4-stroke: How Much Compression?
 

Bought a used Magnum 52 FS. It starts okay, but, sure "spins" easy (compared to a 2-stroke) when starting. It doesn't offer much compression resistance when turning over by hand. Is that "normal"?

RE: 4-stroke: How Much Compression?
 

Compared to what type 2-stroke?

A 4-stroke should have more resistance to being turned over than the same displacement RINGED 2-stroke. An ABC engine when cold would be harder to turn over than the 4-stroke. ( I know of no tapered sleeve ABC 4-strokes... but I haven't seen all engines out there by a long shot.)

Its qute pssible you bought a worn out engine. Its also possible it needs valve adjustment and would be fine. (Its possible its just fine as is...) You need someone familiar with 4-strokes to look at it in-person.

RE: 4-stroke: How Much Compression?
 

If you turn it over with the throttle closed, it will show fairly low compression too.

RE: 4-stroke: How Much Compression?
 

I thought some FS's are manufactured with a ? ring and this shows little compression when not running. I have an ASP 61 FS that turns easily by hand with very little resistance, but runs just fine. The proof is in the running.

RE: 4-stroke: How Much Compression?
 

Yes some 4-strokes have rings. Not familiar with the Magnum.52, so I can't say if its ringed without looking it up. And yes, ringed engines seem to have low compression sometimes when they have no problem at all.

But a ringed 4-stroke should still have more compression than a ringed 2 stroke...

RE: 4-stroke: How Much Compression?
 

Why?

Ed S

RE: 4-stroke: How Much Compression?
 

I thought all 4-strokes have rings. I know all of my YS and OS 4-strokes have rings.

RE: 4-stroke: How Much Compression?
 

The engine is likely OK. I bought a new Magnum 52 FS and it had very little static compression. It started and ran fine. The lack of compression still concerned me so I packed it up and sent it back to Magnum Service Center. They returned it with a test report that said there was no problem. It is a ringed engine and has never had much static compression. As long as it starts and runs OK, don’t worry. Since then I have acquired a Magnum 30 FS and an ASP 30 FS, neither of which have much static compression, but start and run well. I finally don’t sweat it as it must be normal. All of the engines idle slow and smooth and turn up at full throttle within the engine specs. On the other hand, my Saito 80 has real good compression.

RE: 4-stroke: How Much Compression?
 

At cranking speed it leaks through the ring gap. At running speed it doesn't have time to leak through.

RE: 4-stroke: How Much Compression?
 

i own 2 and have seen many...i've never had or felt one that had any static compression