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Sark Advance Shift?

fastjeff

fastjeff

Gold Medal Contributor
Was warming up my 1986 MC 305 hp this morning, playing with the choke adjustment (Q-Jet) to reduce cold motor stalls it's been experiencing for some time. As I was watching the choke slowly open as the motor warmed up, at about 120-130 ' F the motor suddenly increased in rpm by maybe 100 revs. Nothing was touched--it just reved higher (and smoother) all by itself.

Question: Is that programed into the ECM somehow? If so, what purpose could that be since the opposite is what' needed during warm up.

Thanks!

Jeff

Doh! That's supposed to read "SPARK Advance Shift".
 
Jeff, forgive me, but I'm not understanding your thread title!

However, as the choke chimney becomes less restrictive (engine warming up), the fuel/air mixture becomes more lean.
With no other changes to the throttle plate position and/or ignition, a leaner mixture will intrinsically increase idle RPM.



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Last edited:
No ECM with Carbed engines.
If you are refering to timing control module then the answer is no.

Check base timing.
If around 6-8 btdc the bump it up to 10 ish.

Readjust main idle screw to 700 rpm.

See if that helps.
 
.."If around 6-8 btdc the bump it up to 10 ish. Readjust main idle screw to 700 rpm."

I like that. A bit more always timing seems to help a cold, low rpm motor. It's 8 degrees now.

Thanks for the inputs.

Jeff
 
The object here is to allow the lowering of the main idle screw to allow the throttle blades to close more.

This can help in stabelizing the idle circut.
Not always but some times....
 
Right on, Jack! You put me onto what's been wrong with this thing for years--it was retarded (kinda like....) Here's what happened:

When I timed the thing years ago, the idle speed must have been too high--at least 1,000 rpm. It was 'right on the money' (not really) at that speed, so someone else had also made the same mistake. With the motor right on 700 rpm, I retimed it to about 9 - 10 degrees advance, then reset the idle speed and did it yet again. Another bump and she was finally where I wanted it. The motor sounds much 'better' now, and I suspect the cold stalling issue will finally be gone. Sea trial it tomorrow.

Thanks again!

Jeff
 
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