Topic: Throttle Issues '77 CB750/PD41B Carbs

[zalion0]

Hi all, I was hoping to get some pointers. I have a stock '77 CB750 F2 with PD41B carbs which I am gradually building my mechanical experience with but have an issue when quickly opening the throttle, the engine will die/no longer idle. I've recently cleaned (to the best of my knowledge however doubting it now haha) and replaced all gaskets and o-rings. At the moment I suspect its an accelerator pump issue as this relates to the quick throttle action, if i've understood correctly.

The symptoms I have is the bike will be rough starting, so after many attempts it does start but if you blip the throttle the engine dies. However, if you slowly open the throttle the revs will climb but then take a while to return to idle or I hit the kill switch to please the neighbours. The throttle return cable seems fine. All of the mixture screws are set to spec and when I measured the float height as well these were all approx. 14.5mm as well.

I am wondering if it is running a little rich (as it drinks fuel like you wouldn't believe), since it will usually start with the choke all the way in. Since I've got a few days off this week, I was going to do some further investigation. My plan is (to check the following, any suggestions or comments are welcome ).

- Perform the clear tube method for verifying the float height again.
- Confirm the accelerator pump is working (assuming I can do this with the bike running and airbox off?) and squirting fuel when I blip the accelerator.
- Pull the spark plugs and see what's going on.
- Clean the accelerator pump nozzle again.
- Sync the cabs (need to DIY/buy something for that).

[newday777]

Usually it falls into the 'How exactly did you clean the carbs" category?
Did you put in new float valves from Honda or from some aftermarket carb kits?
You didn't replace the accelerator pump? If not, why not?

[TwoTired]

It does sound like an accelerator pump issue.
Did you verify the two check valve balls are free to block flow in only one direction?
Did you keep the flow restrictors in the tubes between carbs?

With the #2 bowl filled with gas, the “posts” at the carb body entrance mouth should shoot a very strong stream into the throats of the carb with each twist of the throttle.   Just have to remove the filter box to see that.

The hanging idle is usually a too lean idle condition.  Did you yank the pilot jets to clean them.  They pull straight out of the carb body when using a parallel jaw pliers.  Tap back in after cleaning.  But it is best to “prove” the idle circuit flows all the way to carb throat and pilot air jet while pilot jet removed.

Make sure the IM Screws are still needle tipped and not broken, leaving tip blocking the pilot circuit exit to carb throat.  (Happens when overtightened)

If you changed the muffler, it might not have enough back pressure for good idle return without making the pilot mixture richer.  Same is true for no air filter or pod filters.

Be aware, that your bike was sold meeting lean burn requirements from EPA to minimize unburned hydrocarbons exiting exhaust at idle.  All stock components were closely matched to achieve this.  Change induction and/or exhaust, and you have a too lean motor.  Hanging idle is a symptom, and the accelerator pump solved the stubbed with throttle twist.

Cheers,

[kerryb]

I was having similar symptoms with a 76 550f which has no accelerator pump.  While struggling with the carbs someone said to double check the ignition system. After replacing two plug caps the engine started and ran much better.  After adjusting the points (again!) It ran even better.
My method to clean the pump nozzles is a piece of music wire (really tiny) bent into a "J" shape, clamped into a hobby knife handle so I can reach into the carb throat and get at the nozzle opening on the back side.  Has always worked...so far.

[zalion0]

So, after getting the carbs off the bike. Cleaning all of the accelerometer pump and pump nozzles. I can now see the accelerator nozzle spray gas into each carb off the bike! Pretty certain it wasn’t doing this when I removed the airborne (but again first time doing this so not convinced I was looking at the right thing! Going to double check the mixture screws again (1.5 turns is factory I believe…) and give it another go.

[TwoTired]

Be aware that the pump nozzles squirt a good strong 3 inches into the carb throats.  If they don’t do this at each station, the bike will still stumble with too much throttle twist.  If they do, then the engine will respond with whatever you do to throttle.

This Assumes a bike with stock induction and exhaust components.

Cheers,

[zalion0]

Looks like that has helped! The bike now revs properly but after a short while the idle stays high. I’ll be getting some new air box to carb rubbers the the current ones are so loose they do not create a good seal.

[zalion0]

I have noticed something as well, the cover for the clutch plate arms is missing. Would this be causing a problem? i suspect this would cause a leak?

[zalion0]

So, I believe I've resolved the first issue with the revs not returning. Despite my cleaning attempts, the slow jet infact wasn't as clean as I'd thought! After learning some more info about carbs and ensuring that I can now indeed see carb cleaner exit the small hole within the carb slide. After this, the revs now return in a timely fashion!

The issue I have is that the bike will now slowly bog down and not idle correctly when the choke flaps are open (so choke is pushed down). I've attempted to adjust the mixture screws from factory spec (So 1.5 ((or 3 turns...))) but 1/8 or 1/4 and there is minimal adjustment.

I am wondering if the air mixture is incorrect (as in I haven't changed the airfilter yet). It looked clean and I did give it a vacuum, the bike idle/revs fine with the choke pulled out. Once the choke is open (so lever is returned to normal position) this (from what i've worked out!) would allow more air in? With the choke in the normal riding position, when attempting to blip the throttle the bike dies instantly or slowly dies with just idle.

Any suggestions before I order a new air filter and sync gauges.

[TwoTired]

State your intake and exhaust component configuration.

[bryanj]

There are tiny O rings on the mixture screws which when bad give odd symptoms.
On the screw should be a spring, small washer, O ring and ALLarenecessary

[zalion0]

Yep; have replaced those o rings and also got the washer, spring and screw in the right order. I’ve got a new air filter due to arrive in the next few days. As given it is 40yr old (looks clean and has been cleaned…) I’d still like to rule this out.

[Merph]

Hi zalion0.
Did you get the problems resolved? I've recently bought a CB750K7 1978 and have exactly the same issues.
I've stripped the carbs and replaced parts, but no luck.
Any advice would be great - its got the same PD41B carbs

[zalion0]

Hi zalion0.
Did you get the problems resolved? I've recently bought a CB750K7 1978 and have exactly the same issues.
I've stripped the carbs and replaced parts, but no luck.
Any advice would be great - its got the same PD41B carbs

Not yet, chatting to someone recently and they believe it is the float height. Which I've measured but going to do a clear tube test soon to confirm.

[Merph]

I used a set of digital callipers to set the float height, after a few adjustments managed to get the readings between 14.4 - 14.6mm so happy with that.
I also read that the part in the aftermarket carb kits may not be fully compatible, so cleaned and polished the old needle valves, jets and air screw and running much better, just needs the air screw setting up , but managed to get a short test ride.
I used the new float valves from the carb kit as the old ones were U/S.
Also replaced the choke assembly as the old one had been damaged and repaired with "epoxy", now works great.
Good luck