I don't have a compression tester (yet).

From the posts I've read, often the vacuum readings are noted as being
indicative.

Fresh rebuild, but having problems with idle and backfires -- would the vacuum readings from the boost gauge tell me the same or close as a compression test?

Thanks

 

There's to many variables that can effect vacuum; idle RPMs, port size, vacuum leak, etc. It's best to get a compression check if you have real concerns. Idle and backfires can be caused by a lot of other factors besides engine health.

 

If your familiar with your car, you can use vacuum as a gauge to know when something is wrong. But its not an accurate telltale and definitely not a compression tester lol. Altitude, weather, heatsoak, tune etc. can affect it.

thewird

 

Vacuum can give a good overall idea of the health of an engine. A compression test can tell you if you have low compression pulses or chambers not totally sealing.

A compression tester runs $30 at an auto parts store. With all the problems you've been having (and threads you've been making...) you really just need to do a compression test. Find out FOR SURE that the engine is good, then continue troubleshooting.

I built a motor for a friend in Tallahassee, ran good for a while, he put some mileage on it then it started acting strange. Idling rough, motor shaking, doing all sorts of weird things. He spent a LOT of time, money, and effort re-doing all sorts of stuff troubleshooting it. I drove over, looked at a number of things, then finally just did a compression test. Sure enough, he had one chamber down on compression. Pulled the motor apart, a side seal siezed in a groove and he was losing compression on one rotor face. If he would have done that test at the get-go, it would have saved a LOT of time.

When I did the test, I just had him crank the motor while I held the bleed valve down on the tester. Saw 2 high pulses, one low pulse - bingo.

A new motor does NOT guarantee a GOOD motor. There's plenty that can go wrong when building a motor, even with the most careful builders and best parts.

Dale