Lean Carburetor Woes

[Yeahrightgreer]

Hey lads, been having an issue with my fuel delivery and thought maybe someone can chime in.

Been having problems with this old Q-Jet for years and finally decided to rebuild it on my ‘88 Dodge P/U with a 360. After soaking/cleaning/rebuilding it runs much better however it has started running extremely lean

When cold and the choke partial it runs great, however once the choke reaches wide open it runs extremely lean and starts to run intermittently. Adjusting the choke and mixture screws make no noticeable difference. Once the engine reaches operating temp, the lean situation intensifies, to the point that once hot, it won’t stall out but if shut off, it won’t restart.

It’s not vapor lock. It will turn over, spark, and start in the “Ignition Start” position and if throttle is held then die when key is released. Mimicking almost exactly as if the “Ignition Run” fuse was blown or a fusible link burnt etc (that’s what I initially thought it was). Once the engine is, it starts again just like normal

The ECU has been replaced, ignition coil, fuel filter, spark plugs, wires, air filter, and all other regular tune up pieces. Never had a problem with vapor lock or fuel overheating


Does it just sound like something as silly as a bad carb to manifold/air horn gasket? That’s what makes sense to me.

Sorry for such novice questions.

 

[Woodruff Carbs]

Sounds like a typical lean Quadrajet. The proper fix is to come down on the idle and main air bleeds, up on the idle feed restrictions and idle down channel restrictions, up slightly on the jetting and squirters.

 

[5wndwcpe]

Sounds like a typical lean Quadrajet. The proper fix is to come down on the idle and main air bleeds, up on the idle feed restrictions and idle down channel restrictions, up slightly on the jetting and squirrels.

English translation - send it to Dave, he'll hook you up.

 

[RJRENTON]

Sounds like a typical lean Quadrajet. The proper fix is to come down on the idle and main air bleeds, up on the idle feed restrictions and idle down channel restrictions, up slightly on the jetting and squirters.

Good advice....but to decrease the main and idle air bleeds is difficult as the origional bleeds were pressed in place during manufacture. And these restrictions may need to be soldered closed and redrilled, using numeric munber drills and a pin vice. The idle feed restrictions can be enlarged SLIGHTLY using the same drills and pin vice. These restrictions may be as small as 0.035" (+/-) or so. Primary jetting increases is easy as there are several suppliers of the jets, primary and secondary metering rods and perhaps primary step up piston & springs.....
Try: www.chicagocarburetor.com or or try Woodruff Carbs as a source for individual parts.
The Q-Jet is sensitive to fuel level because the fuel resevour is small...make sure the float's pull clip is in place to help open the inlet valve quickly as level drops. Try increasing the static fuel kevel SLIGHTLY as the fuel level affects overall mixture richness. Higher fuel levels allows the mixture to feed sooner...therefore slightly richer.
Just my opinion of course.
BOB RENTON

 

[Yeahrightgreer]

Not gonna lie - when it comes to tuning a carb, its like pandoras box for me. Especially this Q-Jet since its unfamiliar to me. Thanks for the insight so far.

Is it even worth finicking with this old thing or just throwing a old Edlebrock 1406 at it?

 

[RJRENTON]

Of course the choice is yours....but the Q-JET is a good carb especially for street use. It would be good to know the number stamped on the carb. There were literally 1000's of Qjet's built. Not all carbs were the same or have the same features but unique to a year, model, engine, emission requirement. Some models had pull over fuel feeds, some adjustable off idle air bleeds and fuel feeds but not all variations had every feature. Jetting, both primary and secondary varied year to year and model to model, which is why you need to be specific as to the model number of your carb.
Stay away from the electronically controlled models, which have an "E" in the model number. The basic model munbers are 4M, 4V, 4MC. M=Manual choke V=Divorced choke MC=Integral choke.
OR Go with the Carter/Edelbrock/Weber design which are readily available and reasonably priced and easily tuned. Just my opinion of course.
BOB RENTON

 

[Yeahrightgreer]

It’s a Rochester design. Model number 17087175.

I have heard from lots that the Q-Jet is a great street carb because it has so many adjustable/tunable parts to it.

Random question - but why would Chrysler of stuck a Q-Jet onto a Dodge truck instead of the Thermoquad they had been using for years up to that point? And even in most applications of the same year? Upon looking it up it looks like they used the Q Jet in very limited applications

 

[Woodruff Carbs]

Because Carter closed its doors in 85’.

 

[RJRENTON]

SORRY, my listings for the Q-Jet don't go up that high, only to 17086----. Perhaps try Woodruff Carbs for more complete listings of any of the variable parts and specs for your listed carb. FYI....some Q-Jet carbs were built, under license from GM/Delco/Rochester Products (RPD), to Carter Carbs, because of capacity limitations/production requirements at Rochester. I believe even Ford used a variation of the Q-Jet on a 429 or 460 cubic inch engine for one or two model years.
HOLLEY also made a "spread bore" design available on the internet used, that you may want to consider for your application. The Carter T-Quad may be worthwhile to consider, but because of limited tuning parts, hampered its acceptance.
BOB RENTON

 

[BSB67]

If it is an original carb, and if this specific condition is new since the rebuild, I would look to see what you did wrong before making physical changes to air bleeds, IFR, and........

 

[Woodruff Carbs]

That carb, in stock form, you will have the idle mixture screws all the way out just to get it to idle, if you are lucky.

 

[BSB67]

But it sounds like a new problem