Vacuum issues....

restomod22 · Apr 11, 2005

I've been having vacuum issues since my rebuild. I cannot find any leaks in the seals of the intake or carb. I hook a vacuum gauge up the the intake (port where old C4 tranny connected) and while at idle the gauge reads around 15. When I hit the gas, the gauge jumps up. Is it possible that my pathetic little Edelbrock Performer intake can't handle the demands of my set up? I recently replaced my short block with a remfg'd 302 bored .30 over, added the AFR heads, cam, and other mods listed below. 9.5 to 1 compression. I will be replacing my carb/intake soon, but would like to solve this problem first. I'm really stabbing in the dark here, but am at a total loss. Could improper roller rocker adjustments affect the vacuum? Say if the valves weren't closing properly due to overtightened rockers?

Thanks for any help!

6Stang7 · Apr 11, 2005

what issue(s) is this vacuum problem causing?

restomod22 · Apr 11, 2005

6Stang7 said:
what issue(s) is this vacuum problem causing?

Well, it's hard to say. The car is running OK. I know I have a carb problem, which is giving me some grief, but nothing I can attribute to the vacuum issues. I only have about 200 miles on my rebuild, so I haven't really run her hard yet. So far the throttle response is good and other than some carb issues, it seems to be OK. I just know that I shouldn't have this problem.

6Stang7 · Apr 11, 2005

hmmm. well, it sounds like you really dont have a vacuum issue then if you are not having anything actting up and not work correctly. you might just be under-carbed. o, and not to get off topic here, but you can go ahead and run the hell out of your motor. the 500 break-in is worthless from the tech articles i have read. the cylinder wall hone is like a file to your piston rings, so if you run your motor hard, you get better seating. my old boos when i worked in a shop built up a 347 race motor, and once we finished it we filled it with moblie1 synthetic oil, broken in the cam, then did a fat burn out and ran the piss outta the engine. has yet to have a single problem.

but, take it as you want it. some people are uncomfortable with doing this, which is understandable.

84convertablegt · Apr 11, 2005

im not, i have 20 miles on my rebuild and ive already beat the sh&* out of it.. kind of... i will wait to put synthetic in it until the first oil change at around 500 miles

6Stang7 · Apr 11, 2005

ya, the whole synthentic oil on the break in is still a big debate. i can understand how regular oil would let the rings wear better, but i dont see synthetic preventing the rings from seating.

Max Power · Apr 11, 2005

Even some of the synthetic mfgs. suggest bearking in on dino.

The only time you sould see vacuum increase when cracking the throttle is when you are hooked to a ported source. Valve, carb and ignition adjustments can all effect vacuum readings. 15" is pretty good, really.

69Rcode_Mach1 · Apr 11, 2005

I will be breaking in my engine by running it hard. You shouldn't run synthetic oil for the break in. The oil is a lot thinner and lubes a lot more. Making the walls a lot more slippery hence making the rings not seat as well. Dino will not lube as well allowing the rings to file better and seat.

84convertablegt · Apr 11, 2005

Max Power said:
Even some of the synthetic mfgs. suggest bearking in on dino.

The only time you sould see vacuum increase when cracking the throttle is when you are hooked to a ported source. Valve, carb and ignition adjustments can all effect vacuum readings. 15" is pretty good, really.

I was also gonna mention that. shouldnt the vacuum level decrease when you rev the motor, not increase. well at least under load i know it would decrease and eventually be 0, right???

restomod22 · Apr 11, 2005

Thanks everyone.

I'm not using synthetic oil for the first 500 miles or so. I have read about breaking in engines with high RMP's to help seat the rings. I've driven the car somewhat aggressively but have yet to get on her. The carb isn't tuned that well or just plain not working effectively. This is my first build and I guess I'm just nervous

Max Power, when you say ported source, the connection I'm using for the gauge would apply? So 15 at idle is normal and vacuum increasing with throttle is OK? I always thought the needle would drop to 0 with acceleration...

Will switching to a larger cfm carb help?

Blown4.6 · Apr 11, 2005

restomod22 said:
Thanks everyone.

I'm not using synthetic oil for the first 500 miles or so. I have read about breaking in engines with high RMP's to help seat the rings. I've driven the car somewhat aggressively but have yet to get on her. The carb isn't tuned that well or just plain not working effectively. This is my first build and I guess I'm just nervous

Max Power, when you say ported source, the connection I'm using for the gauge would apply? So 15 at idle is normal and vacuum increasing with throttle is OK? I always thought the needle would drop to 0 with acceleration...

Will switching to a larger cfm carb help?

15 inches at idle is okay. Depending on the cam and compression ratio, maybe 17 is a little better; but, I would not have an issue with 15. When you rev the engine, it should go up to 25 inches or more. At WOT under load, that's when you see big drop off in vacuum.

restomod22 · Apr 11, 2005

Blown4.6 said:
15 inches at idle is okay. Depending on the cam and compression ratio, maybe 17 is a little better; but, I would not have an issue with 15. When you rev the engine, it should go up to 25 inches or more. At WOT under load, that's when you see big drop off in vacuum.

WOT?

Blown4.6 · Apr 11, 2005

restomod22 said:
WOT?

Wide Open Throttle= WOT

Max Power · Apr 12, 2005

Manifold vacuum should give you a baseline reading at idle. When opening the throttle, it may increase a little if the throttle is opened gently, but it will usually drop a little. WOT should get you close to zero. Under load, it will almost always drop, regardless of the rate of throttle opening.

Ported vacuum should read zero at idle, rise immediately when opening the throttle, and gradually drop off as rpms and load increase. Ported vacuum almost always comes from a fitting on the carb. About the only thing ported vacuum is used for is the vacuum advance on your distributor.

ozstang65 · Apr 13, 2005

restomod22 said:
...I have read about breaking in engines with high RMP's to help seat the rings. I've driven the car somewhat aggressively but have yet to get on her.....

It's not high revs that get the job done, its high load. High revs should be avoided initially. Get the tuning done quickly, then hammer it. You only get one go at the run-in, do it before it's too late.

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