Can anyone tell me if it is okay to run 89 octane in a pair of HP 500's.

It lists recommended octane on the spark arrestor. I think they will eat 87 if I remember correctly.

87 is all they need ortherwise you get a lot of black soot on the transom

87 is fine. 89 is better if you can get it.

not only is it fine, they re made for 87

Anything higher than 87, you will have a black transom and less hp. The higher the octane the harder the fuel is to burn, so when you have unburnt fuel in the combustion camber is never working properly witch drops the HP.

The black soot is from them being jetted "PIG" fat. Lean her out a few numbers and hang on! The ol' black engines come to life taking a bit a fuel out of them.

Yes lean them out and advance the timing to 36degree and run 87/89 octane. My HP500 and a lot of the F1 guys ran them this way.

Dean,

Just how many "numbers" do you recommend from the factory sizes?

Thanks.
Mark

To everyone who replied to my question on octane, thanks for the replies they were very helpful, sorry that it took so long to reply but computers are not my strongpoint.