Hello all, just did a vacuum test on my mostly stock ( went from 2bbl to 4 bbl, olds a4 intake)1972 350 to make sure all is well before I go and blow money on headers and nitrous. The needle fluctuated very slightly from 16 to 16.5 in hg. I am thinking maybe the air fuel mix is a wee bit rich? It's about zero outside and I let it warm up for about 5 or 10 minutes before testing and connected to power brake port. The 16 inch reading seemed a bit on the low side. Any ideas what im looking at here? Any and all second opinions on these readings would be greatly appreciated.

 

I would think 18-20 on a bone stock engine. Did you adjust your air/fuel mixture screws for the highest vacuum on your gauge? Are the screws about equal in turns from the fully closed position? It could be a little low due to the gauge also.

 

I wouldn't be to concerned with what the gauge is reading, just that you get the highest reading when adjusting the mixture screws. Those gauges don't all read the same and as long as the needle is just fluctuating and not vibrating madly I'd be happy.

 

Yep, when my engine was stock it pulled 22". Now with the 217/221 cam it pulls 16".

 

I have a big cam in my 455 and it rocks at idle, runs fantastic though.

 

Something I just thought of - the initial timing will affect the idle vacuum reading. More advance will provide a higher vacuum signal, so check to ensure your timing is set properly.

 

Timing chain condition? Won't late valve timing contribute to lower 16 ish" readings?

 

hey guys thanks for all the great replies, its been hellishly cold over here in chicago and i havent been out to the garage all week long. when i do get out there im going to play with the mixture screws and see how high i can get the gauge to go. will post results when i finally get out there. oh and the timing is set at 14 degrees, two degrees above stock so i know it cant be timing.

 

oh and as far as the timing chain condition it doesnt seem to have any slop in it but when i had the motor apart last year when putting it together i definitely should have replaced it. i was very inexperienced back then and working at a diesel shop this past year has taught me alot but if the mixture screws dont do the trick and a compression test shows that its not worn rings thatll be the next place il look.

 

With your low CR, you could easily bump the timing up to 20 ( or more ). If you think that's too high keep in mind that's where all the 73 and up Olds engines were set at ( or at least most of them ). Of course if you did that you would have to recurve your distributor so your total would still be at around 36 or so.

Are you running your vacuum advance off ported or manifold vacuum?

 

The carb I'm running right now actually doesn't have a ported vacuum port so I'm running it off of manifold vacuum

 

What carb are you running?

 

It's a qjet I'm pretty sure it's a marine carb il get the numbers off of it when I go out to the garage. I've got a Holley 4165 I'm going to be putting on it

 

17050288 is the number off the side of the carb by the throttle lever

 

The second to last 8 shows a marine application I think. Not sure about the "50".