Two Cylinders not firing?

[BlizzakeV0]

Hello, I am just trying to ask some thoughts on my roommates non-gs bike :P Cuz we are all out of ideas...

He owns a newer yamaha and two cylinders are not firing, they are the two cylinders opposite the kick stand (maybe 3, and 4?) so they are not attatched to the same coil. And this has only started to happen recently.

He can get it started, but it runs poorly for a minute or two then floods out and stalls. He can get it going for around ten minutes if he keep the revs up higher, like 6-7k.

He also dumped some seafoam into it to try to shake things loose a bit. And while it's running there a bunch of white-ish smoke coming out, so we think that is from the seafoam.

We have ruled out a bunch of issues, but we tend to think it's carbs. He would have just taken those guys off and cleaned em (they were rebuilt around a year ago) but his engine design is stupid and taking the carbs off requires him to drop the engine (dumb right?).

So I am just looking for a yay or nay on the carb issue so that he doesn't have to spend 1-2 days taking apart his bike if that's not even the issue.

Any thoughts would help! Thanks!

[tom203]

Help us out here- what yamaha?? inline 4 or someother layout ? each cylinder have its own carb? Not firing? By not firing, you mean two cylinders not getting hot, Have you checked for spark on those plugs? Doublecheck plug wires going to correct plugs- easy mistake to overlook.
The white smoke is from the Seafoam, so don't panic-

[BlizzakeV0]

Yeah, two cylinders not getting hot, and the bike is whatever was before the r6, yzf6r, or some other letters of the alphabet. Inline four with four carbs. The plugs and wires have been checked, along with the coils, and the fuel pump.

[MisterCinders]

Are the plugs for 3 and 4 wet or dry after you run the bike?

[BlizzakeV0]

OOo, that would be a good one to check. What if it is wet or dry? Dry means no gas coming in through carbs, so carb problem? Wet means too much gas or not enough spark, so electrical or carb problem?

[MisterCinders]

OOo, that would be a good one to check. What if it is wet or dry? Dry means no gas coming in through carbs, so carb problem? Wet means too much gas or not enough spark, so electrical or carb problem?

Got it in one.

Remember, each cylinder needs 4 things to fire (1) air; (2) fuel; (3) compression; and (4) spark.

For those two cylinders to fail, you probably do not have a spark problem, or if you do, it is a very odd coincidence (bad plugs/wires on two cylinders). Check to make sure the leads are on the right plugs though. If you crossed wires, you might have spark at the wrong time on those cylinders, a bad coil, botched timing, etc. Those problems generally appear in pairs but not the pair you are struggling with (i.e., 1/4 or 2/3 -- not 3/4).

If someone rebuilt the carbs and assembled them in the wrong positions, it might block fuel from one side. Don't ask me how I know this .

As difficult as you say the carb work is (dropping the engine? really?), that may not be it.

Good luck.

[BlizzakeV0]

Thanks! yeah he said he had to drop the engine to get carbs off. I don't know if I believe it, but I didn't want to ask any more questions :P