Subject says it - After installing an A/F meter to help with tuning my CC Q-jet, I noticed that the mixture would go lean instead of rich when applying mild to medium throttle (less than half-throttle) to accelerate, like say from a stoplight or during highway cruise when you want to pass someone. I'm talking only part throttle that the secondaries shouldn't be involved in. This behaviour is also sometimes accompanied by what seems to be a momentary sag in power.

To me this seems like an accelerator pump shot issue. I've got the Edelbrock performance kit (#1992) and am thinking of installing the high perf. pump but don't want to "fix" a problem that doesn't exist. Any other ideas out there of what I need to do?

BTW, the carb is from my LG4 but it's now on top of a 350 which has me PO'ed because I built that B!@%$ to make close to 400 hp and the carb is the bottleneck! One problem at a time...

 

anybody?

 

What primary jets are installed?

 

I'll have to pull the carb off to check (planning on it in a couple weeks), but should be the stock LG4 #77's. If that's true, you'd think they'd be plenty big enough.

 

stock LG4 77's ? Mine stock was 73s...
If you're at light throttle, ie. less then 3/4 or so, that's before your secondaries, and most likely still with enough vacuum to seat your power piston.... So i'd guess your primary jet/rod ratio might be small...
You've got a beefy engine eh? 77 jets.... what rods? if 50M or something, i'd go down to a smaller one....
pump shot could be it, but that'd explain the momentary sag, but after that, the pump isn't what's leaning it, it'd be the main flow area.
Good luck.

 

Hold on, hold on. Back up, guys.

This is a COMPUTER CONTROLLED Qjet. V8 computer controlled QJets like in the LG-4 and L-69 do have #77 primary jets, but they're the weirdo ones with the "tower" on top containg the (equally weird) computer controlled primary metering rods (which usually spec out at .056-.058" on the "fat" (lean) part of the rod and .026" on the "skinny" (rich) tip of the rod.

If it's running lean and there are no vacuum leaks I would suspect something like too low a float setting or improper adjustment of the rich/lean stops and/or IAB valve.

Even if your accelerator pump was not working at all it would only go lean for a split second as you open the throttle- it would not be a persistent lean condition as the motor chugs up through the rev range. The accelerator pump's job is done 1/4 of a second after you move the throttle arm.

 

Right, forgot about the goofy rods, and didn't realize their numbering system was "off"....

Best of luck then!

 

Be careful with Air to Fuel ratio indicators.

I know from my own experience that if you engine goes really rich really fast, sometimes some cylinders will quit firing. This cause lots of unburnt air to exit out the exhaust, and your meter will show a quick lean condition.

I had this problem when I had way to much pump shot.

I have a large (ported) intake plenum, with most of the intake polished. This cause puddeling of fuel sometimes.

So if going richer and richer doesn't help, try going leaner.

 

I understand the limits of the A/F. However, what I'm seeing is not during just a quick transition but a consistent tendency to lean out during light throttle acceleration. Something that the O2 should pick up on well.

Damon - Thanks for the info on the accelerator pump. Didn't realize it was good for such a short duration. I will have to check the rich/lean stops. When I put the carb on the new engine, I had a heck of a time trying to get any adjustment out of the IAB though... Would a larger idle discharge port help???

 

I wouldn't mess with drilling out anything on a cc-Qjet unless there is no other choice and I don't think you're there yet.

Set your cc-QJet stops as follows:

1. Lean stop: Lightly bottom it and then turn it out 4 full turns.

2. Rich Stop: Set it for 1/8" of total plunger travel AFTER you set the lean stop. You check this by removing the IAB, placing a small screwdriver down through the hole on top of the MC solenoid plate (exactly like where the plunger on the bottom of the IAB touches it) and then make marks on the shaft of the screwdriver using the top of the IAB tower as your guide (use a felt tip magic marker with a fine point on it). Push it all the way down, make a line on the screwdriver shaft with the pen at the top of the IAB tower. Let it up and make another mark on the shaft of the screwdriver. Top and bottom of the MC solenoid's travel. Measure between the 2 lines and keep adjusting the rich stop until the distance between them is exactly 1/8"

3. IAB. Put it back in and then set it 4 turns up from fully bottomed-out.

4. Idle mixture screws: 5 turns out from lightly seated.

That's the "baseline" settings I use for a V8 cc-QJet and they're usually close enough I never have to fiddle with them again.

 

You're the man Damon! Thanks for the advice. I'll update with info when I get this adjustment accomplished.

 

(Sorry for the lengthy post, but the info seemed necessary)
Did some measuring and adjusting last night:

Orig.
lean stop: 6-5/8 turns out
rich stop: 1/16"
IAB: 3-1/4 turns out
idle mix screws: 4 turns out

Changed to
lean stop: 4 turns out
rich stop: 1/8" (barely made it, the adjustment screw was pretty much at top if its travel)
IAB: 4 turns out
idle mix: 5 turns out

Don't know that the adjustments really helped yet or not since I found a bigger problem (see #2 below).

Notes:

1. The advice about removing the IAB to measure rich stop travel was VERY important. I originally tried it from the side ports, as Roe's book seems to suggest but the angled load applied to the MC plunger during measurement seemed to give slightly false low readings. Reading from the center worked much better.

2. Out of "curiosity" I turned both idle screws all the way in with the engine warmed up and idleing. The engine kept running!! I'm assuming this is not good or proper function even for a CC carb The idle screws have never had much, if any, effect on the idle since putting this new engine in, but until now I assumed that was because of their notoriously small diameter.

3. Related to the idle, I found this old post: Interesting Quadrajet tuning "gotcha" He describes a problem almost identical to mine: I'm not stinking rich, but running about 800-850 mv idle and cruise and you can smell fuel in the exhaust. Given this and the fact that my idle screws don't do squat, I'm thinking I've got some big, unmetered fuel leak somewhere. (I never epoxied the fuel well plugs.)

I'm beginning to think my original carb is a wash and that I need to find a couple decent junyard specials to work on.