Background: I have a freshly rebuilt SBC 350 with 76cc (1.94-1.5) heads a Weiand dual plane intake Holley 600 cfm with vacuum secondaries and a mild flat tappet hydraulic cam: 224 @.050 duration with .465 lift, the LSA is 112 degrees. I have the initial timing set at 14 BTDC.  

Problem: I only get 14 hg of manifold vacuum at idle (800 rpm), further it won't idle smooth without the HEI vacuum advance being connected to full manifold vacuum port, lastly the idle mixture screws are out 2 1/4 turns.

Things I've done: Had the carb checked on a mule motor, pressure checked the manifold for vacuum leaks and have adjusted the valves (1.52 comp cams roller tip rockers)using both the "spin the push rod" and "jiggle the push rod" techniques...
any ideas  

additionally. all other vacuum readings are normal, to include at 2000 rpm the vacuum is 20hg and at 3000 rpm 21hg; also, I can't drive the car as I'm in the middle of a frame off and the body isn't on the frame...  

i am wondering if you have double checked the timing marks on the balancer???? to make sure they are at top dead center???? there are 3 different positions that it could be...  


after you verify the TDC marks line up with actual top dead center on the pistons..

there are screw in the spark plug ajustable stops... i would pull the push rods on that piston when doing it.. just to make sure you are not going to touch a valve with it... when the valves open...


once verified...      bring the engine around to 8 degrees before top dead center...   pull the dist cap and rotor off... turn the housing till the pickup coil and reluctor teeth are pointing at each other and lock it there... that is where the ignition will fire..

makes you look like a super tech to spin a fresh motor over and it starts on the first go round....  

waynep712 thanks or the reply;  I have double checked that the harmonic balancer and my timing tab are compatible.  I have also double checked my cam timing and alignment.  When I installed my distributor (per David Vizard's bible) it aligned perfectly and the engine fired first try;  starts easily (hot or cold) and seems to run great except for the low manifold vacuum.  check out this guys reply on another post:
http://www.hotrodders.com/forum/low-manifold-vacuum-143356.html  

take the number one spark plug out and stuff a piece of paper towel in the hole to plug the hole tight. dont put it to far in just enough to plug the hole tight. tap the key very quickly. tap-tap-tap you will hear and probably see the towel pop when it does stop you are t.d.c remove the dist,cap the rotor should point at the number one plug wire in the cap. set it at 0degree atdc with vaccum un hooked from dist. and check vaccum if still to low your lift is probably to tall  

Help needed, I have the same problem more or less. Engine (351 Cleveland in DeTomaso) runs fine except rough idle. It idles at 13InHg, when in Drive 10InHg. Rev in neutral to 2000 rpm produces 18InHg. Engine is (I think) stock 1978 2V, I've added Holley 600 vacuum and Summit distributor w/o vacuum advance.

Timing: The earlier timing, the higher idle vacuum. I now have the timing so high that a few degrees more makes the starter struggle to turn it when warm. Balancer reads 10 degrees, but I don't trust that.

Carb: Seems to idle a little better the more the idle screws are turned out, I now have them at 3+ revoultions out. Main jets are 64, PV 6.5

The plugs are light, indicating a lean situation. I've pressure tested the manifold w/o carb, and I've tried spraying around the carb base, no leaks. I've also taken one ignition wire off at a time, all 8 cylinders contribute almost equally. Compression is OK.

Note that there are 2 natural vacuum leaks, the PCV and the hot air choke. I've increased the hole for the hot air choke to get it to work properly, but the rough idle problem was there before.

I have another DeTomaso, a Pantera, with same engine and carb. It idles beautifully at 18InHg, idle screws around 1½ revolution out and plugs a little rich. Jets are 58 and PV 6.5. I've tried switching the carbs, no change. How can two similar engines be so different?

Any suggestions?  

In both situations, your cams are fairly mild, but have higher than stock duration. You could be disrupting your vacuum at idle, because your initial timing is too high, even with alot of fuel. With increased duration and overlap, engines will struggle to mix air/fuel properly at low speed RPM. They will operate more efficienctly at higher RPM though. You could try disconnecting your vacuum advance, and reconnecting it to the "timied" vacuum port. My friend tried this. He took the charcoal canister out of my Malibu, hooked it up to his full vacuum port. It smoothed out his idle, not sure as to what it will do to his performance. He called it a "vacuum canister", also said you can make them out of a coffee can if you had to. It sounds pretty "backyard mechanic" to me, but I'm not sure. My cam is still stock. I also went with a very mild cam for my 283 project engine. If yu guys figure it out, please let me know. This way if I run into the same problem.