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Old 03-17-2012, 06:31 PM
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Low vacuum at idle

OK, I previously posted a thread titled "high rpm miss". I replaced the HEI with a Summit brand and changed out the plug wires with much success. Problem gone!! But, a problem that was always there is low vacuum at idle once warmed up. I never payed much attention because I was so concerned about the other high rpm problem. Motor dead cold I will start it up and have about 7-8" vacuum. Once fully hot and driven for a bit the vacuum drops to about 2", not enough for my vacuum advance to kick in causing crappy idle and I cant adjust my idle mixtures either. Thats probably because I dont have enough vacuum to over power my metering springs? Its got a pretty aggresive cam (specs unknown). My initial timing is at 12 degrees and valve lash has been adjusted from .011, when I got the car, to .025, .015, to .011 with the same issues. Intake manifold gaskets? It runs great soon as it gets past its off the line sputter. Any advice at what to look at next would be great. Thanks again.

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Old 03-18-2012, 06:21 AM
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sounds like the cam timing may be off, i'd check the timing chain and degree the cam while the cover is off.
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Old 03-18-2012, 07:06 AM
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I'm not sure of the cam in it. What would I degree it to? I forgot to mention if I start to give it gas, lets say 1200 rpms it will go up close to 10". If I crack the throttle good and give it a good rev, I watch the gauge climb up even higher.
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Old 03-18-2012, 07:34 AM
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What size cam?

improper air/fuel ratio, low ignition timing, and low idle rpm all cam lower idle vacuum.

It sounds like low ignition timing.

The engine will need at least 25 degrees of timing at idle with a cam big enough to only produce 7 or 8 inhg.

What happens is low timing makes you turn up the idle screw to get the idle rpm to the right level. And too much idle screw will open the throttle blades past the idle transfer slots which makes the engine run very rich at idle. Not so bad when cold but gets out of hand when the engine warms up.

plus a cam that big will require 1000 rpm idle speed.

You will need to reduce the amount of timing in the distributor advance. Don't run over 40 degrees mechanical advance. 25 initial plus 15 mechanical advance is 40. Add another 10 with the vacuum advance if the engine will make enough vacuum.

Last edited by 454C10; 03-18-2012 at 07:49 AM.
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Old 03-18-2012, 07:47 AM
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Not sure of the cam size. I dont think its anything really crazy, all the owner new is that it was a circle track cam. This vehicle went through a few owners that werent the engine builder. I really bought this car because of the clean body. Its mechanical flat tappet too. Makes sense about the idle circuits and idle itself. Should I just start playing with the timing by the vacuum gauge and forget the timing marks? Would this compensate enough if the cam is installed wrong? I really dont care if I blow the damn thing up right now lol.
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Old 03-18-2012, 07:55 AM
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circle track cams tend to have very narrow lobe separation angles (104 to 106 degrees LSA). This makes a lot of valve overlap which lowers vacuum.

I would turn the timing up first, set the idle speed to 1000, and re-adjust the idle mixture screws.

Sounds like you need to change the cam to a street cam (110-112 LSA).
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Old 03-18-2012, 09:52 AM
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Agree about the cam change. I'm just trying to make this fun and livable for this summer. I'm planning on tearing the motor down next winter and starting fresh. Too many unknowns with this combo. My initial is 12 degrees. Maybe 14 or possibly 16 or just watch for the most vacuum?
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Old 03-19-2012, 03:56 AM
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Initial is now at 16 with idle bumped up a little. I readjusted the fuel mixture screws and I'm pulling about 7" now. This seems to be ok for now. Thanks again.
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Old 03-19-2012, 05:37 AM
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try 20 degrees. then readjust the idle mixture for the highest rpm after the engine has warmed up.
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Old 03-19-2012, 05:42 AM
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Thats initial? My vacuum advance should give me 10 and my mechanical advance another 10. Would 40 total be ok?
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Old 03-19-2012, 06:42 AM
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You need to recurve the distributor to shorten the advance curve length.
this will allow generous base idle timing, yet avoid excessive max advance.
limit the mechanical advance to about 10deg.

26deg at idle +10 deg mech adv =36deg at max advance.

its will idle and run much better.
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Old 03-19-2012, 07:10 AM
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Sorry if my questions seem stupid but the 26 degrees, is that with my vacuum advance hooked up? My distributor is summit racing brand hei. It says the mechanical is 10 degrees and the vacuum is 10 degrees also. I appreciate the help guys.
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Old 03-19-2012, 07:50 AM
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Quote:
Originally Posted by danzigs
Sorry if my questions seem stupid but the 26 degrees, is that with my vacuum advance hooked up? My distributor is summit racing brand hei. It says the mechanical is 10 degrees and the vacuum is 10 degrees also. I appreciate the help guys.
No the 26deg is the base initial timing.
vacuum advance is a separate thing and issue.

Get a timing light and balancer timing tape and check the advance curve limit
Does this Summit distributor have the big GM HEI style cap?

26 base + 10deg mech adv = 36deg max ( occuring at 2800-3500 rpm)
This is checked with vac adv disabled.

vac advance 10-12deg max limit at high speed cruise
adjust the rate as required.
Try using ported vac with the circle track cam.

Limiting, or locking out the mechanical advance on a HEI distributor.:
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Old 03-19-2012, 08:12 AM
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It does have the big cap. Do I need to lock out the distributor?
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Old 03-19-2012, 09:10 AM
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Quote:
Originally Posted by danzigs
It does have the big cap. Do I need to lock out the distributor?
ya you may. Big cams with lots of valve overlap (typical circle track cams)
Want so much idle base timing that you might as well just lock out the mech curve altogether and run 36deg fixed at idle.

Don;t need to eliminate the vac adv system thou.
You want to keep and use that for street driving.
It will need to be fine tuned thou.

Locked out street timing + vacuum advance

36deg fixed mechanical

+ 10-12deg vacuum advance at hiway cruise.
Adjust the rate as required. You want the +10deg to come in when the manifold vacuum is high, not all in at idle.

Try it locked out. The motor will show you what it likes.

If its hard to start, when hot use a ignition power interupt switch.
Be sure to have the GM starter motor bracket/brace installed.
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