Mercury 1972 800 Thunberbolt Outboard Octane Question

[krizm200]

I just found the Mercury home page, I think from Australia, that recommends an Octane of 95 for engines larger than 30 hp built before 1980. My engine is a 1972 Thunderbolt 800 80 hp. Just wondering if anyone has experience with this engine since I am new to this, and my engine tends to bog down after running a while when restarting. I have been using 87 octane with stabilizer. I also found a comment in the one of the Mercury engine manuals that lower octane can damage the piston top (actually showed a picture of a burnt spot). Am I using the wrong octane for this engine? Thanks for the help!

 

[Mercurylips]

We are supposed to use regular unleaded and no ethanol. All of the octane ratings on the gasoline, here, is 87 to 89 octane rated. Damned if you do and damned if you don't. Octane enhancement hasn't been necessary so far. I run a 1977 Mercury 500.

 

[merc850]

If you're using the wrong octane so have I for 30 years - regular 87octane (US), the worst thing you can do is use a low quality oil, wrong spark plugs, over-advanced timing and a prop of the wrong pitch. Australia may have a different method of measuring octane than N.A. Remove the plugs and look at the piston tops for damage or carbon buildup.

 

[Faztbullet]

Your motor if it hasn't been rebuilt in the last 10yrs has hi dome pistons which has a higher CR number. Just back the WOT timing off 2 degrees and go have fun.

 

[krizm200]

Thanks for the information everyone! I doubt the engine has been rebuilt in the last 10 years. I do know the compression is 160 LB on 2 cylinders and 155 LB on the other, pretty high for an engine from 1972 (just my guess). I'll read the engine manual on timing to see how it is done. I have not had a timing light in 40 some odd years!

I assume the prop size is correct for the engine, I replaced a 13x19 with the same prop size.

I did think it was interesting that the Mercury home page is listing Octane 95 as recommended for this engine and year. I have read many posts that state 87 is fine.

Does anyone know the right spark plugs for this engine? I was going to replace the existing ones with the same type, but I have not pulled the old ones out yet. That's what this weekend is for! I'm in the USA, by the way. My family goes out on Lake Sammamish.

Mark

 

[merc850]

Buy a tach if you don't have one already to make sure that the engine is running in the right range 4800-5300 rpm.