I just broke in my new engine, and it has poor vacuum at idle and runs like crap till 2,000 rpm.a bit on the combo,400 sb+ 30,ported 487 heads,engle cam ep 22 .491 lift, 238 duration @0.50.I am running my cars original 594 GM intake and 780 carb which were swapped as a set from my original lt-1 [2 days ago]Break in went fine,retorque every thing,just terrible shake at 900 rpm and only 5" of vac[checked gauge works fine] cylinders 1,3,2,4 plugs are black as sin,others ok coloring,Ignition and wires are directly removed from my original engine [motor ran fine just tired]I checked topside for a vac leak, [carb spray and propane method] nothing i'm thinking I'm leaking from beneath the intake but want some in put first.my bolt holes all lined up fine on assembly [I did not have heads shaved don't know if previously done] block has not been decked.I used a napa [victor] gasket as I've had good luck with them in past,[seem to compress better than the "hard" felpro] timing is @ 8 degrees btdc and I can get adjustment out of the idle screws. At 2,500 rpm I have 18" of vac. if heads are shaved is there a formula for "shaving" the intake or rule of thumb?? Where do I go from here??I'm very dishearted about this but thanks again for putting up with this epic Steve!!!!

You need a lower power valve number. If you're idling at 5 inches of vacuum you're below anything that came stock in pretty much any Holley. You need one rated at 2 or 3 inches before it opens.

Eight degrees of initial timing, 5 inches of vacuum, 238 duration @ .050, yep - sounds about right for a shaking combination. Get your hands on a dial back timing light or put a timing tape one the balancer and set total timing (w/out vacuum) at 36*. That will get you in the right direction. Dont fret if initial winds up in the 14-18* range. Unless there has been a lot of incorrect machining, I doubt you have a leak under the intake - which I would replace immediatly anyway with a Eddy RPM or equivelant.

------------------
79 Z28 Bought new, honeymoon ride, same wife, same car.

What Dave said. Stock PV is 6 I think.

That cam's too small to cause vacuum that low. Especially in a 400. You should be pulling at least twice that. I'd check valve adjustment (down too tight and hanging a valve or two off it's seat) as well as plug wire routing and/or other ignition related problems.

This sounds like classic "valves adjusted down too far" symptoms. Idles worse than a Top Fuel motor but clears up and runs reasonably well if you raise the RPMs up.



[This message has been edited by Damon (edited September 06, 2005).]

Damon,Thank you for your help past and present with this motor, the cam is a solid flat tappet btw,I set the lash about 4 thou more than the card just in case things "tightened" up on seating of valves etc,I will recheck lash but I fired it tonite dead cold and does the same thing. Please enlighten me a bit how or why could a "miss" cause low vac [one or two cylinders not pulling their load???]not being smart at all in the phrasing.I never have encountered that [but also never checked an engines vac with a miss.]Does my thinking of a underside leak make sense??? I would think it would push water if that was the case [15# cap that does hold pressure.] I will keep posted on progress later this week P.S, vac gauge is as steady as can be thru the range Thanks again Steve!!!

<font face="Arial,Verdana" size="2">Originally posted by 72'z'steve:
P.S, vac gauge is as steady as can be thru the range </font>

Then its not the valves.

I ran the valves tonite, all were okay, engine starts right up at half turn of key,I was thinking cam timing may be off,but builder "degreed" it yeah right,tru roller with 3 keyways installed "straight up" marks did line up before tin was put on. I was thinking if valve or two was hanging vac would flutter??been nearly 25 years since auto shop.I've pulled small blocks apart that I could skip the timing chain by hand that still"ran" so My thought of cam timing is probably a stretch.I had this engine built by one of the best speed shops in my area so I wouldn't have this crap,getting frustrated Does my intake theory hold reason?? I can get adjustment out of my idle screws which also perplexes me usually the stuff I've had run like crap has not adjustability there HELP!!! My sick tired 30 year old lt-1 ran better than this pig Steve!!!!!!!!!

Lets get back to basics here, makesure your timing is BTDC, Ive seen many an engine having the intial timing ATDC and this will cause low vacuum and crappy performance.

Mabey experiment with the timing and see if it runs better at idle with more intial advance or less, keep giving it more advance or less (whichever one the engine likes more) till it runs its best at idle. If the cam timing is off this test will tell you, if the intial required to keep the engine running good is extreme then there is something wrong with the valve timing. I use this test all the time on engines with timing belts to see if the belt has jumped a few teeth.

[This message has been edited by Joekool (edited September 07, 2005).]

So I guess you're not going to just try and change the PV?

I guess I'll stop by the speed emporium and see if they have one that low, would "leaning out" the mixture give me better vac?? I'm sure 5 is too-low the cam isn't that big really,it can't even pull my power brakes in,I guess I'm jumping to the conclusion something isn't happening at the right time here,I hope it is something that easy Thanks to all so far Steve!!!!

<font face="Arial,Verdana" size="2">Originally posted by 72'z'steve:
I guess I'll stop by the speed emporium and see if they have one that low, would "leaning out" the mixture give me better vac?? I'm sure 5 is too-low the cam isn't that big really,it can't even pull my power brakes in,I guess I'm jumping to the conclusion something isn't happening at the right time here,I hope it is something that easy Thanks to all so far Steve!!!!</font>

The only thing that would give you better vac is to have a different cam. You can always put in one of those canisters for the brakes. I put a 2" high flow one in my car because at idle I'm only getting around 5 or 6.

Well some update, I swapped out the carb to a known good 650, my vac has jumped to 10-12" @ 700-1000 rpm, It still shakes alot at idle. timing seems best at 12 dgrees but pings on moderate acceleration,my front 4 cyls are still black though, ran a comp test 165- 175 most in the 170 area,also smells bad rich,but it runs "better' don't get how the carb can do this,same vac port {on intake] but some progress is made I may just reseal the intake tomorrow using a gasket with the"seals" around the ports see what happens. I used everything from my old engine and didn't change/touch a thing thinking those items can't cause a proble Thanks to all so far the saga continues Later Steve!!!

my buddies ford has internal vacuum leak, its full race and we were having trouble setting it up, got buddie to pull the breather off the valve cover on his side, then i pulled the one on my side and put my hand on it, and it was sucking. maybe try that before tearing the intake off. my street driven 350 has next to no vacuum at idle but thats because of the 244@50 cam thats in it...but it was laying around so what the heck. it works.