Misfire below 7k rpm

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bluesmokeracer
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Misfire below 7k rpm

Post by bluesmokeracer » Sat Sep 08, 2018 11:04 am

All,

Having got my sons 1993 GSXR 400 SP running beautifully, I'm now trying to get his 1992 GSXR 400 running beautifully too!

I have:

Pulled carbs
Cleaned thoroughly
Replaced O-rings on mains, air screw, fuel feed (orange tree thingy)
Pulled slides - checked condition
Replaced 117.5mm mains with 100 mains (its an ex-track/race bike)
Checked pilots - 47.5
Reset needle clips to middle position

Went out for a run, after 5 miles, started to misfire between 2k rpm and 7k rpm, then it runs clean right into the red. Absolutely howls as it has a (too loud) race can.

Rode home. Left it 30 minutes. Started it up and went for a potter - ran clean from 2k to red line - no misfire.

Came back, left it an hour, went out for a proper run. Started to misfire between 2k and 7k rpm again.

Came back, pulled plugs. 1 and 4 sooty black, 2 and 3 normal. Suspect Low Tension side of coil failed.

Bought second hand genuine GSXR 400 coil. Fitted.

Started it up this morning - misfired between 2k rpm and 7k rpm from cold.

Any ideas? I do wonder whether there is an issue from air screw to pilot coming onto the main, hence it howls above 7k rpm.

Or maybe plugs are knackered? Going to fit a new set.

Its just such a pain in the backside pulling the carbs again. Wonder whether one pilot is blocked/crap is still circulating (the carbs were foul inside).

I had thought electrical because on cooling down it worked, and then misfired again after 3 miles. But it is mechanical - carbs?

I'm not prepared to pull parts of the 1993 model to test on the 1992. That's running so well I dare not touch it!

Thanks

moforockband
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Bike owned: GK73a SP CRM250 mk3
Re: Misfire below 7k rpm

Post by moforockband » Sat Sep 08, 2018 11:55 am

It does sound like plug failure to me, I managed to kill a new set in a year with the bike running too rich.
If it's breaking down when hot, it might be worth checking out the pickup coil, as the are notorious for this when hot and failing.
The o ring around the mixture screw must be the correct size and in sound condition also.
What's the compression readings if you have any?

bluesmokeracer
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Re: Misfire below 7k rpm

Post by bluesmokeracer » Sat Sep 08, 2018 2:28 pm

moforockband wrote:
Sat Sep 08, 2018 11:55 am
It does sound like plug failure to me, I managed to kill a new set in a year with the bike running too rich.
If it's breaking down when hot, it might be worth checking out the pickup coil, as the are notorious for this when hot and failing.
The o ring around the mixture screw must be the correct size and in sound condition also.
What's the compression readings if you have any?
Thanks

O-ring around mixture screws are genuine Suzuki and brand new.

Not got compression tester so sadly cannot test.

Pick up is possible, but I do get the sense it's not misfiring on all cylinders

I'll change the plugs with new and see if that fixes it.

Would be nice ...... !!

Thx

bluesmokeracer
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Re: Misfire below 7k rpm

Post by bluesmokeracer » Sat Sep 08, 2018 5:07 pm

I've replaced the plugs and it appears to be running nicely now - but I've only done 3 miles on it so far. Ran clean and crisp. It was peeing with rain, so my ride was limited!!

I've listed the bike for sale on eBay. Item number is 202430717865

Thx


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