Hey there! I have a question again about my 78 Bel Air, this time its about the vacuum advance unit. When I got the car home it ran pretty well, warmed up quick and idled smooth and made good for "78" power, only a slight hesitation off idle and 2 crank attempts when cold starting other than that she runs great. Upon looking under the hood to give everything the once over I discoverd that the thermal vacuum switch for the EGR valve and purge canister is broken out of its brass fitting(not leaking vacuum) and the vacuum hose to the other thermal vacuum switch that controls the vacuum advance unti on the distributor was collapsed, thus not getting any vacuum advance. After replacing the damaged hose, I started up the engine and the idle was way high, wich makes sense due to the increased timming. So I lowered the idle and the fast idle, the hesitation off idle is gone but it takes longer for the engine to accelerate and make power. Would not having the vacuum advance make the engine hesitate?, should I be recieving a vacuum signal to the vacuum advance at idle?, do I even need vacuum advance, dose it save fuel at cruse? I know this is a mouthful and I really appreciate all of your replys, thanks.

Here is a link to an explanation for the vacuum advance.  Seems it helps fuel economy and overheating at part throttle.  Your vacuum advance will kick in as soon as your throttle blades open and will stay until your vacuum falls off with increased rpm. Thats when your mechanical advance starts kicking in . At wide open throttle you will only have your enitial timing at the crank and your mechanical advance. Your vacuum advance will drop to zero.



http://www.stoveboltengineco.com/howto/vac.htm

I think if I understand correctly, the part that you are talking about is temperature controlled and only affects vacuum advance until the engine is warmed up. If there is a hose in and out of the switch I think it is, you can just pull both hoses off the switch, connect them together with a small piece of tubing and your vacuum advance will always function the same regardless of engine temp.

Carl Stevenson wrote:

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I think if I understand correctly, the part that you are talking about is temperature controlled and only affects vacuum advance until the engine is warmed up. If there is a hose in and out of the switch I think it is, you can just pull both hoses off the switch, connect them together with a small piece of tubing and your vacuum advance will always function the same regardless of engine temp.

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I should have read more closely , its a 78. I was thinking 60's.  Your explanation make sense. 

 

Thanks everyone. I believe that the vacuum source is wrong in my application, because I have a vacuum signal at idle. The vacuum lines are hooked to a TVS but I believe someone has messed with it because I have read through other sources that it was used to increase timming and idle speed when the coolant temperature rose above 220f to prevent over heating! Would not having the vacuum advance hooked up cause a hesitation off idle? Where could I locate a vacuum diagram for this engine, 1978 chevrolet 250 six (LD4) with auto trans!

Can you see a vacuum source above the throttle (base) plate on the carb? If so, try hooking the vacuum hose up there.

would a monojet carb have this non ported source, also would not having the vacuum advance hooked up cause a hesitation off idle?