Compression test results in relation to reed valves

[Beno]

years back I was working on a junky Polaris and I was getting low compression I ended up tearing the motor apart trashing the ski only later on to think oh crap . I didn’t have the throttle open…  Well of course it would have had low compression. I was creating somewhat of a vacuum in the crankcase.. and decreasing the mass of air going into the cyl…

Now I’m older… Making newer mistakes

I was just going through in my head over the volume of mass in the crank during intake pressure cycles relates to the compression cycle..

This lead me to this interesting hypothesis.. if you had a bad reed valve. The crank/intake chamber would not correctly pressurize (clearly).. this would lead to a drop in the amount of air and fuel matter going into the cyl.

Therefore in a multi-cylinder system.. a bad reed valve should show up in a compression test somehow?  Or does it really not work that way…..?

 

 

[seakaye12]

Questions about reed valves rank right up there with questions about oil mix and brand!  It’s an interesting question you pose there;  I imagine we will get a lot of theory.  Looking forward to the responses!

 

1 user thanked author for this post.

frankr

[frankr]

Beno, you think too much. Not really.  But you are missing some important facts.  For one thing, you are confusing 2-stroke and 4-stroke basics.  But rather than get into all that, I’ll just cut to the chase and answer your basic question:  Would a bad reed valve affect a compression TEST?  Answer is no.  Reason is, on a 2-strike the cylinder is not filled with air by sucking it in through the intake manifold.  It is filled with air by a pressure charge in the crankcase, caused by the downstroke of the piston.  I think you already understand that and are correct in assuming that the pressure charge depends on the reed valve and everything before that.  But let’s take it a step forward and consider what happens at that critical moment when the air charge is admitted to the cylinder. When the piston nears the bottom of its stroke the exhaust ports open first.  This allows the previous power stroke’s gasses to escape, unless of course the exhaust outlet is blocked.  THEN the intake ports open and a puff of air from the pressurized crankcase enters and part of it blows through the cylinder. This purges the cylinder of the remaining exhaust gasses from the previous stroke.  So, what is in the cylinder pressure-wise at this bare moment in time?  Zero (atmospheric) psi. Or possibly a mere trace of pressure from the exhaust system. By now, the piston has started back up on the compression stroke compressing that atmospheric air pressure to whatever.  and you read the resulting compression test on your gauge at its peak.

So, what happens if no crankcase pressure if a bad reed valve?  Nothing.  The piston is still starting its upstroke with zero pressure.

3 users thanked author for this post.

vintin, labrador-guy, The Boat House

[Beno]

gotcha… well. sounds right.

I have a 120hp merc/Force I’m going to be pulling some reed valves for inspection when the heat relents.
and what I’ll do is to compression test before and during..

If the reeds make zero difference to compression we will be able to confirm the reed effect!

should reeds be touching the base at rest? or should there be a hairline gap?

 

[crosbyman]

the downstroke normaly  refills the cyl  cavity with  air  or  air/fuel to be compressed. …but keep in mind that at the same time  the exhaust ports are also open to atmospheric pressure   and whether or not  air   or air-fuel    actually comes from the downstroke…… the cyl would be  filled anyway  with air or exhaust fumes  if a fresh load never entered via the intake ports….be it air… air-fuel

the upstroke would pressurized whatever    was in the cyl …. regardless of  reeds being good or bad.

Joining AOMCI has priviledges 🙂

[Fastwin18]

So throttle open or closed would not matter as well then?

[crosbyman]

that is how I understand it

Joining AOMCI has priviledges 🙂

[Author]

Posts