Well, until recently I had been timing my 351w by advancing until getting a ping, then backing off enough to eliminate it.

I just installed a new vacuum advance onto the distributor this week; the original had been removed by the PO.
I decided to use a timing light & tach to check the curve etc & finally time the engine.

The dilemma is that with the vac advance disconnected/plugged & on 91 gas, the engine pings badly above a base timing of 1 degree ATDC (now that I am using a timing light & can see).

At first I set the initial timing at 14 BTDC & checked the centrifugal advance, which was all in at 2750 rpm (34 deg total), but the pinging made it undriveable.

I assumed a spun balancer was the problem; so I made a piston stop to check. There was a small discrepancy of 1.5 degrees, but not enough to be an issue.......so I had to progressively retard the timing down to 1 ATDC to eliminate the knock, then connected & adjusted the vac advance.

The truck starts, runs & cools fine with the 'late' timing; but what could be the cause of this?

BTW I'm using ported vacuum; plug electrodes are light brown; no EGR; 2150 carb with #53 jets.

Thanks

 

Wow! That's really retarded! The timing, that is.

Seriously though, how much compression are you running? Other than that, you seem to have covered the bases.

 

Hi Gary, I guess I set myself up for that!

It just occurred to me since I posted, that when the vac advance was removed to disguise the DSII conversion in Ca., the advance springs were probably lightened up/changed as well, maybe too much.

I didn't get to graphing the curve, but the timing lifted fast even though it stopped at 34 deg.

I'm off to investigate.

PS - I don't know what the c/r is, but my SOTP impression is not excessively high. The engine was rebuilt just before I bought the truck; it has E7 heads, no idea on the pistons.

 

That's odd. Mine has a 13? L distributor arm. I have a light and a medium spring in there. I run 11* BTDC timing with my vacuum advance on manifold. I average 30-35* BTDC at idle with it all hooked up like that. Never any problems. Very odd. I do have a problem when I set it to 14* BTDC with restarts. And yes, my timing retard on my DS II box works. I'm also running 87 octane. What spring setup do you have in the distributor? I do have D8AE heads, so compression is around 8.4:1. E7 heads are supposed to have 64 CC cambers. So if it was using stock pistons it would be around 9.1:1-9.3:1. Assuming the block hasn't been zero decked.

 

Tonight I found at least one spring has been changed for a lighter one - similar to a 925D; but also a lot of slack was put into the large spring, & not much tension on the light spring, so the advance was very fast from an idle, causing the knock.

After finding all this I've reduced the slack in the heavy spring & put more tension on the light one.......so tomorrow we'll see.

Don't you ever sleep Matt?

 

Matthew was up late or early, for sure.

Anyway, really light springs can be part of it, and the total amount of advance (34 degrees) more of the problem. But I think the compression is also part of it. All those things taken together and you have to retard the initial timing significantly.

 

Gary, 34 degrees was my total timing (14 initial + 20 centrifugal)........I think that's reasonably conservative as far as the total.

Less initial & more centrifugal would probably be better, but I'll experiment today and see how it goes.

 

My misunderstanding. 20 degrees of mechanical is conservative. I thought it was 34. Duh!

I think the light springs and slack is the issue as you are getting too much too early. Reminds me of the 69 Super Bee I had. It was an auto tranny and I put light springs in the dizzy. Worked fine until it snowed - the car could hardly be driven. Look at the throttle and the advance came in, which raised the RPM, which brought more advance in, which…. So all that happened was that the right rear was going to spin. That was one time I was thankful to not have a limited-slip diff. At least one wheel back there wasn't spinning so it didn't immediately start walking sideways.

Anyway, hope your tests show improvement.

 

Thanks - I'll update tonight.

 

I was up late lol. I have sleeping problems in the winter.

 

Well, with experimental tensioning of both springs & a lot of test drives & recording, I've got the ignition at 12 deg BTDC initial + 22 centrifugal + vacuum on top.

At that, I had to turn the vac. advance back a few turns to stop some mid-throttle pinging, but otherwise no problem.

I graphed the curve as I made changes, & the current setup is almost linear. I don't know if that is good, bad, or of no consequence.....but it's working fine.

Interesting day

 

Good news! Those are reasonable numbers, and now you have vacuum advance. In fact, since you are just shy of pinging you have about all the advance you can handle. It will be interesting to see how your gas mileage changes.

 

Thanks Gary. Do you know of any down-side to the near straight 'curve'?

 

I don't. But all the advances I've seen in books show a curve. That may be due to the nature of coil springs.

 

Just to update -

I've had time to put a few miles on now since installing the vacuum advance & resetting the curve (I also backed the initial timing off a further 2 degrees, to have more of a safety margin for towing).

Previously my short distance, local running was averaging 13.8 mpg; now 16.1......a 16% improvement ; BTW this is Imperial gallons.
Longer distance, open road driving has gained 10%.

I didn't realize the mpg value of the vacuum advance was so significant.