Carb problem or wrong cam selected?

[Mechanic No. 4]

Sure hope you can help me out with this one. Customer has a '70 GTO, 400, non ram air, 4 speed, 7040273 carb, Proform HEI distributer with new plug wires, headers, and a Comp Cams #51-225-4 camshaft. I have never seen this car before. Customer complained that the engine used to run better. To me, is seems kinda like the ignition is breaking up. I checked timing, 20 initial and 32 total. I have 13 volts to the coil. Only has 5 in. of manifold vacuum though. I'm figuring the carb needs to be re-calibrated in this application, but I can't rule out a poor camshaft choice of maybe a lobe starting to go round. The cam spec. card he supplied indicated the 106.0 degree intake centerline was changed 105.0 degrees with a 2 degree key in cam. Any input would be greatly appreciated. Thanks in advance!

[lightning boy]

Can you be more specific? Does it have a mis in it, a stumble? Does this happen just revving it or only under load. He said it used to run better, meaning the cam choice wasn't affecting it then. Is it popping (broken valve spring)? check for a vacuum leak 5in seems low even for a big cam. I'm sure others here will chime in.

[Mechanic No. 4]

Definitely not a consistent cylinder misfire. Has quite a cam "chop" at idle, but will idle at about 1000 RPM. Seems to break up as RPM's rise. Wish I had an old fashioned ignition scope, that would answer a lot of questions. You may be onto something with a broken valve spring there's no popping through carb or exhaust though. The 5 inches of vacuum at idle still concerns me. Would you recommend recalibrating the carb in light of the cam installed? Sure wish I had an opportunity to drive this thing to see how it ran before he dropped it off! Thanks for your input Lightning Boy!

[lightning boy]

No.4 I looked up the specs of the cam. Maybe not the best cam choice, but should be making way more than 5in of vacuum. Something wrong here somewhere. Is the vacuum reading steady or does the needle flick back and forth? Ignition problems tend to get worse when it is under load. (like a bad coil.) Does advancing the distributor make the vacuum increase? if so, verify the timing on the balancer. maybe remove a valve cover and rotate the motor over. See if the rocker arms are fully up/down. You need to find the problem with the motor before you start messing with the carb. Just for the heck of it to you have control of the mixture screws on the carb?
Does the engine respond to turning the screws in/out?

[Cliff Ruggles]

Simply a very POOR cam choice.  Even the factory Ram Air IV engines didn't use that much camshaft in them and the big RAIV cam was ground on a 113 LSA.  The Comp XE284 with 240/246 @ .050" on a 110 LSA isn't going to make the grade.  We also don't know the rest of the engine specs and likely the compression ratio is lower than stock which is a common practice these days "for pump gas".

IF you are only seeing 5" vacuum at 1000rpm's with 20 degrees initial timing the answer is already in front of you.  Not sure of the rest of the engine specs and the actual compression ratio but if you are idling that fast with that much timing and only seeing 5" vacuum it's just NOT going to work.......

[Mechanic No. 4]

Thanks guys. I probably won't be able to spend much more time on that job until Monday. I'll check out your suggestions then. I'm pretty sure the vacuum gauge held steady. I'll stay in touch. THANKS