HEI mechanical advance
Thread Starter
Intermediate
Joined: Oct 2009
Posts: 37
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From: West Michigan
HEI mechanical advance
I have a 1977 Vette L48 that I am trying to get the mechanical advance plus initial to 36*. I have the base advance set at 700 RPM at about 16* and after several tries, I can not get it to advance to 36* only about 28* or so at 2800-3000 RPM's. My latest try is with a Mr. Gasket advance kit with weights and the lightest springs. No change from stock. Is there some kind of stop that is built into the distributor that I am missing. The HEI distributor I am working with is stock. Thanks.
Tech Contributor
Joined: Aug 1999
Posts: 14,377
Likes: 6,377
From: At my Bar drinking and wrenching in Lafayette Colorado
The HEI distributors were built with a huge range of centrifugal advance curves. There were quite a few that were built with "short curves" to keep timing limited for emissions purposes - these short curve distributors had vacuum advance control units with very long curves so that timing at cruise would be advanced for fuel economy.
The "short curve" distributors are now highly desirable for high performance use: A car with a bit of a cam will want about 18-24 degrees of initial timing, but total must still be limited to 36-or-so. By installing a short-curve vacuum advance on one of these distributors, you can use it with great success on a high performance engine.
The centrifugal curve is limited by 3 things:
Lars
The "short curve" distributors are now highly desirable for high performance use: A car with a bit of a cam will want about 18-24 degrees of initial timing, but total must still be limited to 36-or-so. By installing a short-curve vacuum advance on one of these distributors, you can use it with great success on a high performance engine.
The centrifugal curve is limited by 3 things:
- There is an advance stop slot machined into the mechanism. If you disassemble the distributor you will see the stop slot, and you can alter its length.
- The center cam and advance weight geometry is designed in such a way that the weights will actually run out of mechanical advantage at a certain point and cannot advance the timing any further regardless of rpm. In the kits that I was building several years ago I supplied a special set of hand-made weights that would overcome this geometry-imposed limitation and force the curve further. There are no aftermarket weights that I am aware of that will do this - I fabricated weights to accomplish this, and you can do the same with a little work.
- The rotor inner diameter actually limits how far out the weights can "fling": The weights are constrained by the rotor. Often, you can remove some material from the outer edge of the weights so that they can move further before the rotor stops them. Also, there are aftermarket rotors with incorrect inner diameters that will limit the weights' travel. You can put a little grease on the inside diameter of the rotor, rev the engine up, and see if the grease is transferred to the outer edge of the weights.
Lars
Thread Starter
Intermediate
Joined: Oct 2009
Posts: 37
Likes: 0
From: West Michigan
The HEI distributors were built with a huge range of centrifugal advance curves. There were quite a few that were built with "short curves" to keep timing limited for emissions purposes - these short curve distributors had vacuum advance control units with very long curves so that timing at cruise would be advanced for fuel economy.
The "short curve" distributors are now highly desirable for high performance use: A car with a bit of a cam will want about 18-24 degrees of initial timing, but total must still be limited to 36-or-so. By installing a short-curve vacuum advance on one of these distributors, you can use it with great success on a high performance engine.
The centrifugal curve is limited by 3 things:
Lars
The "short curve" distributors are now highly desirable for high performance use: A car with a bit of a cam will want about 18-24 degrees of initial timing, but total must still be limited to 36-or-so. By installing a short-curve vacuum advance on one of these distributors, you can use it with great success on a high performance engine.
The centrifugal curve is limited by 3 things:
- There is an advance stop slot machined into the mechanism. If you disassemble the distributor you will see the stop slot, and you can alter its length.
- The center cam and advance weight geometry is designed in such a way that the weights will actually run out of mechanical advantage at a certain point and cannot advance the timing any further regardless of rpm. In the kits that I was building several years ago I supplied a special set of hand-made weights that would overcome this geometry-imposed limitation and force the curve further. There are no aftermarket weights that I am aware of that will do this - I fabricated weights to accomplish this, and you can do the same with a little work.
- The rotor inner diameter actually limits how far out the weights can "fling": The weights are constrained by the rotor. Often, you can remove some material from the outer edge of the weights so that they can move further before the rotor stops them. Also, there are aftermarket rotors with incorrect inner diameters that will limit the weights' travel. You can put a little grease on the inside diameter of the rotor, rev the engine up, and see if the grease is transferred to the outer edge of the weights.
Lars
Quote:
Originally Posted by lars View Post
Idle fuel is metered and restricted by the orifice in the Idle Fuel Restrictor Tubes. Power piston position has very little effect on idle quality if the carb is idling on the idle circuit. If the carb is set up so lean that it won't idle on the idle circuit, the raised power piston will cause an extreme rich condition through the main metering circuit if the carb is idling on the main circuit.
The 77 carbs tend to run very lean, both at idle and at WOT. You may need to enlarge the idle tube restrictions along with a significant jetting change. As a last resort, you can block the idle air bypass holes to force engine to pull more fuel through the idle and transition circuits.
Lars
Lars, what main jet and primary rods do you recommend? I set the primary rod height like you suggested in one of you papers via the adjusting screw and a dial caliper and that seems to make a big difference. I also rebuild the carb and found several tubes were blocked and one had fallen into the bowl. The primary rods had a fair amount of corrosion on them as well. My vacuum at idle is about 14 inches. Should I get a lighter spring? The stock jets are 77 and primary rods are 52. Spring is #7029529. What size of restriction tubes and where can a person buy those from?
Thanks for all your advice, it's really helping me learn.
