Topic: Jetting fun! 77 CB750F Pd42a carbs.

[lucky]

If it is popping on decell when you rev it up, try opening the mixture screw.
If it pops no matter where you have the mixture screw, it could indeed be an exhaust or intake leak.

[Retro Rocket]

An exhaust leak will pop on decel.
 FWIW. 1080 ran 130,s on his 1080 motor, think he said he tried 135 but went back to 130 . That was with cam, flowed head K1_6 roundtop carbs
 He also ran Kawasaki  smoothbore Mikunis, with 127.5 mains.

Hi Frank, did 1080 get the mikuni's working well..?

[CB750R]

If it is popping on decell when you rev it up, try opening the mixture screw.
If it pops no matter where you have the mixture screw, it could indeed be an exhaust or intake leak.

Dropped down to 125s and added a half turn on the mixture screws.

Bike idles better, less popping on deceleration. Pulls hard. Look forward to getting an A/F graph on this setup.

[Davidov]

Did you ever get a follow-up dyno run? Curious to see what the A/F chart looks like.

[lucky]

I'm running a cb750f2

The jets are
137.5 mains
35 pilots
Needles are f2051f shimmed with a washer underneath and at lowest clip position.

Dude says there's a huge lean spot than corrects and may be a tad rich on he top end.

I'm already maxed out on the needle. And seem to be running very large jets.

Wtf is going on???   It pulled 55 hp

My only mods are a Tintop air cleaner dyna s ignition and coils.  And some very mild clean up of ports when I rebuild my engine.

Dyno guy said its Waaaay off bike runs pretty nice on the road...

So new needles?? Or more main jets?

You keep making new posts.

You did not tell us the thickness of the shims.
Did the shims fit down inside the pocket in the bottom of the slide or do they sit on top of the pocket??

The number on the needle does not matter.

I would go down to a 125 main jet. EDIT:Ooops ,I see on an earlier post you did change main jets.
I think those large 137 mains are spilling over early.

[lucky]

Enrichening the pilot circuit will have no effect on the main jet.


Pilots standard
IMS screws two turns out

Runs well, though isn't as smooth through the carb circuit transitions as a standard bike.

Perhaps a larger pilot would help?

I agree and I would really want to know if when it is warmed up and you give it a quick turn of the throttle does it stumble??? Very important.

(Not a timid turn of the throttle, a quick twist like you don't care!!!)


Or do you have to sneak up on it and be real careful so it does to cough and threaten to quit.

[lucky]

If it is popping on decell when you rev it up, try opening the mixture screw.
If it pops no matter where you have the mixture screw, it could indeed be an exhaust or intake leak.

Dropped down to 125s and added a half turn on the mixture screws.

Bike idles better, less popping on deceleration. Pulls hard. Look forward to getting an A/F graph on this setup.

You want no popping on deceleration.  More on mixture or larger idle jets.
What kind of exhaust? Aftermarket right?


This seems like the SAME OLD STORY to me.
You need #42 idle jets and then the lean spot will be gone at the beginning of the needle position. Don't change the needle .

Do not be tempted drill out to .45mm it will be too large!! The .42mm is what is needed.
I have done extensive testing on this set up. #40 too small #42 just right,#45 too large.

Wait till you how well that #42 works.

Do you have that needle clip lowered one notch? Or just the shim is added or both?

[gerhed]

I gave up on the 4 carb Spring tune up thing a few years ago and got the Cycle-X dual
Mikuni setup--no probs since!