Hello, I have a stock 68 400 A.T. Recently had my carb rebuilt and have been tuning it. I have been able set my timing at 9B, idle speed at about 500, and fuel mixture at about 20in. I tired to get the most vacuum when adjusting the fuel mixture and the needle is able to stay fairly steady at 20in although I'm not sure how steady it should be, there is a slight noticeable movement in the needle but it never really reaches 21 or 19. I'm not too sure what else to look into to fix this bogging issue other than the accelerator pump which I have not yet adjusted because I'm not exactly sure how and what to achieve when adjusting it. I found this picture but I'm having a hard time understanding it, in particular "make sure fast idle cam follower lever is off steps of fast idle cam". Any help would be appreciated thanks.
Is the bog when you have just normal light acceleration or when you just mash the pedal? The fast idle holds the primary throttle blades open until the choke opens to a certain point. You could have a bog if the choke is not working properly and opening up and the choke pull off diaphragm is damaged. Check if the accelerator pump is working properly and if gas is squirting out into the primaries by opening the throttle lever. But don't do this more than a few times on a engine that isn't running or you could flood the engine with raw fuel. This is assuming the carb was rebuilt properly with a new correct accelerator pump and has all its . Proper springs. And also the check ball is installed properly in the carburetor.
Found out today that the bog is actually present with normal light acceleration and if I mash the pedal. I also noticed when I accelerate the gauge reading drops almost past 0 before it climbs back up to 20. I don't think that's good but I don't know what the gauge should read when the accelerator is pressed. I can see gas squirting in the primaries so I think it's safe to say the accelerator pump is working. The choke is open and moves freely when it's warmed up. Although it takes about 5 seconds of turning the engine over for it to when it's cold so choke could probably use some adjustment to improve cold starts. I not well versed enough to know if that's related to the bogging, I would guess it's not. Also I'm not sure what you mean by checking the ball is installed properly. I'll tighten the spring tension and see if there are any changes.
I think your idle is set a bit low. I think the spec is 600 RPM in gear. In neutral that might be 800-850 RPM. I'd try that. The increased RPM will help increase the vacuum signal in the carb...maybe.
A bog off idle usually indicates a fuel starvation issue. It wouldn't hurt to back the idle mixture screws out 1/4 turn. 19" of vacuum is PLENTY. my build runs at 10-11" and powers my brakes just fine.
I'm a hobbyist. Not a professional. Don't be hatin'!
I would highly recommend getting Cliff Ruggles book on Qjets before you go making any drastic changes. You can also check out his forum and ask your question there as well. He is good about answering personally.
Thanks for all the suggestions hopefully I can really get into it this weekend. By the way forgot to mention I sent my carb to Sparky's. I thought he did a good job but now it seems not so much the case but, I have never sent a carb to get rebuilt before so maybe this is just as good as it gets...