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top end misfire

Old 05-21-2013, 03:20 AM
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top end misfire

Hi all, great to be here.

I have an issue with my 98 VTR.

It misfires in every gear at anything over 6500rpm. Doesn't completely cut out, just sputters and pops, until you hook another gear.
I've had the carbs out and cleaned them all out. They looked ok with no gunge or anything obvious. Slides move evenly and smoothly.
I think the petcock diaphragm is good. I had a suck on the vacuum pipe and my tongue stuck to it!
All pipes are in the correct place on the tank i think.
Bike starts from cold no problem and idles well at 1100rpm.
It runs really good and strong up to 6500, with no flat spots or any other hiccups.
I pulled the spark plugs, they are the correct NGK iridium ones. They are both the perfect shade of brown indicating the mixture is good. No signs of weak mixture on unburnt fuel etc.
I noticed the plugs looked like they'd been in there a while, and the gap seemed quite wide. I closed them up to 0.8mm and it seemed to improve things a little bit.
Could this simply be a plug issue? I thought those iridium plugs last for years...
I'm sure its something electrical as the bike runs so well at the bottom and mid range.
Anyone else had this issue? Please help, its doing my head in!!
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Old 05-21-2013, 08:04 AM
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Had you done anything prior to the onset of the problem?
Has the bike ever run properly for you?
Does it misfire at all throttle positions?
Stock air filter?
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Old 05-21-2013, 08:26 AM
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Originally Posted by RCVTR View Post
Stock air filter?
+1.

Also, may try bypassing the kickstand switch. Sometimes at certain rpm's will vibrate and cut out the motor for a second. Probably not the case but it's easy to try.

You check the resistance on your coils?

Also, don't fully count fueling out. At higher RPM's under load it's swallowing the most gas, so you may be getting plenty of delivery at the lower ones but once it needs the full flow it's running out.
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Old 05-21-2013, 09:45 AM
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If you had to re-gap the plugs then they are worn out.

Any gap bigger than .040" is an indication the plugs need to be replaced.
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Old 05-23-2013, 11:39 PM
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Thanks for the reply's

Yes, it seems to misfire at that rpm no matter what the throttle position.

Sidestand switch is fine and not causing an issue.

Bike has always had the misfire since i bought it. It has improved a little with the cleaning and maintenance i have done, but i still haven't found the root cause.

Bike has a K+N filter and Scorpion end cans, both of which would appear to have been fitted some time ago, so i don't think it has anything to do with this particular fault. I don't like K+N's though (more harm than good) so i will be going back to a stock filter asap.

Haven't measured coils yet. How do you do that?? Multimeter?

Worth noting that the bike misfires at the same rpm even in neutral, with no load on the engine.....
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Old 05-27-2013, 01:20 PM
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Update:

New plugs fitted.
This has improved things a fair bit. The misfire is not as severe, but is still there.
Any advice about measuring the coils??
Are they considered problematic on the VTR?
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