Thunderbolt IV Timing mark question

[Profittakr]

I know that there are timing questions all over this forum but I have not read about this.

I have Mercruiser 5.7l, Thunderbolt IV, S/N OD724908, "Engine Rotation LH", 1992 24' Searay BR, Stock size 19P prop.

New-to-me boat and I am having trouble getting to WOT. I am going through the Low WOT Checklist and am now checking timing. Warmed up and at 700 rpm I measured about 12* ATDC - (Not BTDC). All the specs, including the engine cover, say it should be 8* BTDC.

I now see this service document and I am confused. Am I reading correctly in BOLD that this should be set to 8* ATDC now? I am very confused. At present, the boat is timed to 8* ATDC (at the bottom of the timing tab in the picture) and I did gain 400 rpm but still only getting 3400 at WOT. I ESTIMATE that if I advance base timing to 8* BTDC I will pick up about 800 rpm which starts to get me to the low end of the WOT range. Bottom paint is THICK and not smooth at all so would not be surprised if that was a factor as well.

So, basic question is - Am I timing base timing to 8* BTDC (top of timing tab in picture) or to 8* ATDC (bottom of timing tab). I KNOW what the spec says, but this service document has me confused as to which 8* mark to set timing to.

 

[Profittakr]

[No message]

 

[Bt Doctur]

what plug wire are you using to check timing?

 

[Bt Doctur]

The "A: is not ATDC ,the "A" designates the advance side of the tab.

 

[Profittakr]

I may have just answered my own question.

In reading the Service Manual 15 I see the SAME sentence in bold EXCEPT, it has one sentence added stating basically that the RH engines are to be set on the ATDC side of zero. It looks like the service document that I read initially (shown above) left out the comment about the RH engines. I feel more confident tin the 8*BTDC (top of tab) is correct.

Someone please confirm.

 

[Profittakr]

Thanks Doc! I am using #1 cylinder. (front right looking at engine from inside boat). I get your comment about "A" = advance - or BTDC. I was not catching that distinction - even though it says it! Thanks for setting me straight. Sometimes you just need another set of eyes looking at things.

 

[Bt Doctur]

You need the Blue, NOT the Red

 

[Profittakr]

YUP! - Thanks Doc. that is what I was planning to do. That basically tells me that the piston was basically 25% of the way down the cylinder when it was firing - no wonder it was down on power and rpm.

 

[Profittakr]

for my knowledge - What ill effects would setting the timing at the "red" cause besides lower power/WOT? Heat issues, engine damage, etc?

 

[harringtondav]

I purposely did this to a Ford 302 in 1987 to pass NJ annual vehicle inspection's exhaust sniffer. I passed, but the engine got smelly hot. Enough that I shut it down halfway on the 15 min. drive each way. I believe the heat was in the exhaust manifold. Firing on the compression down stoke is not good. I suspect the charge was still burning some when it passed the exhaust valves.