WHAT OCTANE?

[john1569]

I have a 93 60hp johnson which blew a piston. The omc manual says the cause could be detonation from the use of low octane gas .I was using 87.The manual recommends high octane gas.Our local marine store says you should never use anything but 87. I have heard so many different opinions on this . As I dont want to have this problem again can anyone put me right please.<br /><br /> REGARDS JOHN

 

[wilde1j]

Re: WHAT OCTANE?

87 is fine. Your problem had nothing to do with the gas.

 

[DJ]

Re: WHAT OCTANE?

Joh1569,<br /><br />Wildeone is right, you had/have another problem, more than likely, a clogged jet.

 

[Joe Reeves]

Re: WHAT OCTANE?

John.... I'm in full agreement with the above. Make sure that your carburetors are cleaned, and restored to new condition with the rebuilding.

 

[john1569]

Re: WHAT OCTANE?

JOE THE CARBS WERE REBUILT 3 OUTINGS BEFORE THE PISTON BLEW.<br /> WHAT DO YOU THINK?<br /><br /> REGARDS JOHN

 

[Greg Lawrence]

Re: WHAT OCTANE?

dont trust anyone who rebuilds carbs.. unless you know them personally. Ive pulled carbs apart that just got a "carb job" and they were filthy. no o-rings or gaskets changed. best bet is to do it yourself, its really quite easy. you cant get anything right unless you do it yourself ive learned.

 

[john1569]

Re: WHAT OCTANE?

Thanks Greg I think you are right.After we took the engine apart ourselves to have the rebore and pistons replaced we noticed that one of the carbs was missing a gasket.

 

[Dhadley]

Re: WHAT OCTANE?

Was the #2 piston the one damaged? What spark plugs?<br /><br />Good luck!

 

[john1569]

Re: WHAT OCTANE?

The top piston was damaged the most it had a hole the size of a nickel. The middle cyl had slight scarring on the walls and the lower cyl was untouched. The plugs were Champion.

 

[clanton]

Re: WHAT OCTANE?

There is about 5/6 service bulletins this engine and one of the includes carb jets and timimg change. Try search may find them.

 

[Forktail]

Re: WHAT OCTANE?

One point to consider is that upon reboring and running oversized pistons, you should jet up. This is overlooked a lot.<br /><br />The cylinder becomes bigger and thus holds a greater volume of air. Without increasing the jet size slightly, you begin to run a leaner condition....more air with the same amount of fuel as before. Since this engine already has bulletins out on jetting, the little air volume increase with the bore could make a difference.<br /><br />Also, not all 2-strokes run the exact same jet in each carb. I'm not sure about your 60, but sometimes the stator side of the motor (your top two cylinders) run different sized jets than the PTO side. They may have been swithced during your carb rebuild.<br /><br />Hopefully your carb rebuilder is paying for the piston failure?

 

[john1569]

Re: WHAT OCTANE?

Thanks for everyones advice.I am a novice when it comes to engine repairs . The reason I am attempting to do this is the cost of repair at the shop was more than I could ever afford .So I either try to do the job myself or the boat just has to sit there.I had the rebore done bought new pistons and a manual. With my brothers help we shall try our hand at it.Do you think the carb rebuild was the cause of this as it only ran 3 times before the failure.And if so how could I prove this considering the engine is now in various pieces.

 

[BillP]

Re: WHAT OCTANE?

Be sure to check the timing. The adjuster screw can rotate by itself if the locknut gets loose. It doesn't take much turning to change the timing and cause major problems. The hole in your piston could have also come from carbon buildup on the piston crown...which is caused by a too rich mixture. Have a professional mechanic look at your used parts before cleaning them to id the cause. Also, learn how to read sparkplugs and check them every few hours until you know all is well. Then do it once a season.