Acceleration dead spot

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pacofortacos

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Ported vacuum will have no vacuum to the distributor at idle.
Manifold vacuum will have a lot of vacuum to the distributor at idle.
Just have the truck idling, pull the hose off of the distributor and feel if it has vacuum or not.

If ported, then as you open the throttle, you will get vac. to the distributor and advance the timing.
If manifold, you will be at full advance at idle and lose timing as you open the throttle.

Inside the distributor below the rotor, there is the reluctor and pickup plate - there is a specified gap between them.
Also, at that age, you should check to make sure that there isn't excessive bushing wear in the distributor - there should be almost no side to side play for the shaft that the rotor mounts on.
 
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bdarcher

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:)
Ported vacuum will have no vacuum to the distributor at idle.
Manifold vacuum will have a lot of vacuum to the distributor at idle.
Just have the truck idling, pull the hose off of the distributor and feel if it has vacuum or not.

If ported, then as you open the throttle, you will get vac. to the distributor and advance the timing.
If manifold, you will be at full advance at idle and lose timing as you open the throttle.

Inside the distributor below the rotor, there is the reluctor and pickup plate - there is a specified gap between them.
Also, at that age, you should check to make sure that there isn't excessive bushing wear in the distributor - there should be almost no side to side play for the shaft that the rotor mounts on.
Thanks for the info, I’ll check things out and see what’s up. I did notice when I put the new cap on that it looked fairly rusty under there. Can I guy spray wd40 or carb and choke cleaner or something in there and maybe give it a shot of air?
 

pacofortacos

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Only if you really air it out - otherwise when you put the cap on and start it, the sparks inside the cap will make it go boom :)
 
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bdarcher

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Only if you really air it out - otherwise when you put the cap on and start it, the sparks inside the cap will make it go boom :)
So we checked the truck today. It’s definitely setup with manifold vaccuum as there was plenty of suction at the distributor at idle. I also took the cap off and tested things by sucking on the hose and plugging the line with my tongue. Everything seemed to move well and hold under pressure. I checked the reluctor wheel and pickup coil gap. And I think it’s close. It’s supposed to be .008 but I didn’t have a .008 brass feeler gauge, just a brass .010. That slid in and out of the gap fairly tight.
So although We haven’t found the reason for the dead spot we seemed to have eliminated more things.
Wonder if I need to go back to just looking at timing?? Or do I replace that pickup coil? Or do I replace the spark plug wires? Other then those I don’t see what’s left?!
 

turkeybird56

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Sorry, I’m no mechanic. I don’t know if it’s ported or manifold?? Tube come from vac adv and plugs in front of carb on the bottom. And i checked or know nothing of a gap on distributor…..just slapped a new rotor and cap on it.
If you still have points, is the gap set correctly, or do U have reluctor? See video attached> Reluctor around 14 minute mark, but U may want to peruse the whole video.



 

pacofortacos

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Ok, with manifold vacuum, are you setting your timing with the hose unplugged? It is full advance with the hose hooked up.

Do you have a ported vac. port on the carb base? It should only get vac. at part throttle.

Otherwise, try this and see how it feels. Disconnect and plug the hose to the distributor - leave plugged and disconnected.
Set the timing at idle to stock setting plus 5 degrees - so if stock is 12 degrees, set it at 17 degrees. Take it for a ride.
Then try it at stock plus 10 degrees and take if for a ride.
Listen for pinging and see how it feels on both settings.
See if the dead spot is still there or has changed.

Pick up either works or it doesn't most often - there is a mechanical advance setup in the distributor, to make sure it is working just try turning the rotor by hand with the cap off - is should move slightly one direction and spring back to the original position if the weights and springs are working and ok. If it is stuck or a spring off, it could give you problems like you have.
 
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bdarcher

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Ok, with manifold vacuum, are you setting your timing with the hose unplugged? It is full advance with the hose hooked up.

Do you have a ported vac. port on the carb base? It should only get vac. at part throttle.

Otherwise, try this and see how it feels. Disconnect and plug the hose to the distributor - leave plugged and disconnected.
Set the timing at idle to stock setting plus 5 degrees - so if stock is 12 degrees, set it at 17 degrees. Take it for a ride.
Then try it at stock plus 10 degrees and take if for a ride.
Listen for pinging and see how it feels on both settings.
See if the dead spot is still there or has changed.

Pick up either works or it doesn't most often - there is a mechanical advance setup in the distributor, to make sure it is working just try turning the rotor by hand with the cap off - is should move slightly one direction and spring back to the original position if the weights and springs are working and ok. If it is stuck or a spring off, it could give you problems like you have.
Ok thanks. I can certainly check out the movement on the rotor. As for the timing, I’ll have to take it back to the mechanic or another guy more experienced then me!
 
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bdarcher

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If you still have points, is the gap set correctly, or do U have reluctor? See video attached> Reluctor around 14 minute mark, but U may want to peruse the whole video.



It has a reluctor wheel and pickup coil and appears to have the gap set correctly, as best as I can tell.
 
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bdarcher

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Anyone think that adjusting that 3/32 Allen screw in the vac advance canister would be something that could be the culprit of a dead spot and adjusting there might eliminate it??
 

pacofortacos

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It's hard to say without knowing how much advance it currently gives you and how much the screw can limit the amount of advance.

Having a timing light would really help you as far as playing with the timing goes.
 

turkeybird56

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It has a reluctor wheel and pickup coil and appears to have the gap set correctly, as best as I can tell.
K, I just wanted to send U the info to possibly help. In those years I had no Mopar, all GM, mostly Buick.
 

turkeybird56

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It's hard to say without knowing how much advance it currently gives you and how much the screw can limit the amount of advance.

Having a timing light would really help you as far as playing with the timing goes.
Something like this setup:

IMG_0886.JPGIMG_0887.JPGIMG_0889.JPG
 
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bdarcher

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It's hard to say without knowing how much advance it currently gives you and how much the screw can limit the amount of advance.

Having a timing light would really help you as far as playing with the timing goes.
I definitely agree with you. I’m thinking on getting one. I’ve been watching lots of videos and just a little overwhelmed by what looks to be a process….guys talk about ignition timing, distributor timing, mechanical timing, total timing etc
Some adjusts have to be at higher rpm and some do not, some you have to have vac line off and plugged etc, etc
Just not sure I’ll be able to figure it all out?! I’m going to look today and see if I can at least get at that hold down bolt for the distributor on my sons truck, seems like that’s a challenge sometimes too?!
I certainly see where some of those engines that have distributor in the front is a big advantage!
 
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bdarcher

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Thanks. I’m going to talk to my uncle who is a lot more mechanically inclined then me and see what he has. Perhaps I could borrow his ( he used to have a ‘68 GTX )
Or I can always go to local NAPA or Bumper to Bumper and buy something, but I need to figure out if I’m going to be able to learn all this stuff….all these different timing settings and adjust my is a bit confusing!
 

turkeybird56

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YUP, go see yer Uncle. He may have stuff lying around and be able to assist U.
 

pacofortacos

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It really isn't that hard, hardest part is the distributor lock down bolt - I used to have an offset wrench for it - basically is was a long bent box end wrench.
Do it on a warm engine if you can, not a cold one
But you don't want to get it too loose - just loose enough to when it takes just a bit of force to turn the distributor. Slightly snug it back down.

Then start the truck, let it idle, unplug the vac. advance hose and plug it with a bolt or golf T.
Put your timing light on #1 cylinder - ie. front driver side.
Point it at the timing marks and set to spec. - to start with - it wouldn't be unusual to find out that your truck may like a few degrees one way or the other.
Tighten distributor and check timing again to make sure it hasn't changed.

Then try the different tests I mentioned above and see if any work best for your setup.
 
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bdarcher

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It really isn't that hard, hardest part is the distributor lock down bolt - I used to have an offset wrench for it - basically is was a long bent box end wrench.
Do it on a warm engine if you can, not a cold one
But you don't want to get it too loose - just loose enough to when it takes just a bit of force to turn the distributor. Slightly snug it back down.

Then start the truck, let it idle, unplug the vac. advance hose and plug it with a bolt or golf T.
Put your timing light on #1 cylinder - ie. front driver side.
Point it at the timing marks and set to spec. - to start with - it wouldn't be unusual to find out that your truck may like a few degrees one way or the other.
Tighten distributor and check timing again to make sure it hasn't changed.

Then try the different tests I mentioned above and see if any work best for your setup.
Awesome, thanks for the info!
 
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bdarcher

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So, the truck is fixed and running good! Dropped it off with a local retired mechanic. He said the floats were set fairly close but he tweaked them, he said timing was way out of whack, but I think the main culprit to the problem was the springs in the distributor were sticking and not working properly! Seems to be running great now!!
 

pacofortacos

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That's good to hear. It helps if someone has worked on those systems - they are simple for me as I played with them all of the time growing up. Just hard to do from far away :)

There is your initial advance - which is set with the distributor at idle with the vac. advance unplugged.

Your vacuum advance - which gives advance when a high vac. is applied to the canister - usually set for ported vac. so that it doesn't give advance at idle but gives the most advance at light throttle cruise. Though I have setup engines running ported or manifold. Some canisters are adjustable to limit the amount of advance.

Your mechanical advance - this timing advance is checked at higher rpms ( you can actually see it as you slowly rev the motor advancing the timing) with the vac. advance unplugged. This advance is accomplished via weights with spring/s. The rate is adjustable by changing the springs and some aftermarket distributors also have adjustable limits which can limit how much mechanical advance you have. Great setup for fine tuning performance - both acceleration, limit pinging, and mpg.
 
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