Topic: 78 CB550 jetting for 4-2 exhaust

[stevenwr]

I have a 1978 CB550, everything is stock except the exhaust is a Jardine 4-2. I backed my pilot screws out 1 1/2 turns and it seems to be running a little lean. The stock jets are: main 90 & pilot 42. I'm thinking with the exhaust I should rejet.....any recommends for what jet sizes I should go to?

[Cut]

100 for the main jets and 38 for the slow jets.  I too have the same 4-into-2 setup and report that this works great.

[cavi]

before you follow those jetting suggestions, please elaborate more on how it runs now.  I say this because if you go from a 42 pilot to a 38 you will be leaning out even more on the piolot circuit, while going up on the main circuit.  you state it is running lean, where is it running lean?  does it idle okay, how is throttle response from idle all the way up.....

[lucky]

before you follow those jetting suggestions, please elaborate more on how it runs now.  I say this because if you go from a 42 pilot to a 38 you will be leaning out even more on the piolot circuit, while going up on the main circuit.  you state it is running lean, where is it running lean?  does it idle okay, how is throttle response from idle all the way up.....

I agree do not go leaner on the idle jet!!!

[lucky]

I have a 1978 CB550, everything is stock except the exhaust is a Jardine 4-2. I backed my pilot screws out 1 1/2 turns and it seems to be running a little lean. The stock jets are: main 90 & pilot 42. I'm thinking with the exhaust I should rejet.....any recommends for what jet sizes I should go to?

I would need all of the facts before giving you a a recommendation.

1978 CB550.
Intake type?
clip position? Is it adjustable? (Middle position is stock on the 1977).
Does it have an accelerator pump system and nozzles?
4-2 exhaust.
I would use a 105 Main jet.
Idle jets...I would go with #42mm the stock size was #38mm
Mixture 1.5 turns out.

[lucky]

I wonder where stevenwr went?
Maybe he is checking those acclerator pump nozzles it he has them?

[Taybreezy]

Well if Steven is done, I think I will just jump in here.

I have the same bike except everything is stock (which means 4 into 4 exhaust) including my PD46C Keihin Carbs. My current #42 pilot/slow jets appear to be donesky and I need to order new ones. The only ones I could find are made by Sirius Co Inc. and they are "out of stock," and might not be getting more any time soon.

Sooooooo, would it be advisable to jump up to the (in stock) #45 slow jets and just adjust the idle mixture screw accordingly?

[stevenwr]

Hey guys I'm still here, sorry about that. I've been dealing with another problem.
The carbs on a 78 are PD46C, stock air box with stock air filter, jets are stock: main-#90 & pilot is #42. The mixture is 1.5 turns out, the exhaust is a Niche 4-2. Clip position is in the middle. There is no accelerator pump on these carbs.
 I increased my main jet to #92's and the plugs seem good. The bike runs good, it has a little flat spot around 5000rpm.
 The problem I've been dealing with is when I idle the bike shut it off and check the plugs there all the same color but when I head out on the highway at more then half throttle then check the plugs....#1 plugs look rich and 2&3 seem good.