outlawinil

Wanting to check timing on my 500 carbed hp's. Merc says 34 degrees at 4500 rpm right? Read on here where Dennis Moore says 34 degrees at 3500 rpm. Wouldn't that make total advance alot higher? If I set them above 34 at what point do you need to run higher octane?

MACDAD260

As far as what rpm to set total advance, rev it up until the timing no longer advances with increased rpm. Different distributors and modules advance differently but, they all stop advancing at some rpm point. Just make sure you find that point and you will be OK.

Higher octane fuel may help if you advance the timing more than 34* or it may not. Compression ratio has something to do with that as well. Stock Merc engines do not have very high compression so high octane fuel probably won't help much. In fact, there has been some discussion here that indicates that 92 octane or higher fuel may not burn as well as 87 or 89.

obnoxus

Correct. If your compression ratio does not warrant it, 92 octane will hurt performance. 87 burns more quickly.

Mac is right on finding your total advance point.