#4 Cylinder Dead - Can't find Cause

[Brendan W]

I have seen plugs that fired in the open but wouldn't in the cylinder but it wasn't the plugs fault. Generally it's down to a weak spark.You could try pulling #1 and comparing the strenght of the spark - it should be 'fat'. I'd try running it in the dark with the tank off looking for escaping sparks along the ht lead. I don't know if it's possible to swap the ht leads on that coil but you should be able to get the longer one to the short side and see if it still runs on three if you get my drift.

[WingMan71]

No one is going to believe that this works, but I had a similar problem. Carbs spotless. Could not get #4 to fire. So I knew I had spark. I knew I had air. Here comes the part no one likes... Pour 2 teaspoons of raw gas down the spark plug of #4 and put plug in and connect wire. Start the bike and see if it runs a little using #4. I must of missed a little piece of something in that carb, because as soon as I got it to run on all four, it started pulling fuel in. Bike is still in use in my race bike 2 years later...
Curt

Curt,

I'm considering doing something similar, but perhaps a bit safer.

Thought I'd squirt some starter fluid into the vacuum port on the #4 intake boot and see if she fires.

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[tkent02]

The plug wires are long enough, you don't have to take anything apart. Just pull it through to the other side, just wire #1 to the #4 plug and wire #4 going to the #1 spark plug. This takes less than a minute, and will tell you once and for all if it is an ignition or a fuel problem. If the problem goes over to #1 it's ignition. If it stays in #4 it's not.

[WingMan71]

I have seen plugs that fired in the open but wouldn't in the cylinder but it wasn't the plugs fault. Generally it's down to a weak spark.You could try pulling #1 and comparing the strenght of the spark - it should be 'fat'. I'd try running it in the dark with the tank off looking for escaping sparks along the ht lead. I don't know if it's possible to swap the ht leads on that coil but you should be able to get the longer one to the short side and see if it still runs on three if you get my drift.

Brendan,

I haven't yet, but I intend to swap the #4 and #1 spark plug wires and see if the symptom moves over to #1 cylinder. If so, bad plug wire.

Thanks.

[barnbiketom]

I just thought about something...I use a propane enricher tool for vac leaks. you could try propane too as an aid to finding out if #4 is LEAN/no fuel....but discaimer here . I have the right tool but have also used a burn-zo-matic torch on occasions.. but using a torch you can not turn it all the way on.. just crack it open UNLIT to get a small flow.. then maybe let it suck into the sync hole? just another idea here...don't blow yourself up...
I always suspect carbs first but in your case.. ??? if that does nothing, after eliminating spark, get a compression gage!

[Brendan W]

Something like this?

[WingMan71]

The plug wires are long enough, you don't have to take anything apart. Just pull it through to the other side, just wire #1 to the #4 plug and wire #4 going to the #1 spark plug. This takes less than a minute, and will tell you once and for all if it is an ignition or a fuel problem. If the problem goes over to #1 it's ignition. If it stays in #4 it's not.

Did not realize that they were long enough to pull across to the other side.

If so, that makes it easy.

Thanks!

[tkent02]

Something like this?

[WingMan71]

#4 is alive ! ! !

#4 cylinder is ALIVE ! ! !

As suggested a number of times, I swapped the #4 and #1 plug wires.

As soon as I fired it up I knew something was different since I was getting some blue smoke out of the right side exhaust that I had not seen before. (I had squirted some engine oil into the cylinders through the spark plug holes before trying to start the bike the first time.)

So, I figured that with #4 running, #1 cylinder now had to be dead (if it was a spark problem). But no, #1 cylinder was also firing, WHICH MADE NO SENSE AT ALL.

The only thing I can figure is that the last time I ran the bike I used the old carb mechanic's trick of tapping on the side of the fuel bowl with a plastic screwdriver handle in an attempt to loosen a stuck float and/or needle valve.

The only thing I can figure is that the needle valve in #4 was stuck from sitting dry so long after the rebuild and the tapping finally freed it.

I still need to get the valves adjusted, but for now I think I've got all 4 cylinders running.

Thanks everyone for all the help and advice chasing down this dead cylinder.

***

[bellucci]

Big smiles all over the forum...
Curt

[SocialAnomaly99]

Nice! Glad to hear it!

[barnbiketom]

awesome! I've had a number of carbs stay dry after a refresh.. dunno why it happens but it does... once they get gas , all is well!!
CONGRATS.. hope the valves are right on baby!!

[Mutashio]

Hey guys, in an attempt to stir up an old thread I'm having basically the same issues with #4 cylinder except that regardless of which boot I use #1 or #4 it does the same thing. With the boot fully on #4 doesnt fire, if I pull the boot off and just place it on top of the spark plug the cylinder starts to fire! It's almost like something is shorting out my ignition coil??? Any thoughts?

[Rich82GS750TZ]

Spitballing hear based on what I’ve read. Bike originally had resistor caps (I think). Original spark plugs were non-resistor. If you’ve replaced plugs with the more commonly available these days resistor type, you could have 1 too many resistor on a given line?

[Mutashio]

Spitballing hear based on what I’ve read. Bike originally had resistor caps (I think). Original spark plugs were non-resistor. If you’ve replaced plugs with the more commonly available these days resistor type, you could have 1 too many resistor on a given line?

I'm not sure if i follow you...So a little background, the bike had the original ignition coils on when i got it, which i have had issues with over the last 2 years. So last year i did the coil relay mod and now get a full 12v to the coils. Then a couple weeks ago i picked up a new set of coils off amazon, i know someone is going to give me grief about it but yes they are cheapies from china...although it doesnt seem like its a coil issue, as yesterday i went ahead and swapped the 1-4 coil back to the original and it does the exact same thing! I'm very confused how that could even happen unless its a spark plug issue, is that what your referring too @Rich82GS750TZ ?