I just performed a cam/intake/transmission swap on my 1995 350 TBI truck. The cam is a Comp 4x4 XE 250 (250/258 .432/.453 lift), the intake is the Edelbrock Performer TBI unit.

My problem is that I am reading < 5" vacuum at idle (400-800 RPM). Reving the engine to 2500-3000 RPM I read 15" vacuum. The things I have tried:

Checked cam timing with dial indicator
Installed new intake gasket
Installed new TBI gasket
Plugged EGR vacuum port
Plugged brake booster port
Plugged charcoal canister port
Replaced PCV (and blocked port)
Replaced IAC
Replaced MAP (before I attached a vacuum guage)
Replaced O2 sensor
Set base timing to 4 degrees advanced
Tried running with cat/muffler removed (restriction check)
Checked TPS voltage (.47 full closed)

While the engine was out I replaced the temperature sensors, knock sensor, distributor cap, rotor, plugs/wires. Installed new timing chain and lifters with the cam, new oil pump just because.

The low vacuum is obviously causing an extreme rich condition... I have pulled the plugs and cleaned them several times as they get fouled.

The exhaust is not overly hot indicating very retarded timing. Compression seems good also. I have not started tuning the PROM because of the vacuum problem. The only change I made to it so far is to disable the electronic transmission control in the PROM (there are two bits that control this). I had to adjust the idle screw full open to get the engine to idle at all (.97v TPS). The only things I have not replaced are the TPS (checks good through ALDL) and the ignition module (spark seems fine).

I cannot find anything in the service manuals about the distributor needing to be indexed in any way... I set the timing as I would any other SBC: find TDC and line up the rotor with the #1 wire, pull the bypass wire and set initial timing. Definitely not 180 out (verified when I checked the cam timing).

This cam should be mild enough not to have idle vacuum issues. I was really expecting 20-25" vacuum.

PCM is not setting any error codes other than rich O2.

The engine was in perfect condition prior to this project. I really have run out of ideas of what to do next. I am about to go get some more carb cleaner and start spraying it into random places to see if I can find the leak. Any ideas guys?

 

I say your rich condition is bogging the engine down, which is then giving you your lower vacume readings, which then richens the motor more and so forth, till the thing just runs like poo. I would start pulling fuel, and see how the motor responds. I bet your vacume starts coming back quick once you get your fueling corrected.

 

Sounds like a bad vacuum leak. Intake properly torqued with new gaskets? All vacuum lines attached in their proper places? TBI base gasket good and TBI properly torqued? I have run more cam than that in a 305 TBI with the stock tune and still had 15 in/hg of vacuum at 500 RPM idle.

One other thing did you unplug the set timing connector before setting the timing to 4*? If not you have way too little timing advance.

Also are you sure you are not reading vacuum off the ported vacuum connection? Ported is on the driverside. Manifold on the passenger side and the rear.

 

Yeah, pulled the connector when setting the timing. Vacuum reading taken at the EGR port (passenger side). The vacuum guage reading correlates to the MAT sensor values so I'm sure that's right. I am on the second intake gasket... first was a stock unit, this is a Felpro 1205. Carb cleaner around the intake has no effect on idle... no sucking noises coming from the crankcase when I pull the breather tube and fill cap.

Pulling fuel and adding timing actually got it to idle for longer than a few seconds at about 850 RPM. I was able to lean it out until I saw LR transitions, but I had to back off at that point because it started to ping. Vacuum still at about 3".

It's really starting to sound like the cam is not timed right. I will pull the TBI again tonight and check the vacuum ports just to be safe, then pull the timing cover. If that's right then I guess I'll borrow a degree wheel and check the cam grind on each cylinder. I was just hoping there was something obvious that I was missing.

Thanks for the suggestions guys.