I have read that the vacuum reading should be at it's highest when the car is at idle. According to my gauge, my reading is at it's lowest at idle - about 10. When driving it is between 16 and 22. What does this indicate?
Thank you

On my engine, the valves needed to be lashed, needed a new timing chain (they stretch over time, retarding the cam timing, and causing a drop in cylinder pressure b/c intake valve is closing later), then needed to be correctly timed for the ignition event.

After those, my vacuum jumped 10". What is your park/neutral idle speed? If the idle speed is too low, that will give a low vacuum reading as well.

The Vacuum should be between 16-20" @ idle for most cases. You will have MAX vacuum on decleration (coming off a hill or coming to a stop with your foot completely off the throttle).

Does the car run rough at idle? Is the needle on the gauge steady when idling? Or does it wiggle a bit?

~THOR~

_________________
1984 D100 Shorty Custom
Certified Auto Appraiser - RevItUp Classic Appraisals
President - Cherry City Bombers CC
Part of Tyrde-Browne Racing

I would estimate the RPM does not exceed 1000 at idle. It idles VERY smoothly.

Is this a stock engine with a stock camshaft ?



If it is stock, idling that fast can help cover up problems (like a rough idle or carberator ajustments). It really should still hold good vacuum at a 1000 RPM's, but if there is a slight problem it could make the engine run smooth even though it may need some work.

If it is stock, try lowering the idle to around 600 or 700 rpm and see if it shows any rough running. If it does run rougher at lower idle settings then the first things I would check is valve lash ajustment, timing settings, vacuum leaks, then carberator ajustments, and in that order.

1000 rpm is a little high for idle with a stock slant and may be hiding problems.

If it has a over size camshaft then its possible for this to be normal if its big enough to lower vacuum. It would also be normal for it to have more vacuum at higher speeds when going down the road if it has a oversize camshaft. A lot depends on what size.


Jess

Just like LUCKY13 said.

I have personally run my car down to 300 RPM, and it ran smooth, it is completely stock, so I agree that would be a good test.

I also take it that the needle on the vacuum gauge is also steady.?Adjusting your A/F ratio may also affect vacuum at idle but not much.

How many miles on the engine? My car ran smooth as glass even with the stretched chain, so that could also be a good possibility. In which case, the timing sets are just a little over 20 bucks, but you will also need new timing cover gaskets (cover to block & cover to oil pan), timing cover crank seal (kind of optional), a damper pulling tool & installer. It is important to not beat these onto the end of the crank, as damage to the thrust surfaces can occur, not to mention you can damage the end of the crank. There are a few aftermaket tools such as this:

http://store.summitracing.com/partdetai ... toview=sku

that are relatively cheap, and they also work well. I used this specific unit to install mine. It threads right into the end of the crank.


~THOR~

_________________
1984 D100 Shorty Custom
Certified Auto Appraiser - RevItUp Classic Appraisals
President - Cherry City Bombers CC
Part of Tyrde-Browne Racing

The idle was pretty low-about 450 but VERY smooth. I raised it to 700 and the vacuum went to 11. This is a stock engine with 45000 on a rebuild. Thank you all for your replies

It almost sounds to me like you need to have the timing chain looked at. Ignition timing will have a little to do with it, and unless the vacuum gauge is moving around when the RPM is steady, there arent any problems with the valves sealing.

Sounds like a loose timing chain to me.


~THOR~

_________________
1984 D100 Shorty Custom
Certified Auto Appraiser - RevItUp Classic Appraisals
President - Cherry City Bombers CC
Part of Tyrde-Browne Racing