Vacuum Advance
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Vacuum Advance
When I have the vacuum line attatched to the distributer, reving the engine causes it to shudder and knock a little ecsepialy if you try to hold it at about 1500rpm. When I disconnect the line and cap it off, it is as smooth as silk. When it is idling I have no vacuum, and the vacuum increases as the rpm's goes up. If I apply vacuum to the dizzy with the hand pump, the rpms increase. I don't have a timing light, I just set it by ear and feel. Do I just have the timing set too high, or is it a vacuum issue?
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I checked and all of the plug wires are going where their supposed to. I also played with the timing fo about 20 minutes (with the hose off and plugged) and it still runs better without the vacuum advance. In the Haynes manual it shows the #1 cylinder on the dizzy being right above the bottom cap clip, but mine is just below it. When I turn the dizzy clockwise it retards the timing, and counterclockwise seems to advance it. I never get any pnging though, it just starts to stumble when you get to a point. Anyone got any suggestions?
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Looks like there is 2 vacuum sources that combine and supply the dizzy. Both come from the base of the carb (the two closest to the firewall) one is yellow, and the other red. The red one has a check valve on it the yellow does not. They both feed into a "T" and then into the dizzy. She runs really good with the line disconnected, what are the drawbacks to leaving it off? Fuel economy will be very important to this truck's purpose.
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From: Lost
I find it odd that there are 2 hoses teed together for the vac advance. There were some vac advances used that had 2 seperate vacuum fittings, and as such, used 2 lines.
Both those lines that are connected together have the same vacuum behaviour? No vacuum till the throttle is opened? This *could* be part of the problem....
Normally, having the vac advance unhooked will hurt power and MPG, mostly in town.
Both those lines that are connected together have the same vacuum behaviour? No vacuum till the throttle is opened? This *could* be part of the problem....
Normally, having the vac advance unhooked will hurt power and MPG, mostly in town.
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From: Virginia
It does sound like the timing is too advanced. I have had this happen before, and to get it to run with the vacuum hooked up, I had to turn the timing down to like 4 BTDC. It ran like a dog with it like this, so I turned it back up to around 14 BTDC and left the vacuum line off. My next move was to buy an adjustable advance unit, but I never got around to it and sold the truck.
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Interesting, I wonder if the original dizzy has the 2 fittings, and it was changed to one that only has 1 fitting. If I were to just run a new single line, where would the best place to get vacuum from be?
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From: Virginia
You need to run it to the ported vacuum port, the one that has no vacuum at idle, and has vacuum when you rev the engine. I believe you said you already had it there, and that's the correct port. Fiddling with these other vacuum lines is not going to help much.
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