Author Topic: CB750F3 carb Qs  (Read 1554 times)

0 Members and 1 Guest are viewing this topic.

Offline Davidov

  • Hot Shot
  • ***
  • Posts: 284
  • 1978 CB750 F3
    • My CB750F project log
CB750F3 carb Qs
« on: July 19, 2011, 09:53:59 PM »
I'm currently getting my 78 CB750F tuned just right before I get it on the road. I have some questions is anyone can provide some knowledge that would be excellent.
They're stock Keihin PD carbs.

1. After the initial 1 3/4(1.75) turns of the pilot screw. I ended up with them all screwed out about another full 1 turn. Is this OK? Or should they be left at 1.75 out?

2. The lowest idle I can get is about 1500 RPM. I know the recommendation is 1000 RPM, but it just doesn't want to run solid at that speed. I tuned everything, cleaned the carbs, etc.. Is 1500 OK?

3.This sort of relates to #2. I was spraying around the carbs with cleaner looking for any intake leaks. I noticed a gap at each choke plate shaft?

Once again, sorry for all the dumb questions. I figure somebody out there has been in my shoes!
-David

Offline Gearheadgreg

  • Hot Shot
  • ***
  • Posts: 254
  • Jamestown Rhode Island
Re: CB750F3 carb Qs
« Reply #1 on: July 20, 2011, 02:49:32 PM »
Yep even my new rider lawn tractor would not idle when stored over the winter, you guessed it clogged slow speed jet!!! 
Got to Love E-10 gas  >:(  If you did not pull your slow jets and clean them your adjusting screw will not respond much,, If you have stock exhaust and a stock airbox it should run good at 1-3/4 to 2-1/2 turns out,,mine is 1-3/4 and idles @ 1000 rpm solid. don't forget to check the 0 rings on the mixture screws it will run like chit if broke or missing.
2001 Honda Supercharged Valkyrie   <iframe width="480" height="360" src="http://s1218.photobucket.com/user/meloafer/embed/slideshow/"></iframe> Just having some fun

Offline Davidov

  • Hot Shot
  • ***
  • Posts: 284
  • 1978 CB750 F3
    • My CB750F project log
Re: CB750F3 carb Qs
« Reply #2 on: July 20, 2011, 03:19:17 PM »
These carbs have pressed in slow jets, but I did thoroughly spray through each one with carb cleaner. I made sure the accelerator tube(proper term?) was clear.

It's a stock airbox, and an aftermarket muffler on the stock 4-1 750F exhaust. I need to try a few more tricks on the idle.

I'm still confused about being able to see light around the choke plate shafts? I did some intensive research, but pictures aren't as good as real life. Should there be a perfect seal around each one?

Thanks again.
-David

Offline Spanner 1

  • Old Timer
  • ******
  • Posts: 4,092
  • CB 750 K0 ( always thought it was a K1!) + CB750K8
Re: CB750F3 carb Qs
« Reply #3 on: July 20, 2011, 04:19:40 PM »
Seeing light at the choke shafts is not critical IMO..... but not pulling the slow jets is !
Those pressed-in slow jets do come out by pulling/twisting with a parallel jawed pliers... wrap some cloth around under the plier's jaws will help to not mar the sides. I bet the cross drilled holes are all blocked as well as the jet itself..... you can see light thru' it when held up at just the right angle... good luck  :)
If your sure it's a carb problem; it's ignition,
If your sure it's an ignition problem; it's carbs....

Offline Gearheadgreg

  • Hot Shot
  • ***
  • Posts: 254
  • Jamestown Rhode Island
Re: CB750F3 carb Qs
« Reply #4 on: July 20, 2011, 05:45:27 PM »
If you have a slip muffler and the stock headpipe it will lean out a already Factory lean motor, These PD carbs have a accelerator pump to over come fuel starvation for EPA reasons, My 78F 3 was all over the place at idle and popping on decel a couple of weeks ago, just installed  # 38 slow jets and smooth as silk No popping through my slip on pipe with stock header and enjoying the ride  :D
2001 Honda Supercharged Valkyrie   <iframe width="480" height="360" src="http://s1218.photobucket.com/user/meloafer/embed/slideshow/"></iframe> Just having some fun

Offline fatkid

  • you cant undo whats already been done. so just ask
  • Enthusiast
  • **
  • Posts: 107
Re: CB750F3 carb Qs
« Reply #5 on: July 20, 2011, 06:07:57 PM »
hey gearhead nice f man thats a sharp one

Offline Gearheadgreg

  • Hot Shot
  • ***
  • Posts: 254
  • Jamestown Rhode Island
Re: CB750F3 carb Qs
« Reply #6 on: July 20, 2011, 06:16:05 PM »
Thanks Fatkid. I like to look at everybody's bikes so pictures are a must  :)
2001 Honda Supercharged Valkyrie   <iframe width="480" height="360" src="http://s1218.photobucket.com/user/meloafer/embed/slideshow/"></iframe> Just having some fun

Offline Davidov

  • Hot Shot
  • ***
  • Posts: 284
  • 1978 CB750 F3
    • My CB750F project log
Re: CB750F3 carb Qs
« Reply #7 on: July 20, 2011, 09:40:18 PM »
Seeing light at the choke shafts is not critical IMO..... but not pulling the slow jets is !
Those pressed-in slow jets do come out by pulling/twisting with a parallel jawed pliers... wrap some cloth around under the plier's jaws will help to not mar the sides. I bet the cross drilled holes are all blocked as well as the jet itself..... you can see light thru' it when held up at just the right angle... good luck  :)
Ok, thanks. This is the type of response I needed. I did overlook the slow jets, I assumed "pressed" meant a permanent fit!
If you have a slip muffler and the stock headpipe it will lean out a already Factory lean motor, These PD carbs have a accelerator pump to over come fuel starvation for EPA reasons, My 78F 3 was all over the place at idle and popping on decel a couple of weeks ago, just installed  # 38 slow jets and smooth as silk No popping through my slip on pipe with stock header and enjoying the ride  :D
Good to know as well, off come the carbs....again!  :o
(BTW, that's a slick 750F!)
-David

Offline Davidov

  • Hot Shot
  • ***
  • Posts: 284
  • 1978 CB750 F3
    • My CB750F project log
Re: CB750F3 carb Qs
« Reply #8 on: July 24, 2011, 08:08:50 PM »
I couldn't find the original post that I read, but.....is the Honda Manual for CB750 K8 and F3 misprinted about carb types?
I have Keihin carbs stamped PD 42A 105 main jet, 35 slow jets.

This particular manual has those specs listed for a 750K.  ???

Now these two sites list the carb type and jet size properly. Right?
http://www.hondachopper.com/garage/carb_specs/carb_specs.html
http://www.motorcycleproject.com/motorcycle/text/specs.html

-David