When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
It has two heat shield spacers under the carb. Timing is set at 8 BTDC at 700 RPM (have experimented with different settings). Since it is the LT-1 it has the factory electronic ignition (all parts in the distributor including the electronic control module have been replaced). Idle is 800 RPM.
What I have decided to do is to replace the tank (only thing left). In the records I find where a new sock was purchase I can find no record of it being installed. I did some looking around and could not see where the tank has ever been removed,
Do your idle mixture screws respond well? Can you get a rich roll off and a lean roll off? Or only one? Are they the same both sides? Did you adjust the idle mixture via the best vacuum method?
That could all make a difference, you could be on the edge of "too lean".
Second possibility:You could have the problem I had with my 70 LT-1 with the OEM Holley. After it was rebuilt and soaked the hidden adjustable air bleed screws moved, and the two idle mixture screws responded differently. They had to be uncovered and reset. Drove us crazy for a year til we found them. I finally would up sending it to Holley. Even the Holley techs could not find them until they checked the factory blue-prints. These screws are very rare. One was stripped and they had to supply me a new one. The screws are only on factory installed AIR pump equipped 70 & 71 Holleys. LT-1s, LS6s, & L78s. Not sure if it was ever used on anything else.
If the mixture screws do not respond the way you think they should, as above, or especially if they are uneven, dig further. Pics below.
Clue No. 1 This is my 1975 over the counter LS6 454 carb. It does not have the adjustable idle air bleeds. It was never factory installed. If you look closely at the indentation here, you can still see the machining marks from the pointed tool bit. A car with the factory adjustable air bleeds has a very tiny mixture screw installed in this spot, and it is sealed with a blob of "plumbers putty" I kid you not. If you look close on one of those you will see a small rounded indentation with no sharp point and no trace of machining marks
Same LS6 Holley with fixed air bleeds. C= High Speed air bleed. A=Idle and low speed air bleed B= spot where air bleed adjustment screw goes. This is a 454 engine so you can see how large of a diameter the idle air bleed is at "A" . The brass bushing is very thin. IIRC a normal Holley for a 350 has a smaller sized orifice in this spot. However Clue No. 2 for the adjustable idle air bleeds is NO ORIFICE AT ALL IN THIS SPOT! So how does that carb control idle air? The area at B is factory drilled out and tapped into the idle air bleed orifice channel. Something resembling a 50% sized idle mixture screw is inserted, it is brass, has a ROUNDED end, not pointed, because it is for air, and a small eyeglass sized screwdriver slot for adjustment. It's use was for fine tuning the emission levels on the AIR pumped equipped solid lifter engines. They are normally never seen because the plumbers putty both covers them and locks the screw into position.
The problem is that soaking the carbs during cleaning/rebuilding dries out the plumbers putty, and it becomes cracked or loose, and the screw starts moving around and screwing up your carb calibrations!!!!
I sold my LT-1 15 years ago so I have no pics.
SOMEONE WITH A 1970-71-72 LT-1 SHOULD TAKE PICTURES OF THEIR CARB IN THESE TWO SPOTS AND UPLOAD THEM HERE TO COMPARE WITH MINE AND HELP DOCUMENT THIS PITA ISSUE. I AM SURE IT WILL HELP SOMEONE.
These screws drove me nuts. The car would not idle well or run well. I almost trashed the OEM carb over this issue!
Start the car, Let it idle, look into the carb with the air cleaner off. If you see gas pouring into the carb, the float level is off or the float seats are leaking. From the sound of what you are saying it sounds like the car is flooding out.
The other issue could be a vacuum leak. Plug the vacuum line to the vacuum tank and see if the idle improves - if it does it's a vacuum leak. Easy place is to plug it at the check valve.
A weak coil will also cause the issue - as it heats up it will loose power.
Last edited by BLUE1972; Jun 22, 2020 at 11:23 AM.
1. Eyeglass sized screwdriver. Seat of the pants. Pretty hopeless. Car actually ran different from one week to the next as the screws moved. Pure frustration.
2. Holley Tech - rebuilt carb, redrilled/rethreaded replaced one air bleed screw, set by airflow - much better now, pretty good
3. Emissions meter - CO/HC/CO2 - engine not all stock, headers, so too lean, needed further adjustments to fix flat spot in transition zone between idle circuit and main circuit. Perfect
That car was only 10-12 year sold at the time and a two owner and the carb was a mess!
I cannot imagine how badly "out of whack" some of thee carbs are now.
FWIW The Holley Tech did not even know what I was talking about! 1st time for him!
Last edited by leigh1322; Jun 22, 2020 at 12:01 PM.
1. Eyeglass sized screwdriver. Seat of the pants. Pretty hopeless. Car actually ran different from one week to the next as the screws moved. Pure frustration.
2. Holley Tech - rebuilt carb, redrilled/rethreaded replaced one air bleed screw, set by airflow - much better now, pretty good
3. Emissions meter - CO/HC/CO2 - engine not all stock, headers, so too lean, needed further adjustments to fix flat spot in transition zone between idle circuit and main circuit. Perfect
Original problem:
I am having a problem with my 71 LT-1. I drive for a sort distance (maybe 5 miles) and at a light the idle will become erratic. If I let it go it will die but start up with the accelerator to the floor. When the erratic idle starts it will cough when I give it some gas. If I keep the RPMs up (1200), there does not seem to be a problem. The temp is below 160 more like 130-140. What has been done, the original Holley 780 has been rebuilt twice and flow tested twice, fuel at proper levels in both bowls, new water pump, new fuel pump and filter, fuel lines have been wrapped with reflective tape, timing set and rechecked. The air from the fan blows hard at all speeds. I use non Ethanol Premium Gas, inspection in the tank shows no corrosion. This happens in all weather although if you kick in the secondaries the process will happen sooner. I have looked at vapor lock but the car will start up and idle ok after it dies. Any help would be appreciated.
Cure: The original problem has been solved after chasing it for over a year. I want to thank everyone that has made constructive suggestions. After many hours, I found that two plug wires were bad. (#2 and #7). As the engine heat went up so did the resistance thus causing weak spark. This LT-1 runs better than it ever has.
Thank you!
