Topic: CB650 Cylinder 3 only warm, need help!

[eastcoastish]

Hey guys, I'm finishing up a CB650 build and am having a hell of a time figuring out this last big issue. The bike doesn't seem to always be running on 4 cylinders, or at least not completely; the header on cylinder 3 is always about half the temp of the headers on cylinders 1,2 and 4. (Cylinder 3 is around 150 degrees when the others are 300+).

I'm pretty sure it's not electrical, I've swapped plugs, caps, wires and even switched the coils between sides, and cylinder 3 is always half the temp. The plug in the number 3 cylinder is a bit wet though so it looks like it's getting fuel (maybe too much? maybe too little?) I'm running brand new Keihin CR29 carbs, so I doubt (but haven't completely ruled out) it being a carb issue.

I did a compression test and all 4 cylinders were about 140 so that would point to the carbs but not sure why or where to start.

Any thoughts or suggestions would be greatly appreciated.

[scottly]

Is the #3 plug wet and clean, or wet and black? Wet and clean would indicate lack of spark, while wet and black would indicate too much fuel/too little air.

[eastcoastish]

I put new plugs in 3 and 4; 4 is looking pretty good, 3 is definitely black. These are brand new NGK D8EA plugs after a 5 mile ride...

[DaveBarbier]

Can you verify that the choke lever (really enrichening circuit lever) releases the plunger on the #3 carb when it’s lowered? I assume you or someone else spaced the carbs to fit the 650 head. If that plunger is cocked or not aligned properly from adjusting the spacing it can stay in the increased fuel for startup setting. Just make sure the the #3 carb acts the same as the others. 

[DaveBarbier]

Hope the 29’s aren’t too big for your bike. I have the 26’s on my beefed up 650 and it’s great.

[eastcoastish]

Can you verify that the choke lever (really enrichening circuit lever) releases the plunger on the #3 carb when it%u2019s lowered? I assume you or someone else spaced the carbs to fit the 650 head. If that plunger is cocked or not aligned properly from adjusting the spacing it can stay in the increased fuel for startup setting. Just make sure the the #3 carb acts the same as the others.

I read this reply today at work and spent the rest of the day thinking about it. When I got home I took off the tank and opened and closed the lever to see how it actuates all 4 plungers. They all looked good, but once closed, I realized #3 was staying about 3mm open. Incredulous, I checked multiple times and all 4 plungers were fully seated except #3. I adjusted the lever on the #3 plunger, put the tank back on, fired it up and the bike ran smoother than ever and the exhaust headers all read the same temperature!

Anyway I just wanted to say how much I truly appreciate the response and you solving my problem. I spent hours wracking my brain on it and while the solution was obvious in hindsight, who knows how long (if ever) I would have taken to figure it out on my own.  If you're ever in the Melbourne, Florida area, drinks are on me. Cheers! 

[DaveBarbier]

Glad to help! That system Keihin has for “choking” the carbs is pretty strange and a little wonky. Makes sense only if you recognize this system keeps the carb mouth completely clear. A normal choke butterfly system inherently adds turbulence since it’s right in the middle of the air flow. And since the CRs have such nice polished bell mouths they didn’t want anything to get in the way ruining the design.

Nice 650!