Firing on 3?

[Milts]

Long story, wondering if anyone can point me in which direction to go next?

Bought a 1998 Honda CBR900RR project bike (bent forks etc). Finally got to the point where I want it to run, and it only cylinders 1 & 2 go with intermittent firing on #4.

So far I have:
Checked fuel etc is flowing fine (obviously it is, first 2 cylinders go OK)
Stripped and cleaned the carbs. Everything in them looks fine to me (no expert here). Clear holes through all the jets.
Replaced all 4 spark plugs.
Tested the spark on 3 & 4 - looks good.
Shop has checked the compression and says all is fine.

What am I missing here? Can anyone advise as to my next step?

[T.W.R]

Tested the spark on 3 & 4 - looks good.

What do you mean looks good?

yellow or blue spark?

Might be sparking ok when the plugs are sitting free but could be breaking down underload. Check the primary & secondary resistance of the coil should be somewhere around 1.8-2.8ohms(primary) & 10-16 k-ohms (secondary), also arcing distance of the spark should be able to arc out at 7mm or more.

Check the pick-up coil too as seeing the bike has had a bin it could've taken a decent thump on that side of the motor

[Drew]

What do you mean looks good?

yellow or blue spark?

Might be sparking ok when the plugs are sitting free but could be breaking down underload. Check the primary & secondary resistance of the coil should be somewhere around 1.8-2.8ohms(primary) & 10-16 k-ohms (secondary), also arcing distance of the spark should be able to arc out at 7mm or more.

Check the pick-up coil too as seeing the bike has had a bin it could've taken a decent thump on that side of the motor

That's all helpful, but not likely to be the problem.

1 and 2 run on different coils. If #1 sparks, then #4 which runs off the same coil should spark if the HT lead isn't fucked.

Did you pull the jets out of the carbs completely? Clear hole right through is all well and good, but the little holes in the emulsion tube need to be clean too. So ya need to take out the bit with the hex head, as well as the jet (with the flat head screw head) and clean 'em up real nice.

The pilot galleries may also be full o' crap. So some softish thin wire (I use copper for this) wiggled around up there, a can of carb/brake clean, and a compressor are your friend.

Good luck, be fastidious.

[Milts]

That's all helpful, but not likely to be the problem.

1 and 2 run on different coils. If #1 sparks, then #4 which runs off the same coil should spark if the HT lead isn't fucked.

Did you pull the jets out of the carbs completely? Clear hole right through is all well and good, but the little holes in the emulsion tube need to be clean too. So ya need to take out the bit with the hex head, as well as the jet (with the flat head screw head) and clean 'em up real nice.

The pilot galleries may also be full o' crap. So some softish thin wire (I use copper for this) wiggled around up there, a can of carb/brake clean, and a compressor are your friend.

Good luck, be fastidious.

Cheers - I'm wondering if maybe I wasn't fastidious enough. I did pull all the jets out, drop them in a jar of carb cleaner for 10 minutes, and blasted them with compressed air. Might have missed a bit though so that may be the next step if it doesn't come right.

Can't think what else it could be, if it's getting a spark & the carbs seem OK?

And yeah I'd be surprised if it's the coil or plugs, given that plugs are new and as you say coil 1 = cylinder 1 & 4, coil 2 = cylinder 2 & 3.

[T.W.R]

That's all helpful, but not likely to be the problem.

1 and 2 run on different coils. If #1 sparks, then #4 which runs off the same coil should spark if the HT lead isn't fucked.

And yeah I'd be surprised if it's the coil or plugs, given that plugs are new and as you say coil 1 = cylinder 1 & 4, coil 2 = cylinder 2 & 3.

See Drew, Milts does know how to read the ID colletts on the HT leads

Impact damage does interesting things to electrical items that are sitting static so exploring every avenue to remedy a problem is warrented.

if it's getting a spark & the carbs seem OK?.

Getting spark when sitting in the open doesn't mean jack

[Katman]

I did pull all the jets out, drop them in a jar of carb cleaner for 10 minutes, and blasted them with compressed air.

It's not enough to just blow air through the jets.

The galleries inside the carb body need to be checked that they are free off obstruction.

Thing is though, when you blow air in one hole you need to know where to expect to feel air coming out the other end.

[AllanB]

Thing is though, when you blow air in one hole you need to know where to expect to feel air coming out the other end.

ah shit - I'll just leave that one alone I think ......... too easy!

[kiwi cowboy]

Cheers - I'm wondering if maybe I wasn't fastidious enough. I did pull all the jets out, drop them in a jar of carb cleaner for 10 minutes, and blasted them with compressed air. Might have missed a bit though so that may be the next step if it doesn't come right.

Can't think what else it could be, if it's getting a spark & the carbs seem OK?

And yeah I'd be surprised if it's the coil or plugs, given that plugs are new and as you say coil 1 = cylinder 1 & 4, coil 2 = cylinder 2 & 3.

I've posted on KB before about soaking the carbs in white vinegar and it works a treat

[matrox02]

I would pull the HT lead out, Wrap the stalk that goes into the head with a bit of insulation tape,and see if that is making any difference, I wouldn't be surprised if you had some insulation break down on the stalk inside the head, had the same on my CBR250 and it doesnt last all too long, but does help with diagnostics

[Milts]

FIXED! Bike now running mint on all four...

Pulled the carbs out and cleaned the 2 that weren't going again. Left the jets to soak overnight, was ridiculously careful about saturating and then blasting every hole in the carb bodies too. Put it on the bike, no go.
Left it a few days, charged the battery I'd been using to start it, and fired easily... after a few minutes running and some revs, it doesn't need the choke, idles fine, and revs well

Long painful process. But it's nearly there! Just got to sort the front brakes... and the throttle cable (sticky as hell and no idea why).... also put all the bits back in the subframe. But soon!

[Icemaestro]

what did you soak the jets in out of interest? vinegar like suggested earlier?;-P

[Milts]

Nah I still had half a can of carb cleaner sitting around, so sprayed it into a jar and sat them in that...