I was riding yesterday, pulled into a bud's place. Time to leave, and bike won't start. Had been running like a champ--no missing, no pings, pulling strong. Plugs have looked good.

After trying to start for a while, fuel is pouring out the intake (Mikuni). Finally got it started, running on only the rear cylinder. Fuel now spraying and pouring out the carb intake and the exhaust port on the front cylinder.

Put a different plug (not new, but appears to be in good shape) on the front, same problem. Buddy puts a spark tester on, it says the spark is ok. Check the compression, it's 150 lbs plus.

We've got spark (according to the tester), fuel, air, compression--and only one cyclinder firing. WTF?

My buddy thinks intake leak--that doesn't make sense to me. Doesn't an intake leak make it run lean as opposed to not run at all?

Intake valve stuck or busted? We would have no compression?

I think it's got to be spark? Don't trust his tester, and we havn't verified visually. Is it possible that an induction tester might show power in the wire, but still not have fire at the bottom of the plug?

 

It may be showing power in the wire but it may be arcing to ground lower in the wire or at the plug. Checking the spark at the plug tip is about the only sure way to see if there is spark there. Don't take it for granted, check it there and save a lot of headaches.

 

fuel spraying and pouring out the intake? that doesn't sound like spark to me.
I would guess sticking intake valve too but then you wouldn't have the compression.
hmmmmmmm........
all things working correctly even if the front isn't firing it should still be pumping the air fuel through and out the exhaust shouldn't it?

 

Last night I installed a new Crank Position Sensor as those are a known problem with 2004 and 2005 Sporties. If anything, it made things worse. With my Daytona TC-88 ignition, I could get no spark at all, but when I installed the stock ignition module, I got sporadic and weak spark—but not enough to actually start the thing. Notice that the observed behavior is now different—I’m not getting good spark any more—with new plugs yet. But the engine is fully cold now too.

Tonight I played with it some more. I changed the plug wires from the pretty blue ones (Thundervolts from J&P) to the old ugly stock wires.

It started!! Now it ran ragged as hell, and I think it is getting actual ignition only on about every other cycle, but both cylinders are firing. Still quite a bit of fuel standoff outside the carb. I think the “back cylinder only” impression from the first day was the result of the front cylinder flooding out to the point where it did not ignite at all.

About all I’ve got left is the coil, even though I have been told they “rarely” fail. We shall see.

 

If the rear cylinder is running, you know that plug is good. Swap it for the front plug and see if the front fires and the rear cylinder doesn't. Do the same thing with plug wires if you have enough reach to swap them out. If the front is still not firing, my bet is on the ignition.

 

Problem resolved!

A case of several issues, both unlikely, masking each other.

Problem # 1: I removed the coil and had it tested. While the technician said that the resistance tested within parameters, it was very marginal. In short, it wasn’t officially bad, but it was very close to it. Given that the testing is done under very different conditions than typical riding conditions, I chose to replace it.

Re-assemble the bike to the point where I could start it. It starts. Both cylinders fire, reasonabley regularly.

BUT:

It smokes like a bitch. And fuel is still standing off outside the carb! Pig Rich. Hmm.

I find the old CV-40 carb, and slap it on to see how it runs. The bike fires right up, runs well, no smoke. Must be a problem with the Mikuni, eh?

I pull the bowl off the mikuni, check for dirt, check the float, pull the jets—just looking for something unusual. Nothing.

Pull the top off the carb, look inside, work the throttle. Whats that? A black screw, lying sideways, jammed up against the needle. The needle is about an inch higher than it should be as a result, and jammed in that position. As a result, fuel has just been pouring into the engine, at the equivalent of about 75% throttle—at idle.

Inside the Mikuni, there is a screw that anchors a pivoting assembly designed to hold the needle down. Apparently this screw came completely unscrewed, and jammed up against the needle, which prevented the needle from moving up and down with the slide. We eventually did manage to remove the jammed parts—with some difficulty. The needle was also seriously bent. We got everything replaced—with the screw tightened with blue lock-tite. Hopefully this will prevent a repeat.

After restoring all of the original parts, the bike once again runs smoothly.

I would like to thank all of those who helped with suggestions.

 

Stoked you figured it out. Good job at being persistant

 

Quick help I have a similar problem but my bike starts working after 1/4 throttle twist front cylinder kicks in is this the carb? Coil? Position sensor? Jets? Intake leak? Bike runs on one cylinder lots of spitting and sputter weak ignite light in front when it's idle

 

Been having same issue for better part of a year now. Front cylinder dead only at idle. If I take the plug wire off the coil u can't even tell a difference but it won't accel. Runs like a raped ape after about 1500+ but my god it hates going slow. Spits and sputters tries to rock u back and.fourth. Anyways it seems like its not burning the fuel in the front. Gets terrible fuel mileage. Also have the same carb as OP so I'm checking that first thing tomorrow.

 

you got fuel coming out of the carb? sounds like a stuck float. sounds like you need to rebuild the carb