470 Distributor Mechanical Advance

[tmccotter]

Hello!

I've recently acquired and rebuilt a 3.7l 165. All is good at lower RPMs but under load and over 3K rpm I hear a clacking sound which I believe to be detonation. I believe the issue is excessive timing advance from the mechanical advance.

I measured the advance at 500 rpm increments up through 4K rpm and compared it with the timing curve specification. I can see that the advance climbs correctly with the curve and doesn't flatten out at 2K rpm per the specification but rather continues to increase up to up over 40 degrees. Most mechanical advance mechanisms have something limiting the total advance but I can't find any documentation for the distributor.

Does anyone out there know how to limit the mechanical advance? Is there any documentation for these distributors?

This is the first question I've asked but you all have answered at least a thousand questions I have had through these forums..... thanks!!

Tom

 

[Bt Doctur]

If its still points, pull the dist and examine it

 

[Bt Doctur]

Advanced Distributors – Automotive Distributor Rebuilding Services

advanceddistributors.com

 

[Bt Doctur]

I`m not positive but I think the innards are GM 4 /6 cylinder dist parts

 

[tmccotter]

Thanks so much for the speedy reply! It is still points... new points, rotor and condenser were installed during the rebuild. I examined, cleaned and lubricated the distributor during rebuild and didn't see any issues. When I was cleaning it up I didn't break down the advance mechanism so I don't know for sure how it works. Other distributors I've seen have a shim or something that stops the weights from traveling further out past the maximum advance but I don't know what happens with these distributors.

I also looked at the equivalent part from Sierra and Delco and I can't find any documentation for those either.

 

[Lou C]

If it’s like a Prestolite under the point plate there are a pair of flyweights that are retained by springs. When the engine speeds up the flyweights move outward due to centrifugal force and advance the position of the rotor which advances the timing. Then when it slows down the springs pull the flyweights in which retards the timing. So if it’s advancing too much the springs may have weakened. Or corrosion may be causing the flyweights to stick. You can remove the point plate and clean and lube the flyweights and re test. If it’s still too advanced see if you can get a new set of the correct springs.

 

[Scott Danforth]

see how much slop is in the $5 distributor shaft bushing from napa

also, if you didnt set your dwell with a meter, go back to that, then set timing

 

[tmccotter]

Right, the timing does advance as you described. I think it is possible that the springs have weakened; however, the advance follows the curve as rpms are increasing and then just keeps going. I can't find any specs or description of the springs but I will try to find replacements. I did clean and lube the flyweights already, they aren't sticking.

If it’s like a Prestolite under the point plate there are a pair of flyweights that are retained by springs. When the engine speeds up the flyweights move outward due to centrifugal force and advance the position of the rotor which advances the timing. Then when it slows down the springs pull the flyweights in which retards the timing. So if it’s advancing too much the springs may have weakened. Or corrosion may be causing the flyweights to stick. You can remove the point plate and clean and lube the flyweights and re test. If it’s still too advanced see if you can get a new set of the correct springs.

Right, the timing does advance as you described. I think it is possible that the springs have weakened; however, the advance follows the curve as rpms are increasing and then just keeps going. I can't find any specs or description of the springs but I will try to find replacements. I did clean and lube the flyweights already, they aren't sticking. I was really curious as to whether there was something that limited the maximum advance.

 

[tmccotter]

see how much slop is in the $5 distributor shaft bushing from napa

also, if you didnt set your dwell with a meter, go back to that, then set timing

dwell and timing are good. To which bushings are you referring? I looked at the parts diagram and I can't figure out where they are.

 

[Scott Danforth]

Pull the dizzy, knock the roll pin out of the gear, pull the inner shaft, look in the housing. All dizzys have bushings, and napa has generic bushings for dizzys cheap

 

[tmccotter]

I just found this... I may order it and give it a shot...

PerTronix D700700 PerTronix Billet Distributor Advance Spring Kits | Summit Racing

Free Shipping - PerTronix Billet Distributor Advance Spring Kits with qualifying orders of $109. Shop Distributor Advance Kits at Summit Racing.

www.summitracing.com

 

[Scott Danforth]

you dont have a billet distributor, you also dont have a sunnen distributor machine to set the distributor up

 

[tmccotter]

I know... I just want the set of springs and shims.

 

[Scott Danforth]

I know... I just want the set of springs and shims.

and how do they help you with the prestolite distributor you have?

 

[tmccotter]

The kit has several sets of springs. Currently, the centrifugal advance is over advancing. Either tighter springs or something to limit the advance would be the likely solution. For $15 it seems like a reasonable thing to try. As I mentioned earlier I can't find any documentation or replacement parts (nor a parts diagram) of the current distributor so I need to experiment.

 

[Lou C]

Why not look around and see if you can find the original springs that it came with? I was able to find an original set of springs for mine and its 32 years old....

 

[tmccotter]

Why not look around and see if you can find the original springs that it came with? I was able to find an original set of springs for mine and its 32 years old....

I'd like to do that but I haven't been able to find any sort of part number or description of the springs. Mine is 32 years old too. How did you find yours?

 

[Scott Danforth]

The kit has several sets of springs. Currently, the centrifugal advance is over advancing. Either tighter springs or something to limit the advance would be the likely solution. For $15 it seems like a reasonable thing to try. As I mentioned earlier I can't find any documentation or replacement parts (nor a parts diagram) of the current distributor so I need to experiment.

you cant use springs from one manufacturer and expect them to work on another manufacturers distributor and expect them to work. only way to "experiment" is with a sunnen distributor machine.

 

[Lou C]

I'd like to do that but I haven't been able to find any sort of part number or description of the springs. Mine is 32 years old too. How did you find yours?

I looked it up in an OMC parts manual! Found them on another site I can’t post it here but they were still available so I picked up a set of springs, new point plate etc. I also picked up a spare Prestolite for my 4.3... eBay....

 

[GA_Boater]

I looked it up in an OMC parts manual! Found them on another site I can’t post it here but they were still available so I picked up a set of springs, new point plate etc. I also picked up a spare Prestolite for my 4.3... eBay....

Thank you for not posting the link, Lou.

What you can do is post the part numbers if you still have them. All tmcotter needs is new springs and he can Google the part numbers.