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Base Timing To Far Advanced

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old28racer
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Base Timing To Far Advanced

Post #1 by old28racer » Thu Jul 11, 2019 7:12 pm

My base timing on my stock style 300 motor rebuild is set at 18-20 degrees. With the vacuum advance disconnected the mechanical advance will push the advance to about 28 degrees @ 2500 rpm. Connect the vacuum advance and it maxes out at 32 degrees @ 2500 rpm.

The 1971 F250 has the stock rebuilt P&C distributor.

My question is does the base timing being at 18-20 degrees when it should be at 12 degrees affect my mileage? Most all town driving 35-45 mph, 5% freeway at 55-60 mph. Truck starts great on first turn & idles great around 700-800 rpm.

Can provide more information if needed.
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pmuller9
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Re: Base Timing To Far Advanced

Post #2 by pmuller9 » Thu Jul 11, 2019 9:07 pm

When you say "My base timing on my stock style 300 motor rebuild is set at 18-20 degrees",
is that with the vacuum advance disconnected?

Base timing is with the vacuum advance disconnected.

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old28racer
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Re: Base Timing To Far Advanced

Post #3 by old28racer » Thu Jul 11, 2019 10:56 pm

pmuller9 -- Paul this is what I posted, maybe was not clear. I am an old dude. :mrgreen: :bang:

"My base timing on my stock style 300 motor rebuild is set at 18-20 degrees. With the vacuum advance disconnected the mechanical advance will push the advance to about 28 degrees @ 2500 rpm. Connect the vacuum advance and it maxes out at 32 degrees @ 2500 rpm".

I should have said that my base timing is at 18-20 degrees with vacuum advance disconnected & motor at 700-800 rpm. Add mechanical advance only & it is 28* degrees @ 2500 rpm (base 20 + 8 mechanical). Connect vacuum advance and to the (20*+ 8*+4* = 32* degrees @ 2500 rpm.

This is with only the advance side of my current dual chamber connected, the retard side is disconnected.
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Re: Base Timing To Far Advanced

Post #4 by pmuller9 » Thu Jul 11, 2019 11:18 pm

Normally with a low duration cam the initial timing is closer to 12 degrees at idle and maxes out at 34 to 36 degrees at full mechanical advance for a non-EFI head.
The engine rpm needs to be continued till the mechanical advance stops. That could be as high as 3000 to 3500 rpm.
Check it again to see what the final mechanical advance is and at what rpm it takes place.

The vacuum advance needs to be disconnected during the mechanical advance check.

When the vacuum advance is connected it can add 8 to 10 degrees making the total advance above 40 degrees at higher rpm.

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THE FRENCHTOWN FLYER
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Re: Base Timing To Far Advanced

Post #5 by THE FRENCHTOWN FLYER » Thu Jul 11, 2019 11:34 pm

When you say the vacuum advance will push the total to 32* @ 2500 - What is the manifold vacuum at that point? It may not have all the vacuum advance fully in. A better way to check vacuum advance is by hooking up a vacuum pump and plotting the additional vacuum advance vs the amount of vacuum you are applying. That can then be easily plotted and added to the initial and centrifugal at various RPMs - depending on how much load there happens to be on the engine which, of course, will change the vacuum.

My personal opinion is many people crank in more total advance than is needed, and also end up with detonation issues due to too much (unneeded) spark advance. Especially on "built" engines that generate higher cylinder pressures. Many firms that advertise "re-curving" only make things worse in doing just that.
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Re: Base Timing To Far Advanced

Post #6 by old28racer » Fri Jul 12, 2019 8:44 pm

pmuller9 --- Paul, I'm on board with what you are saying. I need to make a few timing changes in the next few days. The base timing at 20* was where it was when I picked up the truck from last owner. All base & mechanical advance tests have been with vacuum advance disconnected & plugged.
The max mechanical advance is 6-7* @ 2300 rpm and it does not go any higher with more rpm. This is with a stock style P&C distributor.

When I am able I will set the base timing at 12* BTDC @ 700-800 rpm.
Then I will re-test the mechanical advance at 2300 & 3500 rpm to see if it is more than 6-7*.
Then I will re-connext the vacuum advance & see what the total (base + mechanical + vacuum) is.

As a side note I really no not think the mechanical & vacuum advance on my current P&C distributor is working to it's best. I have recurved my new HEI distributor and installed an adjustable vacuum canister. The new counter weights & light springs will provide 14* BTDC @ 2500 rpm / 18* @ 3500 / 22 @ 4500 / 24 @ 5000 & above. I set the adjustable vacuum canister at 5 turns out of 10 as a starting point. Hope to get it installed first of August.

FTF --- Greg, I have not really looked at the vacuum advance while in my garage in park. My manifold vacuum is 20" @ 700 rpm. I have done a road test with my vacuum gauge in the cab and this is what I got. Cruising around town at 35 mph = 18"/45 = 17"/50 = 16". Freeway 55-60 mph = 14". Up steep grade at 45-50 mph = 10-to a low of 8". I will try doing some vacuum advance readings with my hand pump at difference rpm & vacuum inches.
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Re: Base Timing To Far Advanced

Post #7 by GPGoverMPG » Sat Jul 13, 2019 7:56 am

So it runs good but the base timing is high. To me it seems that the total advance (no vacuum advance) of 28* is way to low. Seems these motors like 36* to 38* total. So if he is not getting enough total advance it would affect his MPG. As I found you don't get full vacuum advance at WOT and Ignition advance can be over 40* under light loads. My engine is running 18* initial, 18* mechanical, and a possible 12* vacuum. I have not checked MPG but it runs great and maximizing performance should help fuel mileage.
71 F350 4x4 Super Duty 7.3 IDI
72 F250 4x4 4speed 300 I6
72 F100 2x 428 Tri Power
2014 SHO

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old28racer
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Re: Base Timing To Far Advanced

Post #8 by old28racer » Sat Jul 13, 2019 8:12 pm

GPGoverMPG -- Thanks for the reply. I am sure the P&C distributor is not functioning as it should with my mechanical advance toped out at only 8-9*. As I understand if your base timing is at 12* you should be able to add 20-25* more mechanical advance to get 34-36* total. Then my vacuum advance will push it up to 40-45* advance.

Why are you running your base timing at 18* instead of 12*?
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GPGoverMPG
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Re: Base Timing To Far Advanced

Post #9 by GPGoverMPG » Sun Jul 14, 2019 8:28 am

My mechanical advance will only pull in 18* max. I kept advancing the timing a bit at a time until I felt a little performance drop off driving the same route every time. I was doing short full throttle runs of maybe an 1/8 mile in third from 1000rpm up to my 4000rpm shift point. Once it stopped improving I removed a couple degrees to keep it safe. I'm running the Ford DSII ignition with the 5* start retard hooked up although it starts just fine without removing the 5*. Drivability really improved it's a fun truck to drive. The reason I haven't checked MPG is it's a 4X4 with 4.10:1 gears, an np435 trans, and 33x12.50x15s.
71 F350 4x4 Super Duty 7.3 IDI
72 F250 4x4 4speed 300 I6
72 F100 2x 428 Tri Power
2014 SHO

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old28racer
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Re: Base Timing To Far Advanced

Post #10 by old28racer » Sun Jul 14, 2019 1:44 pm

GPGoverMPG --- Thank you for the reply. With you saying that your mechanical advance will only pull in 18* max (18* base + 18* Mech = 36* total). As I stated my P&C distributor mechanical advance will only pull in 9* advance, with 12* base + 9* mech = 21* total, way off from the 34* I am looking for.

I just purchased the truck a year ago and it has a new stock style motor rebuild, 700 miles on rebuild. From what you have said in your reply I think the former owner was only able to get 9* out of the mech advance and was able to push the base timing up to 20* for a total mech advance of 29*. Not 34* but better than 21*.

The truck runs great, starts easy, no ping noise, and has good acceleration. My setup makes more sense now. I need the truck running daily so I think my next move will be to install a new HEI distributor I purchased for my 300 shop build. I have installed light springs in the mech advance so it will make 22* advance @ 2300 rpm & 24* @ 5000. My truck never sees 5000 rpm as it is just a cruiser and I am light on the peddle. If I do put in the HEI for a while that would make 12* base + 22* mech = 34* Total mech advance. This would be right where I want my ignition set at.

The finial setup for my truck would be to rebuild my existing P&C distributor and convert it to a Protronix 1261 Ignitor electronic ignition as I wanted to do in the beginning.
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