I have several 750s but am having a leaning problem that occurs prior to the power piston kicking up the primary rods (if I go with a stiffer spring, the power system will be on at idle due to low vac from a moderate cam).

Anyway, I was told the 1967 carb I have was not designed with the internals that I need to cover the problem I am having.

So, I can work on of my 750s or try to find an 800. I thought the 800 may give better drivability without a chance for the "lean hole" that I have.

Engine dynoed at 430 HP and 470 ft/lbs at the flywheel and I have this in a '68 Chevelle w/ 700R4 and 3.55 gears, 2,400 stall.

Do you have a carb with APT?

How much vacuum at idle?
Idle speed?
Do your idle mixture screws have little or no effect?
Accelerator pump is new, and properly adjusted?
What is the rod and jet combination on the primary?
Carburator is completely stock otherwise?
If you invert the carb, how much of the transition slot can you see on the primary?

800 cfm Qj is fine on a 383. I've run a 800 cfm Poncho carb on my 327 with good results. You might loose a hair of responsiveness compared to a 750 but that is about it.

The QJet flows only as much air as the engine demands. You can find the 750 Qjet on little 4.3L V-6s. The 800 will be fine.

Thanks for the replys. I emailed Lars who wrote the Q jet tech paper on the corvette web site. He said that the carb I have was original equipment on 1967 327s and has been known to cause problems that I have.

But for those interested, this is my problem:

Anyway, I am running a 750 Q-jet (Carb number is 7037213 and I have 39B rods and 74 jets) on a '68 Chevelle with a 383, 9.7:1 compression, Trick Flow 23 degree heads, Comp XE 274 hydraulic cam, Edelbrock RPM, Headman LT headers, 2.5 inch duals with Dynomax mufflers, 700R4, 2,400 stall, 3.55 gears, Accel 300+ ignition w/ ZZ3 (or 4, I don't remember) HEI.

In gear, the car idles at about 650-700 RPM and I have between 11.5 - 12" of vacuum I have the vacuum advance hooked up at idle to help get as much RPM as possible without exposing the off-idle slots more than .035". I do have small holes drilled in each primary throttle blade and am using the venturi by-pass holes as well. My base timing is at 6 degrees (according to Accel, base should be set about 4 deg less than without their box) and, while I have not checked total timing, I think it is around 32-34. I am running an Eldelbrock power piston that kicks in at 10" vac. I used your (Lars') procedure for setting the APT and I am running 39B rods and a 74 jets.


Now, I have not tried to optimize my tune yet because I cannot get past one problem. That is upon moderate acceleration, the engine goes lean, surges and detonates. Moderate meaning that I still have about 12" vac and maybe going from say, 40-55 MPH, for example (does not really matter the speed but same vacuum conditions in 2nd gear and above, the car has the same problems). If I give the car a little more throttle and get below 10" vac, the surge and detonation goes away (this is b/c the power piston is up and allowing more fuel).


It seems like I need more fuel before the power piston moves up. According to Doug Roe (page 14, "off-idle system") it would seem like I could get more fuel during the time I need it by enlarging the "lower idle air bleeds". It is kind of hard to tell exactly what he is talking about based on his diagram because it looks like when the throttle blades pass these bleeds, you will be at about 50% full throttle which is not my case at all. Anyway, here is what he says, "A larger bleed/feed hole can supply more fuel when the throttle blade passes it. Thus, it can be effective in covering up a "lean hole" or "sag" after the throttle passes the off-idle holes/slots and before the main system starts to flow". He goes on to say that enlarging this hole will give leaner near-closed throttle mixtures which could cause another problem I guess.


It seems like I need a power piston spring that is stiffer than what I have that will start the power system at about 12" of vacuum. Problem is that it will be engaged at idle since that is all the vacuum I have at that time.



I know that the car is going lean because I have a wide-band O2 monitor from Innovate and I have hooked it up along with a vacuum gauge to see what was going on. During the acceleration that I described, the air fuel ratio goes way lean, up to 17:1. At cruise, I actually am a little rich (thanks to the 39B rods) and I have a ratio of about 12.5:1. I thought I would try going rich at cruise to try and fatten things up as a potential cure.



I had this problem before I set the APT and that really does not matter because the 10" spring is not going to do anything with 12" vacuum where the problem occurs.

Wow, 74 jet with 39 rod is stinking rich. 74j 49 rod is ballpark ok.
Is this 17:1 reading a spike or is it continuous? Cause it's not that crazy to have that ratio under those conditions.
What I think is happening, is you have too much vacuum advance, and your mechanical is full in at the same time. So you could have 6i + 18v?? + 24m = 48 deg of advance at cruise, which is WAY to much for the Trick Flow chamber.
For your combination, I would have the following setup, 10 deg initial, 22-24 deg mechanical, and only 10 to 12 deg vacuum. I think then you will find that you are pulling a few more inches of vacuum at idle, and you will be able to go back to a more standard jet / rod combination.

The lean a/f ratio spikes around this particular load situation. However, if I give it just a little more gas so that vac drop to 10", the a/f ratio goes back to normal and detonation goes away (this is b/c my power piston activates at 10").

I ordered a mity vac so I can actually check the vac advance. I guess you feel that my mechanical weights are stuck out? I've not checked this yet but I will.

I have made many incorrect assumptions in the past and maybe I am again but it seems like it is a fuel problem since by giving it a little more gas, kicking in the power circuit, the problems go away.

I don't think their stuck, but if you're cruising down the road at say 2500rpm, your mechanical is pretty much all in. And because you're using almost no throttle, your vacuum advance is full in as well. I agree the stumble is a lean condition, but I think if you sort out your timing, you will be able to tune the power circuit in sooner. Also, you're absolutely sure the accelrator pump is adjusted properly.... The rod is in the inner hole, and there is no gap between the rocker tip and the piston.

Carb number is 7037213Isn't that an A.I.R. carb? If it is, then the jetting will never be right. I'd suspect that any 702---- (or newer) would make a world of difference. IMO, stick with the 750s - better throttle response.

Yes, it is an AIR carb which is supposed to be run with the matching AIR distributor. Best bet would be to get a non AIR carb.

Secondly, I would try to set my initial timing @ 12-16 degrees BTDC. That will give you more vacuum at idle.

Yes, it is an AIR carb which is supposed to be run with the matching AIR distributor. Best bet would be to get a non AIR carb.

Secondly, I would try to set my initial timing @ 12-16 degrees BTDC. That will give you more vacuum at idle.
Thanks alot guys! I am searching for a newer 750 now. I think I will skip the 800 as there were plenty of factory big blocks making similar power to me with the 750.

The reason my timing is not advanced is b/c Accel states to retard the timing by about 4 degrees under what you would normally run. They claim their spark is hotter, lights faster and creates more cylinder pressure sooner and without retarding it, you will get detonation. I know I need to play with it some but since I already had detonation, I wanted to keep it low.

Fix the vacuum advance first. I had a simular problem with a Vortec motor I just built. Ended up shorting the vacuum to 6degs but it cured the problem.