Distributor timing curve question
Thread Starter
Drifting
Joined: Apr 2002
Posts: 1,720
Likes: 19
From: Manhattan Beach Ca
Distributor timing curve question
My initial is set to 12 deg BTDC but at full mechanical advance it's going all the way up and maxing out at 40 instead of 36. The engine doesnt like initial to be at 8 deg so what should I do? To avoid detination I really want max to be 34 which would require 6 initial.
I guess I dont understand why the stock distributor is allowing it to go so high. I could try and put a bushing on the little post to shorten the slot but that's so Bubba. Someone had said something about brazing the slot to fill it in a little but that's a lot of trouble.
Are most stock distributors set up this way with 28 degrees total mechanical?
I guess I dont understand why the stock distributor is allowing it to go so high. I could try and put a bushing on the little post to shorten the slot but that's so Bubba. Someone had said something about brazing the slot to fill it in a little but that's a lot of trouble.
Are most stock distributors set up this way with 28 degrees total mechanical?
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
I brazed in a mechanical stop in mine back in the late 70's and bought the spring and weight set to change the advance curve. You might find this interesting.
http://www.chevyhiperformance.com/ho...438/index.html
http://www.chevyhiperformance.com/ho...438/index.html
Race Director
Joined: Dec 2007
Posts: 14,345
Likes: 4
From: Hartselle AL
St. Jude Donor '09
Ditto on getting a spring and weight kit. Cheap and gives you a lot of options in fine tuning the advance. I went through several iterations/combinations of springs and weights to finally get the right setup.
Drifting
Joined: Apr 2007
Posts: 1,970
Likes: 45
From: Cameron Park CA
He'll not only set the timing for you, but will go through the entire unit and fix/replace any worn parts. You'll be amazed how much better your engine will run and respond to the throttle.Cheers,
Pete
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
Other than welding. I bet you could make a stop out of a small screw and nut. Just put it through the end of the slot as a stop. Then just a dab of silicone so it won't fall out.
I had my Mallory Unilite recurved to 16 mech advance total so I could run 18 initial and end up with 34 (Dart or AFR aluminum heads)
My new digital dist. is set by a dial to 15 degrees mech program advance with it all in at 3200
I had my Mallory Unilite recurved to 16 mech advance total so I could run 18 initial and end up with 34 (Dart or AFR aluminum heads)
My new digital dist. is set by a dial to 15 degrees mech program advance with it all in at 3200
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Drifting
Joined: Aug 2005
Posts: 1,443
Likes: 65
From: Wauconda IL
I just went thru the same effort on my 69'
I wanted 34 total with 14 initial so I needed only a 20 degree advance.
Looking in the spec section of some old Chevy shop manuals from the early 70's I found several distributors that had the 20 degree max advance and picked one up off of ebay for like $15.
These aren't Corvette distributors, just SBC distributors but you can mix and match the cam plate and weights from any points style SBC distributor in your Vette distributor.
Even though you can control the total advance by the bushing or the size of the slot in the cam plate, the weights can physically control the max advance as well. This is how I got my 20 degrees.
It may be a little more effort initally but you won't need to fabricate/butcher or weld anything and you can bring it back to the stock config at any time with minimal effort.
I believe I currently have the cam plate and weights from a 72' 307 2bbl SBC with springs from a Crane re-curve kit to give me the 20 degrees at 2200 RPM I was looking for.
Elm
I wanted 34 total with 14 initial so I needed only a 20 degree advance.
Looking in the spec section of some old Chevy shop manuals from the early 70's I found several distributors that had the 20 degree max advance and picked one up off of ebay for like $15.
These aren't Corvette distributors, just SBC distributors but you can mix and match the cam plate and weights from any points style SBC distributor in your Vette distributor.
Even though you can control the total advance by the bushing or the size of the slot in the cam plate, the weights can physically control the max advance as well. This is how I got my 20 degrees.
It may be a little more effort initally but you won't need to fabricate/butcher or weld anything and you can bring it back to the stock config at any time with minimal effort.
I believe I currently have the cam plate and weights from a 72' 307 2bbl SBC with springs from a Crane re-curve kit to give me the 20 degrees at 2200 RPM I was looking for.
Elm
Drifting
Joined: Sep 2004
Posts: 1,959
Likes: 1
From: North Bend Ohio
Thread Starter
Drifting
Joined: Apr 2002
Posts: 1,720
Likes: 19
From: Manhattan Beach Ca
The re-curve kits are specifically for distributors that accept a bushing the correct size that locks onto their post. The posts from the stock distributor need the bushings usually bonded on the post using maybe JBweld or something since it's hard to find one that slips on tight.
Ezo, that makes sense there would be other plates with only 20 and not 28 degrees of advance. I probably would find it hard to find the correct plate though. And I'm not sure you can actually contral max advance with just the weights unless you put super stiff springs in. But that would mess the curve up by bringing the final max in say at 4000 rpms instead of 2800.
Ezo, that makes sense there would be other plates with only 20 and not 28 degrees of advance. I probably would find it hard to find the correct plate though. And I'm not sure you can actually contral max advance with just the weights unless you put super stiff springs in. But that would mess the curve up by bringing the final max in say at 4000 rpms instead of 2800.
Le Mans Master
Joined: Mar 2004
Posts: 7,473
Likes: 1,495
From: Little Rock AR
You can only reduce just so much by adding a bushing. The GM slot is not wide enough for a bigger bushing so once you add the one in the kit, you are done. Any further reduction will need to come from somewhere else. Aftermarket distributors have room for several different diameter bushings but you don't have to buy anything to do this to a stock distributor if you have the tools.
Brazing the slot closed a bit is not bubba at all. What do you think people did before speed came in blister packs? You dissasemble the distributor, heat up the autocam plate with an oxy-acetalyne torch and fill the slot up a bit with some brass brazing rod. Use a dremel or some tiny files to finish the slot final shape then reassemble. If you have 30 degrees originaly and you want to have 20, just fill up 1/3 of the slot. Just old school stuff.
-Mark.
Brazing the slot closed a bit is not bubba at all. What do you think people did before speed came in blister packs? You dissasemble the distributor, heat up the autocam plate with an oxy-acetalyne torch and fill the slot up a bit with some brass brazing rod. Use a dremel or some tiny files to finish the slot final shape then reassemble. If you have 30 degrees originaly and you want to have 20, just fill up 1/3 of the slot. Just old school stuff.
-Mark.
Thread Starter
Drifting
Joined: Apr 2002
Posts: 1,720
Likes: 19
From: Manhattan Beach Ca
stingr you're right. I would do that but I just dont have the know-how or equipment to braze. I only need to reduce 4 or 5 degrees so I think the bushing route should work OK if I can find one with the right diameters.
Drifting
Joined: Aug 2005
Posts: 1,443
Likes: 65
From: Wauconda IL
It's the shape of the weights not the weight of them that controls the amount of total advance. If you look at the top of the distributor shaft itself, you'll see a specific cam shape- This cam shape in relation to the shape of the opening in the weights contributes to the total advance more than the hole in the plate and the bushing. The bushing can only control minor limit changes and is more of a fail-safe stop mechanism to keep the whole assembly from getting too out of control should something happen to the springs or weights.
I needed to go from 32 degrees on my original set-up to 20 degrees. There was no way that bushing was going to be able to accommodate that kind of change.
I actually have the 're-curve kit' bronze bushing pressed on my dist shaft (I did that first before I realized the bronze bushing is the same diameter as the black plastic factory bushing). The combination of the weights from the 307 distributor and the cam on the main shaft is what is limiting my advance to 20 degrees. The bushing right now never makes contact, even at full advance at 5000 RPM (which is still 20 degrees).
I wish I knew how to attach a picture to these threads; I could attach a shot so you can see what I mean.
Elm
I needed to go from 32 degrees on my original set-up to 20 degrees. There was no way that bushing was going to be able to accommodate that kind of change.
I actually have the 're-curve kit' bronze bushing pressed on my dist shaft (I did that first before I realized the bronze bushing is the same diameter as the black plastic factory bushing). The combination of the weights from the 307 distributor and the cam on the main shaft is what is limiting my advance to 20 degrees. The bushing right now never makes contact, even at full advance at 5000 RPM (which is still 20 degrees).
I wish I knew how to attach a picture to these threads; I could attach a shot so you can see what I mean.
Elm
Thread Starter
Drifting
Joined: Apr 2002
Posts: 1,720
Likes: 19
From: Manhattan Beach Ca
I fabbed up my own bushing and JB welded it on. Should work. The thing is if you want to up initial by just 1 degree, then you have to re-do everything over again to bring max mechanical back down a degree.
Seems very messed up and there should be an easier way to go about this on stock distributors.
I'll look at the shaft cam and weight shape ezo, you may have something there. THX
Seems very messed up and there should be an easier way to go about this on stock distributors.
I'll look at the shaft cam and weight shape ezo, you may have something there. THX
Le Mans Master
Joined: Mar 2004
Posts: 7,473
Likes: 1,495
From: Little Rock AR
I fabbed up my own bushing and JB welded it on. Should work. The thing is if you want to up initial by just 1 degree, then you have to re-do everything over again to bring max mechanical back down a degree.
Seems very messed up and there should be an easier way to go about this on stock distributors.
I'll look at the shaft cam and weight shape ezo, you may have something there. THX
Seems very messed up and there should be an easier way to go about this on stock distributors.
I'll look at the shaft cam and weight shape ezo, you may have something there. THX
you are exacly right, it is a pain. I started at 14 initial on the last one I did and had to grind it a bit to get down to 12 to avoid an off-idle stumble that just would not go away.
Keep in mind that you are just setting where the initial timing will fall when idling and maybe have some effect when going off-idle. Peak power and track times are not affected by this mod, only idling and off idle to some small amount. Getting it close is probably good enough in many cases. If the car idles well with the new bushing and does not stumble at tip-in then being "in the range" is probably good. Once you are "on the curve" at say 1400 or 1600 RPM, the bushing is no longer a factor.
-Mark.
Melting Slicks
Joined: May 2002
Posts: 2,716
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From: Sulphur LA
St. Jude Donor '05-'06,'11,'13-'14,'16,'18,'19,'24, '25
from what I've read factory weights and center plates will limit advance to 20*. The only reason to need to fill the slot would be if you set initial above 16* or you use an aftermarket set of weights and center cam that would give you more that 20* machanical.
enzobens is correct
Neal
enzobens is correct
Neal
