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Adjusting Distributor Advance

 
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MikeJinCO  



Joined: 08 Jun 2010
Posts: 937
Location: Maysville, Colorado

PostPosted: Mon Oct 05, 2020 5:45 am    Post subject: Adjusting Distributor Advance Reply with quote

Understanding timing should be about 34 BTDC. I have a MSD 6AL ignition and have been timing it with my old style Sears timing light at about 3000 -3500 rpm. Looking at the Haynes manual the mechanical advance does not come in fully until about 2200rpm of the distributor shaft(1/2 motor peed). I don't run any vacuum advance. Do I really run it up to 4400rpm to set the timing?

Many modern digital timing lights don't work with MSD, either older ones or quite expensive ones set up for the multi spark of the MSD.

Second question, why would they say the advance should peak at 41 BTDC when anything over 38 BTDC is not recommended for CC in piston motors? I don't really understand how the vacuum advance comes into play here.

I have not disabled the mechanical advance as when we tried to take the
distributor apart to get to the springs it did not want to cooperate and I didn't have a spare distributor if it got damaged.

Last race next weekend . Hope to go but some non car issues may have to force cancellation. At La Junta there is turn in a sea of concrete(old WWII airport track) and the October late afternoon sun make finding the turn a challenge.
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Poco a Poco, #033 '78 Vintage racer, SCCA Dp-81
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Fifty50Plus  



Joined: 28 Feb 2008
Posts: 1107
Location: Washington DC area

PostPosted: Mon Oct 05, 2020 10:15 am    Post subject: Reply with quote

Mike,
I have one of those Sears advance timing lights and they are not very accurate.
I set my race car at 41 degrees at 3000 RPM and it runs great with plugs on the vacuum device. We determined that the max power for the engine was at 41 degrees on an engine dyno. All mechanical advance distributors are fully advanced at that engine RPM - some lower.
I ran a 3 race weekend on Labor Day and races 1 and 2 were fun and exciting. However, I felt that my engine wasn't performing at its best on race two vs. the other cars. I checked my timing the next morning with my Sears light at 3000 RPM and it said 48 degrees advanced. So I cranked out 7 degrees to run race 3. It was a dog.
The cars I could accelerate with on race 2 ran away from me.
Moral of the story is that the Sears dial on the light is crap. I should have left it alone.
BTW MSDs put out two spark pulses per cylinder up to 3000 RPM and then only one spark above that.
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1979 924 NA race car
1982 924 NA race car - Sold
1982 924 Turbo almost a PoS
1981 924 Turbo a real PoS, new engine
1982 924 Turbo nice body, blown engine
1972 911 E race car - going to Vintage
Various 944s to become IT-S race car
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924RACR  



Joined: 29 Jul 2001
Posts: 8346
Location: Royal Oak, MI, USA

PostPosted: Mon Oct 05, 2020 10:04 pm    Post subject: Reply with quote

Good point on the Sears timing light - I have the same, but haven't had a method to test it for accuracy. Sounds like I ought to.

I suppose I could dial in whatever advance I want on my Turbo, since it's Microsquirt, and then check the light... or maybe just buy a better timing light? Ironic, since the 924NA is the only car I have that needs a timing light...

I think on the #77 that setting (41) works out to about 12 degrees static advance...
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MikeJinCO  



Joined: 08 Jun 2010
Posts: 937
Location: Maysville, Colorado

PostPosted: Sat Oct 10, 2020 12:17 am    Post subject: Reply with quote

I did an experiment. I borrowed a non digital MAC tool timing light. First I set the timing at 41 with the Sears light, then checked it with the MAC and it at first was very scattered(probably the RPM not high enough so multiple spark) then adjusted it to 41. Then I went back and checked with the Sears and it showed timing at a 15 degree advance mark I had previously put on the front pulley for static timing( with carbs about 10 BTDC and old time advance at altitude of 8300 ft). So I switched back to the MAC and it was 41 again. I repeated the exercise again and it held.

Shows me two things. First, the Sears is probably toast. Secondly, I have been looking at two timing lights an Acton which is a simple dial advance and an Innova digital that shows RPM etc., that Innova says is not good for the MSD, but lots of users say works just fine. I'll look at the Innova closer.
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Poco a Poco, #033 '78 Vintage racer, SCCA Dp-81
'77 924
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