Topic: 77-78 CB750 PD42A Carb Rebuild Info

[Old Okie]

Taking a chance, hoping for a response. Still trying to get this 1978 CB750F to run.
It will start and run on partial choke and I know the accelerator pump is not working.
I have not removed the carbs from the bike, I was advised to avoid removing them if
possible because they are sooo hard to get back on the bike. My question is, the screws
are not budging on the accelerator pump, can I remove the whole float bowl
with the pump attached???  Are the carbs that hard to install on the bike.
I have done lots of twins but this is my first 4 cylinder.  Thanks for the help.

[DHuffman]

Taking a chance, hoping for a response. Still trying to get this 1978 CB750F to run.
It will start and run on partial choke and I know the accelerator pump is not working.
I have not removed the carbs from the bike, I was advised to avoid removing them if
possible because they are sooo hard to get back on the bike. My question is, the screws
are not budging on the accelerator pump, can I remove the whole float bowl
with the pump attached???  Are the carbs that hard to install on the bike.
I have done lots of twins but this is my first 4 cylinder.  Thanks for the help.

You should be able to pull the #2 bowl with them on the bike - just be careful when trying to remove the screws as they can get brittle and strip pretty easy. Did the bike sit a long time without being run? My carbs were in pretty rough shape. The holes in the idle jets were really clogged with oxidation (0.10" guitar string works well for them), and so were the small jets for the accelerator pump that shoot fuel into the housing. The accel jets were really a pain to clean - you have to get at them from the back side, so you can't really get to them without pulling the carbs. The rubber diaphragm under the accelerator pump cover on the #2 bowls usually need replaced too - make sure you pay attention what side the spring goes when putting it back together.

I have an extra #2 bowl, but I can't remember if its from a 41A, 42A or a 42B. So if you end up needing one, let me know - it may work for you.

[Old Okie]

I can remove the #2 bowl pretty easily if I do not have to take the pump housing off before removing the bowl. The screws in the pump housing are very tight and ready to strip out.
I have been told to clean the carbs on the bike. Are they that hard to re-install??? I do not
feel like I am getting anywhere cleaning the carbs the bike. The previous owner said the bike ran about a year ago but I am beginning to wonder. Also the bike has a Martek 440 electronic ignition, I need some documentation for this set up. I printed instructions for the ign from an online source but the document is really fuzzy and mostly unreadable.

[Old Okie]

Slowly getting this thing to run. Starts and runs but will not idle, just dies.
Can I actually clean these carbs and not remove them from the bike. I think I
am OK except for the idle circuit. Tell me about the idle system???

[brewsky]

Go ahead and pull the rack.

Leave the rubbers on the engine.

You will need to remove the slow jets and clean them and all the passages in the carb body. Cut a short section of reinforced fuel line to slip over the slow jets, grab them with vice grips, and twist and pull, (do not rock)

When re-installing, bike on centerstand, heat the rubbers with a hair dryer, use some lube (WD40, silicone spray, ru glyde or whatever), straddle the bike with the seat off, grab an outside carb with each hand, rock the rack up and down while pushing forward till you feel them pop back into place. (tie the centerstand to the frame with a tie down so you don't push the bike off)

For the stripped screws, tap the deformed parts back into place with a punch, get a JIS screwdriver, dip the tip in valve grinding compound, tap the screwdriver into the screw head and give a sharp twist. (a little pre soak with liquid wrench and some heat won't hurt)…...if that doesn't work, drill the heads off.

[DHuffman]

When you pull the bowls, also make sure the brass tubes in the centers don't have any cracks in them. That's pretty common. You could also use a dremel to cut a flat cut across the pump screw heads to try to get them off before completely drilling the heads.

[Old Okie]

Leave the rubbers on the engine, that is what I have been looking for.
I will pull the carbs off the bike tomorrow and begin to clean them.
Using fuel line to remove the slow jet if a really good idea. Removing
that jet was bothering me but now I think I know how to get it out.
This CB750F is new to me and I would like to get it going without breaking
anything important. Thanks for the help. More later.

[flybox1]

If the carbs have not been off the bike recently, or the rubber boots are a little hard, the carbs may take a little persuasion to come off.
straddle the bike, grab the carb ends with your hands, and pivot them up and down, while pulling them out of the rubbers.
Best to cover them with a towel to prevent them going 'thud' against the frame or upper case when they pop out.
Getting them back on is a bit easier if you have a light schmear' of lube on the insides of the rubbers(blue grease, motor oil)

The (slow) pilot jet.  Just pull it out with a light twist.  Marring on the outside of it wont hurt.  Crushing it will.  My originals had vice grip marks from the PO 

If you have any issues cleaning or getting the accel pump circuit to work, feel free to PM me while youre working on it...

[flybox1]

This link has great PD carb pictures.  Just dont take the carbs apart from each other.  There is no need to go that far in your case. 
http://forums.sohc4.net/index.php?topic=146893.0

[bluedude432]

If the carbs have not been off the bike recently, or the rubber boots are a little hard, the carbs may take a little persuasion to come off.
straddle the bike, grab the carb ends with your hands, and pivot them up and down, while pulling them out of the rubbers.
Best to cover them with a towel to prevent them going 'thud' against the frame or upper case when they pop out.
Getting them back on is a bit easier if you have a light schmear' of lube on the insides of the rubbers(blue grease, motor oil)

The (slow) pilot jet.  Just pull it out with a light twist.  Marring on the outside of it wont hurt.  Crushing it will.  My originals had vice grip marks from the PO 

If you have any issues cleaning or getting the accel pump circuit to work, feel free to PM me while youre working on it...

Flybox1, I tried to PM you but I guess I'm still too new to the forums to send messages...  I'll just post my question here with my email if you want to take this topic out of this thread.

Hey, I have a set PD42A carbs on a '78 750F3. The issue I think I'm having is in the 2 Mickey mouse holes in the accelerator pump cover. All the ports seem to be clear on the assembly except the tunnel that connects to the bottom check valve. When I blow cleaner through it, I just get blow back. I'm not sure how to clear an obstruction out of it or remove the check valve cover.

Here's a picture for giggles. It's my understanding that fuel would dump down the hole and return up there center of the check valve? Then it would close when the diaphragm pushes down forcing fuel through the adjacent hole and spray up through the carb float&body (which is clear).

Maybe you can PM an answer? Email jdraymond432@yahoo.com

Sent from my Pixel 2 XL using Tapatalk

[madmtnmotors]

Here's a picture for giggles. It's my understanding that fuel would dump down the hole and return up there center of the check valve? Then it would close when the diaphragm pushes down forcing fuel through the adjacent hole and spray up through the carb float&body (which is clear).

Nope, you're going the wrong way. Push a short piece of 5/16 fuel line over the post in the center of the accel pump bowl and try compressed air the other direction. The accelerator pump diaphragm does not "draw" fuel, only pushes. The accel pump bowl is gravity fed through the other hole that aligns with the hole in the bottom of the carburetor bowl. 

Currently working on a set of PD carbs for yet another K8 CB750 and came back to this thread for a little refresher. I am compelled to set the record straight here and confirm that you were indeed correct. The diaphragm pulls from the #2 bowl through the check ball in the bottom of the pump bowl, and up through the check ball at the accel fuel pump crossover at the back of the #2 carb. The arrows indicate direction of flow through the accel pump bowl:

[flybox1]

Wilbur is correct.  you have to squirt fuel down into the center plug, and force it out the hole you are shown squirting into.....
Give it another try....the other direction.

[DHuffman]

Wait - I'm confused. You're saying that you should be able to blow through the check valve from inside the center of the accel pump cover? I thought it was the other way... (not saying you're wrong - I want educated). Mine was seized, so I took it apart. Doesn't the square plug in the center have a spring and ball that seats underneath that would prevent you from blowing from inside the cover?

[bluedude432]

So I took a torch to it (because heat fixes everything right?) and boy I didn't realize that I could deform the aluminum so easily! Well I blistered the metal slightly but it did give me enough play that I could get the check valve and spring out. The ball in there was completely rusted in. When I finally pried it loose I nearly lost it flying across the garage.

Good news it all went back together and it completely works like a charm. A little blistered from a bit to much torch heat but all in all its sealing alright for now with no leaks. If I can ever find one I'd like to replace the float bowl since I also broke the over flow time working on it.

Thank for all the great help!

Sent from my Pixel 2 XL using Tapatalk

[Old Okie]

These are PD42A /QA7 carbs. Carbs are off, no problem there. I am cleaning #2
since it should be the hardest. The pump diaphram is hard as a rock and there
was no rubber gaiter on it. All the clamps were loose like someone had been there
before. I see the check valves and understand how they work, Are the check valves
removable and should I?? The accel pump jets will be hard to clean, are they removable?? 
1 low jet is out but there is still a brass thing down in the hole??? On the low jet I have
pulled out, should it be clear and open straight through from end to end??

[DHuffman]

. I see the check valves and understand how they work, Are the check valves
removable and should I??

Mine was stubborn, so I got a small pick and went in around the sides of the square plug and was able to work it out. Scored it up pretty bad though in the process, but I don't think it affect functionality. I was able to see that the small ball was corroded and stuck to the pump cover body and got it free. Id try soaking in cleaner, and blowing compressed air through it before going that route though.

The accel pump jets will be hard to clean, are they removable??

Yes they are removable, but I'd avoid it at all cost. Mine were pretty plugged too but some guitar strings and a little patience and I was able to clean the hole. You can bend it in a hook and get at it from the front, or go in from the back if the carb assembly is off the bike. I never needed to remove the accel pumps on mine.

1 low jet is out but there is still a brass thing down in the hole??? On the low jet I have
pulled out, should it be clear and open straight through from end to end??

Yes, there is a brass seat in the body.

On the low jet I have
pulled out, should it be clear and open straight through from end to end??

Yes you should be able to see through it, but the hole on the small end is very small. Also make surethe ones up and down the side are clear. Go to your local music shop and buy some 0.009" and 0.010" guitar strings. They work good for cleaning them. Careful you don't expand the holes though - it doesn't take much to make them not work properly, or so I've heard.


Here is a good rebuild video for reference
https://youtu.be/4WSP1lUll2w

[Old Okie]

I'm here, listening and learning. The tiny hole in the carb body
in front of the slide, what does it do???  I will need a accel pump diaphram
and some small "O" rings. Order from?? or check my local dealer??

[flybox1]

4into1.com
theoringstore.com
get buna-n or viton o-rings only....all others will break down in gas.
The only advantage of viton is it has a higher temp tolerance.  Good for engine internals...

[Old Okie]

Time for a progress report. Progress has been slow but I am getting back in the groove andgetting things done. Carbs are off the bike and I think they are clean and ready to be put together and back on the bike. Got the accel pump kit from 4 into 1, it has the missing "O" ring for the pump to carb body spot. All 4 slow jets were totally plugged, what size is the hole in a stock slow jet, #35???
The only uncertainty is the tiny hole in front of the slides, where does it go??? These carbs are not that bad to clean, I left them all connected to the supports. Should finish the carbs and start the bike this week, next week, tires and brakes. The weather has finally changed from summer to fall, temps in the low 80s rather than hith 90s.  My shop is unbearable most summer days and my eye sight is so poor I can not work at night.  I appreciate all the good advice, I could not have came this far with all the help I have received from the forum members.  Mort later. Okie

[flybox1]

yes, #35s are stock, but this creates a lean condition at idle.  Most K7/8 players go to a #40 pilot jet. This makes it easiest to tune teh lean out.
This was also remedied by adding the accel pump. 
When the IMS, pilot jet, and accel pump are right for each other....it makes for some very fun green-light starts.

[Old Okie]

Carbs are clean, putting things back together. Took the battery ground cable off and now I do not know where it was attached other than the battery. Where does the battery cable ground to??

[flybox1]

to the frame.  upper rear engine mount.

[madmtnmotors]

to the frame.  upper rear engine mount.

+1

I usually put the cable end between the engine and the bracket (on the long bolt) so the connecter is touching the engine. 

[Old Okie]

Ready to put the carbs back om the bike today. Not sure the accel jets are clean enough, carb cleaner goes through all ports, some more than others. Due to my poor eyesight I can not see the openings in those jets. Could I remove the hoses that connect the ports and clean them better that way??? I would have to replace the hoses because they are 40 years old and very hard anbd stiff. Suggestions???

[flybox1]

Yes, you can remove the tubes between the carbs and spray into the brass barbs to get a shot thru the accel jets.  Be careful though....the connector tubes each contain two brass restrictor tubes inside.  these are necessary to the system as they take up volume. Be sure to clean them and put them back in the tubes.

Dont put your carbs back on until you test the whole accel pump system (with fuel in the bowls) assuring it works properly.
It needs to be working properly for easy cold starts and great off the line acceleration...