Topic: 77-78 CB750 PD42A Carb Rebuild Info

[brewsky]

What am I doing wrong??? I can not get the air box on.
It does not go far enough forward to meed the mounting bolts??
Is there a trick??….

I'm waiting on this answer too!
The orientation of the rubber boots seems critical, and there are some indicator marks I believe.
The carb rack/air box boot install seems to be my rubik's cube moment also.

[Erny]

What am I doing wrong??? I can not get the air box on.
It does not go far enough forward to meed the mounting bolts??
Is there a trick??….

I'm waiting on this answer too!
The orientation of the rubber boots seems critical, and there are some indicator marks I believe.
The carb rack/air box boot install seems to be my rubik's cube moment also.

Whats exactly the issue? You took out rubbers from airbox and don't know how to install them back? I did that on my K7, I replaced all rubbber (between carbs and airbox) - I should have pics from that, I guess it is the same on K8

[Erny]

I found these 2 pics - stock boots before replacement

[flybox1]

are you sure you slid the velocity stacks into the airbox far enough that the airbox is locked into the slots on the stacks?
 and....are the carbs fully seated into the boots to the head?

[2wheels]

Are you leaving all 8 hose clamps loose (very loose) while installing the upper air box?

[Old Okie]

I will look at this again today. All 8 clamps??
I expected to just push it on and be done, not necessarily so??

[TwoTired]

I will look at this again today. All 8 clamps??
I expected to just push it on and be done, not necessarily so??

Old rubber hardens and shrinks.

Have you tried treating the interface areas with wintergreen oil and solvent?  It both softens and swells the rubber. Treatment timing is an unknown variable though. So, you have to babysit the treatment.

Cheers,

[Old Okie]

And the answer is, No change. The air box was a pain to install, it would not go forward enough to fit the support bolts. The rubbers are soft and I lubed them generously. and finally pushed had enough to get it on. 
     What I have done so far.. cleaned the carbs twice, air screws 1 turn out, float bowl levels checked with the plastic tube method, acccel pump working, new spark plugs,  Checked compression, 120, 120, 100, 120, Has electronic ignition, timing left as was, it ran at some time, checked valve clearances and probably more things that I do not remember. Checked for air leaks, none found.
No change at all from the first start up. Well maybe a little. Will start, easily, will idle about 1200 rpm, then rpm s go up to 2500. I loosen the idle adjust screw to get the rpm s back down then will not idle. I can maintain a prettty good idle with the throttle but it still revs up at will.  From the beginning not much change.
If I set the idle screw to get it to idle at 1200 it will idle for a few seconds then rev up, if I set the idle screw low enough to keep it from revving it dies, I can keep it running with the accel pump though. When it is idling it sounds pretty good...for a few seconds.     This is beginning to feel like a carb linkage problem???
I did not take the tops off the carbs or mess with the slides, have not synchronized the carbs. Suggestions, I am sure I am not the first one to have this problem.

[flybox1]

It does fit. 
I bet you have something installed incorrectly...but its hard to say what without pictures.
take and post the following pictures

Carbs installed - the engine side boots where the carbs fit on.
Airbox intake rubbers
Top half of the airbox installed only, and specifically the misalignment of the mount holes.

[madmtnmotors]

And the answer is, No change. The air box was a pain to install, it would not go forward enough to fit the support bolts. The rubbers are soft and I lubed them generously. and finally pushed had enough to get it on. 
     What I have done so far.. cleaned the carbs twice, air screws 1 turn out, float bowl levels checked with the plastic tube method, acccel pump working, new spark plugs,  Checked compression, 120, 120, 100, 120, Has electronic ignition, timing left as was, it ran at some time, checked valve clearances and probably more things that I do not remember. Checked for air leaks, none found.
No change at all from the first start up. Well maybe a little. Will start, easily, will idle about 1200 rpm, then rpm s go up to 2500. I loosen the idle adjust screw to get the rpm s back down then will not idle. I can maintain a prettty good idle with the throttle but it still revs up at will.  From the beginning not much change.
If I set the idle screw to get it to idle at 1200 it will idle for a few seconds then rev up, if I set the idle screw low enough to keep it from revving it dies, I can keep it running with the accel pump though. When it is idling it sounds pretty good...for a few seconds.     This is beginning to feel like a carb linkage problem???
I did not take the tops off the carbs or mess with the slides, have not synchronized the carbs. Suggestions, I am sure I am not the first one to have this problem.

This discussion sounds familiar, from another thread IIRC. You talk about adjusting the "idle screw" without mentioning the choke. What results are you getting at different choke settings? There is a "fast idle cam" that comes into play as the choke is closed. As the engine warms up the idle will rise and moving the choke to the open position gradually takes this fast idle cam out of the equation when the choke is fully open, allowing the idle to rest on the idle set screw.

I could not get an answer to this question in the other discussion (different member if memory serves).

You "cleaned the carbs twice" and then you say "I did not take the tops off the carbs". What was the extent of the "cleaning"?

[Old Okie]

Sorry I did not mention the choke. I start the bike with the choke on and fast  idle is working. As the bike warms up I can ease the choke off and the bike will idle for a few seconds, 10 or 15, then the rpm goes up and stays up. If I loosen the idle adjust screw the rpm s come down but then the bike wants to die, I can keep it running if I keep blipping the throttle. Air box on or off makes no difference. This is the way the bike has ran all the time I have worked on it. What should  the idle speed be?? I set me CX500 and my CB400 at about 1200.

Cleaning the carbs was done by removing carbs from the bike, removing the float bowls and removing slow and high jets, running wires of appropriate sizes through all holes and passages that I could find,using carb cleaner and compressed air to blow out the passages. I did not totally dismantle the carbs.

Could the intake vacuum be pulling the choke closed and activating the fast idle???  Grasping at straws here.

[madmtnmotors]

I have had issues with the choke plates sticking closed or partially closed. With the airbox off look at the choke plates as you work the choke cable (bike does not have to be running). There is no "direct connection" between the choke cable and the choke plates. The PD "keyhole" carbs use a spring between the choke linkage and the choke plate rod. Any resistance between the choke plates or the choke rod and the carb body can cause the plates to remain partially (or fully) closed, even when the choke knob is all in.

The fast idle cam is part of the linkage and is not part of the equation when the choke knob is all the way in. If intake vacuum is pulling the choke plates closed then something would be wrong. This would not happen if the carbs are properly assembled.

[Old Okie]

Sooo...if the vacuum pulled the choke plates closed it would not activate the fast idle.
I am going to run it again tomorrow but I have no idea what else to do.  When the rpm goes up after idling, sometimes I get some rough running?? Like it is running between the slow and high system. Could this in any way be ignition related???

[madmtnmotors]

Again, the choke plates are independent of the linkage. No, if vacuum pulled the choke plates closed the fast idle cam would not be part of the equation. If intake vacuum is pulling the choke plates closed then your carbs are not assembled properly. Check for proper operation of the choke plates as I described. Are they binding or sticking closed or partially closed when the knob is pushed all the way in?

If they are moving freely then look elsewhere for the problem. Surging/high idle sounds like a vacuum leak to me, but you should run through all the 3000 mile tune up steps to rule out anything else after you thoroughly eliminate the possibility of a vacuum leak.

[madmtnmotors]

I am pulling close to 10,000 rpm regularly with my K8 dragbike that is sporting PD carbs and I have yet to pull the choke plates closed with intake vacuum. The choke plate pivots on its center. As the choke plate closes there are equal forces on each half of the choke plate. One half of the plate has force trying to close it while the other half has an equal force trying to open it. By pivoting around the center of the plate it is in perfect balance and cannot be "sucked closed" by intake vacuum no matter how hard you try.

[flybox1]

I am pulling close to 10,000 rpm regularly with my K8 dragbike that is sporting PD carbs and I have yet to pull the choke plates closed with intake vacuum. The choke plate pivots on its center. As the choke plate closes there are equal forces on each half of the choke plate. One half of the plate has force trying to close it while the other half has an equal force trying to open it. By pivoting around the center of the plate it is in perfect balance and cannot be "sucked closed" by intake vacuum no matter how hard you try.

Nice explanation, mad.

When was your last vacuum sync?
what type of sync device do you have?
Has the bikes exhaust ever been removed?


...and just for clarification, when the bike idles high and you are trying to get it set at a comfortable idle, are you taking off the choke first, and THEN setting the idle set screw to hold it at 1200rpm?  If different, please explain.

[Old Okie]

Not sure when the carbs were synced, it was running when they parked it. you know the story. I have a set of gauges for a 2 cylinder but have a four
 cylinder set available.I jhave some new info, I think. The small part of linkage on the right side of carb 2 may be part of the problem. When I rev the engine with the throttle the revs stay up and slowly come back to a fair idle. If I push the small tab at the right side of carb 2 the engine revs quickly and slowly returns to idle. (see photo)
When I rev the bike with the throttle this tab moves forward and slowly returns to rest on a stop on the carb bracket. What does this tab do and why does it seem to be the problem.

Starts fine with no choke, a twist of the throttke gives it enough gas to start on. No choke used today. It all works better without the choke. I guess that is progress??

[flybox1]

Vacuum sync the carbs....ASAP.  This needs to be done to remedy your hanging idle...

If hot, yes, it will start without the choke.
If it cold starts without choke, than your idle mixture is too rich.

[madmtnmotors]

Have you removed the airbox and verified the position/operation of the choke plates?

[madmtnmotors]

What does this tab do and why does it seem to be the problem.

Why do you interpret this behavior as a "problem"? That part of the linkage activates the accelerator pump and the behavior you describe indicates that it is at least working, or at least working to some extent. When you push on this linkage you are giving the engine a extra shot of fuel. 

[Old Okie]

I now understand about the tab that works the accel pump and I suppose the slow return of the tab is the pump drawing fuel into the pump chamber. This bike belongs to a family member, he has an uncle who is experienced with 4 cylinder Hondas and the uncle may finish this bike. I have spent too much time and energy and forsaken my other duties while trying to get this bike to run. The other party will likely continue with the info we have developed on this forum. I appreciate all you guys have done for me and I will report progress at it happens.

Where does the little black hose dangling from the carb go???

[Old Okie]

Well, you are not rid of me yet.  While tinkering with this bike I discovered that cylinder 3 and 4 are not running as they should, after some investigation I think I have some bad spark plug caps, more on that later. My question today is, does the 1978 CB750F SuperSport have tube type tires?? I started putting on the new ires and discovered that the old tires have tubes in them. The rims are not marked tube type or tubless.  So what did this bike have originally??

So some of my carb problems may be electrical??

[madmtnmotors]

Well, you are not rid of me yet.

Keep at it!

I think I have some bad spark plug caps.
So some of my carb problems may be electrical??

All part of the 3000 mile maintenance/tune up/inspection

Does the 1978 CB750F SuperSport have tube type tires??

Yes

[flybox1]

So some of my carb problems may be electrical??

HAhahahhaha....can I quote that?!?!?! 

You can run tubes or go tubeless with comstar rims.  The tire will dictate. 
Plug caps should meter at >5000ohms.

[Old Okie]

This thing hs taken a funny turn.
After all these years of mounting my motor cycle tires, I quit. I mounted 1 tire, pinched the tube and made a hicky or 2 on the rim. It would not be so bad but it is not my bike. Now for the hard part, How do I get the scratches off that beautiful Com Star wheel. A eegular Com Star from 1978. I turned 81 today, getting old is not for sissies, This week I have had a colonoscopy and an endoscope down my throat and an injection in my eye. The good news, I am pretty healthy. Back to the wheels, how do I get those scratches off that wheel???

Our discussion about tube type or tubeless does not matter now, I ain't mountin' any more tires but thanks for the help.