Best way to verify ign timing
 

Last season I put a timing light on my engs for the first time since I got the boat.
Starboard eng was reading unusually high at abt 42 degs at idle and would adv to abt 45 degs at 3k rpm.
Pulled the outboard plugs and didn't see any signs of detonation.(static compression is at 10.25. 93 oct only)
Found rust in the MSD dist. Cleaned it out and installed adv kit. Mech adv works good now.
I set the timing the normal way so I was all in at 3k rpm with 34 degs total adv.
Went for a test drive and found my top speed had dropped by 7-8 mph.
Starting to think maybe the damper has slipped.
I was thinking about buying a degree wheel an a piston stop to verify if the damper had slipped or not. but not sure how well it would work out with such a tight bilge.
My back up plan is to swap dampers between the engs, but that's not much fun either.
Question is: Is there an easier way to verify if the damper has slipped?

EFi or carb ? Please say carb. :)

Go get an anti-fouler from the auto parts store they screw into plug hole and plug goes into them. (They are also good for getting cars past emissions when cat converter is going bad) tap a thread in the small hole in anti fouler and screw a bolt about an inch long into thread. You can also drive the center out of a plug and use that, but I think more work. You now have a piston stop. Install in cyl 1 with balancer timing mark mebbe 30 degrees from tdc. Put a wrench on balancer turn crank till piston hits stop. Mark balancer at tdc mark on timing cover. Reverse crank till piston hits stop again. Mark balancer again. Marks should exactly straddle orig tdc mark. If not, halfway point between marks is tdc.

Still running some old crappy Holleys (4010s). Thinking vacuum?

Good idea using an anti-fouler. Trying to avoid pulling the plugs if possible. The inboard 4 are just too much fun..

You could use 6 instead of 1 for tdc on the starboard engine.

the keyway lines up with TDC mark in most cases

When you changed total timing from 45 to 34 degrees,,thats a lot,, you lost horsepower, and speed, set it to 36 degrees total and go run it, will most likely pickup a mph or 2.

He shouldn't see much diff between 36 and 34. I think he was nowhere near 45 when light said so. Woulda been pinging like crazy and running hot. I think he was close then and now 34 is really in the 20's.

Wouldnt it be an absolute pig getting on plane if it was in the 20's?

are you using a digital timing light?

We dunno whether he was timed right at 45 or what. I think it would be better in the 20's than in the 40's. I just checked one of my balancers. It is about 20 to 30 degrees from keyway.

Make sure you went off the right mark on the harmonic balancer. I ran into that earlier this year, my harmonic balancer has 2 indentions on it and I was using the wrong one getting screwy timing readings. Once I realized what happened it all made sense.

^^^^Yup. And I've run into some engines with two timing tabs mounted to engine...make sure you use the right one. :)

BTW: back in the day...with inefficiently designed large domes on pistons and the OE inefficient combustion chamber, it wouldn't be uncommon to run from 42 all the way up to 48* BTDC for total timing on BBC's at the race track. Sounds crazy, but it was almost the norm back then. Obviously newer marine is different, but wanted to state that fact before anyone thinks that timing figure is unheard of.

Oh, and sometimes people without advance timing lights will make a paint stripe on the balancer at 8-10*BTDC so they can line that up with the big indention (0*) to se it more clearly.

This can mix up the next person of course. So, make sure if it has a painted line on the balancer, it is actually on the 'cut or grooved' line on the balancer.

I've got a drawer full of old spark plugs that I broke the insulator off of, tapped the ID and put a bolt into to serve as a piston stop for TDC reference. When I have a new motor in front of me to work on, and it involves anything related to timing (ignition, cams, etc) the first thing I do is verify the TDC mark on the damper and degree tag. Some have been off a surprising amount. Knowing that you have accurate TDC indicator is good.

Knowledge is power.

Thx for all the good replies!

Good to know I can use #6 cyl, however I would still have to pull all the plugs to turn the eng over manually, especially with a high compression eng.
Wonder if I could just use a bolt with the same threads as the spark plug? Are plug threads special??

I'm using an analog adv timing light. Had it for years and seems to be working fine.

The damper has yellow paint at TDC (in the groove).

Only 1 timing tab on these engs.
The heads are the large rectangular port Merlins. Not state of the art by any means but not ancient either.

I like the idea of comparing the key way to the TDC mark. I can compare with the port eng. Will need to take a look and see if the key way is even visible with the limited space in front of the engs. Even if I can see the key way, I'm not positive I can eyeball ~ 6 degs of slippage at such an odd angle.

It is a 14 mm spark plug thread. O2 sensors use it too.

Went out and eyeballed the situation again looking to see if I had a chance of being able to see the key way. Can't tell for sure until I remove the crank pulley but it doesn't look good. However, I did notice the crank pulley has a raised line cast into the metal that lines up perfectly with the TDC mark on the damper.
Checked the other eng and sure enough it has the same raised line and it's also lined up perfectly. Anyone know about this cast line on the crank pulley? I believe it's the original pulley from when it was a gen-4 365hp Mag (1990).

Anyway, this makes me feel a little better. Maybe these engs do like a lot of advance. The prev owner specifically told me not to change the timing but didn't say why. Don't know if the MSD distributors were rusted or not when he had it. I'll double check the timing and maybe bump it up from 34 to 36 degs and see how it goes.
I took the boat out by myself and made a WOT run. Both engs are at 4800 RPM and it still gets into chine walk territory so it will be plenty fast enough for the family over the 4th.