I have a eldebrock proformer carb #1910 ( 800 cfm i think) any ways, it has a hesitation problem. I have adjusted the secondaries the way lars describes in his paper. When you floor it while driving down the road it does fine. When youre sitting at a light and floor it it bogs down for a second and then take off, sometimes it back fires. I work with a someone that only runs hollys he says I have to much carb for my car and that I am running it to lean (I have the screws set at 2 1/2 turn out, I have a 454, duel exhaust, no cats and a 14x3 round k&n filter which all fits under a stock 77 hood but thats another topic)what should I do to fix the hesitation? I set the car using a vacuum gauge. I run at 700 idle and am pulling 14 on the vacuum gauge? am I to lean? how can I tell? the vacuum dosn't increase if I richen the mixture?

thanks for the help

 

I'm by no means a carb expert but here's my 2 cents.
I have a holley double pumper on mine and it does the same thing. I'm pretty sure it has too much carb on it. I am going to get a smaller carb and see how that works. Probably go with a Edelbrock 600. Not sure yet when I'll get it but I'll let you know how it works out for me. I'll spend two to three hundred and probably sell my old one if it works out.

 

OK, I'll take a shot with my limited knowledge. (BTW, my knowledge is Lars derived).

Idle mixture screws have minimal effect off idle. The secondaries work off demand so at the stop light, they are probably not doing much. My gut feeling is that the stock jetting for the carb is too rich. Confirm this by removing the pin from the accelerator pump arm, then tap the gas (I do it on the carb). This will disable the accel pump. If there is no bog and the engine doesn't die, then you are running too rich on the primary side. Time to rejet. Bit of an iterative process but good way to waste an afternoon. :jester

I would also get the full tech paper from Lars and compare the Edelbrock jetting with the jetting for a comparable engine to the one you have. Carb manufactures tend to jet rich out of the box to avoid destroying engines with a lean condition.

My $.02 worth.

BTW: Hard to overcarb with a Q-Jet since the secondary air valve only opens on demand (the spring loaded part).

Worst case, a quick drive to Denver to see Lars will fix the problem.

:cheers:


[Modified by SteveG75, 11:05 AM 12/14/2001]

 

Steve75 has the right idea: You have an off-idle problem indicating that you're either too rich or too lean. The backfire through the carb indicates too lean.

But before you go chasing the carb, be sure to get your timing set up right. Timing problems often feel like carb problems, and you can't solve your timing issues by tweaking the carb...

If you have an adjustable light, set up your total mechanical timing for 36 degrees and verify that it's coming in nice and smooth. If you don't have an adjustable light, do a quick-and-dirty setup by setting your initial timing at idle without the vac advance hooked up to about 14 degrees. Then rev it a little and verify that the advance is at least working (the timing line should move as you rev it). Hook up your vacuum advance to a ported vacuum source so you get steady, unchanging timing off idle.

Once this is done, take a look at the carb setup if it still hesitates. You'd be hard pressed to overcarburate an engine with a Q-Jet, so toss the idea that the carb is too big. Do like Steve says: knock the pin out of the accel pump lever and pull the lever off (do it with the engine running so you don't have trouble starting the engine back up). Then drive the car up and down your driveway or on a non-busy street. If the problem goes away or improves dramatically, you're too rich. If it gets even worse, or makes no difference at all, you're too lean or you have an inoperable accel pump.

That should give you a good starting point. Try those things out first, and if it doesn't work, let me know, and I'll give you a half-dozen other things to play with.

 

Looked up some numbers when I got home.

All jetting is primary side only

Edelbrock #1910 .074 jets .050 rods
1971 454 Vette w/manual .077/.049
1973 454 all w/ manuall .077/.048

So you are running leaner bot at WOT and cruise than the old 454's. Looks like Lars hit the nail on the head (running too lean). Once again, "we're not worthy" :cool:.

FYI: My 350 with the mods in sig is running .074/.047 primary combo and getting 18-20 mpg on the highway with great response. So I am running a little richer than you at cruise and the same at WOT.

Hope this info helps.

 

Do yourself a favor. Send the carb to lars I just got mine back today and the car runs like brand new. The best $150 I've ever spent! So good that I'm selling the 1901 I bought because my original didnt work! See below.

 

Steve -
Excellent info and observations on the jetting combos. The 74 main jet size should be adequate at WOT for most engines, so I would drop the rod size down to about what you're running to fatten up the mixture at idle & cruise only - this will probably solve the problem. Kinda' fun using that carb listing chart to "see" all the metering areas at cruise and WOT, isn't it?

For those of you who are new, I have a little Q-Jet tuning paper available to our Forum members. This paper has a listing of most of the popular Vette/Chevy Q-Jets and their stock jetting, broken down into actual "metering areas" at cruise and at WOT. Makes for some good comparisons for doing jetting changes. Steve is making good use of this chart when he's talking about the how the carbs are running either richer or leaner at cruise and at WOT. We used this chart and the techniques in the paper when we tuned and set up Steve's car, so Steve has become pretty darned good at using the data and the methods for troubleshooting. Keep spreading the "Q-Jet Gospel," Steve! If anyone needs or wants a copy of the paper, just drop me an e-mail:
V8FastCars@msn.com

Mick -
Great to hear that the carb is running well for you!! Actually, great to hear you have a carb at all...! (we got a little concerned about Mick's carb: I sent it out to him via 2-day Priority Mail, and it didn't show up for 11 days.... we thought it had been installed on a USPS Mail truck).