Intermittent #2 Cylinder

[Commodus]

Hi, I have what is apparently a common problem...I see several other threads on this, but none of them seem to contain my solution.

The #2 cylinder on my 1982 Suzuki GSX750S Katana is only firing intermittently. It will fire, not fire, sometimes back-fire a little; sending a little tongue of flame up the vacuum line. When removed, the plug is black and wet.

Things I know:

Compression is OK.

Petcock is OK.

Spark is OK.

So that leaves carbs right? Well, I've partially dismantled the problem carburetor and sent compressed cleaning fluid through the circuits, it seems to go everywhere you would expect it - I can't find a clogged circuit. I initially just assumed the pilot fuel jet was clogged because I knew I had some rust in the tank and was running without a fuel filtre briefly. I was so sure that's what it was, I went ahead and Kreemed the tank and installed the filtre - so clean gas. When I dismantled the carb, however, I could find no trace of blockage...also it happens regardless of fuel position, which seems strange to me. All circuits are apparently suffering the same problem.

Am I missing something really simple here? I sure hope so! I even went ahead and leaned/richened the pilot air screw apparently without change, but since it's intermittent...

I do notice that it seems to run more consistently when I have the choke on? So I guess that means it's somehow way too rich...

Help me out!

Thanks in advance!

[chiphead]

If it runs better with the choke on, that means you are running lean. Using the choke richens the mixture. Perhaps spray for vacuum leaks.

[tfb]

Anytime ANYTHING is up with #2 cylinder, the first thing you should suspect is the fuel tap, or the vacuum line that runs from the rear of the tap to #2 manifold.

If it is sucking air, this will cause it to run lean on that cylinder. Check the vacuum line for any splits, cracks, holes etc.

Co-incidentally, it could be a leaking manifold on #2. As per chiphead's advice, spray to detect any leaks.

[brveagle]

I'm gonna go in the opposite direction you are thinking and say that it's your coil/plug/wires.

If the bike was running lean on #2, it'd be white and dry.. not black and wet. I think you're losing spark and causing it to foil the plug. The backfire is unused fuel exiting the system.

[Commodus]

Okey doke, I'll check the vacuum line and the intake. The petcock seems to be working okay, it flows on prime and stops on run...is there anything else to check?

And yes, of course, running with the choke on would indicate too lean, thanks for that correction. So that would point to a possible vacuum leak...

Thanks for the responses guys!

[tfb]

If the bike was running lean on #2, it'd be white and dry.. not black and wet.

Good call, brveagle. Always pays to take all of the info. in hand!

[Commodus]

Ok, well no vacuum leak. I imagine spraying starter fluid about the intakes would tell me if there was one, right?

So too rich or too lean? I guess it has to be too rich if the plug is wet...

So confused! I think I may admit defeat on this one. I have an appointment at my local bike shop where my excellent mate and mechanic will solve the problem for me. I'll make sure I post the solution on here for you all to peruse, add it to the knowledge base, so to speak!

[Commodus]

OK, so the tech found that the needle valve orifice was dirty, and perhaps interfering with its operation and that was maybe causing it to flood out...borne out by the fact that the float bowl did run over briefly while we were working on it.

So, cleaned it out (along with the rest of the carburetor) and bolted it back up...presto it runs! On all four!

Except by the time I got it home (and after letting it sit overnight) I had exactly the same problem again. So, still fixated on this needle valve issue I decided to replace it, as the plug was still showing wet (though the bowl was not leaking, so I suspected I was just busying myself).

No change, obviously.

Anyone have any ideas? It's going back to the shop on Monday...

[txironhead]

This is just a guess, but it might be a valve seal leaking oil into the cylinder. Don't know exactly how to check that, though.....

[tateG]

Have you checked the spark plug boot? I had my #1 cylinder start fouling on me, started intermittently, then just wouldn't fire at all. I pulled back the rubber boot that covers where the wire connects to the hard plastic connector and the end fell off. I screwed it back into the wire and my problem was solved. May not be your problem, but worth checking.

[Commodus]

Both these last suggestions are excellent!

The valve one is probably where I am heading my troubleshooting efforts next, as I have mostly exhausted the carbs.

I have indeed checked the spark plug boot and have even replaced it and did find some corrosion (sort of a greyish/greenish powdery stuff) around the connection from the boot to the wire. So I cut the wire a little higher to get a nice clean connection and applied some dialectric grease and that did certainly make for an even stronger spark. I will be doing this to the rest of the cylinders when I get this problem fixed!

[SqDancerLynn1]

Two sugestions Replace the O ring on the float needle seat if you haven't done so. Next PLUG the vacuumk fitting on the #2 carb & use the petcock on prime for a test ride. DON'T forget to turn it back to the ON position when done. I would still bet the petcock is bad

[Commodus]

Nope, the petcock is OK and the float valves are workin (and sealin) just fine...well, now anyways.

The problem turned out to be a coil...

It wasn't grounding properly - so of course when I removed the plug to check the spark, I grounded it to the head and fixed the problem! Until I reconnected it that is!

So I guess the threads into the frame weren't clean enough, or something, after the powdercoating. So we just ran some grounds directly to the head and no more problem!

Well, OK, a few problems with the carbs, but they're well within my mechanical ability to sort out.

This one had me scratching my head for a long time!