1990 FZR250 misfiring

[Driftmonkey]

Picked up a bit of a mistreated 3LN FZR a few days ago. 1990 with 40,000kms Been fiddling with It for the past few days trying to sort it out but it starts misfiring REALLY badly at about 13,000rpm. Runs beautifully all the way up until this point. Doesn't do it as bad in higher gears.

Ran a tank of 98 with a fair amount of carb/injector cleaner through it, checked plugs, cleaned tank out, charged battery up, and done a tonne of tidying up cleaning around the bike but I am stumped. I've done a bit of searching and no one seems to come to a definitive answer and I don't have the means to go out and buy a few new coils tcis and new sets of carbs with no certainty the problem will be fixed.

TLDR; typical 'help thread', I need to know an elimination sort of process to pinpoint the problem.

[Driftmonkey]

EXUP is operating as it should as is the fuel pump.

[kiwi-on-wheels]

Check your earths, i had one break off on my cbr and it wouldnt rev over 10k. Also ran like s**t.

Might be worth running a couple of new earths?

[Driftmonkey]

Read somewhere while searching that spraying water on the extractors could give an indication of which cylinders are misfiring. So did that and the water dissipated instantly from cyl 1 and 2. 3 and 4 took a while with 3rd taken the longest. gunked up carbs not supplying enough fuel at top end? will sort some new earth's out on the mean time. Cheers for suggestion.

trying to avoid taking the carbs out cause I know there's going to be perished rubbers that'll need replacing all over the show.. why did I buy an old bike..

[FJRider]

Read somewhere while searching that spraying water on the extractors could give an indication of which cylinders are misfiring. So did that and the water dissipated instantly from cyl 1 and 2. 3 and 4 took a while with 3rd taken the longest. gunked up carbs not supplying enough fuel at top end? will sort some new earth's out on the mean time. Cheers for suggestion.

trying to avoid taking the carbs out cause I know there's going to be perished rubbers that'll need replacing all over the show.. why did I buy an old bike..

Rearrange the same plugs in different cylinders. Then do it again. If the observation follows the plugs ...

[Driftmonkey]

Pretty much isolated the problem to cyl #3 and #4. Pulled leads from each cylinder and the bike died when 1 and 2 were pulled, bike ran still but noise changed a bit when 3 and 4 were pulled. Will get new plugs asap, least that'll cross one thing off the list.

[Driftmonkey]

[FJRider]

Pretty much isolated the problem to cyl #3 and #4. Pulled leads from each cylinder and the bike died when 1 and 2 were pulled, bike ran still but noise changed a bit when 3 and 4 were pulled. Will get new plugs asap, least that'll cross one thing off the list.

If you have two coils ... one will cover 1 - 3 ... the other 2 - 4. Possibly a short in the leads from 3 - 4. If it died with 1 & 2 pulled ... 1 & 2 were the good ones.

[Driftmonkey]

Got home later after work and decided i had enough of the misfiring. straight away pulled carbs out. Needles in carb 3 and 4 were blocked. Along with the 3rd spark plug being finger tight. Just in the process of putting back together crossing my finger and toes that it'll go now. Fair amount of junk in the carbs but it's cleaned out and it idles much better. Here's hoping. Tank and fairings left to put on

[Driftmonkey]

STOKED.

[mossy1200]

STROKED IT.

Good for you. Now get back to tuning your bike.

[Driftmonkey]

Good for you. Now get back to tuning your bike.

Hahaha, ya ya. All done now after sitting around for a good while with a grin on my face So I guess water on the extractors really DOES give you an idea of where to start looking for problems.
What blew my mind was that the carbs weren't even hard to take out. Was only a 2 hour job pulling fairings tank carbs cleaning reassembling and I'm no bike guru by ANY means.. clearly.


For everyone elses sake that might come across this thread (because there is so many bloody threads on this sort of shit but no one updates with a solution). My solution was a combination of things

• Switch to 91 octane (was using 98 .. )
• Remove carbs and clean needles
• remove and clean sparkplugs
• flush tank
• disconnected, sanded surface and reconnected earths + added two more

[ducatilover]

If you have two coils ... one will cover 1 - 3 ... the other 2 - 4. Possibly a short in the leads from 3 - 4. If it died with 1 & 2 pulled ... 1 & 2 were the good ones.

Noooooo FJ, ya silly ol' bugger Cyls 1&4, cyls 2&3 are pairs.

STOKED.

Good to see you sorted it out.
Don't run it on 91 though.

[Driftmonkey]

Noooooo FJ, ya silly ol' bugger Cyls 1&4, cyls 2&3 are pairs.


Good to see you sorted it out.
Don't run it on 91 though.

So I should continue to use 98? I stopped by Botany Honda motorcycles for a little bit of advice and they said the highest octane rating out when my bike was made was 87 octane, and that I want to use a fuel as close to that as possible.. correct orrr

[ducatilover]

Our octane ratings are different to the ones used overseas. Japanese fuels are much higher quality. Hence why most 45hp 250s do not manage to make the 45hp they once did when they come to NZ.
I would run 98, it is more detonation resistant, and with a carboned up 250/4 pulling 18,000rpm, you need the best fuel you can get.

I would not listen to anyone who gives such fucking stupid advice.