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About 5 years ago I bought a "custom" QJet from a carb shop in SoCal. The carb was for my completely stock rebuilt '68 427 390hp automatic car. I've never been able to get the idle smooth. The idle mixture screws would only impact idle quality and vacuum when they were turned all the way in (yes, the motor would die). The exhaust had a puff puff out of the tailpipes which I believe indicates a lean mixture. I've set the timing everywhere between 12-18 degrees advanced. Short story .... I pulled out my original (not original to the car) QJet and compared the base plates thinking something has to be impacting the ability of the idle circuit to work. I found that the primary throttle blades had a hole drilled in each of them (see pic). I puta qwik carb kit in the old QJet, put it on and the motor and it idled almost 100% perfect. The idle circuit works good and the drivability is way better. Opinion - will the holes in the primary throttle blades cause a lean condition, both at idle and driving? I think yes. You?
About 5 years ago I bought a "custom" QJet from a carb shop in SoCal. The carb was for my completely stock rebuilt '68 427 390hp automatic car. I've never been able to get the idle smooth. The idle mixture screws would only impact idle quality and vacuum when they were turned all the way in (yes, the motor would die). The exhaust had a puff puff out of the tailpipes which I believe indicates a lean mixture. I've set the timing everywhere between 12-18 degrees advanced. Short story .... I pulled out my original (not original to the car) QJet and compared the base plates thinking something has to be impacting the ability of the idle circuit to work. I found that the primary throttle blades had a hole drilled in each of them (see pic). I puta qwik carb kit in the old QJet, put it on and the motor and it idled almost 100% perfect. The idle circuit works good and the drivability is way better. Opinion - will the holes in the primary throttle blades cause a lean condition, both at idle and driving? I think yes. You?
Typically those holes are for a car equipped with a radical cam not for a stock engine. Did the shop that did the work know what engine the carburetor was being used for?
In your case the idle transfer slot will not be correctly exposed due to the holes.
Can you switch the blades with the other carburetor?
Yes, I actually filled out a spec sheet with all the motor info, referencing "stock" manifold, camshaft, etc. I may switch out the throttle blades. We'll see how my old Qjet does for a while. I have been chasing the poor idle issue for a long time + a lean condition. I don't have a lot of faith in that carb right now.
Thx,.
From: At my Bar drinking and wrenching in Lafayette Colorado
Drilling holes in the throttle blades on a Q-Jet shows that the builder had no idea what he was doing. The Q-Jet has a built-in idle air bypass system that takes the place of having to drill holes in the plates, and throttle opening at idle can be tuned using this stock system on the carb. Drilling holes in the plates will certainly screw up idle quality and tunability, since the t-blade holes will allow excessive airflow through the primary venturies at idle, causing fuel to discharge from the main discharge nozzles. This will cause the idle mixture screws to need to be set almost fully closed, while still producing almost no control over idle mixture and idle quality. It is highly likely that the "builder" did other inappropriate "tuning tricks" to the carb if he did the hole-drill trick. I'll bet there are incorrect mods to the idle air bypass system as well as bad mods to the main and idle air bleeds. The carb pictured appears to be a 1976 small block carb, which makes it an M4M carb. Your '68 takes a 4MV carb, with a massively different calibration and setup. But, yes - those holes are screwing you up.
Lars
Thanks for the comments Lars. I had a suspicion that if I posted something in reference to a QJet I may hear from you. After seeing the holes in the throttle blades I am confident the overall carb is the root cause of tunability and drivability issues. This is a fresh motor with approx 200 miles on it. A lean condition is a death sentence. I don't need to burn any holes in pistons, etc. The plugs are white! I'll be checking them again soon, anticipating that the porcelain will have some color to it. Thx,
I had a Holley carb that had the primaries drilled we hit it with a tiny bit of weld then ground it smooth with a Dremel. Perhaps you could start there.
I had a Holley carb that had the primaries drilled we hit it with a tiny bit of weld then ground it smooth with a Dremel. Perhaps you could start there.
Thx for the comment. I’d probably look to replace the throttle blades, but in this case I’m shelving the carb. It runs lean …. I just don’t trust it.
👍
From: At my Bar drinking and wrenching in Lafayette Colorado
The M4M carbs (like the one you have pictured) are what we call "Large Main Air Bleed" carbs. These carbs have no restrictors installed in either the upper or the lower Main Air Bleeds (MABs), so they typically have very large primary jets to allow adequate (i.e., "marginal") fuel flow at cruise and WOT operation. These carbs typically have #77 primary jets in them, and still run lean enough to pass late 1970's emissions. Compare that to an original 1968 427 Q-Jet with "Small Main Air Bleed" configuration, which uses size 71 jets, even for the big engine. The Small MAB carbs for the small-block engines used #66 jets, and they ran perfectly. If you look at your carb, you can see if it has been modified into a "small MAB" carb to make it suitable for your use. If it is still a "large MAB" carb, it will run lean.
This is a "small MAB" 1969 427 carb. Note the brass insert with the .047" diameter hole in it next to the screw hole. This limits the amount of air that will get "sucked into" the main fuel stream, allowing use of smaller jets to produce a richer air/fuel ratio:
Compare that the a "large MAB" M4M carb. These carbs have no MAB restrictors, and the MABs are sized at .120" - this allows a massive amount of air to "bubble into" the main fuel discharge stream, causing a lean condition for a large-bore performance application. Notice that there is no brass restrictor in the holes next to the screw holes. If your carb looks like this, it will have a lean condition for your '68 427 application:
If it has restrictors installed, it likely has other mods done to it as well, but those mods should be inspected and evaluated to see if they are a good match for your application, and if they have been properly done. Based on the drilled throttle blades, there's a good chance that things have not been done properly...
From: At my Bar drinking and wrenching in Lafayette Colorado
Originally Posted by glackore
BINGO! This looks to be a large MAB M4M carb.
It sure is, which means it's not set up well to support your 427. Very poorly done mods. I'll bet all the internal bleeds are set up wrong, too... you might be best off using that carb to practice your hook shots into the garage trash can...
Lars
Originally Posted by MelWff
Lars can confirm but it also appears to have the wrong screws. Should be countersunk head screws.
Yeah, the "pro builder" pulled some screws out of the barn for that one..! Assuming the same level of attention to detail was given to the rest of the build, it is obvious why the OP is not having a very good experience with that carb...
Lars
QJet Tuning Issues
