Hey guys I'm looking for a base line to compare. I just installed a Comp Thumper Int 521 Exh 507 235/249 Duration. I think I have a vacuum leak, anyone have this cam or one close and how much vacuum do you have at idle?

My friend is a machinist and talked my wife into going with the thumper cam in her 69 when we rebuilt the engine. I knew from the start it was way too much cam for the combination but its what she wanted. You have to be careful with the brakes, sometimes they will give you a good scare. I havnt put a guage on it yet but its definately not the best cam for vacuum accessories/brakes.

7 to 9 "hg @ 800rpm. sounds like muffeled firecrackers in your collectors (really cool sound) running the small Roller thumper in 10+:1 compression 350. it's a real scream above 1800 rpms. you'll like it if you have it matched to your compression.

Hey joe, Do you have power brakes?

Sorry for the Hijack...

no, 4 wheel manual discs.,

maybe good for referance my 327 with a comp 274xe, 10 to 1 comp, victor jr manifold, and 600cfm holley pulls 9.5 vac at 750 rpm and no problem with the 9 inch power booster.:yes:

Those cams won't give you much vacuum. I had the small Thumpr in my 396, best I could do was 9" vacuum at 900rpm. 4.5 Power valve, 12 deg initial timing and B-28 vacuum can on the Pertronix. If you are out of tune you may only see 7" or so.

Thats kinda what I thought, I must have a vacuum leak. I have a really nice sounding idle at 600 rpm but I am lucky if I get 5". I have been told to change the timing to at least 20 deg inital and recurve the dist. Will this increase the vacuum? I have been over this motor with a fine tooth comb looking for a vacuum leak and keep coming up empty. Next is to remove intake and check fitment to heads. GM zz383, GM Fast Burn Aluminum Heads, Edelbrock Performer RPM. Any other ideas? Thanks for your help

Thats kinda what I thought, I must have a vacuum leak. I have a really nice sounding idle at 600 rpm but I am lucky if I get 5". I have been told to change the timing to at least 20 deg inital and recurve the dist. Will this increase the vacuum? I have been over this motor with a fine tooth comb looking for a vacuum leak and keep coming up empty. Next is to remove intake and check fitment to heads. GM zz383, GM Fast Burn Aluminum Heads, Edelbrock Performer RPM. Any other ideas? Thanks for your help

Agree with the above, all these Thumper cams are is noise makers, long ago we used to imitate these sounds by pulling manual choke cables when cruising thru the drive inns, we were kool as long as we did not choke motor so bad it would die on us and have a hard time restarting .

You could try to move up timing but that in turn will effect your total advance. You could try moving up idle to 800 or so.
If you have not yet done so, try the WD40 trick at all manifold/carb sealing points to see if the fumes speed up idle or even try to cover the carb airhorn with you hand and see if idle increases, if it does, you found the problem but where is next.

One other thing, I just put together a short block zz383 or a friend, problem here was that the short block did not come with the cam the long block had, Scoggin Dickey had Lunati grind us on as the factory cams were on back order. Friend already had the fast burn heads on a 350. He tells me that he had to get a GM intake gasket to fit the Airgap manifold to seal right on these heads. Something about the thickness of the gasket.

use a soft and easy to compress intake manifold gasket (edelbrock makes a good one). i have never had any luck sealing a SBC with a hard steel reinfored intake manifold gaskets with aluminum heads and aluminum intakes.

you should be able to idle it up to 800rpms with a proper initial timing of 14 or so degrees BTC. at 600 rpm's 5" is not too far off. Is it a thumper, or the mutha or the big mutha thumper? if its mutha or big mutha, 5" may be it!

Mutha Thumper, I spoke with a guy at Summit yesterday and he said GM Performance makes a special thick intake gasket (50 bucks) that will work. I removed the intake and really could not see anything. I'll install the new one when it gets here and see if I can get more than 5. Thanks everyone!

600rpm is way too low for a cam with 230* @.050.
Needs to be at 900-1,000rpm idle in gear, or at regular idle for a manual.
Get a vac gauge and tune the carb to the highest reading, your initial timing should be at least 18* 20* would probably be best.
If you're using vacuum advance, adjust the can, or get the correct can for your vac reading, or it will never run right.