Too much Vacuum Advance

[69GMCLonghorn]

I recently bought another project. It's a bare bones '70 Chevy 1/2 ton with beyond lethargic 307/Turbo 350. I've driven 6 bangers with more power than this thing has. Anyway, I checked the base timing and it was at around 8° BTC, so I figured that was at least in the ballpark. Then I checked it without the vacuum advance hooked up and it was somewhere AFTER TDC. It has the stock 2 BBL Rochester on it. It seems to be getting full vacuum at both ports on the carb at idle. Currently I have it set at 8° BTC with the vacuum advance disconnected, which equates to about 18° with it hooked up.

The other side effect is when I fire it up when it's cold and the choke comes on, the vacuum drops and retards the timing and it stumbles until the choke warms up and turns off.

Is this a carb that about ready to crap on me? Or is there some other reason for this?

[airdale94]

If your idle is to high you will get vacuum on your ported port. If your carb. needs rebuilt and consequently you have your idle screw in far enough that your throttle plates are pass the bleed for the ported port just to get it to idle, then yes you will have 2 full vacuum ports. You shouldn't be losing that much vacuum with the choke on and fast idle cam engaged. I would say your choke is to rich.

[geezer#99]

When you set your base timing at 8* did you disconnect and plug your vac advance?

[LONGHAIR]

10º from the vacuum canister is not too much, it's pretty common.

Are you sure that you are experiencing a significant vacuum drop when the choke/fast idle is on? Have you actually tested for this?

[69GMCLonghorn]

The way the P/O set the timing was 8° with the vacuum advance connected. There was ZERO power. I set it at 8° with it disconnected (and plugged). Now it has a reasonable amount of power for what it is (or isn't). The idle adjustment doesn't seem screwed in very far at all, basically about where it should be. But it does make sense that if this was in too far, I'd start having port vacuum at idle. I'll do a little more research this weekend and see what other details I can come up with.

[geezer#99]

Hook your vac advance to manifold vacuum, then turn your idle down and adjust your mixture screws. It'll run better like that.
Take a pic of your carb if and when you can.
Someone might see something to help you.

[Zeke's Garage]

If your 307 was like the one I pulled out of my Chevelle wagon, it probably had the nylon teeth coating on the cam gear of the timing set, and they might all now be in your oil pan, causing some crazy timing issues.

Otherwise hook up a vacuum gauge and check for any jumps and variations in the vacuum. The only way the vacuum will drop in the manifold, is if something allows outside air in, like opening the throttle, or a significant leak somewhere. So block off ALL vacuum sources while you're troubleshooting.. power brakes, trans mod valve, vacuum advance on the distributor. If you're still seeing drops and jumps, it's time to check your valve train and verify your static timing.

Good luck with it!

Shawn

[airdale94]

Originally Posted by Zeke's Garage

If your 307 was like the one I pulled out of my Chevelle wagon, it probably had the nylon teeth coating on the cam gear of the timing set, and they might all now be in your oil pan, causing some crazy timing issues.

Good luck with it!

Shawn

Good call!

[69GMCLonghorn]

Well, my plans of delving into this problem a little further were derailed today. I started driving and suddenly had SMOKE billowing out from under the dash! After a few minutes of being parked, it was OK and I made it home. But nothing screams "GET THE F*** OUT NOW" like a cab full of smoke! Time to start tracing wires and see what was cooking. Whatever it was, it was pretty serious.