Running on 2 cylinders with choke on

I have a 1991 CB 750 Nighthawk with 11K miles. Once warmed up, the bike runs very strong, although it hesitates a little bit when hitting throttle fast from idle. The problem is that when I start it cold with full choke on, it will only run on two cylinders (first and second from the right). Also, even with full choke on, the idle sits at just about 1k rpm (shouldn't it be more like 2 or 3K?). I am unable to advance the throttle without the engine dying unless I let it warm up on those two cylinders for a few minutes. Even then, it takes a lot of feathering the throttle to get it to reach higher rpm without stalling. How do I know it only runs on two most outer right cylinders? After running it for a few minutes with choke on, the exhaust pipes on the left side are completely cold and so is the head while the right side is hot. I am thinking the choke or air/fuel mixture must be off, but I am not sure what I can adjust. Would you recommend carb cleaning/rebuild service?

Spark plugs are probably fouled. Replace all 4 plugs. Full choke should have bike running 2000 rpm and moving up toward 3000. My 1992 is cold blooded even in AZ on cold days it is hard starting. Are you getting some January riding in Illinois??

Thanks Biebs, I checked the plugs, they are clean and properly gapped, so I don't think that's it. I also checked compression and it's equal in each cylinder. Not much riding in January for me (as in any), but I started the bike up yesterday and it behaved exactly the same as it did when the weather was warmer (I had it in a warm garage). I am just looking for a solution to be ready for warmer days. I think the two cylinders that remain cold aren't getting the proper fuel/air mixture with choke open, but I don't see how I can adjust the choke setting for those two.

What was the compression?

Also consider these DOHC bikes have hydraulic lifters (no valve adjustments) so oil pressure is needed to pump up lifters open the valves. Choke is one thing but valves have to be open to let the mixture in. So what weight oil are you using?? Are lifters gummed up?? Also ignition coils/wires. Many different variables affect cold start up. As you stated run fines when it is warm. I feel you are just fighting the nature of the beast.

dirtdigger said:

What was the compression?

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I don't remember the numbers exactly, but I know they were within 5% of each other in each cylinder. All 4 spark plugs looked very nice and even, too. If there was an issue with compression or valves, the bike would like run poorly at higher rpm and when warmed up, but mine is a beast all the way to redline once warmed up. It only has 11K miles and sounds very good and idles perfectly when warm. It's almost like it's not getting any fuel (or spark) when cold with choke on into two of its cylinders.

Biebs said:

Also consider these DOHC bikes have hydraulic lifters (no valve adjustments) so oil pressure is needed to pump up lifters open the valves. Choke is one thing but valves have to be open to let the mixture in. So what weight oil are you using?? Are lifters gummed up?? Also ignition coils/wires. Many different variables affect cold start up. As you stated run fines when it is warm. I feel you are just fighting the nature of the beast.

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I am running Honda's own oil as specified for this motor (I bought the oil at a Honda Motorcycle dealership, too). I was thinking something was off with the choke because from what I could tell (YouTube videos), other Nighthawks idle high with choke on and can be throttled up even when cold, while mine won't idle above 1K rpm with choke on and there is no way for me to advance the throttle until the motor warms up. It starts immediately when cold, but only runs on 2 cylinders till warmed up (sounds like a V-twin). As stated before, idles great on all 4 cylinders when warm and choke off and runs like a cheetah through the gears to triple digit speeds every time. I tried no-ethanol fuel and always highest grade, but nothing seems to make a difference.