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Tuning questions timing and carb

jcskokos

jcskokos

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67 coronet 500 4 dr, 383 2bbl carter single exh 727a/t.
Installed a new chain due to some serious stretch, adjust trans bands, and rebuilt carb.
Everywhere I've looked has different recommendations on timing advance on these for some reason from 0*tdc to 32*btdc. I've found a few recommendations for setting with a vacuum gauge. So far I've hooked it up to the (unused) brake booster port on the manifold and had it at a sloppy 15" vacuum at probably around 12*btdc and ran out of daylight and fuel. Put new fuel in and now it's being moody about starting back up.

I am asking if there is a straight forward step by step to getting it dialed in on both the timing and carb adjustments.
 
jcskokos said:
67 coronet 500 4 dr, 383 2bbl carter single exh 727a/t.
Installed a new chain due to some serious stretch, adjust trans bands, and rebuilt carb.
Everywhere I've looked has different recommendations on timing advance on these for some reason from 0*tdc to 32*btdc. I've found a few recommendations for setting with a vacuum gauge. So far I've hooked it up to the (unused) brake booster port on the manifold and had it at a sloppy 15" vacuum at probably around 12*btdc and ran out of daylight and fuel. Put new fuel in and now it's being moody about starting back up.

I am asking if there is a straight forward step by step to getting it dialed in on both the timing and carb adjustments.
Click to expand...
Ok...assuming your all stock with a functioning vacuum advance distributor, I would start with 12 degrees at idle with the vacuum advance blocked off. Cap the manifold vacuum port on the carb and the vacuum on distributor. Then connect the vacuum advance (dist to manifold vac)and bring rpms to 3000. Set timing to 32 degrees total. 12 initial and 20 with advance. This is just a ballpark and should be safe and run fine. This is assuming no vacuum leaks ect. Dial in carb after timing is satisfactory. Tuning takes time. Look at factory specs to get you there. Just a place to start. When tuning the carb you want the highest vacuum reading at idle and back down a touch. Check the plugs for color and deposits. Go from there. Good luck. This is just what I would do. Process of elimination and timing first. Timing then carb.
 
I think it's helpful to also know how much cranking pressure the engine has. If the compression is somewhat low the engine is probably going to like more timing based on my experiences.
 
That brings up another issue.
Half of what I've read says to set all timing and metering in drive, other half in park or neutral. Similarly it's also split on leaving the vacuum advance disconnected or reconnecting it
 
jcskokos said:
That brings up another issue.
Half of what I've read says to set all timing and metering in drive, other half in park or neutral. Similarly it's also split on leaving the vacuum advance disconnected or reconnecting it
Click to expand...
Mmmm. If you try set timing while in drive you'll need a helper to keep a foot on the brake or the car will try to leave. Tuning in drive is a carburetor vacuum thing with an auto trans. Vacuum readings in drive to determine correct power valve or brake issues. Assuming your have a stock motor shouldn't have to worry about it. Set timing in park or neutral with park brake and then move to carb. Saftey first. Adjust mixture screws for highest idle or vacuum if you have a gauge and back down a touch. Adjust idle after mixture screws are happy.
 
Stock is easy.
It gets a bit more involved when the engine is modified.
Either condition is well within the abilities of a reasonably competent person.
Regular, NON high performance 2 barrel cars using the stock points distributor had a fairly conservative timing curve. They were set at about a 5 degrees advance at idle and the mechanical advance in the distributor was in the 22-24 degree range. This meant that full advance at WOT would be still under 30 degrees. This worked fine for these cars.
When you crank the distributor to get it to 12 degrees at idle, that pushed the total timing to 36 degrees. That can be a problem with iron heads on low octane gas.
Several ways to work around that though....
 
I mean, as far as I know it's -a- stock 2bbl motor. The cars ID plate shows it came originally with a 318 motor. I got it that way and daily drove for a good 4 or 5 years but the engine always had a pinging issue under high load. We chalked that up to a stretched chain and as I mentioned that was just replaced with a new double roller set to 0* cam offset.
The main reason I wanted to ask about the drive or neutral condition is I know I would get a good rpm drop when placed in gear before was parked.
 
I would not blame pinging or detonation on a stretched timing chain. Late valve timing, sure but not spark knock.
 
jcskokos said:
I mean, as far as I know it's -a- stock 2bbl motor. The cars ID plate shows it came originally with a 318 motor. I got it that way and daily drove for a good 4 or 5 years but the engine always had a pinging issue under high load. We chalked that up to a stretched chain and as I mentioned that was just replaced with a new double roller set to 0* cam offset.
The main reason I wanted to ask about the drive or neutral condition is I know I would get a good rpm drop when placed in gear before was parked.
Click to expand...
No worries. I set timing by ear to get in the ballpark. Kern is right about the amount of built in advance on points. Just play with it until it seems to run good then put a light on it and see what it likes. Have fun. Good luck.
 
jcskokos said:
I would get a good rpm drop when placed in gear before was parked.
Click to expand...
I have seen this with a high performance cam but not so much with a milder cam or a stock engine UNLESS there was a vacuum leak, carburetor tuning problems or something amiss in the distributor.
 
I should clarify: because of the slack in the chain we were assuming that was the reason for needing a lot of advance before. Setting it below 15* would make a hell of a backfire trying to accelerate so advance at idle was set pretty high so once vacuum was kicked in I suspect it was well over the maximum allowable, thus the pinging. I want to say the slack gave the cam around 15-20* of retard.
 
Wow...That is a LOT of slack then!
I once had a 318 in a 74 Duster that was GUTless ! After checking all sorts of things, I tore the front down to find a newer chain in there but it was THREE teeth retarded....That is over 24 degrees!
 
Yeah, I was surprised it hadn't jumped teeth yet. It probably helps that I drive it like a grandpa and the person who owned it before was the original owner.
 
As mentioned, tuning takes time but it's worth the effort. Besides a timing light, it is best done with a vacuum gauge and a tach. Your initial will likely be around 12-14 btdc with the vacuum advance disconnected and plugged. Monitoring the tach and vacuum gauge will tell you a lot, setting at the highest rpm and vacuum reading should be near where you want to be. Check total mechanical advance at around 2500 rpm and you will likely be at 32-34 all in. If it starts ok there, you're where you want to be. Now connect your vacuum advance to manifold vacuum (I know a lot won't agree with this) but do it. You will see an immediate increase in rpm. This gives you the advantage of easy starting and more timing at idle. When you hit the throttle, the vacuum advance will decrease as the mechanical kicks in giving you the best of both worlds. Then tune the carb. Timing first, then carb.
What I have given you is how I tune my car, if you disagree, I respect that but it works well for me.
Here is an article on the manifold vacuum:
http://4secondsflat.com/Ported Vs Constant Manifold Vacuum.htm
 
So I would set timing with the dist disconnected still for good idle and start and then just plug her into constant vacuum source with, theoretically, no more adjustment needed? Then play with carb, then do all again a second time, yes?
 
jcskokos said:
So I would set timing with the dist disconnected still for good idle and start and then just plug her into constant vacuum source with, theoretically, no more adjustment needed? Then play with carb, then do all again a second time, yes?
Click to expand...
Theoretically, yes with a little luck. If the intitial is good but you are off on total, it could require a change to the distributor. A really trick way of doing that is available from FBO (advance limiter plate)
http://4secondsflat.com/Mopar Mechanical timing limiter plate.htm
 
You want as much timing as you can get WITHOUT detonation! Once you check everything, give her a good run and listen carefully, you do not want to hear valve knock. Every engine is different and will be timed according to what it likes.
I have never tuned one in drive
 
Last edited:
I've never seen replacing a stretched chain fix any problem. The degree change from a stretched chain is minuscule. The retarding of the cam that comes with a chain stretch will reduce cylinder pressure and would be less likely to detonate when everything else is right. If you have an ignition timing issue, it has nothing to do with the cam or chain.

If its stock ish, maybe start with the factory service manual for your starting point on your tune..
 
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