Curvy rear cylinder cuts out.
 

I have a 1999 sv650s with a problem. after a few miles the rear cylinder
cuts out and the fuel consumption rises alarmingly. I have changed the rear carb, removed the rear head to check the valves, no marks or damage, replaced the coil, replaced the igniter, even checked the wiring, no breaks.
But it still cuts out. any body come across this before.
after a while it will restart ok.

Re: rear cylinder cuts out
 

How have you determined that it's the rear cylinder cutting out?

Have you been able to check for spark at the time it cuts out?

Jambo

Re: Curvy rear cylinder cuts out.
 

By the time I get the seat off and the tank lifted the plug is sparking.
and the bike runs OK. Also rear plug changed, even if I let the bike run
and get the engine mad hot (water out of the radiator overflow)
it will not fail. Only when being ridden.

Re: Curvy rear cylinder cuts out.
 

Missed one question, rear cylinder exhaust pipe cool,front cylinder hot

Re: Curvy rear cylinder cuts out.
 

Sounds more like a fuel starvation problem to me...especially if it runs again after a short time...

Re: Curvy rear cylinder cuts out.
 

OK,
I've seen a couple of similar issues before. You have an intermittent fault, which, as you've discovered can be a bit of a pain to find a solution to.

You say you've swapped the rear carb over, have you replaced the cab bodies entirely (the pair) to a known good set?
How quickly does the bike get back onto 2 cylinders after you stop? If you slow down will it come back in, or is it a case of once it's cut out, you need to stop and let the bike cool for it to come back in?

My understanding is you've replaced:
Rear coil, rear spark plug, ignition control unit. Did these all come from a properly working bike?

If you carry a spare spark plug would you be able to pop the rear HT lead off and check for spark before the bike started working again?

Jambo

Re: Curvy rear cylinder cuts out.
 

It's not unusual for rear coils to fail as they get quite hot, but you say you've changed that. Is the replacement okay?

Re: Curvy rear cylinder cuts out.
 

I replaced the complete read carb for a known good one same for the rear coil, igniter etc.
when it cuts out it does not come back if I slow down.
One thing it did do though was if I wound the throttle open it came back
in at about 90MPH in top (6500 revs). I am thinking sticking carb slide.
By the time I have the seat and tank off it is running OK again

Re: Curvy rear cylinder cuts out.
 

When you say you've replaced the rear carb, they come as a pair.
They are bolted together.
have you swapped for a pair of known good carbs?

I think the first thing to try an establish is whether it's fuel or electrical. Where are you based?

Jambo

Re: Curvy rear cylinder cuts out.
 

If it was a sticking carb slide I can't see it doing the same thing with two different carbs.