Distributor timing adjustments

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GPGoverMPG
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Distributor timing adjustments

Post #1 by GPGoverMPG » Wed Apr 18, 2018 2:47 pm

So I'm working on the motor still in my 1979 4x4 4speed truck. It runs good but I don't like the low oil pressure at idle so I'm building my replacement. It's mostly stock the only changes being EFI exhaust (no muffler-way to loud), Chevy rockers, and a 2v carb off a 1964 289 mustang. I have way to much total timing advance. Initial 6* + Mechanical 18* + vacuum 22* for a total of 46* is there really this much adjustment available in the vacuum advance unit or is it really just the wrong one. I think the total advance should be around 36*. I'm using this motor as my test mule to make sure everything is working right before installing new motor. It seemed to run Ok in the driveway but now it's plated and i'm driving it some and it's not right does not pull well past about 3500rpm. Thanks in advance for any help.
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71 F350 4x4 Super Duty 7.3 IDI
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THE FRENCHTOWN FLYER
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Re: Distributor timing adjustments

Post #2 by THE FRENCHTOWN FLYER » Wed Apr 18, 2018 3:08 pm

That doesn't seem too much. I know some V8s like the 351M - 400 with slow burning heads and low compression had up to 55*. Remember you'll never have total mechanical and total vacuum at the same time.
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Re: Distributor timing adjustments

Post #3 by GPGoverMPG » Wed Apr 18, 2018 8:01 pm

When checking timing at 2500rpm I get 46*. Seems like I should only see about 36*. Where do I fix this? I'm using an MAC tool timing light with the timing dial on it to check this. Thanks
71 F350 4x4 Super Duty 7.3 IDI
72 F250 4x4 4speed 300 I6
72 F100 2x 428 Tri Power
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Re: Distributor timing adjustments

Post #4 by GPGoverMPG » Wed Apr 18, 2018 8:16 pm

So I'm using ported vacuum for the distributor vacuum advance. Ported vacuum increases with throttle opening so there would be times when you see both mechanical and vacuum advance at their max. Or do I have something wrong here?
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86f250straight6
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Re: Distributor timing adjustments

Post #5 by 86f250straight6 » Wed Apr 18, 2018 9:29 pm

Do you still have egr? Egr is meant to have additional advance which could ping if its been removed.
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Re: Distributor timing adjustments

Post #6 by GPGoverMPG » Thu Apr 19, 2018 8:05 am

No EGR. Really simple set up. The timing light is indicating 46* total timing at about 2500rpm. I think it should be around 36*. I'm going to try adjusting the vacuum advance today and see if I can remove 10*.
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72 F250 4x4 4speed 300 I6
72 F100 2x 428 Tri Power
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THE FRENCHTOWN FLYER
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Re: Distributor timing adjustments

Post #7 by THE FRENCHTOWN FLYER » Thu Apr 19, 2018 9:48 am

When you are revving to 2500 you do not have any load on the engine. Of course you will have some mechanical advance and vacuum advance under those conditions. But you're not moving or accelerating a vehicle when you do that, are you? Under load the vacuum is reduced and thus vacuum advance goes away. Try it with the vacuum advance hose disconnected / plugged. You will see what the engine sees when you are accelerating.

Side note: The engine was calibrated for EGR, which allows for more advance, due to the fact that the intake charge is diluted with residual exhaust, which slows the burn rate (to reduce NOx). Removing EGR often leads to over advance and detonation, unless timing is retarded at part throttle. At WOT, EGR is inoperative and of no consequence in limiting max power.
Last edited by THE FRENCHTOWN FLYER on Thu Apr 19, 2018 9:53 am, edited 1 time in total.
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Re: Distributor timing adjustments

Post #8 by CNC-Dude » Thu Apr 19, 2018 9:52 am

On that old of an engine, I would confirm true TDC with your timing mark on the balancer and timing cover to make sure they haven't been changed or mismatched in the past.
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Re: Distributor timing adjustments

Post #9 by GPGoverMPG » Thu Apr 19, 2018 6:09 pm

I've used a piston stop and verified top dead center. The balancer is fine. i will connect a vacuum gauge with a tee in the line going to the vacuum advance from the carbs ported vacuum outlet and drive the truck. Under full throttle i'll monitor the ported vacuum. My base assumptions are as follows: at idle manifold vacuum is high ported vacuum is low, as throttle is applied manifold vacuum goes down and ported vacuum increases. My thought was ported vacuum is based on airflow in the carb and sustained full throttle results in increased airflow and the ported vacuum will not go back to zero.
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72 F100 2x 428 Tri Power
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Re: Distributor timing adjustments

Post #10 by GPGoverMPG » Fri Apr 20, 2018 4:33 pm

So, first off all my assumptions were wrong. At idle manifold vacuum is high, ported vacuum is low. Partial throttle manifold vacuum is less and ported vacuum is high, At full throttle manifold vacuum is low and ported vacuum is low. So Frenchtownflyer is correct again. Thank you very much. I have now fixed my ignition problem. My book says to set initial timing at 6*btc. For my application that's crap. I set initial at 18*btc with the vacuum advance disconnected and I now get 36* WOT advance. I limited the vacuum advance to 10* and the truck drives great, idles better, and starts nicer. Free horse power. Thanks again.
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Re: Distributor timing adjustments

Post #11 by Lazy JW » Fri Apr 20, 2018 5:27 pm

GPGoverMPG wrote:... At idle manifold vacuum is high, ported vacuum is low. Partial throttle manifold vacuum is less and ported vacuum is high, At full throttle manifold vacuum is low and ported vacuum is low...


Mmmm... once you open the throttle enough to uncover the ported vacuum port, that port pretty much = manifold vacuum.
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Re: Distributor timing adjustments

Post #12 by GPGoverMPG » Fri Apr 20, 2018 7:00 pm

Yes, but not really. I didn't bother putting numbers on here because it will be different for everyone. It's not very black and white I still need to play with my vacuum advance stop. So on my truck mechanical advance starts at 700rpm and it's all in at 2500rpm that's 18*. So at 1500rpm partial throttle I MAY have 9* mechanical and 8* vacuum advance which is 17* plus the 18* base for 35* light load and a lean slow burn fuel mixture. It seems a bit hit and miss. I'll need to run some fuel through it (butt dyno) and watch temps. The problem being you can't see whats going on.
71 F350 4x4 Super Duty 7.3 IDI
72 F250 4x4 4speed 300 I6
72 F100 2x 428 Tri Power
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Re: Distributor timing adjustments

Post #13 by Lazy JW » Mon Apr 23, 2018 4:00 pm

GPGoverMPG wrote:Yes, but not really...


Ok
"The White OX" 1974 F-350 300-6, Stock single exhaust, Carter YF, T-18A, Dana70 w/4.11, Flatbed dually w/dump bed. "Where no oxen are, the crib is clean, but much increase is by the strength of the ox" (Proverbs 14:4)
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