Topic: Rejetting for an 836cc upgrade

[scroggins5000]

Here's the run down:

1978 CB750F
      
Bike has 836cc F2 piston kit installed from Dynoman

Oval type K&N brand pod filters

Yoshimura replica exhaust.

I currently have stock Keihin carbs from the CB750F with 105 main jets. I need to find out where I need to start on rejetting the carbs for this motor. Can anyone give me a recommendation on what jets I will need, and where is the best place to buy them?

[Old Scrambler]

Assuming those are pumper carbs...........I'd go to 115 and stay with Kiehin jets available almost anywhere that Japanese bikes and parts are sold or serviced.

[scroggins5000]

I sent an email to dynoman.net to see what they recommended and this is what they said.

"We would recommend that you order 115, 120, and 125 jets to get you started. We would start with the 120 jets and see how it runs. Then go 1 size smaller and one size larger to see what works the best. All engines are a little different, but if you start rich and you will be safer, as a lean mixture can damage your engine."

Unless anyone would see a problem with this, I think I'll go this route. Not sure If I want to order from them, but they are 4.50ea there. Not bad...

[eurban]

Just make sure that your carbs are spotlessly clean and adjusted correctly with all systems functioning BEFORE you start evaluating the main jet size, main jet needle position and perhaps (not likely) any changes to the idle jet size.  120 sounds like a good starting point and you will likely need to shim the main jet needles just a bit to richen the low to mid throttle carburetion.  You might consider ordering up a set of new adjustable needles (specced for the 77s).  I got my set from David Silver Spares.  These new needles will come with new needle jets which are the "bushings" that needles ride in and out of. This insures proper metering from the main jet needles.  Have fun!

[scroggins5000]

Cool. Anyone else want to add their share on where to start the jet sizing?

[Don R]

Sometimes increasing the airflow will make a carb more efficient. So a conservative increase would be a start. My older 836 has 1970 4 cable carbs and likes 120's but I believe that is what came in these carbs.

[wick]

IM running- a 150 main /on my 836-kit- with pods. wot- zero- bogg- cb750k2- screams- oh` & cycle-x- 4/2/1- pipe-

[odiewan]

I would think you need to go considerably bigger than 130 as 120-130s are the range for a stock 750 with pods and a 4 into 1.