81 CB750K Cylinders 3 and 4 Run Lean

[oldsoulcole]

I have an '81 CB750K that I recently re-jetted to accommodate for my 4 into 1 exhaust. I installed #72 primary jets and left my secondary jets alone. I did this because I was not lacking power in the higher RPM's (4,000 and higher). But before re-jetting I had a lot of hesitation anywhere below 4,000 rpm and could barely keep it running. After re-jetting nearly all of my hesitation is gone and takes off from a stop very smooth. It even idles! ;D But the problem I am having now is that cylinders 3 and 4 are running lean. I sprayed carb cleaner around the intake boots to see if any were leaking and got nothing. Just so you know where I am at I have the stock airbox, all 4 carburetors have been rebuilt and cleaned, synced, and new air box - carb boots installed, all cracked vacuum lines replaced, float heights are within spec, pilot screws are out 1 1/2 turns out, new exhaust gaskets installed, engine is timed correctly. I honestly do not know what else to do to try and solve this problem.

Will turning my pilot screws in cause my air/fuel mixture to become richer because that will be restricting more air to that cylinder? Any suggestions is greatly appreciated. I've done everything I can think of at the moment.

 

[SkullCrusher]

How do you know they're running lean? I assume by reading plugs? Does it run good? pilot screws will adjust mix at idle and low throttle openings. What rpm range are you running lean in?

 

[barnett468]

in addition can you post photos of the plug tips . . use flammable brake cleaner with the long nozzle and spray gently for the air leak testt . . if you are near sea level you should g up on he mains 1 size with headers irregardless of how it runs . m if there is not air leak simply jet the lean cyls 1 jet richer in the lean area no big deal.f

 

[oldsoulcole]

The pictures go in order from Cylinder 1,2,3, and lastly 4. I am going off of what the plugs are telling me and the difference in heat. If you put your hand by cylinder 1 and 2 head pipes they feel normal. When you do the same by 3 and 4 they are much hotter. Recently I have also re-jetted and am now running #75 mains. This definitely made a difference because now the bike has zero hesitation at low speeds and rpm (10-25mph/ 1,500-4,000 rpm). It feels just as it should to me all the way through the powerband it pulls and it pulls HARD. I also sprayed both sides of my intake rubber boots (airbox to carb and carb to intake manifold) and I found zero leaks. I have tried making adjustments with the pilot screws and it makes little to no difference from what I have found. So will it not throw things off if I am running #75 mains in cylinder 1 and 2, then jump up a few sizes in cylinder 3 and 4? Also, the weather has been really bad lately and I have only really been able to run it in the garage. I would like to take it around for a few laps on a nice day and see if it does better. Because last night after I put the new jets in, it was raining and late I took it around the block once and when I pulled into the garage 3 and 4 didn't feel hot like before!! They felt nice and cool like cylinder 1 and 2 but the plugs said otherwise. The plugs showed cylinder 3 and 4 as still running hot.

 

[barnett468]

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you did not use flammable brake cleaner for your test, therefore your test is inconclusive, its that simple, but if it idles smooth i doubt it has a leak.

jetting your cylinders so they all have the same afr will make it run better and more even . . running them uneven like they might be will not.

dont ask me why these two look different than the others, i dont know for certain, but it sometimes happens . . i just have never seen it happen to this degree . . could be the carbs or could be the header design . . it is what it is.

by the way...the first two plugs are a bit rich and/or it is burning oil which looks more likely to me.

i would go up 1 size on the main on the lean ones.

if your needles are adjustable i would raise them all up 1 clip . . if not, i would increase the low speed jet 1 size.


this is the only way to know exactly what your gas level is . . they must all be fairly close.


..................here's an example of it a little too high . . it should be just below the solid black line in your case.

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.................................................here's another and you can see the is level way too low.

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