Topic: Issues with a 1977 cb750 fuel issue.

[Paintedseat]

Ok. It is a 1977 cb750. Dyna S, valves set, and carb rebuilt by previous owner. I'm pretty sure it is lean. The boots are sealing and it has the factory air box. It will not take full throttle. It will if I have the choke fully closed. You can see in the video

[Paintedseat]

Help.

[Paintedseat]

All the plugs look about like ths

[TwoTired]

Sounds like it falls flat when you give it throttle....

If so, that's likely an accelerator pump issue.

The pump is at the bottom of #2 carb bowl.  There are two check valves that allow it to operate, and there are nozzles at the carb thraot entrance.  When you twist throttle the nozzles ought to shoot 4-6 inches.

You should review the Honda shop manual supplements regarding adjustments related to that pump.

Cheers,

[Paintedseat]

I was think that was what it was. Is there any way to test them without removing CARBS?

[TwoTired]

You can see them squirt if you remove the filter box top and look into the carb throats.

There are many parts to the accel pumps system.  Some can be addressed with the carbs in place.  Might be able to get at what's faulty without carb removal.

I'd start with removing the #2 carb bowl.  You can test, inspect for pump function at that point.  You can also look into the reservoir to see general inards clean condition.  Might tell you a general cleaning is needed.  Or that it is just the pumps system needs attention.

Cheers,

[Paintedseat]

Is the pump vacuum activated or throttle?

[Airborne 82nd]

Is the pump vacuum activated or throttle?

.
It works off your throttle and some linkage not vac. I think TT has it. The setup is in the manual Have you ever had the carbs off? Did it work right before?

[Paintedseat]

I bought 2 months ago and ago. Rode it 4 miles home and I have been working on it since. I was doing a bunch of wiring and cosmetic stuff. Took it for its first ride 2 days ago and that's when I discovered the issue. It runs great besides the full throttle issue. I plan on riding it some more and see if a few more miles will help clean up some issues. It hasn't been ridden much in the last 4 years. The PO said the carbs were cleaned, synced and a few weeks before I bought it. But heck if I know. I pulled the carbs off 2 weeks ago the soak the boots.

[sixthwisconsin]

The accelerator pump check is a good first step. If its bad it will cost you $30 bucks to replace. Ethanol gas will eat them up so if you can get non ethanol, use it. Do you have any idea what jets are in it?

[Paintedseat]

No idea on jets. I am going to just rebuild it I guess. Most kits come with the accelerator pump I assume

[sixthwisconsin]

Nope...
The accelerator pump is a separate purchase. You don't need rebuild kits just the gaskets and reuse the brass jets but you said the PO just rebuilt?

But lets look at this. TwoTired suggested checking the Accelerator pump function first so do that. If it is functioning properly, I would check to see if your jets are properly sized for the set up you are running. Stock intake but what exhaust? Where are your air screws set? I would check them all and see where they are. Have you balanced the carbs? Your plug looks OK but maybe a little lean? Hard to tell. I understand that there is a reddish light coming from the right and the plug is not actually red!

[seanbarney41]

I would make triple sure your boots are viable too... absolutely no leaks!  These 77/78 boots are particularly problematic.

[johans]

The 78k that we just finished a complete rebuild/restoration on had a acc. pump issue even after rebuilding the carbs.( the bike sat for 30 years ) There are a couple of good articles on taking it apart and checking the small check balls and cleaning the passages ( see link below ). The carbs must be perfectly clean inside. Check the acc. jets after you rebuild the carbs on the bench. Saves alot of r&r in case of a problem. Just use a fuel supply hanging above the carbs and check the spray from the pump stems into the bore after whomping on the linkage. No dribble , just a nice strong stream. The last blockage i had was on the #4 carb and i took a can of contact cleaner and placed some small shrink tubing ( sleeved 2 pcs from a harbor freight assortment kit) on the red stem of the can and slipped the other side over the acc. pump jet in the carb throat. That cleaned out the jet perfectly. i would advise doing that to all the acc. pump jets for each carb during the rebuild . There are also restrictors in the small fuel lines that join each carb which are critical to getting that nice built up pressure through the pump jets. The acce. pump is critical to off the line power. I'm glad the 76 SS i'm doing now does not have them =)   I'll try to locate the pic of my contact cleaner setup i did. Also , buy a new pump with diaphragm , its cheap insurance and make sure the holes in the rubber align up well. I had to enlarge mine with a hole punch.

http://www.hondachopper.com/garage/carb_info/accelpump_valve_repair/page1.html

[johans]

Pic of cleaning out acc. pump nozzle on carb