How much mech adv can distributor have?

[beck]

Hi Everyone, decided to put a rebuilt carb on the 340 ,rebuilt by Woodruff carburetor, exact replacement, change out the plugs that came with it,ngks that were all carbon up, and set timing. Just want to note that l checked timing before I did anything and it was at 5* and pulled the old carb and found vacum adv was plug in at manifold vac and the vac choke spring into ported vac.I do not know anything about this engine, seems stock,j head,stock intake,maybe mild cam.
Set initial timing at 14* and go to check mechanical and it came in at 52* around 2400 rpm.Plug in vac adv and motor shakes bad at 1200. played around with timing and 14* seems to be best but notice timing mark moving up and down about 2* to 4 *.I hook vac gauge for tuning carb ,and that is also bouncing
So 14* and 52 mechanical at 66 all in at 2400 . Does 14 -52 give me 38* of mechanical? Does this sound right?
And is timing and vacum bouncing related to distributor.

 

[jeff alder]

@halifaxhops

 

[Oldmanmopar]

Total mechanical advance travel is around 40 degrees. I usually weld up the slots half the travel and always end up with 20 total. I do this and use one light spring and one medium spring. This allows the engine to start at 15 and go to total 35 at 2500 welding the inside of the slot. If you weld the outside it comes in quicker at around 1500-2000. When this is done I eliminate the Vacuum advance. I usually then use a lean burn pickup solid plate and a cover for the advance window as seen in this picture.

 

[halifaxhops]

The bouncing should not be timing related, I would start by looking at all mechanical things that can affect this. Google Vacuum chart. There are some out there they will get you in the area.

 

[halifaxhops]

 

[Newbomb Turk]

There is no way in HELL that engine wants 52 degrees of timing at 2400.

You keep messing around and you’ll knock the head gaskets right out of it and probably worse.

The fact that the timing is all in by 2400 says the advance springs are way too light.

What distributor is it?

 

[4spdragtop]

 

[beck]

I do not want 52* or did not expect that.I do not know what type of distributor it is ,looks stock. I did not even pull the cap off to look yet.I assume that the person who had before me was trying to hide something setting it at 5*.I was just asking how much mechanical advance can a distributor supply

 

[Newbomb Turk]

View attachment 1716378322

That’s a factory distributor.

The answer to your question is it depends.

It depends on what part number is on the distributor.

It depends on someone not modifying it. Or modifying it.

You can pretty much make the curve about whatever you want within reason and to explain within reason is beyond the scope of typing it out on a forum.

You’ve already learned that in the vast majority of engines there you don’t need manifold vacuum advance.

The best advice I can give you is contact @halifaxhops and send him the distributor and let him put a curve in it for you.

That’ll be money well spent.

 

[Bewy]

I would remove a plug lead & measure the resistance. If it is 8000 ohms or more it is the junk carbon type lead that break down & cause all sorts of issues [ like engine shaking ]. Replace them with spiral wound [ also called heli wound ] leads, many brands, all are good.

 

[RustyRatRod]

First, are you 100% SURE you're reading the timing correctly? Not 99.9%, but 100%. "Usually" I've found that factory style distributors have "around" 20 degrees advance. I've not seen much more than that, so that's why I ask if you are sure. Also, for an engine as mild as it sounds like you have, there's no need to run the vacuum can off manifold vacuum. You should use ported. Probably a good starting point for a stock or mild engine would be say 14-18 initial with a total of about 34-36 degrees all in by 3500. This is excluding the vacuum can. You may have to limit the mechanical advance by welding and filing down the distributor governor slots and it certainly does sound like you have way too light of springs in it.....that is IF you are reading it correctly. I would double check to ASSURE that you are. Don't take anything for granted.

 

[beck]

Just following up, I double check my timing and set base at 20*.as soon as I gave it gas it jump up to 50 - 52. Around 1500 rpm.Vac adv plugged and .idle at 850.I am guessing it's locked up ,I pulled it and going to work on it tomorrow. I keep you posted.Thanks to all

 

[Scody21]

Once you get the distributor figured out and that’s all correct, if your timing is bouncing around a few degrees, your timing chain is most likely junk….

 

[AJ/FormS]

If the magnetic pick-up is connected in reverse polarity, the timing can be set for a normal idle, but as soon as the rpm starts to climb, the strobes will randomly jump all over the place.
Small block pick-ups are different from BBs. I think they bolt in, but polarity is wrong. They are color-coded.
The SBM has one orange wire.
IIRC, the BB has a violet wire, not sure.
The other wire is usually black or gray.
These are the only ones I have seen, there may be more.

The wrong-colored pick-up can be made to work, just by changing the polarity. I had to do this on the side of the hiway once, when my ignition system quit. I noticed the color thing but thought nothing of it. When the car wouldn't rev properly after the switch, that had me scratching my head. No worries, I just cut the plug off with a generous amount of wire on each side, split it, stripped it, twisted it together, reset the timing and drove home. Next day I hunted up a pick-up with the right colored wires same as the dead one.
Then I hunted up another correctly colored one, for the glovebox.
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Part-2
As for your other questions;
>the max mechanical advance cam I have seen is 32*.
I imagine there has to be a cam for 36*, for engines that were factory timed to TDC. IDK this.
> the max vacuum advance-can I have seen was 17* I partway trimmed the stops off mine to get 22*. Another member got 24*. I kindof think 24 is the limit.
> at idle, the mechanical advance should not yet be functioning.
> at idle, neither will the VA on the sparkport.
> at idle with the VA on manifold vac, you could get anything depending on how the diaphragm in the device is damped, and how much vacuum your engine is pulling.
> Whatever you set your Idle-timing to, with the VA disconnected. It should hold that to at least 800 rpm. If it comes in sooner, some engines can have idle stability issues.

> Just so you know; on my engine, the Idle-timing is set to 12>14.
It starts advancing at 1000, and by 2800, the light reads 28*.
With the VA hooked up, it reads 50*.
All-in is not until 3400, and that brings 6 more mechanical degrees..
I have a dash-mounted, dial-back Timing device, that has a capacity of 15 degrees. I can add some, to all, of that. It is adjustable; so I have it set for up to 6 advance/9 retard.
Thus, my minimum timing is 5*, and max at 3400 could be 62* for cruising. But with overdrive, she cruises at 2240 on about 53 to 59 degrees.

Most stock period SBM engines I have worked on, would accept an easy 25* from all sources, at idle. Some, up to 35*. However; good luck driving it off idle like that. Usually the transfer slot is so far out of sync, that the engine will not tolerate throttle-opening very well, usually ending in a stall.

And if you have a manual trans, with hiway gears, good luck driving slow.