bouncing vacuum at idle help
 

here is a video i shot today of my problem. the idle vacuum bounces very fast at idle. when driving it is very steady. specs of the engine are:


chevy 350 lunati cam ( .458 lift both sides, 110 LSA, intake center line 106, adv duration 284/284, duration @ .050 218/218) holley 600 cfm carb
edelbrock performer rpm intake, TFS 195 cylinder heads.

a quick run down of what is going on in the video, sorry for the horrible quality it is a cell video.

vacuum on cold start bounces from 8-12

engine rpm cold start out of gear 500 rpm

driving at 20 mph steady ~16"

vacuum at warmed up idle out of gear bounces 12-14

rpm warmed up out of gear ~750

vacuum warmed up in drive 9-12

rpm warmed up in drive 600

here is the video.....

Re: bouncing vacuum at idle help
 

what could be causing this? i am trying to adjust the mixture screws but it is difficult when they bounce like this.

Re: bouncing vacuum at idle help
 

A sticking valve is normally the culprit when the vacuum is bouncing that badly. I had it do the same thing one time with a worn out lifter. What happens is the vacuum signal for most of the cyl. is normal, but when the cyl with a bad valve is on the intake stroke, the vacuum signal changes from normal. Pull the valve covers and run it, you could have anything from a bent push rod to a bent valve.

Re: bouncing vacuum at idle help
 

thanks for the advice. what all should i look for when running without valve covers? bent pushrod should be easy to see but what about a bent valve? anything else to look at? i will be checking compression and valve adjustment also.

Re: bouncing vacuum at idle help
 

A bent valve wont seal to the valve seat, meaning you wont have compression on that cylinder.

Re: bouncing vacuum at idle help
 

Did a compression test last night...

1** 160

2** 160

3** 170

4** 165

5** 178

6** 170

7** 165

8** 170

Re: bouncing vacuum at idle help
 

since they are all withing 10% of each other i would say that my rings and valves are good to go right?

Re: bouncing vacuum at idle help
 

and this was on a cold engine. dont think the neighbors would like a 1 am alarm clock...

Re: bouncing vacuum at idle help
 

I have almost the same cam in my 305 in the Corvette and it bounces the same way. Makes a touch more vacuum than you though. Probably a tuning issue rather than a mechanical one. With the 110* LSA and 218/218 @ .050 its not going to have a low smooth idle with a ton of vacuum. It will have a moderate idle at 750 rpm and it will definately have some lope.

What is your timing set at? Is your vacuum advance hooked up? Is it hooked to manifold vacuum? Have you tried screwing in your idle mixture screws, bringing them out evenly 2.5-3 turns and starting over?

Re: bouncing vacuum at idle help
 

See this link:

http://www.secondchancegarage.com/public/186.cfm

A vacuum gauge is one of those forgotten diagnostic tools.

Re: bouncing vacuum at idle help
 

yeah i have been talking to lunati about this too. they say the flicker of the needle is fine due to the cam but the vacuum is low, should be at 16" or so at a idle rpm of 850 out of gear. my timing is 12-14 initial with the advance disconnected. when it is hooked up i have it on the ported vacuum. i will adjust my mix screws again tomorrow.


i have seen this before. i think i just need to get my mix screws dialed in

Re: bouncing vacuum at idle help
 

Just messed with my timing and screws. Raised initial advance to 16, played with the screws and got it pretty steady at 14.5-15" at 850 idle. I think that's the best I am going to get it. The bouncing calmed down some too.

Re: bouncing vacuum at idle help
 

. Your cam timing, pistons design, quench distance, and compression ratio will also affect vacuum...

Re: bouncing vacuum at idle help
 

I think what I have it at now is pretty good. Got more vacuum out if it and it is steadier.

Re: bouncing vacuum at idle help
 

Vacuum advance should be MANIFOLD or FULL vacuum advance and it should be limited to 10* @ 10 in/hg. Use either an adjustable can or a 1978 L82 corvette unit, probably listed at an autoparts house as a 350 with special high performance package.