Problem...

-
T

threefortyduster

2XSM
Joined
Sep 19, 2004
Messages
165
Reaction score
23
Location
St. Petersburg, FL
Ok, so I am at the point where I need to act like I have never learned anything about engines, tuning, etc. My car has an issue and I cannot for the life of me figure it out.

Specs: 340, X heads, Edelbrock LD340 intake, 650 Speed Demon, 3200 stall converter, 727, 8 3/4 with a 3.91 Sure Grip. It is in a Duster that weighs around 3200 lbs. The cam is aftermarket, but the exact specs aren't known. It has electronic ignition, new plugs/wires/brass cap and rotor. I removed the power brakes and went to manual brakes, the water pump is driven by an electric motor and it has an electric fan. I am using a stock mechanical fuel pump, all new fuel lines and a fuel cell. No air filter at the moment.

The issue is this: The motor idles fine (timing set at 10 btdc) and runs great at idle, obviously firing on all 8. I have my idle set at about 1050 rpm. The problem occurs when you mash the throttle from an idle or with a previous load on it (i.e. staging, etc). The motor will not rev up for the life of it without a horrible stumble and then it likes to puff back up through the carburetor. The vacuum advance is working, as you rev it up slowly, you can see it advancing like it should, but it will only rev up slow. If you floor it, it nearly shuts off and then comes on like a bat out of hell.

Throw out any possible ideas, or if you need other information, let me know, and I'll get it to you. Thanks in advance.
 
Sounds like the carb is lean and not enough pump shot or the squirters are wrong size. What size squirters are in it and what are the jet sizes. Large cams and converters like lots of jet.
 
That is most likely a lean condition of the carb. Not enuff pump shot. Try a faster riseing cam on the carb to get more pump shot at throttle opening.

The advance could also be tooo much.

Your idle speed is way to high and the engine is useing the carbs main circut to run on.
 
Either the accelerater pump (late or no squirt) or the venturi bleed tubes (clogged or dropped from casting) can cause that.
The "Blah" 2,3, "Veroom" song. A favorite tune of the quadrajet. LOL
 
BJR Racing said:
Sounds like the carb is lean and not enough pump shot or the squirters are wrong size. What size squirters are in it and what are the jet sizes. Large cams and converters like lots of jet.
Click to expand...

Its got 68s in the primaries and 72s in the secondaries for the jets. I haven't changed the jets from stock, could be an issue. The carb has been rebuilt, and the guy did it right back to stock stuff. As for the squirter, its all stock per 650 SD. The accelerator pump cams seemed to be sized right as it does squirt any time the throttle is rolled open, so it may be a squirter or jet issue.

I'll lower my idle down to about 850-900 tonight and find a jet pack to start tuning. Thanks a lot, any other input still welcome.
 
the guys hit it on the head, it's the squirter size. first off with the motor OFF move the carb to full open and hold it there with something or someone. now take a .016" feeler gauge and see if it slides between the pump lever and pump cam easy. if it does you good to go. if it doesn't adjust the pump spring until it does. next start by installing larger squirters until the stumble goes away rapping it by hand. next take it out to some back road and try it out, if it still stumbles increase the squirter size until it's gone.
 
I have a similar set up in my dart except it's a 360. I run 72 and 83 jets w/a 50cc pump in the rear. Front acc pump 30cc. When I'm racing I have a 38 squirter in the front. 34 rear and orange pump cam. I also adjust the front pump where there is play at the pump arm at idle, that gives you a bigger shot when you dump it... I leave on my converter 15-1800. Timing 18-20* @ idle 36* @ 2400. I'd replace the fuel pump with an aftermarket one. I run the carter 6270 or 6290. It's the street/race 7 psi pump.
My 2cts
 
threefortyduster said:
Its got 68s in the primaries and 72s in the secondaries for the jets. I haven't changed the jets from stock, could be an issue. The carb has been rebuilt, and the guy did it right back to stock stuff. As for the squirter, its all stock per 650 SD. The accelerator pump cams seemed to be sized right as it does squirt any time the throttle is rolled open, so it may be a squirter or jet issue.

I'll lower my idle down to about 850-900 tonight and find a jet pack to start tuning. Thanks a lot, any other input still welcome.
Click to expand...

I'd put the jets up to about 74/76 in the front and 83/85 in the rear and move the squirter to about a 35 and 40 in the rear.
Then the way that I always adjusted the accelerator pump is to open the throttle wide open and you want to have .020 left before the pump lever would be stopped or fully depressing the pump on the bottom of the bowl. This will give you the full pump shot and not damage the pump. If you run out of travle on the adjusting lever then move the pump cam from position 1 to position 2, this will give you a faster pump shot with less travel. The adjustments would be the same reguardless of the cam position.
 
Thanks a lot guys...as for the back road, those don't exist here, so I'll just piss the neighbors off some more :)

Once I get out of class and can go home, I'll take a look at all of what was mentioned.
 
Out of curiousity. why such a high idle? I think you are lean, but I also think the carb is not set right on the basics. Get the idle to a normal level... If the cams not a huge race unit, it should idle at less than 850 with no effort. If it can't, find out why. Buy a vacuum, gage too.. It will help you se tthe carb properly, and tell you some idea of what the cam size is.
 
moper said:
Out of curiousity. why such a high idle? I think you are lean, but I also think the carb is not set right on the basics. Get the idle to a normal level... If the cams not a huge race unit, it should idle at less than 850 with no effort. If it can't, find out why. Buy a vacuum, gage too.. It will help you se tthe carb properly, and tell you some idea of what the cam size is.
Click to expand...

Most of the reason I still had the idle set high is because it doesn't have to be low for streetability or anything, it goes everywhere on a trailer. As of now, it is about 900 rpm, and it runs just as well at idle there as it did higher. Like I said, idling is not a problem just that hard pop.
 
The reason I ask is, the transition circuit is designed to go from idle to main jet. If you're already on the transition, you may be confusing the carb a little. Leading to issues with the accelerator pump timing. A nice lean idle circuit makes a snappy transition. Also, your power valve acts on vacuum, and you are creating extra by idling high.
 
You must log in or register to reply here.
-

Back
Top