[walladm]

I have a 1996 CBR F3 that is only hitting on cylinders 3 and 4. This all started after I did a compression test, before that, the bike was running great. (Why did I have to mess with something that was working well? ... another topic) So everything was fine, I pulled the plugs, checked the compression (which was all even), then replaced the plugs and attempted to start the bike. At that point, I only had #3 and 4 generating heat.

The spark plugs, tested out of the engine, all seem to work normal on their respective wires, although they seem a little weak. (I don't have Honda experience to compare them to..) I have compression. It's getting gas, but I'm not getting power from #1 or #2. I removed the exhaust manifold, among other things, in an attempt to figure this out. Occasionally I'll get a backfire on 1 or 2, which means there's spark at some point, but at startup they will blow out unburned fuel mist.

Here are the items I've checked:
-Flipped spark plugs from the working cylinders to the dead cylinders, no change.
-Flipped ignition coils, no change. (one coil fires #1 & #3, the other fires #2 & #4, so it really shouldn't have an effect anyway, since #1 and #2 aren't hitting.)
-All resistance tests on the coils are normal, including the plug wires
-The pulse generator has normal resistance

I know cylinders 1 and 2 are getting gas, because the plugs are damp after I run the engine on the two working cylinders.

What could I have knocked while pulling the plugs that would cause this? I have unplugged and cleaned most of the wire connectors for engine management.

Is it possible that I fried the spark unit when I tested the compression? Would a wanky unit cause two cylinders to drop out but fire the other two correctly?

The past few days have been really frustrating, and I'm at a loss of how to approach this repair. I GREATLY APPRECIATE ANY IDEAS!

[Ammocbr]

I would just ride it...It will still outrun a Harley. j/k.

How much compression are you getting on those cylinders since you did the initial check. Has it gone significantly down?

Could something have dropped in the cylinders when you had the plugs out?

Did something crossthread when you were putting in plugs or gauges?

Good luck.

[denzee]

You said replaced the plugs as in new? If you didn't already, replace the plugs with brand new ones gapped properly and try again. Are the plug wires numbered? Check the firing order again after putting the coils back where they originally were so as not to confuse things. Do this and get back to us. Good luck walladm.

[walladm]

Thanks for your replies... To answer a few new questions:

I did test the compression after the problem, and all cylinders were pretty even, everything was over 175 psi, just a hair shy of 180.

I have not replaced the plugs, however I have flipped plugs from the good cylinders to the bad cylinders several times without luck.

After reading other forum postings, I am wondering about the possibility that I flooded a vacuum line, or that there is some sort of vacuum problem that is causing cylinders 1 and 2 to just flood out. Does that sound possible? My Clymer manual is pretty vague about what type of connection exists between carburetor 1 and 2. Any ideas there?

When I was cranking to do the compression test the fuel pump would have been running...

Here are a few of the posts I am referring to :
Cylinder not firing
and on another cbr forum...
Not running on all cylinders!


Work is going to prohibit much tinkering until Sunday. I want to take a fresh look at it and start from the beginning (again..) in a few days. Thank you for the ideas.

[denzee]

I think those threads refer to fuel injected bikes. Yours is carbs correct? Plus you said it was fine until you worked on it. Does the problem change at all when you put the choke on while running?. To get it straight walladm, the misfiring cylinders remain at 1 and 2 even when you switch everything (coils, plugs, wires etc.) around. If you had the tank off doing your work, how about fuel supply to cyl's 1 and 2. Line kinked or floats sticking?

[walladm]

So after last night's post i went back and tried to look at the bike with new eyes, starting with the basics. I'm glad, and somewhat embarrassed, to admit that I solved the riddle of how you can have spark, fuel and compression without proper ignition: backwards spark plug wires to plugs 1 and 2. The really embarrassing part is that I labeled the plug wires wrong, and kept putting them back in the same locations. For whatever reason, I had in my head one coil fired cyls 1 and 3, then the other coil 2 and 4. Actually, the left coil fires cyls 2 and 3 while the other fires 1 and 4. ugh.

At least it's a cheap fix and I had a good laugh when I figured it out.

[denzee]

Even the best mess up walladm. No biggy!