Hi All,

Great forum. I am helping my brother in-law tune his olds which has the 400 cu in. motor with the higher hp rating. The motor is all stock and has new ignition components, rebuilt original carb (q-jet) and all potential obvious vacuum leaks were checked and double checked. The problem is low vacuum at idle (5-6 inches) and a sluggish throttle response off-idle. Higher rpms are fine once the timing advances. Base timing is set at 8 degrees which gives a total advance of about 34 degrees at 3,000 rpms or so (vacuum advance disconnected). If the base timing is increased at idle and the throttle plates closed more, vacuum will rise to about 7 inches but then the motor will detonate at higher rpms. My question is what should the idle vacuum of this motor be? It appears that there is not enough idle vacuum to trigger the vacuum advance on the dist. If I suck on the vacuum advance hose it will raise the rpms and advance the timing. Could the vacuum advance canister be wrong?

Thank you,
Jeff

 

I have always checked for vacuum leaks by spraying electrical contact cleaner around connections and junction points with the engine running at idle. If the rpm increases the engine is running on the contact cleaner and you have found a leak. Maybe you already tried that but I thought I would throw it out there. Not sure what the vacuum should be.

Good luck,
Chris

 

To answer directly, yes there could be a leak in the vacuum cannister. It sounds like you have paid attention to most of the tune up specs, what about dwell? Welcome to CO.

 

Thanks guys. The dwell is at 28. I didn't notice a vacuum leak at the canister when applying a vacuum to it. We got our hands another w-30 specific distributor and will give that a try. Is it possible that the factory cam would probuce so little idle vacuum?

 

Attached is picture of the car following paint. I'll get a more recent better photo soon.

 

What is idle RPM? Bigger cams idle at higher RPM's. What fuel are you using? I would think you need more initial timing and Octane boost to stop detonation. I would say about 13deg BTDC at 1000 rpm maybe a little more. Gas today is crap as far as Octane.

 

Idle RPM was about 700 or so. I agree about the fuel. Prior to tunning the tank was emptied a filled with a 1/2 and 1/2 mixture of Cam 2 and premium gas