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Old 06-17-2012, 08:28 AM
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Duraspark distributor mechanical advance issue

I have a rebuilt Duraspark distributor (from a local parts house) installed in my mild 351W.

The first thing I did when I got it was take it apart to verify the reluctor arm settings - it's currently set to the 10L slot, which by my understanding means 20 crank degrees of total mechanical advance. I set the initial to 14 giving a total advance of 34 (vacuum is disconnected).

The issue is that the distributor only seems to advance 10 degrees, maybe a bit less, no matter how high I rev the engine. I don't have a tach under the hood but I'm running it pretty high with no change to the advance.

Is there a way to tell what rpm range the advance begins/ends at? I figure it either is all in at way too high an rpm, or it begins at way too low an rpm, so some of my initial is actually advanced (if that makes any sense).

Thanks for any advice.
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Old 06-17-2012, 08:33 AM
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How about changing the advance springs to lighter set for testing to assure the advance is able to move and at a lower rpm?
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Old 06-17-2012, 08:59 AM
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Quote:
Originally Posted by Shaker455
How about changing the advance springs to lighter set for testing to assure the advance is able to move and at a lower rpm?
I only have the springs that are in there, although I'm planning on buying the Mr Gasket recurve set if I can find one. Would removing one of the springs let me test similarly? (ie, remove the heavy spring and see if I get full advance?)
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Old 06-17-2012, 12:15 PM
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Quote:
Originally Posted by isaac338
I only have the springs that are in there, although I'm planning on buying the Mr Gasket recurve set if I can find one. Would removing one of the springs let me test similarly? (ie, remove the heavy spring and see if I get full advance?)
I don't see why not. Be prepared for it to possibly advance very early. Test in the driveway, I wouldn't risk driving the vehicle for worries it will detonate like that.
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Old 06-17-2012, 12:25 PM
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Quote:
Originally Posted by cobalt327
I don't see why not. Be prepared for it to possibly advance very early. Test in the driveway, I wouldn't risk driving the vehicle for worries it will detonate like that.
Fair enough. I'll give it a shot and see what happens. I'm chasing what sounds like it could be detonation under WOT acceleration which is why I started looking at timing to begin with!

Thanks for the help.
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Old 06-19-2012, 03:06 PM
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Well, go figure, I realized when you put it in drive the timing goes down to around 10 degrees BTDC (probably what causes my stalling-in-drive issue), so I suppose that's where my advance is going (ie at my 1000rpm idle, the distributor is already 10 degrees advanced). Looks like I'll be after one of those Mr Gasket sets sooner rather than later.

Are there any ways to make the mechanical advance come in a little later without changing springs?
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Old 06-22-2012, 10:17 PM
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set the upper limit

Try marking the damper with the desired 34 mechanical total - then rev the engine (vacuum can disconnected) and lock your clamp with your mark at the TDC marker. When it drops back to idle, the difference between the timing there and the 14 static you planned for (to get 34 total) will be the amount of "early" mechanical advance.

You didn't mention the vacuum advance but it needs to be disconnected and plugged at the carb while you are setting up the mechanical.

Inside the distributor, when you have the stator plate removed for access - find the lighter of the two springs and slightly bend the support for one end to increase the amount of preload in it. The light spring is the one that is kicking in "early" and you can try putting a bit more tension on there.
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Old 06-23-2012, 02:00 AM
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Quote:
Originally Posted by isaac338
Well, go figure, I realized when you put it in drive the timing goes down to around 10 degrees BTDC (probably what causes my stalling-in-drive issue), so I suppose that's where my advance is going (ie at my 1000rpm idle, the distributor is already 10 degrees advanced). Looks like I'll be after one of those Mr Gasket sets sooner rather than later.

Are there any ways to make the mechanical advance come in a little later without changing springs?
As mentioned, the spring anchor point can be tweaked but the cure is probably going to require a spring selection/change.

Have you tried using manifold vacuum for the vacuum advance? That will sometimes bring in enough timing at idle speed- even when in gear- to prevent the bottom from dropping out of the idle timing. It's worth trying.
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Old 06-23-2012, 06:59 AM
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Quote:
Originally Posted by GREENBIRD56
Try marking the damper with the desired 34 mechanical total - then rev the engine (vacuum can disconnected) and lock your clamp with your mark at the TDC marker. When it drops back to idle, the difference between the timing there and the 14 static you planned for (to get 34 total) will be the amount of "early" mechanical advance.

You didn't mention the vacuum advance but it needs to be disconnected and plugged at the carb while you are setting up the mechanical.

Inside the distributor, when you have the stator plate removed for access - find the lighter of the two springs and slightly bend the support for one end to increase the amount of preload in it. The light spring is the one that is kicking in "early" and you can try putting a bit more tension on there.
Vacuum has been disconnected since I put the engine together.

I'll try that trick, but I think it's a combination of the light spring coming in too early and the heavy spring coming in too late. I turned down the idle until the timing stopped changing and set the initial that way; I still can't get it to advance more than ten degrees. I did order the Mr Gasket kit, though, so hopefully that'll fix it up.

Quote:
Originally Posted by cobalt327
Have you tried using manifold vacuum for the vacuum advance? That will sometimes bring in enough timing at idle speed- even when in gear- to prevent the bottom from dropping out of the idle timing. It's worth trying.
Any time I connect the vacuum advance (to the ported vacuum port on the side of the carb), the idle increases and it starts pinging - I haven't even checked if the vacuum can is adjustable, and I've just left it disconnected and capped. Should there be vacuum at that port at idle?
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Old 06-23-2012, 01:00 PM
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Quote:
Originally Posted by isaac338
Vacuum has been disconnected since I put the engine together.

I'll try that trick, but I think it's a combination of the light spring coming in too early and the heavy spring coming in too late. I turned down the idle until the timing stopped changing and set the initial that way; I still can't get it to advance more than ten degrees. I did order the Mr Gasket kit, though, so hopefully that'll fix it up.



Any time I connect the vacuum advance (to the ported vacuum port on the side of the carb), the idle increases and it starts pinging - I haven't even checked if the vacuum can is adjustable, and I've just left it disconnected and capped. Should there be vacuum at that port at idle?
If you have enough vacuum at the ported vacuum port to cause the vacuum advance to operate, that's telling you the primary throttle blades are open too far or the carb is assembled wrong (wrong or torn gaskets, etc.).

You can either get an adjustable vacuum can (Crane, etc.) or you might be able to fab a limiter to keep the vacuum advance from giving you too much advance. But that will only work if the tip in vacuum is correct; if the vacuum advance comes in too early (too high of a vacuum signal), it can cause the engine to be over advanced.

All that's needed from the vacuum advance is usually around 10-12 degrees. The tip in point is where the adjustable can comes in handy. It can help to keep the vacuum advance from coming in too soon. Exactly where to set it is trial and error until you hit a happy medium.

But the bottom line is the timing curve needs to be properly set up, and it sounds as though you do not have enough initial timing to keep the throttle blades from being opened too far. I would be double checking how much initial you have, and I would be looking for an adjustable vacuum advance can and a spring kit to dial it all in.

Here is a page on the GM HEI. The sections on advance are universal.

Be sure the damper outer ring isn't slipping, causing a false reading. DETERMINING TDC will allow you to be sure the timing tab and damper are correctly indicating TDC.

MAKE A TIMING TAPE to see what the total timing is, w/o needing to use a dial back timing light. You can also buy a timing tape, get one that matches the diameter of your damper.
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Old 06-24-2012, 05:25 PM
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find TDC

(1) Take Cobalt's advice and go about "proving" your TDC marks. This is very suspect when your numbers don't add up..
(2) If you can remove the cap and the rotor will manually move about 10 and readily spring back - the mechanical advance is probably working.

Don't let the following get in your way......
Ford and replacement Duraspark II controllers (with the blue strain relief) have the starting retard feature(-6) - usually have a white wire connected at the starter solenoid. It should only get a 12v signal when the starter is engaged. Be aware the retard feature works whenever the controller detects the +12v signal on the white lead - starting or not.
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Old 06-27-2012, 04:41 PM
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Quote:
Originally Posted by GREENBIRD56
(1) Take Cobalt's advice and go about "proving" your TDC marks. This is very suspect when your numbers don't add up..
(2) If you can remove the cap and the rotor will manually move about 10 and readily spring back - the mechanical advance is probably working.

Don't let the following get in your way......
Ford and replacement Duraspark II controllers (with the blue strain relief) have the starting retard feature(-6) - usually have a white wire connected at the starter solenoid. It should only get a 12v signal when the starter is engaged. Be aware the retard feature works whenever the controller detects the +12v signal on the white lead - starting or not.
Thanks for the tip with the white wire. The control box is an original Duraspark so I think I'm ok there.

The timing marks were rusted away on the balancer so I made a timing tape some time ago. I tried to make a piston stop today but for some reason no matter how far into the chamber I ran the stop, the piston wouldn't hit it. Must have to do with the plug angle. I did the old dowel-down-the-plug-hole trick and it appears my timing tape was installed like 8-10 degrees retarded.

So, I guess I need a new timing tape, installed in the right place, and my distributor recurve kit is in the mail.

I advanced the initial to about 26 degrees (which should be around 16 accounting for the tape error), and I couldn't get it to idle less than 1200 rpm. I think my secondaries are open too far so I guess I'll have to take the carb off (again!) to close them up a half turn. I took it off yesterday and realized I missed one small gasket under the choke horn, giving me a big old vacuum leak.

The car still dies in gear.. getting closer though.

Thanks again for everyone's help.
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Old 06-27-2012, 09:37 PM
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Secondary screw

I should have read all of the posts more carefully - is this a Holley 4160 carb with the secondary idle screw? A suggestion from a confirmed "sidehill" mechanic - replace the little secondary stop screw with a longer one that has no head - that extends just down far enough to get on it with a pair of needlenose pliers while the carb is mounted. I've tried a variety of "proper" fixes (socket setscrews etc.) - none beat the hilbilly way.

Another trouble I've run into - the PCV's internal orfice being large enough that when the engine is operating at high vacuum it pulls enough air to run up the idle speed. The Holley primary needs placed at the right point in the transition slot and left there - and if the secondary throttle stop is wound down out of the way and it still is getting too much air - put an orfice in the PCV line - or get a smaller one for a four cylinder.

Good luck with your tune-up. Get the electrical dialed in first - then tune the carb to match it. I think I'd be setting it up to have about 10-12 initial - and 24 mechanical advance - for 34-36 total. If you've got the "white wire" on there it will retard to 6 electronically when starting which will be just about right. If you've got "good" gas there - try to get all of the advance in by 2500 rpm - if its flakey - go for 3000.

Last edited by GREENBIRD56; 06-27-2012 at 10:01 PM.
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Old 06-27-2012, 10:17 PM
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Quote:
Originally Posted by GREENBIRD56
I should have read all of the posts more carefully - is this a Holley 4160 carb with the secondary idle screw? A suggestion from a confirmed "sidehill" mechanic - replace the little secondary stop screw with a longer one that has no head - that extends just down far enough to get on it with a pair of needlenose pliers while the carb is mounted. I've tried a variety of "proper" fixes (socket setscrews etc.) - none beat the hilbilly way.

Another trouble I've run into - the PCV's internal orfice being large enough that when the engine is operating at high vacuum it pulls enough air to run up the idle speed. The Holley primary needs placed at the right point in the transition slot and left there - and if the secondary throttle stop is wound down out of the way and it still is getting too much air - put an orfice in the PCV line - or get a smaller one for a four cylinder.

Good luck with your tune-up. Get the electrical dialed in first - then tune the carb to match it. I think I'd be setting it up to have about 10-12 initial - and 24 mechanical advance - for 34-36 total. If you've got the "white wire" on there it will retard to 6 electronically when starting which will be just about right. If you've got "good" gas there - try to get all of the advance in by 2500 rpm - if its flakey - go for 3000.
Thanks for all the help. The carb is a 3310-6, which I believe is pretty similar to a 4150/4160. It has one curb idle screw, the secondary stop screw, and idle mixture screws on the primaries only.

I adjusted the secondaries down a quarter turn and now it's idling well at 800 rpm, and only drops a small amount in gear (instead of dying completely). Still has vacuum at the port on the side of the carb, but I'm waiting for the recurve kit to get here before I fiddle with it any more.

Cheers!
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Old 06-29-2012, 08:51 PM
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Which port?

I believe the 3310-6 is a later version of the 750 cfm carb used on a lot of older high perf Fords and Chebbies. It won't have a rear metering block - but will have the "dual feed" center fed fuel bowls on both ends? The "timed" vacuum port of that carb is up on the passenger side of the primary metering block - not down on the throttle plate. Should have no vacuum at that port - if - the primary throttle blade is properly placed on the "transfer slot" in the venturi bore.
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