Timing - Need opinions on next step

[1972Blaze]

Sorry if you saw this in a different thread. I decided to start my own to get some additional opinions.

I have a 1972 truck that calls for and has a 1112047 distributor. Factory specs on the truck are as follows:

12* initial timing (manual says 8*, decal on fender well says12*)
18* max centrifugal advance
15* vacuum advance (@15.5"Hg)
Dwell 29-31*

Actual specs:

12* initial
18* max centrifugal advance
17* max vacuum advance at 17"Hg (15* at 15"Hg, measured using miteyvac)
Dwell 30* (distributor is in good shape, properly shimmed with no excessive play)


The truck is sluggish, especially on initial acceleration. I can advance the initial to 16*, but that does not help much. I feel like the centrifugal advance of 18* is just too low. The engine is a factory crate replacement from about 1982. I do not know what specs apply to this engine.

I am thinking about setting up the distributor for 24* centrifugal advance, staying with 12* initial to start and using a new VAC of the original spec.

Does this sound like a good plan or should I go a different direction? Is the extra on centrifugal advance going to serve me better than increasing the initial and relying on a couple more degrees from the VAC?

One last thing, the advance curve is pretty close to what the manual calls for, but I have a little work to do on it. It comes in a little sooner below 2000 RPM and a little slower above 2000.

Thanks.

[Ironangel]

Whats the total? In other words, whats the timing at 3000 rpm? What rpm does the timing stop advancing? Note both rpm when advance stops and at what degree. My 350 is set at 15 initial, and 35 total, all in @ 3400 rpm.

[1972Blaze]

All in at very close to 3500 RPM (maybe slightly under), it is at 30* with initial set at 12*.

I can move the initial to 16* and be at 34. Is that enough? Would I be better off with less initial and more centrifugal? 18 centrifugal seems low to me. But maybe I am thinking too much from a high performance perspective.

[geezer#99]

What do you use for checking timing?
A tape, a dialback or a digital light?

[1972Blaze]

Originally Posted by geezer#99

What do you use for checking timing?
A tape, a dialback or a digital light?

dial back. I also made sure the timing mark was correct using a piston stop.

[geezer#99]

Might not be a timing problem, could be carb.
I've seen metering rods stuck before or a secondary air valve set up wrong.

Do you got pics of your motor. Might help. Air cleaner on and of works.

[Ironangel]

Originally Posted by 1972Blaze

All in at very close to 3500 RPM (maybe slightly under), it is at 30* with initial set at 12*.

I can move the initial to 16* and be at 34. Is that enough? Would I be better off with less initial and more centrifugal? 18 centrifugal seems low to me. But maybe I am thinking too much from a high performance perspective.

I just read the other thread and for the sake keeping things simple, disregard the last comment in the other thread. Geezer99, CableGuy, Storm, guy's that have been around for years are the experienced voices. Now with that said you indicated "sluggish" performance. Has this motor ran better in the past, has something changed? A 1982 crate motor was indicated in the other thread, is this a fresh rebuild or a newly installed motor with unknown history? I guess I'm just trying to weed out the fact that some 350's are just dogs. I'm going to ask the same question Geezer did, why havent you converted to an HEI? Thats always good for a couple of pony's...I'm going to turn you back over to Geezer now, he is the resident timing expert.

[geezer#99]

Originally Posted by Ironangel

I just read the other thread and for the sake keeping things simple, disregard the last comment in the other thread. Geezer99, CableGuy, Storm, guy's that have been around for years are the experienced voices. Now with that said you indicated "sluggish" performance. Has this motor ran better in the past, has something changed? A 1982 crate motor was indicated in the other thread, is this a fresh rebuild or a newly installed motor with unknown history? I guess I'm just trying to weed out the fact that some 350's are just dogs. I'm going to ask the same question Geezer did, why havent you converted to an HEI? Thats always good for a couple of pony's...I'm going to turn you back over to Geezer now, he is the resident timing expert.

Don't go away!
More opinions are always helpfull.
I suspect it might not be timing related.

[68c10airstream]

Is the vacuum advance hooked up to a manifold vacuum source (below the throttle blades, or ported vacuum source (above throttle blades)?

Manifold vacuum drops off as you accelerate, and ported vacuum is no vacuum at idle and when you open up the throttle it pulls vacuum stronger and stronger.

I would start with factory specs and hook uo the distributor vacuum advance can to ported signal. With the engine shut off, look down the throat of the carb and crack the throttle open somewhat slowly and you should see fuel squirting down the carb throat. If not the performance will be poor.

[geezer#99]

Originally Posted by 68c10airstream

Is the vacuum advance hooked up to a manifold vacuum source (below the throttle blades, or ported vacuum source (above throttle blades)?

Manifold vacuum drops off as you accelerate, and ported vacuum is no vacuum at idle and when you open up the throttle it pulls vacuum stronger and stronger.

.

You do know that once the throttle opens then you have manifold vac on the ported vac port. Ported is just controlled (by the throttle plate) manifold vacuum.

[1972Blaze]

Originally Posted by 68c10airstream

Is the vacuum advance hooked up to a manifold vacuum source (below the throttle blades, or ported vacuum source (above throttle blades)?

Manifold vacuum drops off as you accelerate, and ported vacuum is no vacuum at idle and when you open up the throttle it pulls vacuum stronger and stronger.

I would start with factory specs and hook uo the distributor vacuum advance can to ported signal. With the engine shut off, look down the throat of the carb and crack the throttle open somewhat slowly and you should see fuel squirting down the carb throat. If not the performance will be poor.

I have it connected to a ported source. I tried it on a manifold source and it really made little noticeable difference so I put it back. The vacuum on the ported source ranges from 0-20"Hg and comes in pretty fast.

[1972Blaze]

Originally Posted by 68c10airstream

... With the engine shut off, look down the throat of the carb and crack the throttle open somewhat slowly and you should see fuel squirting down the carb throat. If not the performance will be poor.

I have not thought of that. I will check it out tomorrow. Are you thinking accelerator pump issues?

[68c10airstream]

yes accelerator pump issues. When they age (like us and get older) it dribbles out instead of a strong squirt. As the air rushes into the carb on acceleration the fuel is heavier and the squirt is supposed to be an immediate response to throttle movement. It takes a bit for the heavy fuel to get through the jets and blend with some air and flow up the emulsion tubes and out the venturi block and down the throat of the carb

Quadrajet carb?? i would be suspicious of a slow responding/weak spring under the metering rods plunger. The plunger can get varnished up and drag and the spring is notorious for getting weak from mileage/backfires during it's life. When you accelerate manifold vacuum drops off and this plunger is tied into this circuit. The spring tension and vacuum are kind of a balancing act and when working correctly when you accelerate the plunger raises and lifts the metering rods up out of the floor jets (main metering) allowing more fuel to go through the floor jets (richer).

[68c10airstream]

i wanted to make the difference between manifold and ported vacuum descriptions clear and easy to figure out.

[Ironangel]

Originally Posted by geezer#99

Don't go away!
More opinions are always helpfull.
I suspect it might not be timing related.

I aint going anywhere...The OP indicated that he did have a 4MV Quadrajet, I have 4MV's on both of my 72's. Might be as simple as rebuilding that carb? Accelerator pump sounds like a viable possibility to me, I think we've ruled out a timing problem. Sounds like that truck has done alot of sitting and with today's so called gasoline it wouldn't be surprising at all that old fuel plus an old dirty carb would lead to a leaking accelerator pump well... Let's rebuild a Quadrajet..."Greatest Carburetor Ever Created" in my book!

[Ironangel]

Originally Posted by 1972Blaze

I have not thought of that. I will check it out tomorrow. Are you thinking accelerator pump issues?

Possibly, yes...Any background on the carb? Last time if ever was it rebuilt?

[1972Blaze]

Originally Posted by 68c10airstream

...With the engine shut off, look down the throat of the carb and crack the throttle open somewhat slowly and you should see fuel squirting down the carb throat. If not the performance will be poor.

I get one decent squirt opening the throttle. The second squirt is pretty weak. Then barely a trickle on the third. Is this normal? I have a Holley 4160 on another vehicle that I get a really solid squirt at lease twice.

Originally Posted by Ironangel

Possibly, yes...Any background on the carb? Last time if ever was it rebuilt?

It was an older carb I sent to have rebuilt. It had been out of service for some time. I had it done by a very reputable source in the Corvette community (Lars Grimsrud) in February of 2015. His feedback was that it rebuilt well. He had to replace a couple of things on it. There was no note of anything special with the accelerator pump, so I assume everything checked out and he used whatever the standard parts were related to a standard rebuild. The truck cranked up as soon as the carb was on and primed.

[cadillac_al]

Your carb should be fine if Lars built it. The accelerator pump shouldn't have worn out yet either. I would actually be hesitant to take it apart. I'm not saying it can't happen and some one in a million thing couldn't have happened to it. Your accelerator pump test does seem to be on the weak side. I usually look for one good squirt and don't really try a 2nd pump so I don't know if it is normal or not for the 2nd pump to be weak. It may be normal after all. Is there a reason you don't want an HEI distributor? They are very cheap and reliable nowadays. I wasted way too many hours of my life messing with points in the old days and swore off them 30 years ago.

[storm9c1]

"I am thinking about setting up the distributor for 24* centrifugal advance, staying with 12* initial to start"

That's reasonable. And with this extra mechanical timing, I recommend vacuum advance with no more than 10 degrees. Attached to full manifold vacuum. Otherwise you may ping while cruising.

I also agree with others, this isn't going to help if carb is questionable. But I like to start with timing and move on to carb after that. I also agree to ditch the points and thank us later. If you do this, don't forget to regap the plugs.

What is your vacuum reading at idle? It would also be nice to know the compression ratio of this engine and cam profile. You may be trying to squeeze blood out of a turnip.