Thread: How much vacuum at idle

Tweet

Hey guys,
Thought Id through a vacuum gauge on my 327, never used one before. She's got an incredibly rough idle, sitting there not in gear at about 750rpm she just about wants to die. Now its gotta cam and pro top line sports heads so this may explain but well see. The cam is a compcam 212-12-2 and the manufacturer says the following it has a duration of 280/280 and a lift of .480 and .480. Now doing a vacuum test full manifold vacuum not in gear in park at 700rpm I have 2 on the vac gauge and the engine lopes really bad its a reasonable 2 maybe up and down by a half, at a 1000rpm she smooths out quite a bit and has a good 9 on the vac gauge at 1400rpm shes a solid steady 14. Now the car doesnt run on petrol its pure LPG my initial timing is currently set at 8 btdc at 12BTDC she runs a little bit hot and backfires under hard accel. Is the above readings simply the type of cam I have or is there something else. The book "generic" for vacuum setup talks about at idle approx 20 for the vac. Anyone got any ideas,
Thanks Steve

Not surprising she idles rough with only 2" vacuum! Are you sure you're using the right port for the guage?

Originally posted by simso
Hey guys,
Thought Id through a vacuum gauge on my 327, never used one before. She's got an incredibly rough idle, sitting there not in gear at about 750rpm she just about wants to die. Now its gotta cam and pro top line sports heads so this may explain but well see. The cam is a compcam 212-12-2 and the manufacturer says the following it has a duration of 280/280 and a lift of .480 and .480. Now doing a vacuum test full manifold vacuum not in gear in park at 700rpm I have 2 on the vac gauge and the engine lopes really bad its a reasonable 2 maybe up and down by a half, at a 1000rpm she smooths out quite a bit and has a good 9 on the vac gauge at 1400rpm shes a solid steady 14. Now the car doesnt run on petrol its pure LPG my initial timing is currently set at 8 btdc at 12BTDC she runs a little bit hot and backfires under hard accel. Is the above readings simply the type of cam I have or is there something else. The book "generic" for vacuum setup talks about at idle approx 20 for the vac. Anyone got any ideas,
Thanks Steve

ARE YOU SURE YOUR GAUGE IS CORRECT?

You should get between 17 and 22 inches of mercury at 1000 rpm at sea level.

The vacuum gauge we have on our van is from a old steam plant built in the '30 easy to read
cruising at 10 in Hg
hank

Yep definetly the right port for vacuum, large brass union right hand of carby on the manifold between 6 and 8, it does however also run the pcv at this point as well so I dont know if that might be why its indicating low could be part of the problem as well. Vac gauge works fine, so any ideas, is this classified as a large cam, I personally dont know only went by the performance places recommendations. The vac gauge needle is pretty steady except down at the 2 and as I said before it fluctuates maybe 1/2 each way, should I be doing the test in park or in drive, does it make a difference
Happy with the performance of the car just curious as to why such low manifold pressure
Thanks Steve