Recently bought this car and it came with the Edelbrock Q-Jet replacemnet carb. I have the receipt and its a 795 cfm, electric choke, about 4 yrs old. I have the original Rochester, CA, correct #'s for this car that I eventually want to sent to Lars and hopefully use.

But for now..... trying to sort things out....oil leaks, bent wheel, leaking TH400 etc. Engine idles OK, rich but it idles, runs good overall. But when accelerating.. it starts to break up, what I think, is the point the secondaries are trying to open. When I back off its OK, push the throttle some, same thing!

Just installed new AC plugs, wires are new, checked points and dwell, timing, distributer seems OK. I read in another thread by Lars that there should only be 1 filter, car had 2 (in the carb and an inline) removed one....no change???
Any ideas.... would be helpful
Mark

 

When you hit the accelerator does it kinda stumble then eventually get gas flowing and move fine? Then when you let off the throttle and hit it again it stumbles then goes again? If so, check your float level. It needs to be at .420" I believe that was the issue with mine was that the float was way too low.

 

The EDL replacement Q-Jets are jetted extremely lean, and this will cause a bad sag going into WOT and the secondaries. Bump the primary jet sizing up at least 2 sizes and see how it responds.

 

Lars thanks for chiming in...the engine does not stumble or sag it almost dies....sounds like bad misfiring, no backfiring though, engine looses RPM . When I back off it's OK. Almost like it's out of gas. I can run in first or second as long as I want until that point and no problem. First thoughts were the fuel pump was bad. Not so sure about that now.
I plan on sending the original Rochester to you. Just like to get this one running. Part of the sorting out process

Which te better carb? the Edelbrock or Rocheester?
thanks
Mark

 

 

Bad HEI module causes same effect. Accellerate slowly and it is OK but quick on the throttle cause a misfire (module can't fire the rich mixture)

 

Your symptoms are typical lean condition symptoms. Richen it up.

The two carbs are identical and will perform the same if correctly jetted.

 

Thanks I'll give it a try!!!
Mark

 

Lars..What is the fix for a rich idle. Have tried different idle mixture screw settings but doesn.t seem to help much.
Mark

 

Almost sounds like the problem I had with mine. Ended up spending over $500 getting it fixed by having a dyno shop work on it. Was in need of it being fixed asap so I had to go that way.

Technically the edelbrock one would be "better" because it's new, so it'd need less fixing to make it right. Otherwise they're exactly the same. If you have open exhaust you definitely need to bump up the jets a few sizes because stock it's WAY too lean. With my stock exhaust on my '81 it was actually fine with the stock jets, but with the exhaust opened up it became ultra lean and made my plugs all white.

Essentially bump up the jets a couple sizes (stock is Edelbrock 1973 I believe so 1975 would be 2 steps up), probably want to step down a size on the rods on it (Edelbrock 1941 is the size I went to), also check your float level and make sure your choke pull off is working correctly. Make sure your vacuum advance on your HEI is plugged in to the correct port and go from there.

It seems like this may be a common problem with the Edelbrock q-jets.

 

Thanks to everyone for your help!!!

I've been studing Lars paper on Corvette FAQ on tunig the Q-jet. So it seems if I have a rich idle and a lean condition at WOT then I need to go to larger primary jets for WOT. Then go to larger metering rods (one step more than what I change on the jets) to lean out the idle and low speed operation.

I mean it is running so rich the back of the car will be covered in soot after a few days of driving. It is worse on the drivers side

It must be so lean at WOT When the secondaries try to open the car slows down and misfies / pops out the exhasut!!!

Lars paper
http://www.corvettefaq.com/c3/QJetTuningPaper.doc

 

First of all, how is your ignition advance setup? Also, what kind of motor is this?

BigBlockk

Later.....

 

1972 Base 350, TH 400 w/308 rear, no mods. Igition is stock...

After doing some more research. I see that this Edelbrock carb is M4M, which is 1975 and later. So I think the primaries might be OK. But the rods are too big for idle and cruise. Not OK for WOT. I suspect the primary rods do not have the .26 "powere tip", more likely it is the .035 tip. This would cause a lean conition at WOT.

Also this is a 795 CFM carb, way too big in my opinion for my setup. Also .048 (1941) is the smallest rod available and theseate the shot primary rods. Carb came with .050 (1943), .073 primary jets and DR secondary rods
Mark

 

I had the same problem on a 77 L-82 carb. The plastic eccentric under the rear metering rod wishbone was broken. There wasn't enough of it left to tell what the original shape looked like. I bought a few different shaped replacements and got a pretty good response from the third one I tried.

 

Interesting!! Is this part \ inside the carb? I haven't pulled he carb off the car and it's been many years since any Q-Jet rebuilds for me.

One problem with this and any other old carb is that after many years and owners you don't know what's been done or what's missing. You don't have a reliable template or model to follow
Mark

 

Just another thought of maybe why the cab is rich at idle....Would not having a PCV valve conected to the carb have any effect on the carb vacuum... internally and cause the rich mixture?

 

ttt

 

First off, you're correct, the edelbrock you have is based on the M4M late model q-jet. The primary rods are the 35s and not the 26s that we'd like. Unfortunately only the 35s are available (also known as the "M" rods used for trucks.)

As for it being too much CFM, you needa study up on the q-jet some more! It's more of a variable CFM because the primaries are small and secondaries are gigantic. The secondaries don't open right away and the majority of your flow comes through the primaries. This is why everyone says to leave the secondaries alone and focus on tuning the primaries. I'm not the expert on the q-jet but do a google on the quadrajet and read some of the sites about it and you'll understand why that, although it's rated at nearly 800 cfm, it's still an excellent carburetor for small block V8s.

First thing to do on your edelbrock, which has already been stated I believe, is jump your jets up at least two sizes. I had to jump up 4, which is the max available from edelbrock at .077" (pn# 1977).

 

Kalway thanks for your imput. I understand the Q-jets variable secondaries feature, but I am concerned that if I jump the primary jets up. Will these make my already overly rich idle and low speed operation worse? W/O the .026 power tip and limited choices in metering rods and the larger power tip. That's what I mean by this carb maybe to large.
What if I went to a larger jets...say the 75's (#1975) and larger rods, say a .054 (#1947). This would a 2 sze increment increase as compared to a one aize increment increase with the jets. The normal spread is .033 between the jets and rods on the factory setups. In doing so this would richen the top end operation of the primaries and MOST importantly, lean out my idle and low speed problem!!!
Thanks to everyone for their imput. This has been a learning experience. I was ready to toss the Q-jet and go with a Holley Spreadbore. The Holley's are much simpler to work on, but after getting acqainted with the featues of the eQ-jet. I have changed my mind. This carb has alot more to offer in performance, everyday driving and eceonomy.

 

I've contemplated ditching the q-jet many times myself, but then I think of all of the nice features it has and decide it's still a great carb, it just takes a bit of tinkering. Although I think if I ever ditch out on the q-jet, the only reason would be the problem you're seeing which is getting the primary metering rods. Once I finish my exhaust work I'm probably just going to go to this carburetor shop nearby and let them dial it in using a 5 gas analyzer to see how lean/rich it is.

I'm not expert on carburetors so I can't really recommend what jets/rods to use, all I can say is you definitely need to use bigger than the .073 jets it comes with.