Reply to "351C typical vacuum readings"

Most likely is the power valve at this point.

If you are using an automatic choke, make sure that the choke is set right in the middle of the dial on "index" and that it is working correctly.

If you try to start the engine by blocking the choke open it will raise the possibility of the engine backfiring, particularly if the air temp is cold.

If it does backfire it is probably from that, i.e., the choke making the starting mixture too lean and depending on the timing of the cam events, make it backfire just a little, or a lot.

Don't dump just gas into the carb instead of starting fluid.


You can confirm a bad power valve by carefully draining the fuel from the carb through the lower fuel bowl screw holes, then removing the primary fuel bowl and metering body.

If there is evidence of fuel in the vacuum chamber, if it is wet, between the metering block body and the main body of the carb, then it is leaking through the power valve diaphram.

That is a blown power valve.


Verify that you do have a checkvalve in this carb. It will be press fit into the throttle body in the vacuum hole that provides vacuum to that chamber from the intake manifold.

It is most likely that you do not have one BUT these check valves are so sensitive to backfire that even if you do, it can still blow out the power valve.

OR you can just try another Holley carb that you know is ok?


The best vacuum at idle with that cam is going to be in the 12-14 inch area, depending on how much initial timing the engine has.

It probably is going to need to be 16 to 18 degrees. That you need to decide using a vacuum gauge once you are up and running dependably.

Also verify the fuel level in the carb. Even new Holley carbs can have that set too high. I set that so that the fuel is right at the bottom of the vision port.

Fuel with even just 10% ethanol will cause the float to be set too high. Holley unfortunately does not take that into consideration and sets the floats like it was 1970 with leaded gas.

Why? I don't know but it just makes this all more complicated.

You need lots of experience with Holleys which means usually lots of grey hair. Too many mechanics are "fuel injection" guys and don't know this stuff until they wind up with a carb fire. Then that burns off their eyebrows and their grey hair.

Once you have confirmed that the power valve is good, I would recommend that you use a spray can of engine "starter fluid" sprayed into the carb.



In my cars that tend to sit for too long to retain fuel in the carbs (which is most of them) I've installed and electric fuel pump IN ADDITION to the mechanical pump.

What this does is prime the carb (or carbs). It just takes about 20 seconds to fill the bowls and makes it much easier, less stressful and safer to start a "dry engine".

I recommend that to everyone. It helps as a safety factor also.