Almost There - Need Help

[EdM]

As I wrap up my first week of retirement I have had some time to play with my car (73 q code 4 speed Mach 1). The previous owner had the original engine rebuilt in 2005 and the car has 5k on it since the build. It has 9.5:1 CR, an Offenhauser 360* intake, a Comp Cams hydraulic cam with 218* at .050" intake and exhaust, .494" lift at 110* CL, a 750 Holley DP, Hooker long tube headers and a Mallory Unilite. It ran OK but was a pain to start, idled rough and seemed sluggish pulling. I swapped the non-choke Holley out for a Quickfuel HR 680 CFM with vacuum secondaries. Initial timing was set at 8*, the vacuum advance was connected to ported on the carburetor, all by the previous owner. I worked on the timing setting the initial to 13* with 37* total all in at 2500 fps. I swapped the vacuum advance to manifold vacuum and set it to 10* at idle so 23* "total" at idle. The engine idle smoothed up, the vacuum is a steady 13" and the exhaust smell cleaned up. The car now pulls much stronger and cruises at highway speed with a very light throttle and what seems like a quieter exhaust note. Zero detonation in a number of tests. Really nice. That said, I have a flat spot or stumble just off idle or just off lugging (low RPM) if in a higher gear. I have thoroughly checked for vacuum leaks and can find none. Suggestions?

 

[Jeff73Mach1]

Your carb has an accelerator pump. The reason it is necessary is that when you open the throttle, vacuum drops. A carburetor doesn't inject fuel into the engine normally, just when the accelerator pump circuit is activated. The rest of the time fuel is drawn through the carburetor as a result of the pressure differential between the venturi and normal air pressure.

So an accelerator pump is necessary to start a non running engine and to fill in the momentary loss of vacuum when the throttle plates are opened. You may need to make a small adjustment to the timing of the accelerator pump shot or the profile of the cam that operates it in order to eliminate that flat spot.

I prefer manifold vacuum over ported vacuum myself, but 23 degrees total advance at idle seems a bit much to me. I'd pull a couple degrees out so you are all in at 34 degrees and see if that helps with your problem.

 

[rocket366]

IMO

Vacuum advance connected to manifold is "Vacuum Retard".

At idle when vacuum is high you have full advance.

As soon as you open the throttle vacuum drops and timing retards (which will

help with pinging), until centrifugal advance kicks in.

I think there is your stumble.

Adjusting the accelerator pump may take care of it but you will be

adding fuel to cover it up.

I dont use vacuum advance at all. I put in a weight and spring kit to get all

the advance in at about 3K and the motor screams, one of the best mods

I have done however I dont have a concourse car.

Tricky Subject

Good Luck

 

[TommyK]

I agree with all the points Jeff made. I would think with that cam it would make more than 13 inches of vacuum with 23 degrees of initial timing. Might want to check for vacuum leaks and verify TDC on the balancer to make sure the timing reading is accurate.

Adjust the pump arm so that the squirter squirts fuel at the slightest movement of the throttle. Is there a pink pump cam on it now? If so and the problem persists try a white or blue cam. They provide a much more aggresive initial shot.

 

[Jeff73Mach1]

At idle when vacuum is high you have full advance, this allows heat to escape the combustion chamber when the engine is under no load, not having this level of advance at idle is a big contributor to cars that overheat

As soon as you open the throttle vacuum drops and timing retards until centrifugal advance kicks in or your vacuum advance returns. At low rpm reduced advance helps to build low rpm power by increasing the length of time combustion can occur, so this is desirable

Adjusting the accelerator pump keeps the supply of fuel available during the transition. Your carburetor is always transitioning between circuits-that's its job!

 

[rocket366]

I lost a lot of sleep and spent 100 bucks in gas for test drives

doing the ported vs manifold advance trying to figure it out with

an engine with a cam and not much vacuum across the RPM range.

Advance at ported vacuum engine came alive at 4K + but was too late.

Advance at manifold engine was lazy and had no power.

Researching racing engine mods I went with all mechanical advance

and I've been extremely happy with the performance!

The mechanical advance kit was only a few dollars and the best money

I spent for sure.

(Let it be known that I S-Canned the original distributor for a Chevy


type HEI for the 351C)

 

[EdM]

At idle when vacuum is high you have full advance, this allows heat to escape the combustion chamber when the engine is under no load, not having this level of advance at idle is a big contributor to cars that overheat

As soon as you open the throttle vacuum drops and timing retards until centrifugal advance kicks in or your vacuum advance returns. At low rpm reduced advance helps to build low rpm power by increasing the length of time combustion can occur, so this is desirable

Adjusting the accelerator pump keeps the supply of fuel available during the transition. Your carburetor is always transitioning between circuits-that's its job!

Appreciated, those were my next moves. Wanted to be sure I was not missing something. I will play some more on Monday. Thanks again.