85cc 2 stroke psi

[DaveyDave]

Just wondering what the psi should be on my rm85 top end? Its at 155psi i dont know is that good or bad

[Hedgehog 33]

I'd say that's pretty good probably not far off what it started at, but I've never owned one so I can't say for sure.

[mlatour]

Unfortunately I don't know what the average compression should be on an 85cc

but as Hedgehog 33 said that is likely very close to having a 'fresh' top end.

(taking in consideration the comp. gauge is accurate too)

 

A good rule of thumb for the recreational rider as to when to refresh a 2 strokes top end

(I'll take a 125 as an example as I'm more familiar with those numbers)

 

is to take a compression reading once a new piston & ring are installed and properly broken in.

Say your new top end gets 180psi, by the time the engine looses 10-15% of it's 'fresh' compression value,

in this case when it falls to about 155psi-160psi it's time for a refresh.

Take a few readings during the season to monitor the drop.

As an example I get about 20 hours on my recreational MX'ing YZ125 before loosing 10%.

 

A serious racer will want to refresh top ends much sooner, perhaps at only a 5% drop to always have a strong performing engine

and as preventative maintenance to avoid component failures from sustained high rpm use.

Edited November 12, 2015 by mlatour