Learn me: Diagnosing and tuning and Edelbroke carb

I know the correct answer is to take it off and use it as a doorstop, but for now I'm stuck with it. I've never had a race car that I couldn't hook a laptop up to so the world of mechanical fueling is new to me.

I have a 500cfm Edelbroke carb atop a Chevy 305 H.O.

I can get it to idle all day long, but I put it in gear and the idle just drops and it will die. I find myself left foot braking just so I can keep my foot on the gas just enough to keep it alive.

Under WOT conditions it runs great, with the exception of some intermittant hesitation when I step on it. It sputters a bit, then takes off like a rocket.

Anyone have any idea where I should start with this heap?

Trash in the jets. Pull them out and blow them out with air or use a very fine wire to clear them. May want to replace the fuel filter as a preventative.

Check spark timing too. Also, is the engine hot when this happens? I have a 550 Edelbrock w/ electric choke on a 350 and I found it to be extremely sensitive to heat soak from the intake after the engine warmed up. WOT, pulls like a train, but with a hot engine the darn thing just wouldn't idle in gear without giving it gas or cranking up idle speed. I put one of those plastic phenolic spacers between the carb and intake, and now it idles all day in traffic, in gear, in summer. Maybe that will help you too.

The thing about carburetors, automatics, and idling is that the idle speed has to be set in gear. If you have a car that idles nice at 700 rpm in neutral, and add the drag of the car in gear, it's a pretty good bet that it'll stall. Your 700 rpm needs to be set with a warm engine in gear. It may be (appreciably) higher than that in neutral.

Also, initial timing is important to how the carburetor works. If you don't have enough, it can cause problems. Too much, and it'll spark knock.

snailmont5oh wrote: The thing about carburetors, automatics, and idling is that the idle speed has to be set in gear. If you have a car that idles nice at 700 rpm in neutral, and add the drag of the car in gear, it's a pretty good bet that it'll stall. Your 700 rpm needs to be set with a warm engine in gear. It may be (appreciably) higher than that in neutral. Also, initial timing is important to how the carburetor works. If you don't have enough, it can cause problems. Too much, and it'll spark knock.

This is the case. Plus modern gas is terrible. I do want to say that 90% of all carb issues are really timing issue at least IMO.

Set your idle speed in neutral around 150-170 higher then you want it in gear. Then block the wheels and set the parking brake and put it in drive and check again.

Off idle stumble is accelerator, varnished up jets or emulsion tubes. Same with Vaccum leaks around the base of the carb. If it idles well in neutral and you have good strong vacuum signal then its the above or the timing. Other issue is that you have the throttle plates a little open and are not using the idle circuits to well idle.

Personally I would check that the vacuum advance if you have one does not have a ruptured diafram as well with a Mittyvac or with some mouth suction.

The carb was rebuilt less than 15 miles ago and the fuel filter is brand new.

Problem existed both before and after rebuild.

Stumble off idle like you said in your first post is accelerator pump circuit. Climb up in there and see how much is coming out when you pump the carb manually. You should see a healthy squirt. You could also have the vacuum signal hooked up to the wrong port on the carb.

WOT and idle are easy to tune for, its the transitions that suck. You can get a motor to run at idle with a hole in a bucket dripping fuel into a motor and it will idle.

Also you really need to check your timing and check ported and manifold vacuum to see if you have some issue with the motor. DO NOT trust the timing marks unless you have confirmed them with TDC or built it yourself.

I once chased a off idle acceleration stumble for months and through multiple carburetor rebuilds and cleanings, because I knew the problem was in the idle circuit or accelerator pump.

Then a old timer suggested throwing a set of plugs in it. I told him the plugs only had 3-4K miles on them. He suggested changing them anyways. I had a old set of plugs in my tool box and threw them in. Problem solved.

I now start all carburetor troubleshooting by changing the spark plugs.

A good 90% of fuel problems are ignition problems. Install fresh plugs (Use plain, cheap, copper plugs), check timing and advance mechanisms (and as mentioned, check the timing marks are actually accurate), check for good hot spark (bad coil or wiring, HEI needs a lot of power), etc. Don't be like me and chase your tail for days/weeks on a few different occasions learning this the hard way.

Plugs are new, less than 60 miles on them. The vacuum ports are all in the correct spots.

I'll be picking up a timing light today after work.

Any progress?

None yet. I drove the car to work on Wednesday, then took the tops off and drove it around a while afterwards. once it got up to temp it didn't want to stall as bad. So I think I'm gonna open up the choke a little bit and see if that helps.

I also picked up a nice timing light and a vacuum guage. going to see what I can't do on saturday.

I have to take her on a 100 mile trip on Sunday so hopefully I can get it figured out.

If the plugs have seen any flooding or excess fuel (heated boil over etc) the plugs could be at fault. every MFG of plugs has deleted a step in the process since EFI came around. Glazing the center porcelain is not needed with efi as the mix is closer to ideal. Motorcycles fight with short plug life when out of tune. You might also try closing the gap up .005 and see if the symptoms move in rpm band or are gone.