Rear cylinder won't fire...

[Dave T.]

My V won't fire the rear cylinder. At idle it sounds like only one cylinder is firing.

Things that are done: carbs sync'ed (18" of mercury at idle), new spark plugs gapped correctly, 110-120 psi compression, new standard piston rings.

When I let off the throttle at rpm the rear pipe spits out flame and sometimes backfires. I pulled plug and it is firing with a nice orange spark. I also turned the idle mix screw in and out to get the fastest idle (on the rear cylinder only).

Any suggestions for me anyone?  :P

[Lucky]

Flames..,COOL!!  just kidding.
some stuff to chew on:

first, your spark should be blue, not orange.  check to see if you have a blue spark up front.

how did the plug look? wet, sooty, oily, etc, what color?

when the carbs were sync'ed was it 18" on BOTH cylinders? they should match.  you want them even, not nessacarily high.  it their both 10" thats ok, for example, as long as their about even.

--Lucky

[Al Shipperbottom]

Check out your Yamaha Vision resources first. But it does sound like your TCI ignitor module. It will probably be a bad transitor that fires the rear cylinder. It may look ok once you follow instructions on how to open it up(you will find this info in the vision ignition FAQ). The transistor may appear to be ok but it can be really corroded at the solder joint and one of the terminals may just fall apart.

[Lucky]

That had occured to me, but he has spark, it's just weak...

[Dave T.]

Well, I gave the V a second shot, no dice yet. I pulled the rear carb top off and looked for gunk in the rear carb tank, looks OK. I ohm'd out the rear coil and got close to the Haynes manual specs. I set the rear pilot at 2.25 turns out. After pulling the carb top I poured in some carb cleaner and then the junk started blowing out the rear cylinder. I pulled the plug and it was carbon fouled. Before it just looked like it wasn't firing (like you pulled it out of the box). I cleaned it with Yamaha contact cleaner.

I did have 18" at both cylinders (the purpose of syncin', after I synced them). I haven't checked for orange spark. I will get a new plug and try again. I also have exhaust leaking from the rear cylinder where my MAC's connect to the piece that goes to the cylinder. I tried to tighten them to the max but they are still leaking. This may be why there is less exhaust at the outlet compared to the front cylinder.

I do have a spare TCI module and I will swap and see what happens. If these don't work, I'm gonna check the valve clearances again and go through the valve timing. I may have a stretched cam chain, leaking rings (I did have a scratch on my cylinder after honing that they didn't get out, I ordered oversized pistons and rings, but I found out they were not make anymore!), leaky valves (I just had a valve job!)

I have been near the end before with my V and came out with an acceptable result. I hope I can go 2 for 2!  :(

[QBS]

Stop!  Before you go any further make certain that you have clean, excellent and bright and shiney spark coil connections.  Your exhaust leaks are probably caused by exhaust cut/eaten copper sealing rings and/or exhaust cut/eaten steel exhaust discs.  If this is so, no amount of tightening will fix the problem.  The only fix is new parts which are still available from Yamaha.  Cheers.

[mbaur]

I also have a small leak where the rear header pipes connect to the Y piece. To get this straightened out I just ordered new copper gaskets and they must be made out of some gold / platinum alloy judging from their price: $10.94 a piece  :o

But I really doubt that this has anything to do with your rear cylinder. My V ran just fine without exhaust at all. A bit on the loud side though ;)

- Markus

[jasonm.]

The pilot screws work best somewhere between 3.5 and 5 turns out. 2.5 is lean. If you have good spark , you may have a carb problem or a cam timing error..