RPM question / problem?

[joho5]

1989 Evinrude 70hp

My research shows me this motor should top out at 5,500 rpms at WOT.

When I run this thing its topping up around 5,900, or close to 6,000.

The previous owner has this motor bored slightly and new wiseco pistons installed, and I didnt know if this had any sort of effect on the rpms, or is my tach possibly off?

I had someone tell me this is very dangrous on a motor and could ruin it, and he said the problem was more than likely the prop, needs more pitch.

any ideas, or what should I do?

 

[domwebhost]

Re: RPM question / problem?

Hmm, I am not too familiar with outboards but in my youth I used to rebuild 80-125cc dirt bike engines all the time and bored and honed cylinders up to .040, I wouldn't think that it is dangerous as long as he didn't bore it out too far (or if he did bore it out too far, he at least sleeved it). The only danger that I can see is perhaps extra heat being generated by the bigger displacement which might equate a cooling problem which may be rectified by a slightly colder thermostat (if warranted). As far as the RPMS are concerned I am not sure if your tachometer is off, or if the change in displacement changes the value of top end RPM. Perhaps more seasoned outboard members can lend their expertise

As far as the pitch is concerned check out http://boatpropellers.iboats.com/propeller-tutorials/rpm/?**********=387569201&*******=566768207

 

[tashasdaddy]

Re: RPM question / problem?

with todays fuels compared to 1989, you are in a good rpm range, just don't run it quite wot. it will run cooler and better there, with todays fuels. i say you are fine.

 

[film495]

Re: RPM question / problem?

this is just a plug theory, but should he run a cooler plug, because the additional rpm will make more heat?