you pull the V/C cover
 
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