www.iFourWinns.com

My engine manual and the placard on the motor states that my engine (1991 OMC 5.7l carbed w/elec ignition f/factory) needs to be at 8 degrees BTDC.

The directions out of my OMC engine manual for checking/setting timing are very basic and traditional. You know, check and confirm idle speed (I did) at 650RPMS, check timing, adjust if necessary. There arent any things that need to be unplugged to check & adjust timing, etc... like some of the newer engines.

My timing was at 12 BTDC when I checked, so I moved the distributor to make at 8 BTDC, as spec'd from OMC. When I do that, it starts to idle real rough and the idle drops about 150-200RPMs. It just doesnt sound right. At 12 BTDC it purrs.

My question is this: Do I adjust it down to 8BTDC and then turn the idle UP? Wouldnt that just advance the timing (if I turned the idle up?)

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1997 Four Winns Vista 278
T - 4.3l EFI
Kohler 4esz Genset
379 hours as of 8/8/21

When you retard the timing from where it was you almost always see a drop in idle speed and you will sense a rougher idle because of that...I would not leave the timing advanced 4* from where it's supposed to be because of the risk of detonation esp if you are not using 93 octane fuel...I'd set it where they say and then adjust the idle...you may need to set the mixture too if you can't get a smooth idle at 600 rpm...keep in mind the dog clutch Cobra will not shift right if the idle is too high...500-600 in gear in the water is the official spec but I usually set mine at 600 on land running on the water hose with the prop off and the drive in foward gear....then when checking it on the water it's pretty close...

I set my '88 4.3 OMC with the 4bbl carb at the specified 6* BTDC and since the only marine gas I can get here is 93 octane I never have a problem with detonation...

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88 Four Winns 200 Horizon
4.3 OMC Cobra-4bbl
2002 Walker Bay 10/2012 Suzuki 2.5
2008 Walker Bay 8

1998 Jeep Grand Cherokee 4.0/Selectrac
2007 Jeep Grand Cherokee 5.7 Hemi/Quadradrive II

Hey Lou, Thanks for the reply!!

I have the cone clutch king cobra ('91).

I have been using 89 octane since I bought the boat without incident... I didnt realize the recommended timing from OMC was for 93 octane!

I suppose I will go home today, keep it at 8 BTDC and adjust the idle speed. I'd rather not mess with the mixture adjustment, but I suppose if I have to, I have to.

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1997 Four Winns Vista 278
T - 4.3l EFI
Kohler 4esz Genset
379 hours as of 8/8/21

Actually if you look in the owner's manual you will see what's recommended, I think that 89 was also recommended for mine too but 93 is all I can get at the gas dock. Some engines need for octane increases with age because of carbon build up and the same can happen if the fuel mixture gets too lean...you may be fine with the 89 as long as you don't have too much carbon build up, have the mixture set right and keep the timing where it should be. One easy way to tell is after a good run take a look at the spark plugs...a nice brown-tan color is good, white is too lean and black is too rich...
Because marine engines run at high loads all the time watching for detonation and lean fuel mixtures is really important...even more so with 2 stroke outboards...

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88 Four Winns 200 Horizon
4.3 OMC Cobra-4bbl
2002 Walker Bay 10/2012 Suzuki 2.5
2008 Walker Bay 8

1998 Jeep Grand Cherokee 4.0/Selectrac
2007 Jeep Grand Cherokee 5.7 Hemi/Quadradrive II

You can turn the idle back up to specs and then turn the low speed mixture until it runs good keep going until it runs rough then back to the smooth setting, helps make adjusting easy, dont use your baaots tach either, use the timing light.


Mike

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Mike
2007 Horizon 200 SS
5.0 GXi SX drive.