Hey everyone,

Trying to tune this Q-Jet is driving me crazy. I haven't driven my Vette much in the last year, but recently I got motivated to work on it again and decided to get the engine tuned up.

I have a ZZ4 engine in there with the original Q-Jet from the L-48. I have followed Lars' method of Q-Jet tuning, except I also installed an air/fuel gauge in the header collector instead of doing the time-to-distance or speed-at-distance. Here's the problem: I've tried jets from 74 to 78, and primary rods from 39 to 45, and still get the same damn result regardless of the components in there:

Rich at idle, lean while crusing, rich at WOT (secondaries locked out or not). Some knocking at high engine loads.

I found that the highest manifold vaccum at idle is a little rich, so I'm not too worried about that. But nothing I do seems to affect the mixture or performance (besides more or less knocking, I think). Haven't played with the secondary circuit yet.

Any suggestions what could be going on? Thanks.

Dave

 

Why is your car knocking? That should be more due to ignition than carb tuning. Are you getting no more thatn 35-36 degress total advance? With or without the vacuum advance? I assume you sarted with the difference between the primary rods and jets and tried incresing the difference gradually. Have you tried tightening the secondary air valve spring to take the secondaries out of the system to insure they are not opening too early and maskings a primary problem. Have you discarded the Q jet fuel filter in favor of an in-line one.

 

Thanks for the reply. The timing is set up for 36 deg at 3,000 rpm, but an overly lean condition can cause knocking, so they are related. I haven't swapped fuel filters or tightened the secondary air valve spring... might be something to try.

 

I have a 79 L-48 all original. It was rich and rough running at idle. I advanced the timing to about 8deg BTC and richen both mixture screwsto obtain max idle, then turned down idle screw to desired setting. My engine runs much smoother and stronger with car in gear and foot on brake, about 650 RPM and most of rich exhaust smell is gone. I should mention that the first thing I did was have my Q-Jet rebuilt by a guy who did carbs for a living. He knocked out the metal tamper proof caps covering the mixture screws. I think that metal cap crap started in 78 or 79.

 

What kind of A/F figures do you get? Are they all between 0.85-0.95 or are we talking about much wider scale?

 

Lars always says that the majority of carb issues are really ignition related.

 

Something else to look at is the gas in the tank. You say the car has been sitting for a year so I assume the gas is at least a year old? If so it's possible the octane rating has lowered enough to cause problems especially if the timing is a little off. A combination of the two is sure to cause problems. Just a thought.

ltlevil

 

Thanks guys. My air/fuel gauge is the Nordskog analog type, so it doesn't give actual numbers unfortunately. I assume it's a pretty small range but who knows.. The timing is set to factory specs, so I'm not sure what else I should do with that. I've put a couple tanks of gas through it already so the fuel should be okay.

Dave

 

The narrowband air/fuel guage like that is only accurate at 14.7:1. Other than that, all it can tell you is lean or rich. It's not good for tuning, as it could read just a little lean and it is really way lean or visa versa.

 

Yeah, I agree. It's not ideal but it's all I've got.

I guess the real issue that I'm having trouble with is that nothing I do will turn the "lean" crusing mixture to "rich" or the rich WOT mixture to lean. There is some other problem with the carb/engine that is totally masking the effects of jets and primary rods.

Thanks for all the responses! You guys are great.

 

Dave -
Try changing your power piston height to make a change to your cruise mixture - raise it to richen up cruise without affecting WOT. You should still have enough range on the idle mixture screws to lean out idle with the higher piston setting.
Your narrow-band will not show whether or not you're too rich at WOT - it will only show that you're rich as compared to 14.7. Since you want to be running about 12.5 at WOT, you do want the sensor to show rich. Give the car a few WOT passes through at least 3 gears, then shut down immediately and pull a few plugs to see if you're really rich at WOT. If you're sooting the plugs after several WOT passes, you're rich. If not, you're not.
Lars

 

Used Q jets are cheap and really not that hard to rebuild as long as they don't need rebushing. I have 3 similar small blocks in 3 of my chevy's, and when I suspect a carb problem, I'd do a carb swap to separate the goose from the gander. Now I keep an extra rebuilt Q jet on the shelf for just such a purpose, and y'know, 4 out of 5 times the problem wasn't the carb.

 

Thanks guys, I'll definitely give those suggestions a try.

Dave

 

and let us know what you find

 

I'm using the same gauge and trying to set up my Edelbrock right now.

http://forums.corvetteforum.com/show...437&forum_id=3

 

Have you thought of just swaping in the demon carb that is already tunned for the zz4 out of the box?

not a challenge i know but it seems someone already did the tunning with the jets for you this way

 

Knocking at hi engine loads sounds like too much adv. Yes way way too lean will agravate it also and lars nailed that fix for u.

U really need to take notes when u make changes and make only 1 change at a time. But u may need to go back and reduce initial timing and or install stronger/thicker dist springs until the knocking stops. It takes a lot of effort but u can drive the car at certain rpms and then stop reduce timing in small steps to stop the knocking - build your own timing curve, maybe even limit ur adv by installing bushings on the dist plate posts.
Actual in-car road testing is optimum method - just takes forever. Usually though the timiming is adv until knock starts then backed off just less than 1* (if possible) at every 100rpm (if possible). Map ur timing curve with dial back timing light then install springs and bushings to match.

But it does take some doing. I recall years i found a long slow upgrade to use for test runs and made a loop to a small strip mall to hide behind and work in back to change springs. Cops never found me/no tickets though i banged the gears and chirpped the tires for days.

IMHO that narrow band should make the job much easier once u get familiar enough with it. Double check the instructions as the lights should tell u where u are within 1 point or maybe even 1/2 pt A/F.

I think ur moving in the right direction here and just need to sort out the finer details - mech adv, A/F meter, carb pwr piston and rods.
Soon u'll be a tuning pro. cardo0