66 L-72 Timing
I just got my L-72 started & running. It is still on jack stands but I wanted to get the timing set up properly. I have done alot of reading on timing but would like to consult with the experts. Duke & others have recommended changing from ported vacuun to manifold vacuum using a small tee at the choke pull off vac line & an appropiate change of the vac can to meet the "2 inch rule". I changed the vac line hook up and installed a B-26 VC1765 (starts to pull @7, all in @14). I set the initial timing to 8*BTDC, RPM @ ~750, with vac hose removed & plugged, & centrifical weights taped to prevent movement. I then reconnected the vac hose & retested, timing jumped to 15-16* BTDC, RPM jumped to 1000-1100 RPM, & vacuum jumped to 17-18". Removed tape & repeated procedure with & without vac line connected. There was no difference in the initial timing & virtually no timing movement with a moderate increase in RPM with line plugged. Vacuum was at 10-11". I then ran it up to about 3500 RPM (vac line connected) & the timing went to 40-41 BTDC, Vac @ 20". Should I run the RPM up higher to see what the full advance is, or wait til I have it on the road. I'm worried about denotation & the loud side pipes make that hard to detect. It starts with just a touch of the key, and no dieseling when turned off. I have added 93 octane with no addatives. In the late 80's I used to mix Turboblue (105 octane, I think) 1/2 to 1/3 with 91 octane premiun. Am I in the ballpark? Is this the right vac can? Do you think it will run OK on 93? Any suggestions or comments are appreciated.
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Re: 66 L-72 Timing
Glen -
Timing readings with the vacuum advance connected at any rpm above idle are meaningless; there are too many variables affecting it. The "total timing" number you're looking for (34*-36* BTDC) is simply the sum of initial plus centrifugal, with the vacuum advance disconnected and plugged. Develop your advance map with the VAC disconnected, then connect it (to manifold vacuum, not "ported") and verify that it's working, providing the spec advance, and is pulled all the way to the stop at normal idle and vacuum level. -
Re: 66 L-72 Timing
The OE centrifugal map runs out at 5000 revs, so you will have to rev it over that to set total WOT timing with a timing light.
Of course, maybe someone modified the dist. in the last 45 years. That's for you to determine. Also determine if the interior of the dist. is clean and the centrifugal mechanism operates smoothly.
If you set the initial at 8 degs. you should be okay. When you then connect the VAC the engine speed will increase.
Now go through the idle speed/mixture adjustment procedure with a target idle speed of about 900, and manifold vacuum should be about 14-15".
You can then check the total idle advance and it should be at least 24.
DukeComment
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Re: 66 L-72 Timing
Glen,
Check your carb vac adv port at idle, I would expect it to be pulling at idle and get the numbers Duke mentions. I did this last year and had similar results as you mention, inclusive of the side pipes.
There are two things to check, first your dwell, if your distributor is solid, not worn wobbly shaft the dwell will remain stable, at idle where its set up to 3000+rpm. If you see it wavering...time for a rebuild.
Second thing that I found, and it was posted here a year ago, is that my 66 L36 original distributor weights were sticking, even though I had lubes them. I saw this with the timing light on reving up the engine the advance was stuck and then about 4000 would really let go, you'd get good advance and the engine would really come to life. I used lighter springs - replacing those originals but I did keep those weights that were not well recognized as the originals versus the repop weights.
Sounds like you are close, good luck!Comment
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Re: 66 L-72 Timing
Ron,
The L72 is a bit of a different beast than the L36 you have. The vacuum is ported unless the carb has been modified. It is best to convert to full time vacuum advance. Secondly, TI comes along for the ride with the L72, so there is no dwell setting because there are no points.
JoeComment
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Re: 66 L-72 Timing
Thanks for your response. We had discussed my distributor in previous posts. Rebuilt TI dist. with new B-26 (VC1765) can, (It had a 360-12 can) No band, thought to be from a 67 400 but not confirmed. I have probably a dozen sets of spare weights, none have numbers, all seem to have the same shape, size & thickness. But, the weight set in this dist is different, they have an 11/64" hole drilled in the end of the weight & what appears to be a stiffer spring than many of the spring set I have. Are they original to the dist, who knows! I think my advance was about 17* BTDC @ 800-850 RPM. & vac @ 12-13". I'll go back & re-check the settings and report back.Comment
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Re: 66 L-72 Timing
This is a TI so no dwell. Weights do not appear to be sticking. I put some lighter springs in, but did not test, the springs were so sloppy, ie. the springs did not even hold the weights tight in the home position. I'll try that if all else fails.Comment
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Re: 66 L-72 Timing
It very possible that you may wind up reinstalling the "heavy" weights again.
Installing softer springs is an old trick from the 60's and early 70's that usually never worked. Mr Gasket made big dollars selling that junk.
All the weekend "race engine tuner experts" immediately install softer springs because it's easy and anyone can do it.
Don't fall for all that BS.Comment
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Re: 66 L-72 Timing
Most OE centrifugal curves are pretty lazy and low end torque can be considerably improved with lighter springs assuming the engine doesn't get into detonation.
Installing the '64-'65 SHP/FI weights/springs in my '63 327/340, which brought all the centrifugal in at 2350 instead of 4600 made a HUGE difference. Since it's pulling a 3.08 axle through a CR trans it spends most of it's time below 3500, and it was much stronger in normal driving at low to medium engine speed, and the more aggressive centrifugal curve also improved fuel economy.
DukeComment
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Re: 66 L-72 Timing
Here are the timing advance readings at various RPM levels, with the vacuum advance connected. RPM 8-900/17*, 1500/30*, 2500/35*, 3500/40*, 4000/45*, 5100/51*, 5500/52*. So, should I try to get the centrifugal all in at a lower rpm? (I've used lighter springs in the past on my SD-455 but it has a much lower compression ratio & runs just fine on 93 octane). I'm leaving for the summer so I can't road test til at least Sept. On another note, before testing, I flushed the radiator and block & tested the 180* thermostat. The stat is OK. The engine has a rebuilt OEM WP a new pressure cap has been installed, But it still overheated. Outside temp 97* today. I think it is time for a new Dewitt Direct Fit radiator. In addition, I noticed that the temp gage shows about 45* less than the temp sending unit when monitered with an IR thermometer. Thanks every one!Comment
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Re: 66 L-72 Timing
You need to redo the job with the VAC DISCONNECTED and plugged.
Did you visually/manually check the centrifugal mechanism to be sure it operates freely?
Also, when reporting the data you need to specify the initial timing EXACTLY and make sure you measure it at a speed below where the centrifugal starts, or tie up the centrifugal with a rubber band to measure it. By subracting the initial out of your readings, you have the centrifugal curve and can compare it to OE specs.
Lighter springs will improve low end torque, but the limiting factor is detonation. Other L-72 owners who have done this might be able to give you some guidance. The '67 L-71 had a more aggressive centrifugal curve. It was all in at 3800.
The VAC should be tested separately. With the centrifugal tied up use a Mighty Vac and timing light to apply vacuum to the VAC and compare to specs.
DukeComment
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