1970 l46... it has always started and run well with no issues but frankly not allot of driving, after body, paint and interior I went into some mechanical maintenance... all fluids and full tun up, started and ran great.
from there i set initial timing at about 9 degrees and dwell at 30 degrees... still ran great but noticed a backfire carb pop after shutting down from a longer ride.
I then set total timing at 36 degrees at about 2700 rpms which landed initial timing at about 12 degrees and still 30 degrees dwell.
still starts and runs great but another backfire pop after shut down today.
I decided to just check where air mixture screws are set and one is at 5 turns out and the other 4.5 turns out which sounds like allot to me from past experiences as well as some reading.
I am going to order a vacuum gauge but what should I start with as far as initial turns for both sides... im reading about 3.5 out from seated as a starting point and likely going in from there

 

Those timing specifications sound pretty poor for a high compression engine.
I'm at about 10.5-1.
18 initial, 35 total , all in by 3K RPM.
11 degrees vacuum advance. All in at 12 inches of vacuum. Engine idles at 14.5 inches.
no pop at shut down nor any run on.

 

I wouldn't monkey around with those carb screws at all until you have the correct tools on hand. Especially since it runs well as is.
your ignition timing specs for a high compression engine on modern fuels sounds way off base to me.
My engine has slightly lower compression, and I'm idling with 29-30 degrees of timing. And I assure you it runs fantastic and cool. Your low timing at idle, low RPM, could also give you overheating issues.
Oh, your dwell sounds fine.
You don't say how much timing your vacuum can is adding or at what vacuum level it's all in. This is very important on a street driven engine.

 

Additional timing from a vacuum advance will help. Probably 10-12 on top of your initial.

If that still isn’t enough you’ll have to shorten your mechanical advance allowing you to run more initial while still getting 36 total.

Then mess with the carb. Yes, 3 1/2 turns is a good starting point.

 

Ok thanks... can I get some clarification to my "poor numbers" as its what's been recommended by many here as well as almost all the threads im reading.

After full tune up I set to 9 degrees initial timing and 30 degrees dwell on the points last weekend... then I did a total timing to 36 degrees at 2700 rpms yesterday which landed the initial timing (idle ) at about 12/13 degrees and it kept the 30 degrees dwell so im at 36 total, 13 idle and 30 dwell.

I will do a vacuum gauge test test tomorrow when I get the gauge... im at about 3.5 turns out on air screws now to start with for tomorrow's vacuum test... im running about 750 rpms smooth at idle warmed up... will set screws at best vacuum I can get tomorrow.

The car has always started after 2 hits of the peddle even after a week, idles well, runs out and pulls strong with absolutely no issues, sputter, bog, stall etc. I can drive for a while, shut down and starts right back up and runs about 190 degrees temp consistently... just been getting a little backfire after shut off only after long drives... im hoping the air/fuel settings that were at 5 turns out were contributing to that... will see tomorrow I guess.

Frankly its a slight backfire and with the side pipes it could be from exhaust or carb... if it does persist after tomorrow's vacuum setting I will pay more attention.

If im starting with bad timing numbers above, should I be looking for different base numbers in timing and dwell?

 

thanks... at 36 total im at about 13 initial at 750 rpms idle, ive always thought the 36 total was more important and let initial land where it does.

 

Thanks... I wanted a good place to start with the screws before vacuum gauge tomorrow, frankly runs as well at the 3.5 turns out as it did at almost 5
As I turn in a turn or two from the 3.5 out I was starting to drop idle so just left it at the 3.5 for a starting place tomorrow.

It doesn't run hot at all, fluctuates around the 190-195 mark.

Will check and adjust from vacuum gauge tomorrow and post back in.

 

You are on the right track.
A good mixture screw setting is a slow roll in, until the rpm drops ~50, then 1/4 turn out.
On the emission era cars they would just leave it there with a 20 or 50 rpm drop.

I would carefully test and/or tune the vacuum can advance. That is the one that determines about 75% of your driving feel & issues.

With today's gas, you may no longer be able to run a 100% factory vacuum can, especially with an 11:1 engine.
Learn to tune that. The rest sounds good.
You will likely have to buy or make an vacuum can advance limiter, to hold it's contribution to advance down to about 11*.
Some of them add18 or even 24*. Just too much.
10* seems to be a good number.

You want it to be all-in at 2" of vac below the idle vac level.
And then start to come off as vac drops.

Those two things should tune the can for you.
Then make sure it does not ping lightly anywhere at any part throttle setting.

 

Well im only getting about 13 inHG of manifold vacuum.
as I play with the air/mixture screws around the 2-5 turns out im really not getting much fluctuation in that 13 number.
There are no apparent vacuum leaks, I dont have power brakes and im using headlight/wiper vacuum port... next I will check for manifold gasket and carb gasket vacuum leaks.

 

I can't find any vacuum leak around the manifold base nor carb base...

I set the mix screws at about 3.5 turns out, I took the car out for a long drive and as usual runs out excellent through gears and acceleration, no issues, runs cool around stop and go as well as 65 mph.

When I got home and shut it down no pop after shut down so hoping the 5 turns out was contributing to that.

If my timings and dwell or good and im not having an issue... if this did in fact eliminate the pop after shut down and im running well and at good temp should I just leave it?

The car definitely has had a cam put in at some point but dont know the specs... its quite lopey at idle and always get a comment on that lope... is it possible I just have a cam yielding lower vacuum?

Or should I pursue this low vacuum?

https://youtu.be/bwJcKl2hRew?feature=shared

 

I would pursue your low vacuum. Like what was said above check and tune your vacuum advance.

 

Ok thanks all... I will pursue how to "check and tune" my vacuum advance

Just ordered a hand vacuum pump

 

A bigger duration camshaft will make it lope more and drop the idle vacuum.
That sounds like what you have.
Your cam basically sounds like a typical hot-rod street cam, the kind that is pretty max'd out before driveability suffers.

An L46 cam has 224*/224* duration @ .050" and about 15" vac at idle.

Based on your lope and the drop in idle vac to 13", I would guess 6-8* more than the stock L46 cam.
Say 228-232* @ .050 depending on exhaust split. lobe centers, etc.
It should rev higher and peak in the low 6000s.

However the increased overlap will lean out the cruise circuits of your factory Q-Jet, from idle to 3000.
Due to the lower vacuum pulling less fuel.

I still say you need to test and tune your vacuum can.
It is especially important now that you have changed the idle vacuum.
It controls most of your normal driving manners.

If the carb part throttle circuits are now jetted too lean, the mixture screws will not have good control.
You should be able to get both a lean-rpm-drop, and a rich-rpm-drop, if the circuit is calibrated correctly.
Otherwise you need a carb guru to correct it.
The lean-roll-off & back out 1/4 turn should always be your best idle, and low speed mixture control setting.
Regardless of how many turns-out that is.
With a different cam, a factory "X" turns-out setting will no longer apply.
Now if your lean-roll + 1/4 turn winds up at 5 turns out, a lot more than factory, that is because of the cam, and that setting will richen that idle & cruise circuit.
At 5 turns out I would be shocked if you can get a rich-roll-off, so your idle/cruise circuit is likely running lean. Now does the 5 turns out help enough? That is the big question.
In many cases the mixture screws can not correct enough, and then you will need different jetting, on the idle/cruise circuit.

A re-jetted Q-Jet should be able to handle a 13" idle cam very well.
But if the cam had any lower vac, it would struggle, no matter what, and you could tune it easier with a Holley. But that is not your case. Do not be talked into it. A Q-Jet is a great carb, just many do not know how to tune it.

 

thank you Leigh, I know I'm asking allot of questions and rather than agravate here I am trying to lean on prior threads through Google search, certainly reading info on cam overlap vs vacuum.

I ordered a hand pump and will absolutely test my vacuum advance canister, from there I will look into whats next steps but believe the car has been this was for some time and again absolutely no apparent issues... even the old plugs that I removed all looked excellent.
I will have more time to play with that vacuum number vs air/mix settings once I get back from travel later this week, of course making sure my canister holds vacuum first

Thanks again