Old outboard compression

[djeady]

Anybody have any idea what the compression should be on an old 1.8 hp Eaton Viking/Elto Ace outboard. Can't be too high with the poppet style carb, but it seems really weak.

 

[F_R]

Sorry, I don't know what the number should be, but just sayin' the poppet has nothing to do with the cylinder compression.

 

[RCO]

I have never checked compression on that particular model, but most of the 30's-40's motors I have played with will run at 60+ lbs, rare to see one hit 80 lbs . What is your compression at now?

 

[djeady]

Couldn't get the gauge I have to read anything. Finger test says probably less than 10 lbs.

Re the poppet style carb, when I press the prime button which raises the poppet, I have zero compression. So I think there must be a relationship.

 

[F_R]

Obviously 10 is not enough.

I'm not going to get into the old argument about reeds, throttle position, etc again.

 

[djeady]

Sorry if this is a dumb question, but are there really reed valves on a 1940s motor - my guess is no. I'm also not sure how throttle position would affect compression, but I am open to being educated.

 

[gm280]

Sorry if this is a dumb question, but are there really reed valves on a 1940s motor - my guess is no. I'm also not sure how throttle position would affect compression, but I am open to being educated.

djeady, I honestly don't know. However, If there were, you would be able to see them behind the carb. That is where they usually are located. I will say with an engine that small, they could have also used a ported crankshaft to supply the fuel into the cylinder. Most all 2-cycle model airplane engines are built that way. And some of them have more HP then yours as well. :noidea:

 

[djeady]

OK, guess next step is to strip it down and see what's going on. Definitely nothing behind the carb, and I looked at a diagram of an Elto PAL which is a similar motor and it looks like it just uses engine vacuum to draw in fuel.

 

[ondarvr]

They all use engine vacuum to bring fuel into the carb, that's the only thing that does it. The reed valve just stops the fuel and air from blowing back out of the carb on the downstroke of the piston.

 

[djeady]

I've looked at the diagrams of similar motors at http://aomci.org/forum/viewtopic.php?t=157 and I don't see anything like a reed valve or other mechanism. I think the poppet is what stops the blowback in this engine design.

 

[F_R]

There are no reed valves on that engine. The poppet valve does the job.

Back to what I said, every time a similar question comes up, a big argument starts about the effect of reeds, whether the throttle needs to be blocked open, and a host of other nonsense. The fact is, a two-stroke begins its compression stroke with the intake and exhaust ports wide open to atmospheric pressure. Unlike a four-stroke.