I have a 68 SS396 with a 69 396/375 hp motor. It is a 4 speed. I just replaced the carb with a Edelbrock 750 Performer and it starts cutting out at about 5,000 RPM's and will still climb but very slowly. I adjusted the idle screws with a vacume gauge (I know this does not effect the high end) and got rid of the gas smell at idle (they were really on the rich side). I checked the total timing and it is about 34 degrees at 3,000 rpm's and runs just great below 5K. Do I need to put in smaller or larger secondary jets? I am lost. It has a HEI ignition and if I remember, it has R44 plugs gapped at .65? I replaced those last year. The old carb was also an Edelbrock but was only a 650 cfm. Boy does the carb make a difference but only up to 5K. Should I replace the plugs again with the new carb. The plugs only have about 400 - 600 miles on them but the carb sucked and it would dog around. Any ideas?? :confused:

What kind of coil/hei system are you running?

At higher RPMs it may not be firing fast enough to get a good combustion. You might try playing with the springs on the mechanical advance. Maybe a 6al ignition box?

I just have the stock HEI ignition. Why would that cause problems now and it never did when the engine was new and stock? And how would the advance springs make a difference?

depending on when your timing comes in full, you may be firing too early creating the same explosion in a bigger area. This would mean less compression and less power.

I'd try some new plugs to see if you're lean or rich. Drive around for a little while below 5,000 and then check em'. Then go for a few trips above 5,000 and check em again.

At the higher RPMs you are probably putting in a lot more gas, which would necessitate more spark. The ignition boxes and a high performance coil would be the way to go if you are running rich above 5,000 but not below. They give you 3 to 4 times more spark than the stock setup.

It wouldn't hurt to play with the jets a little. even mess with the float levels. You may get more of a response over in the performance section.

HOpe this helps.

Thanks JakeS! I put one over in the performance section. I will go ahead and change the plugs and maybe bump the total timing up a few degrees. When I get enough spare change, I plan on getting the MSD 6 ignition.

.065 plug gap? Sounds like you're really taxing that Hei coil to me.

Originally posted by Chris Puddicombe:
.065 plug gap? Sounds like you're really taxing that Hei coil to me. Chris has a good point. Try gapping at .045.

I am going to replace the plugs (any suggestions?) and will make sure the gap is at .45

I run an MSD 6 on my 71 Malibu, with a Flame Thrower coil, AC Delco R45TS plugs, & Bosch plug wires. The only problem I have is the number one getting some crud built up on one side of the electrode, probably due to oil. However, I had an Edelbrock 750 & would not wish those carbs on anyone. I lost all kinds of power, the car would begin to nosedive halfway through second gear, & it got lousy mileage to boot. I was running a pressure regulator set at the recommended 6.5 psi, with a Holley high volume pump. As soon as I switched back to a Quadrajet, everything ran perfectly. The power drop off might be due to low fuel level in the bowls. Try adding the .110 needles & seats to see if that helps. As for bigger jets, I believe that they are a preset size, just like the Quadrajets & utilize the power rods. You can purchase different power rods at Checker Auto Parts. But, I would actually recommend switching to a Speed Demon carb - you will be amazed at the performance gains they give you.