There have been some posts recently regarding vacuum advance and timing. I have my idle set at about 750rpm. I noticed that when I dissconnect the vacuum advance hose(and plug it)it doesn't seem to decrease. When I apply vacuum directly to the vacuum advance the speed does increase. Does this mean I don't have the idle mixtures set properly to apply sufficient vacuum to the advance mechanism at idle?
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Re: '67 BB VACUUM ADVANCE AND IDLE
Rob,
Tell us where your vacuum line is connected at the carb ( ported or manifold vacuum source ) .
Wayne -
Re: '67 BB VACUUM ADVANCE AND IDLE
It's a tripower set up, connected to center carb base.Comment
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Re: '67 BB VACUUM ADVANCE AND IDLE
We've been around the horn several times on this issue with the '66 L-72. They all have ported vacuum advance. I believe the reason was for CA emissions. The '66 L-72s ordered for delivery in CA did NOT have AIR pumps like all other Corvette engines. My theory is that due to low overall volume, Chevrolet certified the L-72 for CA emissions with the ported vacuum advance and sold this configuration in all the other states.
These discussions have never extended to the '67 L-71 (I assume that is what you have, but could you verify if it is a L-71 or L-68), but my suspicion has always been that L-71 also has a ported vacuum advacne.
I recommend that L-72 owners convert to full vacuum advance, and those that have report better idle quality, and less tendency to overheat, which is the expected result. Better fuel economy should also result, but most owners don't keep close track of fuel economy.
DukeComment
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Re: '67 BB VACUUM ADVANCE AND IDLE
I am working on an L68. The vacuum advance hose goes to the base under the carb that attaches it to the manifold. There's a lot of info in the posts regarding this, I'm just trying to find out if my vacuum advance is working the way it should. My engine has all stock components, except for being 30 over.Comment
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Re: '67 BB VACUUM ADVANCE AND IDLE
Rob,
From your description it sounds to me like your source is manifold vacuum and I would think that a L-68 should be drawing enough vacuum at idle to make a noticeable difference in the idle speed when disconnected unless there is some kind of emissions control between the carb and the vacuum advance can. If you have a straight line from carb to vacuum can check to see that nothing is blocking it. If not I would T in a vacuum gauge to see how much vacuum you have at idle. I don't know the specs for your motor but I'm sure someone else here will.
WayneComment
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Re: '67 BB VACUUM ADVANCE AND IDLE
Thanks, I'll try that. It idles nice and smooth, it just surprised me that the rpm didn't increase with the vacuum advance hose reconnected.Comment
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Re: '67 BB VACUUM ADVANCE AND IDLE
Rob -
Tee the distributor vacuum advance hose into the center carb's choke pull-off diaphragm hose (which sees full manifold vacuum) instead of the current carb base connection and see if the idle increases when connected there and decreases when disconnected. I think you'll find that the existing carb base connection is to ported vacuum.Comment
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