Carb issues. Dieing at low rpms

[Tannerm]

Just installed a 670 street avenger on my 347 with built c6. Ran pretty good from friends house but started acting up a mile from home. I started noticing it when I was cruising down a street and when I'd let off the gas it would slowly drop in rpms. When I put the gas back on again, it was an abrupt transition. As I slowed to a stop at a stop sign, the rpms continued to drop like normal but when it reached the 1200rpm range it began to stumble. As soon as I stopped movIng, it died. From then on, about a 1/2 mile from home, I had to feather the gas when slowing and pop it into neutral with some gas applied (2000rpms) when stopping at a light or stop sign. I made it home finally to have it die once more as I rolled into the garage.
- What could this be? So random and happened on the fly. Ran great up until this issue. We test drove it several times today with no issues like i stated above. After several test runs we were able to adjust the mixture screws and idle screw to where it runs smooth and won't die when popped into gear (idle 900rpm, in gear 750rpm) or with a/c on (650rpm). Now it won't start and stay on without assistance.
- Specs are as follows:
Holley 670 Street Avenger (fresh rebuild, stock jets and squirter, 3.5 power valve for my big cam), Holley blue pump.
- I'm gunna try a fuel filter replacement tomorrow in hopes that this helps. We did this carb swap due to the old demon 750 double pumper being too much carb, and leaking like a sivv. This Holley ran 20x better at idle, punching from the hole, roll on throttle, and just about every way; better.

[garycrist]

Almost sounds like a classic case of vapor-lock. Make sure you cut open the fuel filter and see what you find. The tank sock can also cause as well as exacerbate the problem.

[Baron Von Crowder]

I'd look for a vacuum leak first.

[kingjason]

Almost sounds like a classic case of vapor-lock. Make sure you cut open the fuel filter and see what you find. The tank sock can also cause as well as exacerbate the problem.

If the tank sock is clogged it will run like total shit and not really get up to any speed with out backfiring and sputtering. I almost set my 79 TA on fire because of this issue until and old drag racer up the street suggested that. If it was clogged up enough to affect the low it damn sure would affect the middle and high.

Easy easy way to test tanner is to pull the feeder line off the pump, open gas cap, hit with compressed air and blow some fuel out the filler, hook it back up and it would run perfect iffff that was the issue.

Sounds like trash in the carb. How good is your filtration up until it gets to the carb?

[Damnittsteve]

he also gas no pcv

[Damnittsteve]

If the tank sock is clogged it will run like total shit and not really get up to any speed with out backfiring and sputtering. I almost set my 79 TA on fire because of this issue until and old drag racer up the street suggested that. If it was clogged up enough to affect the low it damn sure would affect the middle and high.

Easy easy way to test tanner is to pull the feeder line off the pump, open gas cap, hit with compressed air and blow some fuel out the filler, hook it back up and it would run perfect iffff that was the issue.

Sounds like trash in the carb. How good is your filtration up until it gets to the carb?

carb was just rebuilt , no trash in it.

[kingjason]

carb was just rebuilt , no trash in it.

Yeah but say Tanners got a filthy ass rusted up tank, and no decent filtration to the front. Just sayin... LOL


Maybe after you guys set it up the throttle linkage was kinda sticky and did not alow it to set all the way down on the idle adjustment? Then after he drove it a tad now its idled down a bit more. Dunno interent diagnosis are always fun.

[Tannerm]

Lol yes they are... Idk just woke up going to go try a few things including the only filter I know of on the truck. Its jsut after the fuel reg, in between that and the carb on the line. Its a red aluminum shaft thats about 2 inches long. The tank looks fairly new. Jim thinks it was a float getting stuck. Its just the damnedest thing... Perfect until im ALMOST home and bam! It runs like poo.

[Tannerm]

Ok. I tapped on the bowls in hopes to loosen any stuck floats, and I played with the idle screw. After several attempts, it would always do the same thing. It'll start and die if it's under 1k. Above 1k with the throttle in, it runsgreat. But once I let go of the pedal, it drops big time.

[Damnittsteve]

shoot the damn thing!

[Baron Von Crowder]

Ok. I tapped on the bowls in hopes to loosen any stuck floats, and I played with the idle screw. After several attempts, it would always do the same thing. It'll start and die if it's under 1k. Above 1k with the throttle in, it runsgreat. But once I let go of the pedal, it drops big time.

Start spraying bbrake cleaner around.

[Tannerm]

I did something similar baron. I used carb cleaner in those little vacuum holes on the venturies (total of 8), and took out the idle air mix screws on both sides and shot more c cleaner in there too. After that I hit all those above with compressed air. It started and had to warm up a bit but was able to take foot off gas and it idle normally. I played with the idle screw and air mix screws a bit till it was idling at about 950rpm. Only issue now is when I pop it into gear it drops to about 6-650rpm. Seems a bit drastic in rpm change IMO. It dips to about 500 and corrects itself to 6-650rpm and stays there. Pop it into park and it goes back to 950. In park, i click on the ac and it again drops to about te same rpm as if I was putting it into gear. When in gear, and I turn on ac, it barely does anything to the rpm range.

[80coupe]

buy a new one problem solved!

[Damnittsteve]

buy a new one problem solved!

speak for yourself

[80coupe]

speak for yourself

i am i rebuilt 2 carbs finally had to go buy one!