Topic: 1978 PD42A carb rebuild and initial jetting advice

[lxcoupe89]

I need advice for Main jet size and pilot jet size specs are below. I appreciate any advice you can provide.


I'm new to the forum and inexperienced with carb tuning. I have cleaned and rebuilt carbs. I have a basic understanding of how they work but I haven't done any tuning.

I'm building an old Denvers Chopper from the 70's. I want it to run good and have a crisp throttle but max hp isn't my goal.

1975 CB750 stock motor
1978 PD42A carbs
open drag pipes (I have short ones now to get it running but will put longer drag pipes on it
Old Bread Box air cleaner on its way (also have a set of K&N Pods)

I plan to block off three sides of the bread box and leave the bottom open

I have dipped the carbs in Pine sol/water for 2-3 days and thoroughly rinsed them
cleaned all jets with torch tip cleaner or strand of copper wire
Sprayed all passages with carb cleaner, all seem to be open
accelerator pump nozzles are clean and spray with carb cleaner

I plan to:

clean the 2 ball check valves if I can can get them apart
Lube the throttle and butterfly shaft bushings with vaseline
assemble the carbs
stake butterfly screws
set float heights
bench sync
check fuel levels with clear tube
confirm accelerator pump function

Did I miss anything?


I already bought the following parts from z1 performance Keyster Brand parts

Carburetor Kit - CB750 - 1978 - Intermediate   SKU: SIKH-1157

Bowl Gasket
Top cover Gasket
some orings
needle valve
Slide Needle and jet (non adjustable)
110 main jet
Pilot Jet Needle (w/ spring oring and washer)



Accelerator Pump Diaphragm Set - CB450 - CM450 - CB750  SKU: KL18-2807

Diaphram
spring
rubber boot


I'm trying to figure out what pilot jets and main jets to start with so I can buy them before I put the carbs together.

I have done some reading and i see conflicting info. With similar combinations to mine some say to increase the pilot jet size to 42 others say that the stock 35's  are good

I appreciate any info you can provide

[flybox1]

42/115, with fuel screw at 1 turn out.

[lxcoupe89]

Thanks for the feedback, Should I start with 1 shim on the needle?

[lxcoupe89]

42/115, with fuel screw at 1 turn out.

I ended up going with 42/120 Keihin Brand Jets

[flybox1]

Complete an idle plug chop to help you dial in your mixture screws.
You'll want your plugs on the lighter side of tan as you have an accel pump.

Once that is done, go on a run and see how it accelerates up to redline as you roll on the throttle in 3rd or 4th gear.
If at any point the bike stumbles, you will need to isolate the throttle position where it occurs, and adjust that part of the carb for better mixture...
If your jetting is correct, you will have responsive acceleration all the way to, and thru redline,

[lxcoupe89]

Complete an idle plug chop to help you dial in your mixture screws.
You'll want your plugs on the lighter side of tan as you have an accel pump.

Once that is done, go on a run and see how it accelerates up to redline as you roll on the throttle in 3rd or 4th gear.
If at any point the bike stumbles, you will need to isolate the throttle position where it occurs, and adjust that part of the carb for better mixture...
If your jetting is correct, you will have responsive acceleration all the way to, and thru redline,

Ok Thanks. I'll do that