Inliners International Forums General Discussion Engines dist vac advance

Fellow inliners, do you use ported or manifold vacuum for the distributor advance?

My motor is a 292 w/1bbl rochester, and the dist is hooked up to ported vacuum. A co-worker suggested advancing the base timing from 4* to 8* and hooking the carb up to manifold vac.

Also, what kind of mileage should I expect with a 292 w/monojet, powerglide, and 3.73 rearend? Carb probably needs a rebuild, or at least a new acc pump. I got 12MPG on last tank with stock timing.

I've love to install a 700R4 for the lock up convertor and better gearing.

I love how easy the monojet on my 250 was to rebuild. Quite proud of myself for rebuilding a complicated carburetor. After seeing a few rochester 4 barrels I'm not so sure. My car will pull 20MPG pretty easily but I have the 2.56 gears. Depending on what speed you cruise at it might not make that much of a difference. I run a 250 and monojet. I'm thinking your car should do 15MPG.

[This message has been edited by Greg (edited 10-16-2003).]

Manifold vacuum will not give you better fuel economy and may even make your engine ping when you let off the gas.

I bet your Rochester needs tuning. I get 18- 20 mpg with my 261 in my 4.11 geared 1/2 ton truck. I was getting about twelve when I figured out that the power valve was stuck. Do you have the right spacer installed?

I'm getting about 10mpg from my 250 with headers and electronic ignition. It's in a '68 C-10 1/2 ton with 3.73 gears and a 3 speed. I don't know what's wrong. She runs good. I only see one port on the carb and it's maifold vacuum. The only thing is, it doesn't advance the timing. I can get 20 inches of maifold vacuum from the manifold, but the vacuum from the dizzy's port seems really weak. I'm at a loss. BTW, that figure is mostly highway driving (70mph @ 3300rpm) with proper warm-up, not gassing it, tailgate down, with a one barrel. Base timing is at 8-9 degrees advance with the dizzy recurved. Any guesses.

I've always been told to hook the vacuum advance to ported vacuum. That's how I had it hooked up on my 327. I'd do it the same way now if I could. Thanks for any help, and I hope I'm not hijacking the post. John.

Yes, you're hijacking the thread, but I won't hold it against you .

I dunno if I have the right spacer installed. I don't know anything about Monojets and very little about chevys in general.

I set up the timing, idle, and mixture to be at spec, and it passed emissions. The carb does need some work, and I will be rebuilding it soon.

Highway speed varies from 55 to 60MPH mostly.

[This message has been edited by cruiserman (edited 10-17-2003).]

The vacuum advance needs to be connected to a ported outlet...The vacuum advance unit is designed to give the spark a quick catch up just as the throttle plates open .Then the mechanical weights take over...On an inline you can see if the lever is moving or not while reving the engine a little..Momojets have a lot of different outlets for emissions that can be pretty dead at different times..A vacuum gauge is a handy tool..Use it to find an outlet that shows a vacuum reading just as the throttle plates open..Make sure the line to the distributer has no leaks..Connect the line..rev the engine a little..If there is no movement the diaphragm is probably leaking..Chevy six distributers were not the best of design so it is wise to pull them out and check for bushing wear and to make sure the advance weights are good.. Al...

The rig develops 20" of manifold vacuum at idle. Ported vac revs up with throttle, which is the port to which the dist is connected. I need to check dwell, and I'm going to play around with the base timing. The motor vibrates at idle in gear, and I think it's not happy @ 4*.

cruiserman-- Do you have any idle problems with your engine? I had my 250 at 4* and she didn't want to idle at all. I usually had to keep her choked for about 5 minutes before I could un-choke her and she would idle without cutting off. My timing is between 8*-9* right now, and I just choke her to start and leave it for about 5 seconds then un-choke. I was told (and read) that 8* works well with the chevy sixes. I agree with inlineidiot about checking out the dizzy, and making sure everything is in good working order. Hope this helps some. John.

It does seem unhappy at 4* advance, especially in gear at idle. Down the road is OK, but I'm thinking of bumping it up to 8*.

eric

It wouldn't hurt to try it. I was kinda confused when I was trying to figure out what base timing would be good, bec. my Haynes manual said 4* and I heard 8* from people on this board. I tried 8* and it fixed some of the problems I'd been having. Have you thought about switching over to electronic ignition? I know changing and adjusting points comnstantly is annoying. Just some thoughts. Good luck. John.

Yup, got the HEI parts waiting to be installed. I thought I'd get the basics sorted out before I switch to electronic.

The dist.Vac. Needs to be metered Other wise it will end up sucking a hole in it. And Your Vac advance will no longer work.There should be Very Little Vac. At Idle. And get stronger as rpms increase. Hope this helps.
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NovaMan/Twisted6/Larry

I have a caprice (heavy car) and since here in El Salvador, mechanics only know Japanese cars, they hooked up the vaccum lines wrong when the carb was changed. My grandma bought a rebuilt carb about 2 years ago, a rochester monojet. Back in those days I wasn't too interested in cars so i didnt care much.
When the mechaninc installed it, he hooked up the vac. advance to a manifold vaccum and the vaccum line was clogged. I drove the car for about 2 weeks and noticed i was only getting about 8 MPG and I said wow something must be wrong. So i hooked up an unclogged vaccum line to a ported vaccum and capped off all of the manifold vaccums which were uncapped. I also changed the vac. adv. in the distributor. I did all of this thanks to the advice of my fellow inliners.
Now, after changing the oil, checking the timimg, and hooking up the carb correctly, I get 12 MPG which I consider to be low, but i get more power now. I believe the tranny is ok, haven't checked the muffler to see if it is clogged, but what do you think could be causing this? All of my driving is city driving since there are no highways in El Salvador. I keep my foot off the gas for as much as i can and the car according to a site should have 2.73 rear gears. Do you think maybe the carb was jetted for a truck or something and that could be causing the low gas mileage? How can I check the jets without having to take the carb apart?

I'm not sure you could do that with a monojet. But I don't have a lot of experience tuning. I rebuilt my original one so I know it was setup properly.