Topic: 77-78 CB750 PD42A Carb Rebuild Info

[punch455]

Hey everybody, just wanted to say that I've been lurking for a couple months now and figured I should probably CONTRIBUTE for once... Well here's a quick intro: Bought this bike a few years back, a 1978 cb750F, and it never really ran very well. I always knew it needed carb work, but I was hesitant to take the jump and really get in there and open them up.  But because of everyone on this site, I got the guts to tear them down last October and I'm am really glad I did.  You wouldn't believe some of the stuff I found going on in there.. but that's another story!  So, I've had the carbs off all winter, and they've been updated with new gaskets and o-rings, #125 mains, and #42 slows.  Cleaning them was a PITA but the stuff I've learned in the process because of everyone's info has made it really worth it.  So, before I put the carbs back on the bike, I figured I'd take a few pics and label them with the main parts, and a brief description of what you should see when cleaning them, just to provide a really basic starting point to convince others to get in there!!

First, the intake side of the #2 carb:



1 --  Accelerator Pump Nozzle -- You should see fuel squirting out of this litle brass post into the carb under quick throttle application.  This was probably the HARDEST part to clean.  I used a high E string from an old guitar and that worked pretty well.
2 --  Slow circuit -- Spray carb cleaner into this hole and watch the slow jet located in the underside of the carb body.  You should see fluid squirting through the brass slow jet.
3 --  Main Circuit -- Spray cleaner through this hole, and make sure you see fluid exit from the main jet located in the underside of the carb body. 

Underside of the #1 Carb:



1 -- Float pin --  Works with the float to maintain a constant level of fuel in the float bowl.
2 -- Main Jet -- Screws into an emulsifier tube, which screws into the carb body.  When I opened up this carb for the first time.. this WAS LAYING IN THE BOTTOM OF THE BOWL. Wow. Unscrew and make sure it's clean.
3 -- Slow Jet -- Pressed into the carb body and comes out with a gentle tug of some pliers. These are also very hard to clean because of the small passageway. Make sure they are PRISTINE.
4 -- Pilot Jet/Needle -- A pin, spring, washer, and O-ring that is basically a fine tuning air/fuel mixture screw. Make sure the taper on the needle is clean.

Well, thanks again to everyone, and if anyone has ANY requests for pics in the next few weeks before they go back on the bike, please don't hesitate to ask!

[upperlake04]

 Good post Punch. As you found out, they're not too difficult, you only have to learn one, the others are the same.
 Why did you go with the #42 slow jets instead of the stock #35s?

[punch455]

Well, I know the 77/78 ran notoriously lean from the factory, and that adjustments can be made to the idle with the pilot screws, but I've found out over the years working on cars that it is always easier to adjust for a rich condition rather than a lean one.  I'm wondering if I should change to a little hotter plug now, considering I live in the city and don't go on long trips that often..

[eurban]

Nice pics and write up.  I do however think that you are making a mistake with the 42 pilots.  Jumping up to 42s is a huge step from a #35 and I don't think you are really correct with your assessment that these carbs are lean from the the factory.  I would venture to say that most of these 30 year old machines will have impairment to their idle circuits and will require a very thorough cleaning, perhaps contributing a cold blooded perception.  Without some coaxing, few will perform the required step of pulling and cleaning  the pressed in idle jets along with the passages behind them. . . Have you modified your bike?  Typically the decision to go to larger pilot is based on actual performance observations.  If when turning out (counterclockwise) the idle mixture screw from the factory baseline starting point (between 1.5 and 1.75 turns or so) idle and off idle performance continues to improve as you approach about 4 turns out (the end of the screws effective adjustment range) then the next step larger size pilot jet would be in order.  In my personal experience with the PD carbs both in stock applications and in performance ones, the stock 35 pilots are well up to the task with the sweet spot on the idle mix screw ranging between 1 and 3 turns out depending on the MODs (my 836 kitted bike actually likes a setting of about 1 turn out) You have choosen a pilot that is essentially 3 steps larger.  I would expect that your results will be poor.  BTW, where did you find a #42 pressed in pilot??

[punch455]

Thanks alot for the suggestion! I got them from Siriusconinc.com.. I saw them on there for a pretty decent price - 16 bucks I think - and just went ahead with them.  I didn't realize the jump was so big.  See, I haven't ridden the bike in over a year and a half, and don't really remember where the adjustments on the pilot screws was when I tore these down.  The bike is bone stock as far as air delivery and exit is concerned.  I still have the 35s, maybe I should stick with them.. Has anyone else had any experience with a larger slow jet?

[lucky]

I have a 4 into 2 into 1 header with( all metal baffle) muffler from Cycle Shack.
I have velocity stacks.
I am using a XXXX jet. EDIT #42
Mains are #125's.
With two turns out on the pilot air screws it is still lean.
I have one 0.22 shim under the non adjustable needle. Still lean.

Am am going to #42 pilot jets and a second shim under the slide needle.
I will let you know what happens.

EDIT: The #42 pilot jet was the best choice after all.
IT is just a little less than .003 thousandths larger.

Also one shim under the needle was correct with that shim sitting on top of the pocket in the bottom of the slide which was .012 thousandths deep making it a .034 thousands upward movement of the needle.

[jadedjackal]

This is a great post!! Good job on the photos and descriptions. I've spent numerous late nights reading the gagillion different threads on carbs and found myself getting more confused than informed because there are so many different opinions, also I realized a feed on a 750k0-6 carb issue doesn't always help me understand the issues with my k8.
This helps a lot. Thanks 

[bollingball]

Punch Good post I also think you will need to go back to #35 jets. No big deal you still have them.
 I would suggest while they are still off the bike that you set up or check your full bowl levels with the clear tube to 2-3mm below the bowl gasket and set the gap on the accel. pump rod to allmost zero. Don't forget the choke idle cam adj. It is so much easier to do this off the bike on a test stand.

Ken

PS Can you tell me how to incert those numbers in a photo?

[Imago]

Great thread! I'm just about to tear into mine (1978 CB750F) and this will be VERY useful.

Mines running fine, but won't idle without full choke this spring so the first thing I'll clean are the slow jets.

Can the slow jets be cleaned without pulling or am I asking for trouble if I don't?

I'm going to remove the carbs based on some other advice... but was wondering about removing these particular "pushed in"
jets.

[mrrch]

Pull them out to clean them, they are not very hard to remove at all.

[lucky]

Hey everybody, just wanted to say that I've been lurking for a couple months now and figured I should probably CONTRIBUTE for once... Well here's a quick intro: Bought this bike a few years back, a 1978 cb750F, and it never really ran very well. I always knew it needed carb work, but I was hesitant to take the jump and really get in there and open them up.  But because of everyone on this site, I got the guts to tear them down last October and I'm am really glad I did.  You wouldn't believe some of the stuff I found going on in there.. but that's another story!  So, I've had the carbs off all winter, and they've been updated with new gaskets and o-rings, #125 mains, and #42 slows.  Cleaning them was a PITA but the stuff I've learned in the process because of everyone's info has made it really worth it.  So, before I put the carbs back on the bike, I figured I'd take a few pics and label them with the main parts, and a brief description of what you should see when cleaning them, just to provide a really basic starting point to convince others to get in there!!

First, the intake side of the #2 carb:



1 --  Accelerator Pump Nozzle -- You should see fuel squirting out of this litle brass post into the carb under quick throttle application.  This was probably the HARDEST part to clean.  I used a high E string from an old guitar and that worked pretty well.
2 --  Slow circuit -- Spray carb cleaner into this hole and watch the slow jet located in the underside of the carb body.  You should see fluid squirting through the brass slow jet.
3 --  Main Circuit -- Spray cleaner through this hole, and make sure you see fluid exit from the main jet located in the underside of the carb body. 

Underside of the #1 Carb:



1 -- Float pin --  Works with the float to maintain a constant level of fuel in the float bowl.
2 -- Main Jet -- Screws into an emulsifier tube, which screws into the carb body.  When I opened up this carb for the first time.. this WAS LAYING IN THE BOTTOM OF THE BOWL. Wow. Unscrew and make sure it's clean.
3 -- Slow Jet -- Pressed into the carb body and comes out with a gentle tug of some pliers. These are also very hard to clean because of the small passageway. Make sure they are PRISTINE.
4 -- Pilot Jet/Needle -- A pin, spring, washer, and O-ring that is basically a fine tuning air/fuel mixture screw. Make sure the taper on the needle is clean.

Well, thanks again to everyone, and if anyone has ANY requests for pics in the next few weeks before they go back on the bike, please don't hesitate to ask!

What about the #2 carb accelerator pump check balls?

[Tugboat]

Great thread!! @bollingball that's an AWESOME carb stand!!

[weltzing]

For the large throttle cable spring on the top right side, how many coils is it set to? I took mine apart when I was cleaning them, One time around seems pretty good, but I'm not sure.
Thanks

[Blasbo]

Bookmarked!
Thanks, carbs are next on the list after I get my 836 kit put in and cases buttoned up, etc.

[mono]

I have a 4 into 2 into 1 header with baffle muffler.
I have velocity stacks.
I am using a #35 idle jet.
Mains are #125's.
With two turns out on the pilot air screws it is still lean.
I have one 0.22 shim under the non adjustable needle. Still lean.

Am am going to #42 pilot jets and a second shim under the slide needle.
I will let you know what happens.

Any update, Lucky?  I'm planning a similar setup on mine eventually.

Also, does anyone have a link to a good exploded view of these carbs?  i got a rack that was in some seriously bad shape and had to fully disassemble it.  i did this assuming that there was some online documentation, but my google-fu must be weak today because i haven't found any good diagrams.

- mono

[caferacer750]

very nice post

[AintNoEasyWay]

This thread has helped me a lot with rebuilding my '77 41as. Really think it would do well as a post in the Carb FAQ thread along with everything else there.

[garygnu]

Very helpful info!  Thanks for posting this.

[martin99]

Great post!

Reading Lucky's contribution, just thought it would be an idea to remind folk that shims don't apply on the '77 PD41s, as the needles are adjustable - stock positions top clip groove on the K, second groove down on the F. Hopefully this will help, not confuse!

[flybox1]

dont leave out the accel pump circuit, the brass tubes between the carbs in the accel pump circuit, and the fast idle cam 

[greddm]

Thanks! Just getting ready to tear into mine and couldn't find a good post for the 77-78 carbs.

[lucky]

Great post!

Reading Lucky's contribution, just thought it would be an idea to remind folk that shims don't apply on the '77 PD41s, as the needles are adjustable - stock positions top clip groove on the K, second groove down on the F. Hopefully this will help, not confuse!

Good catch oldskoolnuts!

[Tugboat]

I'd like to add:

Spanner1 discovered that the little brass bung underneath the accelerator pump nozzle can be gently pressed out using a pair of pliers (put some tape on 'em to keep from messing up the brass!). I'm referring to the round brass part just above the right float post, to the left of the top screw hole.


Once you press out the bung, the nozzle will come out so that you can fully clean it.When you put it back in, make sure the little grooves line up so that fuel flow is not obstructed.

[lucky]

Great thread! I'm just about to tear into mine (1978 CB750F) and this will be VERY useful.

Mines running fine, but won't idle without full choke this spring so the first thing I'll clean are the slow jets.

Can the slow jets be cleaned without pulling or am I asking for trouble if I don't?

I'm going to remove the carbs based on some other advice... but was wondering about removing these particular "pushed in"
jets.

You need to remove them to clean them and look though them.
Use a .010 electric guitar high E string. The use carb spray and compressed air after. Guard your eyes!!

If you do not remove them and clean them there is a chance you will have wasted all of your time and will have to completely remove the carbs a second time.

[greddm]

dont leave out the accel pump circuit, the brass tubes between the carbs in the accel pump circuit, and the fast idle cam 

What is the fast idle cam?