dist. mechanical advance ajustment?

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j.d.duggan

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pretty much stock 340,bought new spectra premium dist.trying to get some fuel mileage out of this thing and think vac. advance will help.I have msd box,if it maters.my engine likes to start and idle at 10* btdc.The dist. I have is a ch04,spectra #.the problem is,it has 30* of mechanical advance in it.trying to keep my total to 34*,I have to set my initial at 4*,does not like this for starting.my vac. can on this dist. has 22* in it.I think if I could set my total mech. adv.to 24*,then set my initial to 10*,I should have 34 total and 56 cruising.Am I thinking correctly? If so The real ? is how to limit the total mechanical to 24*:BangHead:
 
Welding the advance slots and filing or grinding to appropriate length seems to be the most common method. Here is a chart for slot length that should get you close.
Dist deg. Crank Deg. slot length
6 deg 12 0.340
7 deg 14 0.357
8 deg 16 0.375
9 deg 18 0.390
10 deg 20 0.405
11 deg 22 0.420
12 deg 24 0.435
13 deg 26 0.447
14 deg 28 0.460
15 deg 30 0.475
16 deg 32 0.490
17 deg 34 0.505
18 deg 36 0.520
There are also these plates that some folks use.
How to limit mechanical advance in a mopar distributor, tuning for street, strip or all out racing, cure that rich stinky idle, win races
Mopar Distributor/Mechanical Advance Timing Limiter | eBay
 
pretty much stock 340,bought new spectra premium dist.trying to get some fuel mileage out of this thing and think vac. advance will help.I have msd box,if it maters.my engine likes to start and idle at 10* btdc.The dist. I have is a ch04,spectra #.the problem is,it has 30* of mechanical advance in it.trying to keep my total to 34*,I have to set my initial at 4*,does not like this for starting.my vac. can on this dist. has 22* in it.I think if I could set my total mech. adv.to 24*,then set my initial to 10*,I should have 34 total and 56 cruising.Am I thinking correctly? If so The real ? is how to limit the total mechanical to 24*:BangHead:

Just to add a little, you can get that limit plate directly from Don at www.4secondsflat.com for 22 shipped.
They are his product.
When I need one (I do them for other people) I just Paypal 22 even to sales@4secondsflat.com and in the notes on the payment put in "Chrysler small block timing limit plate" and the shipping address.
 
anyone have any pics on how to take the dist. apart and reassemble?the plates look way easier,but I would hav to set my timing at 16* to get to the 34 if I used the 18* holes.
 
Shop manual for starters. Go to MyMopar there are lots of them there for free...........and a few of them there came from some of us right here

This help

Vacuum Chamber install
 
pretty much stock 340,bought new spectra premium dist.trying to get some fuel mileage out of this thing and think vac. advance will help.I have msd box,if it maters.my engine likes to start and idle at 10* btdc.
if there had been a nonsmog 340 in 68 it probably would have had initial timing around 10* 600rpm
The dist. I have is a ch04,spectra #.the problem is,it has 30* of mechanical advance in it.trying to keep my total to 34*,I have to set my initial at 4*,does not like this for starting
usually less timing makes for easier starting but doesnt idle as well at low rpm.
my vac. can on this dist. has 22* in it.I think if I could set my total mech. adv.to 24*,then set my initial to 10*,I should have 34 total and 56 cruising.
it depends on what the cruise vacuum is and the mechanical timing at highway cruise. Mecahnical advance should not be all in until at least 3000 rpm.

Vacuum advance: How To Limit and Adjust Chrysler Vacuum Advance Cans Go to the very last picture for MP instruction sheet.

Reducing the allowable advance: Tack a weld the inside of the slots. result will be like discussed here: Erratic Timing
Doing the outside makes it much more difficult to work with vac advance and getting the high rpm slope that helps at the top end.

Changing the adv start rpm, and how many degrees out the second spring joins in is here: Erratic Timing
 
340 timing was a bit different than a 273/318 in the linked thread.
So here's 68 Plymouth. Dodge was sometime spec'd a couple degrees different.
This is the high compression - a late lower comp 340 prob had a slightly different curve but either way this gives you a ballpark that should be close.
upload_2019-3-29_9-16-46.png

Interesting. Comparing it now to the 273 HiPo - they are very similar. :)
 
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anyone have any pics on how to take the dist. apart and reassemble?the plates look way easier,but I would hav to set my timing at 16* to get to the 34 if I used the 18* holes.

This is the basics even though they are showing a points distributor the basics are the same.
They say "lift the cam off" but in actuality you need to pry it off with two flat screwdrivers.
Note the roll pin location in relation to the rotor so you can put it back on the same way.

How to Rebuild and Reinstall Car Distributors

This is another good source.
https://www.hemmings.com/blog/article/the-lost-art-of/
 
Thanks grimreaper,that was one of the most informative,and easy to follow articles I've seen on this subject so far.
 
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