aldot, most side-valve small engines have a compression ratio of 6 to 1, while most OHV small engines have a compression ratio of 8 to 1. This is the biggest single reason for the much lower fuel consumption of the OHV ones. However it means the OHV engines need decompressors. The decompressors usually work by bumping the exhaust valve open momentarily, halfway through the compression stroke. Above 600 rpm the decompressor disconnects itself.

As long as it is a pushrod engine, there is a simple technique for doing a compression test which Honda specified in the workshop manual for the GXV120 - it does not seem to be in the manuals for the other pushrod engines, which were introduced later. All you do is remove the exhaust pushrod, then carry out a compression test in the normal way:

[Linked Image]

Personally I do not like the idea of loading the recoil starter that much - it is pretty hard to turn the engine over without the decompressor - so I rotate the engine with a large back-geared electric drill on the flywheel retaining nut.

The Kawasaki engine has a similar compression ratio to the Honda, IIRC, so pressure reading with the pushrod removed should probably be similar. Pressures measured with the decompressor working may differ greatly from one make or model of engine to another, however, because of differences in the exact place in the compression stroke where the decompressor operates.

Just to make this thread a complete geek-treat, this is how the Honda GXV120 decompressor works:

[Linked Image]

It just flips a little steel pressing onto a position against the cam lobe to bump the exhaust valve open. The centrifugal weight you can see flips the pressing out of the way above 600 rpm. (Note this camshaft is from a GXV120 - the only small Honda that uses a metal camshaft, AFAIK.)

Because most engines with decompressors use a method something like this, you can check whether they are working on OHV engines just by removing the rocker cover and watching the exhaust rocker while slowly pulling the starter rope. There is an easily-visible "bump" movement of the rocker in the middle of the compression stroke. This verification that the decompressor is working may be useful to people with Intek engines, since the decompressor on those does not work if the exhaust tappet clearance is as little as 0.001" above maximum specification..