Not firing on #4 cylinder

[Anonymous]

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.

[Anonymous]

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?

[KEITH KRAUSE]

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.

[Anonymous]

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.

[Anonymous]

#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

[Anonymous]

Did you ever switch number 1 and 4 plug wires, they fire the same and that would eliminate the spark issue. I had the similar issue but it was a dead plug.

[Anonymous]

Alright, I screwed with it some more this weekend. To answere the previous questions, yes, the plug is wet and it has been fouling out. I have been changing different plugs. Also, I have swapped the #1 and #4 plug lead nothing happens and #1 still runs fine. So Saturday I checked to make sure there wasn't extra fuel coming out of the #4 and all was well there. I pulled the valve cover and everything there was fine. So I was at my wits end and I had an idea. I pulled the screw for the merc stix causing a major air leak, wouldn't you know it, it started to fire. however not without a lot of smoke and some popping. So I fooled with that for a couple of minutes and it stops firing, so I changed the plug and put the screw back in. Then I opened the butterfly valve so that it would get more air, still wouldn't fire. Anyways, I went the opposite direction and went full choke making it really rich. The little whore fired again then after a couple of seconds fouled the plug. By that time I had had enough torture for one day and went home. So here is where I am at, I am pretty sure that it isn't engine problems, pretty much 100% sure it isn't electrical, so all that leaves is the carburetors. Are there any holy men out there that I can take my carburetors to and have them blessed? Any Suggestions?

[KEITH KRAUSE]

The most likely cause of your problem is a poor carb synch. Don't underestimate what vacuum imbalance can do. I consider myself pretty good at synching carbs by eye. I always do my best to make the final vacuum tool synch easier/faster. Everytime I hook up the tool, it shows that my bench synch was not good enough. If the vacuum level is too high (compared to the others) that cylinder will run rich. The carb synch is the easiest thing to check before going back into the carbs. Of course, you need the tool first. If the problem still exists after a vacuum tool synch, then it's either too much fuel or not enough air atomizing the fuel before entering the carb throat.
If it's too much fuel, it could be float level, sticking float, ruptured float, poor sealing float valve, leaking or missing gasket or o-ring for the float valve seat, incorrect size jet(s), loose jet(s), mixture screw adjustment, worn jet needle, leaking o-ring around needle jet, and the carb synch.
If not enough air, it could be a dirty/blocked air passage or air jet, dirty needle jet/bleed pipe holes, mixture screw adjustment, and the carb synch.
It could also be the diaphragm/spring assembly installed wrong or binding.

[Anonymous]

Well Its back into the carbs, I know that they need to be vacum synched, but they have to run on the bench synch in order to do that. So, next time I can get to it I will pull them again, recheck all jets, I have multiple spares so I will swap out as much as I can in order to eliminate possibilities.

[KEITH KRAUSE]

Well Its back into the carbs, I know that they need to be vacum synched, but they have to run on the bench synch in order to do that. So, next time I can get to it I will pull them again, recheck all jets, I have multiple spares so I will swap out as much as I can in order to eliminate possibilities.

Yes, carbs can run on a bench synch, but if the vacuum is too high in one cylinder compared to the others, the richer mixture will eventually foul the plug. If you have access to a vacuum tool, hook it up and note the vacuum level of #4. If it's about 3/4" or more higher than the others, that's most likely your problem. If the level is close or even a bit lower than the others, you can stop right there if you like and concentrate on other parts in the carb.
If not the vacuum level, it must be something I mentioned before.
Let me add to my list under "too much fuel"..
Be sure you re-assembled the diaphragm/spring assembly with any jet needle spacer(s) in the correct order, etc.
Also, be sure the choke plunger to #4 is completely sealing when down.
And if you had those rubber plugs over the pilot jet entrance, did you remove them? Some say their bike ran bad when they tried removing them.
Lastly, did you replace the pilot jet with another? On some Mikuni carbs, there are two different pilot jets available. They come in two lengths, and they also come with a different number of holes in their tube. Be sure your #4 pilot jet matches the others exactly. Needle jets can be confused in this way too. Use only Mikuni jets, not DJ or some other brand.