Looking for some help. My '70 LT-1 was running rough and back-firing pretty badly when accelerating (through the exhaust, not through the carb). I checked my timing and it was off...so i adjusted that to 36 degrees which made a big difference. I also did a float bowl adjustment (holley carb) as i found the fuel level in the front bowl was low.. i raised them both so fuel is right at the bottom of the screw hole and visible, just barely dripping out when the screw is out. Last thing i did was an air/fuel adjustment on the carb. I tightened both screws all the way then backed them out 1 1/2 turns. WIth a vaccuum gauge hooked up at the carb i tried to make some further adjustment but didn't really notice a difference making minor adjustments so i left it at about 1 1/2 turns. with the vacuum gauge hooked up at the carb it was reading just under 15 lbs which seemed low...but perhaps i'm not hooking it up properly. Anyway, after making these adjustments i adjusted the screw at the throttle to set the idle at about 750 rpm.

Driving the car the last couple days after these adjustments it seems to run better and less backfiring (it did happen once or twice though but much better than it was). the car idles nice when cold but after driving it for a while i find that it occasionally stalls at red lights. It starts right back up and idles fine again at 750 rpm when warm. eventually though it will stall again when stopped at a light or stop sign. It doesn't do this every time i stop when the car is warm....but even once is too much when you're in traffic

I tried making small adjustments to the air/fuel mix on the carb and double checked the timing and other stuff mentioned above with no success. Any advice on where to go from here?

 

Valve Lash? ...just a thought.

 

Open the idle up 1/4 turn see if it is better, than another 1/4 richer.

It sounds like the carb has junk in it's circuits.. run some Chevron Textron through it.

 

If it is backfiring do you think you blew a Power valve?

Happened to me last night, car was cold and I opened it up a little too much and it popped... then under slight acceleration up a hill at about 2000 rpm it just died... after about 10 minutes it almost fired but I could smell alot of fuel and no where except in the carb....

Pulled the carb and then pulled the lower bowl screws to drain the bowls... front bowl full and REAR BOWL nearly dry...
This indicated to me the rear PV is toast... PV holds closed with Vacuum if blown they just dump fuel into the engine.

Replaced BOTH PV's just for good measure - reinstalled fired right up, and after a minute clearing the engine and exhaust of the RICH RICH mixture, it runs great.

 

Sounds like you are doing it right - If the car gets little use or has repeatedly sat for some time I think the fuel dries up and the passages get all gunked up. Might be a good time to give the carb a good soak, blow out the passages, replace power valve(s) and a new set of gaskets.

Set the main idle speed screw so the transition slot is barely exposed and forms a "square" when you look at it from the bottom of the base plate. If you have this setting too far open, the mixture screw adjustment might not have any effect.

Fuel bowl levels must be set first before anything else is adjusted.

Mix screws start out 1.5 turns out from lightly seated and should be responsive if the throttle blades/idle speed screw are set right as above.

 

I have a 1970 LT-1 short block in my 1969 and have had it for many decades.
You can only adjust total timing to 36 degrees if you are NOT using the original factory stiff springs.
You should not be setting your idle speed to the factory setting of 750, set it to 900.
With the engine running if you turn the mixture screws all the way closed does the engine stall, if not, the power valves should be replaced.
Have you examined the condition of the contacts inside the distributor cap and the condition of the distributor rotor.
I assume you still have the original factory TI ignition, no points?

 

Thanks guys..these are all good suggestions. I'm going to start with the easiest thing which is to re-adjust the air/fuel mix to run more rich and bump the idle up to 900 as suggested above and then if that doesn't work move on to some of the other suggestions.


Yes it does


Correct


Also, i'm not thrilled with the vaccuum reading i'm getting and wondering if it's accurate. Where is the best place to take the reading? Coming out of the carb the small vacuum line has a T ..one side going back to the carb, one going to the distributor and one going to the smog system. do i want to hook up the compression gauge directly to the carb, or is there a better place to take the reading.

 

Since it is a Holley you want to take your idle vacuum reading from a manifold source. This means connecting to a vacuum port that is connected to the carburetor base plate. If you are already getting a reading of 15" Hg at idle where you currently have it connected then you probably found a manifold vacuum source.

Not saying this is a carburetor cleanliness issue...but I once had a contamination (rust powder from an old gas tank) issue that made my carburetor behave in VERY wild/erratic ways. If you suspect cleaning will help then definitely do it.