Distributor Advance Springs

[Roger S.]

Are there any negative effectives to using two different strength springs in the distributor to control centrifugal advance?



In order to get the idle timing to 12 BTDC when using the two medium springs I must turn the distributor which lowers the rpm and decreases the idle quality. I can open the carburetor throttle plates to increase the idle rpm and have better idle quality, but then I start having minor amounts of dieseling when the engine is turned off.

Thanks
Roger

[Steven S.]

Re: Distributor Advance Springs

Mixing different weight springs is standard practice with those kits, shouldn't be anything to worry about!


Steve

[Stuart F.]

Re: Distributor Advance Springs

Roger;

What year/engine combination do you have?

Stu Fox

[Duke W.]

Re: Distributor Advance Springs

Mixing springs is okay, but I don't recommend the heaviest and lightest. This allows a total of five curves.

It sounds like you have ported vacuum advance. Change it to full time, but you probably need a new VAC. There are three that cover the entire range of OE engine configurations, and this has been covered many times in the archives.

Duke

[Roger S.]

Re: Distributor Advance Springs

The Corvette is a 1973 L-82 with automatic.

The advance numbers are without vaccum applied to the distributor. I do run manifold vaccum. With the vaccum attached to the VAC the advance goes from 12 deg @ 700 rpm to 34 deg @ 1020 rpm. The VAC is a B28 and is new. I have measured when it starts/stops it movement. It starts moving at 5"Hg and finishes moving at 10"Hg. The engine idle vaccum at 700 rpm without vaccum supply to the distributor is about 13-1/2" Hg.

The Accel kit comes with three spring stiffnesses. I am running one medium and one heavy stiffness spring. The lightweight springs allowed too much centrifugal advance at idle to the point where I could on get 14 deg advance above idle speed.

Roger

[Duke W.]

Re: Distributor Advance Springs

With an automatic idle speed/idle mixture/vacuum should be set/measured with the transmission in Drive. Be sure to chock the wheels when you do this!

The B28 VAC is correct for this application.

The centrifugal should not start until at least 200-300 RPM above this speed, and observed total idle timing with the VAC connected should be steady and the sum of initial plus full vacuum.

Duke

[Stuart F.]

Re: Distributor Advance Springs

And don't blip the throttle.

Stu Fox

[Stuart F.]

Re: Distributor Advance Springs

For what it's worth, on my 63 L-76 w/ AFB, there is a fine line between the initial advance setting and the carburetor idle speed setting where in if you raise your idle speed by advancing the distributor, then try to correct it by closing the throttle plates with the carb idle speed screw, you change the relationship of the throttle plates to the low speed transfer slots (closing them down) in the throttle bore. In some cases, this can lead to a low speed hesitation. That is a general rule of thumb for Carters and Holleys in production applications. Installing an aftermarket camshaft can change the relationship significantly.

Stu Fox

[Terry M.]

Re: Distributor Advance Springs

And on early C3s with Holley, removing the emissions equipment (particularily the TCS system) can have the same effect. Been there, done that, and got that dirty T-shirt.