I had a similar problem, it was the power valve.

 

What are your ignition timing specs?? Initial advance, how much mechanical advance adds, and how much vacuum advance adds if you are using it??.

What is the ignition system?? Points, HEI, or an aftermarket CDI box??


A brief description of what parts are in the engine would also help.

 

-+you need to drop down to 6.5 power valve , as stated above check your timing , do u have vacuume advance

 

Yes on vacuum advance but I limit it to about 10 degrees with a restrictor. I'm running 12 degrees initial + 20 degrees of mechanical (or is it called centrifugal?) + 10 degrees vacuum for a total of 42 degrees. I see about 22 degrees at an idle with the vacuum advance connected. I'm using medium springs and the mechanical advance starts at about 1000 RPM and is all in at 2400 RPM.

Please help me understand Power Valve performance? To size it isn't the rule to use 1/2 of the idle manifold vacuum? As mentioned I see about 18 " Hg at an idle, I would think a 9.5 Power Valve would be correct. Also I assume the Power Valve opens to let more fuel enter the engine when the vacuum is reduced to the numerical number of the valve. If this is correct I can see that going to a smaller valve will reduce the amount of fuel entering the engine as the throttle would need to be opened more to reduce the vacuum and open the valve. Does that sound correct? Thanks... Al

 

Check the power valve diaphragm for tears first, consider returning jetting to original....then check float levels. I would doubt, at this point, if timing is an issue but timing on the FACTORY 383 can vary a little depending upon what cylinder head is being used. The fast burn heads usually requiring the least, but intake manifold may have quite an impact as we found out on the dyno with a ZZ383. Is fuel pressure around 6 lbs or do you need a regulator to bring it down, were previous carbs of known quality, did they perform OK on another motor? Just tossing out some possibilities.

 

Are those a .060 gap plug? if so I would get rid of them and go with something R45LTS, do you have a MSD 6 on the car? it will help clean the plugs up at idle that's what that plug looks like to me. Very rich at low speed, what list number Holley is the 600?

 

Check your float levels. You can change jets till ur blue in the face. If your float level is too high you’ll be pig rich regardless.

Also with your Holley… you need to check the transfer slot. If you have the throttle blades too far open at idle and expose too much of the transfer slot, you’ll be idling on the primary circuit. Also when you have too much transfer slot showing you’ve now lost the transition slots ability to do its job in providing fuel as you come off idle. Now you’ve got a hesitation that seems like it should be fixed with an accerator pump nozzle…. Thats where you start to chase your tail…

Also if you really want to know the EXACT power valve that is correct for your carb… you need to hook a vacuum gauge up and tape it to the windshield or get a long enough hose to get it in the car. Now at cruising speed see how much vacuum you’re pulling. Now get a power valve 1 or 2 numbers less than that. This will ensure you’re not running around on the enrichment circuit.

 

Thanks to all for the advice, here are some answers to the questions;

Auto Trans? - It is a three pedal car, standard transmission.

Are those a .060 gap plug? if so I would get rid of them and go with something R45LTS, do you have a MSD 6 on the car? it will help clean the plugs up at idle that's what that plug looks like to me. Very rich at low speed, what list number Holley is the 600? - The plugs are R45LTS gaped at 0.035", standard points & condenser, the carburetor is a 3043 series as found on 1965 L79 engines, I believe the list is 4150.

Check your float levels. - I've done that. I adjusted the float level such that with the sight screws removed little or no fuel exits from the holes.

Also with your Holley… you need to check the transfer slot. If you have the throttle blades too far open at idle and expose too much of the transfer slot, you’ll be idling on the primary circuit. Also when you have too much transfer slot showing you’ve now lost the transition slots ability to do its job in providing fuel as you come off idle. Now you’ve got a hesitation that seems like it should be fixed with an accerator pump nozzle…. Thats where you start to chase your tail… - Is the check for this to screw the idle mixture screws all the way in and if the engine stops the throttle blades are correct?

Also if you really want to know the EXACT power valve that is correct for your carb… you need to hook a vacuum gauge up and tape it to the windshield or get a long enough hose to get it in the car. Now at cruising speed see how much vacuum you’re pulling. Now get a power valve 1 or 2 numbers less than that. This will ensure you’re not running around on the enrichment circuit. - OK I can do that. To clarify if the cruising vacuum reading is 15" what power valve should I be using?

 

Rule 1 is ignition first then fuel. I would be checking the points, condensor and coil. Any weak areas there and you"ll never burn the fuel.
Have you checked valve lash? Too tight and you'll have issues but with your vacuum readings this is not likely.

 

a 6.5 power valve would be pretty close and that seems to be what a lot of people are running

 

stay with the 9.5 P/V you really need to see what your vacuum is when cruising on the hi way;has NOTHING to do with idle vacuum. To me it looks like weak spark? a blown P/V would make the car barely run so you can cross that off,are the mixture screws working/sensitive to adjustment? i would try a fresh coil if carb is working as it should. you'll get it!