I've put together a 340 with KB hypereutectic pistons, top ring at .027 gap, should be a 10 to 1 motor and the thing is sick. Cranking compression is 120 after several turns of the engine and vacuum is about 4 in. in gear, maybe 10in. at 1000 rpm idle. It has only a few miles of street driving so I wonder if the rings have seated yet. Or maybe I somehow put the cam in wrong, even tho I degreed it I could have had a brain misfire! All I know is it is not very powerful. The combo is almost all edelbrock, new 340 heads, intake, carb, and the cam is the one they list as 308 duration and .508 lift I think, but the duration at .050 is less,, like less than 240 so the cam is not that wild. I put this engine together for a neighbor but refuse to give it back until it runs like it should. edelbrock says 417 hp with this combo but I think 250 is more like it. Any help is appreciated.

I am running the Performer RPM in my 340 with 10.5:1's. My idle vacuum is about 9 InHg at 1000 rpm so you don't sound too far off. Compression on mine is only 120 with plugs in , plugs removed is 180.
Your rings still need to seat, motor is quite new.
Did you break the cam in properly ie 2000+ rpm for 20 minutes? My cam does not even begin to make power until 2000 rpm, and 3000 is more like it.

That's going to need about 15-20* of initial advance to run right.

Is the cranking compression cold? Needs to be hot or the big ring gaps will bleed off a bit.

hi
i think you are right 120 lbs is low !

i would expect 145 or more with 10-1 comp .

i say move it up 4* and re test the comp .

idle, power , vacuum will improve .

what is your idle speed and inital timming set at ?

Where did you degree the cam too? Whats your initial timing at? No, your rings havent seated yet, but I doubt your cranking comp is going to come up to more than 140 or 150 after it does, but thats plenty for it to run good. It sounds like you have the eddy Perf. RPM cam in there, that thing is probably going to like 16 initial, I know mine did. But even at 12 initial you should be pulling more than 4 inches of vaccuum, that cam isnt that big.

Sorry for the delay in answering but I was on Vacation. The cam was broken in properly- 2000 to 2500 rpm for 25 minutes. Compression check was done cold so I will try it hot. Initial timing is 15 degrees but I paid more attention to getting 36 total with vac. advance plugged. The cam was installed straight up and cam out at 106.5 degrees when 107 would have been exactly what Edelbrock recommends. I emailed their tech and he said I adjusted the valves too tight as I went 1/2 turn after the slack was removed on the pushrod and he was adamant that 1/4 turn was the most it should be and to my surprise it helped a little. He never mentioned anything else but I will call again. Thanks for helping and keep the input coming. This thing is worrying me too much.