[thadleybaxter]

looking for knowledge...Holley carb tuned to my best, following instructions yet it stalls when putting into drive??? Cant seem to keep running on lower idle speeds either. Could it be Intake manifold leaks or vacuum or???

 

[dana44]

First thing you need to do is get a vacuum check to see what power valve you have and what you need. The power valve is what helps give that little bit of gas through a slot that is just at the edge of the butterflies on the primaries, and the way to determine what power valve you need is, at idle, check the vacuum. Lets say you have 16 inches of vacuum at idle. Take the number and divide by 2, which gives you 8, then add 2, meaning you want a 10.5 power valve. What this means is it will give just a tiny bit extra gas when the vacuum drops to 10.5 inches and keep the idle from falling out and dying.

Is the rest of the engine relatively stock other than the carb?

 

[thadleybaxter]

engine is completely stock...carb was rebuilt a few years ago in the lower desert of AZ, and now up at 4500' elevation, 3000' above the build elevation still in AZ. and how do I check said power valve?

First thing you need to do is get a vacuum check to see what power valve you have and what you need. The power valve is what helps give that little bit of gas through a slot that is just at the edge of the butterflies on the primaries, and the way to determine what power valve you need is, at idle, check the vacuum. Lets say you have 16 inches of vacuum at idle. Take the number and divide by 2, which gives you 8, then add 2, meaning you want a 10.5 power valve. What this means is it will give just a tiny bit extra gas when the vacuum drops to 10.5 inches and keep the idle from falling out and dying.

Is the rest of the engine relatively stock other than the carb?

 

[dana44]

Two things right now. Remember, an engine requires leaning out at altitude, less air, needs less fuel, so that is one thing, putting in gear makes the engine run richer, altitude does the same thing.

First, check the vacuum of the engine with a vacuum gauge. Watch the needle for even holding of vacuum. Follow directions for calculating power valve size.

The second thing is the needle adjustment, which, if you said it has been followed, should be set at real close to 1.5 turns out after being bottomed out gently so as not to damage the needle ends. If the adjustment is done with the engine running, or if you can turn the needles in without the engine dying, they are too fat and need to be changed two sizes smaller (two digit number on the side of the jets). If the carb was rebuilt and you are at altitude now, chances are they need to be downsized. If, for example, with the needles set and the engine warmed, if you get the needles down to half a turn out and she still runs, downsize. If it dies when they get to half turn out, they are OK.

The power valve is located on the metering block. Drain the front bowl (I use a rattle paint can lid to catch the gas when one of the bottom bolts is removed, catch the fuel so it isn't all over the engine. Gently remove the front bowl and you will see the valve on either side of the brass jets, (15/16ths socket or wrench), unscrew it and read the number printed on the wrench sides of the valve. Follow the sizing of a new one.

Now, the reason for dying when putting in gear is the vacuum is dropping and the power valve is kicking in and flooding the engine, or not kicking in until the idle drops too much for the power valve to catch it, which is why it has to be adjusted properly, especially now that it is at such high altitude. Best place to check the vacuum is from a vacuum source at the base of the carb, not the distributor line, should be a nipple on the bottom front of the carb base.