Help with timing marks

kaliona

Petty Officer 2nd Class
Joined
Sep 27, 2008
Messages
135
165HP mercruiser I6. Set my idle mixture screws to get a smooth idle at 700 rpm. Set my timing to 6 deg BTDC and the engine seemed to run fine. I used a vacuum gauge to get the highest vacuum when setting the mixture screws and the needle was holding at about 15 to 16 Hg. which indicates "Late Ign. Timing" (according to what's printed on the gauge). So I advanced the timing, the gauge went up to 17" and seems to run smoother. I reved the engine up to 2000 rpms just for a few seconds to see if it would miss or ping but it was fine (I didn't want to rev it too long because I'm doing this in the driveway with the hose flush kit). Locked down the hold down bolt and called it good. But I'm worried it might be too advanced. My timing pointer is hard to read and I'm guessing I'm at 12 degrees, maybe 14 degrees BTDC. Attached is a pic of where the line on the crank is in relation to the pointer. Am I reading this wrong? Should I rely on the vacuum reading and leave it as is? Or change it back to specs of 6 BTDC? Thanks for any input.
 

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kaliona

Petty Officer 2nd Class
Joined
Sep 27, 2008
Messages
135
I did a little more research on setting timing with a vacuum gauge. After I get the highest steady vacuum reading, I'm supposed to retard the timing until the vacuum gauge reading drops 1/2 to 1" to prevent pre-ignition. Maybe that will put my timing at least a little closer to specs than it my pics show.
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
Set it at 6. Those vacuum gauge set ups are for CAR engines. They aren't exposed to the same loadings as your boat engine, and a vac gauge sets too advanced. You'll blow holes in the pistons. Even if you can't hear any pinging, it's happening. Also, as you are running unloaded, no pinging... Put a load on the engine (ie, in the water running at 20knots), whole different ball of wax...And you still won't hear it...

Chris.....
 
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