1977 Mercruiser 140 timing

jackball42

Petty Officer 3rd Class
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Apr 2, 2011
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I have a 1977 Starcraft SuperSport with a Mercruiser 140 I/O. When I mash down on the throttle it hesitates and even dies occasionly unless I pump the shifter and sometime backfires through the carb. I have been through the carb and verifed the float settings are correct and the accelarator pump is working correctly. The timing spec is 6 degrees BTDC and the idle spec is 650-700 in gear. To get the 650-700 in gear spec the idle while on the muffs must be about 775-800 and this causes the mechanical advance to advance the timing. I backed the idle down, while still on the muffs, to 700 and then adjusted the timing to 6 degress. It no longer hesitates as bad and does not back fire through the carb.

The dwell is at 32-34 degrees per spec.

Am I all wet in my thinking and will retarted timing cause the symptons I was seeing? I also saw another thread where Mercruiser changed the sped to 8 degrees at some point. I am running 91 octane gas because the 87 octane around here has 10% ethanol in it.
 

Don S

Honorary Moderator Emeritus
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Aug 31, 2004
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62,321
Re: 1977 Mercruiser 140 timing

Do you have the idle mixture adjusted properly? Sounds like you may be a little lean.
 

jackball42

Petty Officer 3rd Class
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Apr 2, 2011
Messages
76
Re: 1977 Mercruiser 140 timing

I have adjusted the idle mixture to the highest RPM plus about 1/4 of a turn. Prior to readjusting it smelled a little rich.
 

JerryIrons

Petty Officer 2nd Class
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May 27, 2010
Messages
125
Re: 1977 Mercruiser 140 timing

What happens if you advance the timing a little bit just to diagnose? Ie 8 or 10 degrees as a starting for base? Also check your advanced timing with a gun to see how far it advances. As Don said check your mixture for sure.
 

jackball42

Petty Officer 3rd Class
Joined
Apr 2, 2011
Messages
76
Re: 1977 Mercruiser 140 timing

I fiddled with the timing and got the best operation out of it by adjusting to 6 degrees at 700 rpm but didnt advance it any more than that. When I get it in the water next time I will check the timing advance with the gun to see how far it advances at WOT.

I will fiddle with the mixture screws too.
 
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