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View Full Version : Need Help - 82 Honda CB900C Misfire Cylinders 3 and 4



Exit39
05-08-2016, 10:23 AM
I had my '82 CB900C on Ebay for a couple weeks; was asking a bit high to see who'd bite. Didn't sell, no big deal. At the end of the auction 2 weeks ago, I had a local friend become interested. So I haven't relisted it just yet.

However, as luck always has it now I have been battling a misfire in cylinders 4 and 3. Knowingly, one coil fires cylinders 1/4 and 2/3 fire on the other. On start up it will backfire and barely get running (won't stay running) on 2 cylinders, #1 and 2. Cylinders #3 and 4 are stone cold on the header pipe just out of the head.

Look funny? Not your typical coil gone bad seeing as it's one cylinder on one coil and one on the other coil. After the misfiring started which was about 2 weeks ago, I decided to upgrade the ignition system (something that was originally planned, but never accomplished). The original coils were weak at about 2.6-7 ohms. So I installed two new coils from Dyna Coil (3ohm) along with Dyna's 7mm wires and new NGK DE8' gapped at about .025, and two new CDI units from Procom. --- What happened? Nothing. Same misfire on the same cylinders, 4 and 3. I also switched the plugs at the CDI's and it's still the same two cylinders misfiring. Get this --- Spark is beautiful on all plugs when I test them by using a screwdriver to touch the rotor and the spark advancer together. One coil fires 1 and 4 together as it should, and the other fires 2 and 3 as it should. Strong spark from those new Dyna's. So spark seemingly is not the issue. They are not on the same coil, which is driving me nuts.

I also checked timing, static because I can't keep the bike running. Timing is spot on. Then I checked the air gap at the spark advancers; set at .4mm on both.

Cold compression tests were at 155psi across, +/- 7psi. Nothing wrong with that, and remember that's cold. No bent valves, etc.

Battery is strong, no issues there. Charging system is fine. I also checked resistance from the kill switch, that's fine.

Even though I rebuild the carbs I removed them again and thoroughly went through them paying particular attention to the pilot jet and pilot air jet. Those were fine and unclogged. Needle jet and mains are perfect and unclogged and the diaphragms and slides are operating perfectly. I tested the fuel floats and they are not cracked or damaged, operating as they should; verified with clear tubing out the overflow nipples and lined up along side the outsides of the fuel bowls. The fuel bowl needles were also replaced when I rebuilt the carbs originally, and they are not sticking or damaged.

Holy crap, I'm at a loss. I'm missing something simple, I know it. Experienced guys, please chime in with maybe something I've missed, advice or options.

http://i736.photobucket.com/albums/xx9/exit39/Honda%20CB900/0409161001.jpg

WarMullet
05-08-2016, 3:13 PM
Oye mate,

You take cylinder 3/4 plugs out and see them spark? May have bad plug wires or faulty plugs.

Exit39
05-08-2016, 4:08 PM
Oye mate,

You take cylinder 3/4 plugs out and see them spark? May have bad plug wires or faulty plugs.

Did that...
Spark is beautiful on all plugs when I test them by using a screwdriver to touch the rotor and the spark advancer together. One coil fires 1 and 4 together as it should, and the other fires 2 and 3 as it should. Strong spark from those new Dyna's. I laid all four spark plugs in the wires on top of the valve cover bolts and ignited them like I said. Thanks though!

Tattooo
05-08-2016, 4:56 PM
Have you checked for an intake leak??? Is it getting gas???? When was the last time you adjusted the valves??? I have had several 1100Fs and that was a common problem with them...

I just read your first post.... I'm sure it's getting gas..... But adjusting the valves are important.....Also how are you intake rubbers???

Exit39
05-08-2016, 6:56 PM
Have you checked for an intake leak??? Is it getting gas???? When was the last time you adjusted the valves??? I have had several 1100Fs and that was a common problem with them...

I just read your first post.... I'm sure it's getting gas..... But adjusting the valves are important.....Also how are you intake rubbers???

Well, first of all let me start by saying I'm a dork. Stepping back in my mind I tried to place where this really began. A couple weeks ago when I first noticed a weak spark issue, I naturally checked the plugs, wires, coils and CDI boxes. The OEM coils and one wire were weak. That's when I decided to go ahead and upgrade the coils and CDI boxes. Now, stay with me on this one as I also have an 80 CB750F waiting to be brought back to life. Well, I remember grabbing the NGK wires off that bike and installing them on the 900. If you haven't put my brainfart together yet, I'll explain in detail. I installed the wires on the 900 in the same order they were on the 750, a bike I bought last year that didn't run. AND I copied the same thing when I installed the Dyna wires the other day. Yeah, got it yet? The wires were installed in the wrong order on the 750. I got the 900 wires mixed up from the coils to the engine. I haven't fixed it yet as I'm tired and gettin' to bed for work in the morning. But tomorrow evening I'm pretty confident I'll be a happy camper. Sometimes, the simplest of things can just ... yeah, you got it. :banghead:

Tattooo
05-08-2016, 7:06 PM
Yep I bet your right.... A lot of times if you go back to where you started that is where your problem is..... If that isn't it but I bet it is get back with us....

Those are great bikes..... The Fs are better but... LOL