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-   -   2 stroke compression question (https://www.offshoreonly.com/forums/general-q/367651-2-stroke-compression-question.html)

sailtexas186548 07-24-2020 01:05 PM


Originally Posted by underpsi68 (Post 4749844)
Picture with 2 plugs is day that problem started. Picture with single plug has 3 minutes of run time. Changed both plugs twice and fouled right away. I have to find other plugs. Top cylinder was definitely worse than bottom.
https://cimg2.ibsrv.net/gimg/www.off...61f0fb8012.jpg
https://cimg3.ibsrv.net/gimg/www.off...85f0463f57.jpg

I was thinking issue with spark as well? It does spark but how do I test if the coils are working correctly?

bottom plug looks a little steam cleaned, maybe leaking HG. When they get old they can get tiny pin holes and start passing water

mike tkach 07-24-2020 01:15 PM


Originally Posted by sailtexas186548 (Post 4749861)
bottom plug looks a little steam cleaned, maybe leaking HG. When they get old they can get tiny pin holes and start passing water

i agree,a little water is getting past the head gasket.a new gasket should fix the problem.

underpsi68 07-25-2020 12:08 PM

I have a new hg and thermostat ordered. Hopefully they will be here the beginning of the week. I'll reassemble and do a leak down test.

I am also running an aftermarket set of reeds. I'm going to put the stock reeds back in as well.

I'm just curious why the compression is low?

Thanks for all the help everyone!!! I'll report back after I run it.

mcollinstn 07-25-2020 01:45 PM

I'm not sure why none of your photos show BOTH cylinders, but what little I can see shows one plug looking like it is brand new and the other looking pretty typical for a mid-hour 2 stroke plug.
Wish I could see both piston crowns and both combustion chambers.
My guess is you overheated it and got water in the cylinders.

The cheap route would be to run a flat file across the cylinder deck and the head, pit on a new head gasket, torque it in the correct sequence to the correct torque, and then with plugs out and throttle open - recheck compression. If its still under 115 you will be looking at total disassembly. If compression is back up, inspect/replace water pump impeller and make sure there is no debris in the thermostat area. If you don't run in cold weather, you can remove the thermostat completely. Check the water "pee stream" frequently Anniston for changes in the sound of the exhaust (low water flow usually changes the exhaust note).


underpsi68 07-25-2020 02:53 PM

This is the top cylinder
https://cimg5.ibsrv.net/gimg/www.off...852e549eec.jpg
https://cimg0.ibsrv.net/gimg/www.off...48e6731d9d.jpg
https://cimg1.ibsrv.net/gimg/www.off...ab36373100.jpg
https://cimg4.ibsrv.net/gimg/www.off...dce54ba08b.jpg

underpsi68 07-25-2020 02:56 PM

bottom cylinder
https://cimg6.ibsrv.net/gimg/www.off...864285c2e9.jpg
https://cimg1.ibsrv.net/gimg/www.off...891cfd63b9.jpg
https://cimg8.ibsrv.net/gimg/www.off...cdb0659c49.jpg
https://cimg4.ibsrv.net/gimg/www.off...c2e4f893ad.jpg

underpsi68 07-25-2020 02:59 PM

I have to find the old plugs. After installing new plugs they would come out all black/glossy instantly. The bottom plug wasn't as bad, but still fouled. I'm thinking the water was cleaning the plug off.

underpsi68 07-25-2020 08:00 PM

Well the plot thickens.

I removed the plate on the side of the cylinders to take a look. I found a piece of aluminum that corroded off the exhaust port wall that was blocking the flow. Water was leaking into one cylinder.

Looks like it not worth repairing. The cylinder itself is $1200 to replace.

I really don't want to go to a 4 stroke because of the weight. I don't have a trailer for the raft and carry the engine from the basement to the truck. This engine weighs about 80lbs which is about the limit of what I want to lug around. A 15-20hp 4 stroke is about a 100lbs. Problem is all the 2 strokes are getting older and could be the same issue.
https://cimg7.ibsrv.net/gimg/www.off...049b090937.jpg
https://cimg0.ibsrv.net/gimg/www.off...4597fad013.jpg
https://cimg2.ibsrv.net/gimg/www.off...545d93aff4.jpg

mcollinstn 07-26-2020 02:15 AM

Yes, you are right.. Nobody makes ultralight 2 stroke outboards anymore. It's a shame. For what you describe, I would look into a powerhead or block.

Complete powerhead -
https://www.ebay.com/itm/Yamaha-Outb...-/254663384100

Above motor may have salt rot in it. I suppose you could ask the guy to pull off the exhaust chest cover and send pictures...

Or, here is a very pretty bare block. You might want pictures of the cylinders..

https://www.ebay.com/itm/15HP-63V-15...-/273383360024

oconnor marine 07-26-2020 10:48 AM


Originally Posted by underpsi68 (Post 4750049)
Well the plot thickens.

I removed the plate on the side of the cylinders to take a look. I found a piece of aluminum that corroded off the exhaust port wall that was blocking the flow. Water was leaking into one cylinder.

Looks like it not worth repairing. The cylinder itself is $1200 to replace.

I really don't want to go to a 4 stroke because of the weight. I don't have a trailer for the raft and carry the engine from the basement to the truck. This engine weighs about 80lbs which is about the limit of what I want to lug around. A 15-20hp 4 stroke is about a 100lbs. Problem is all the 2 strokes are getting older and could be the same issue.
https://cimg7.ibsrv.net/gimg/www.off...049b090937.jpg
https://cimg0.ibsrv.net/gimg/www.off...4597fad013.jpg
https://cimg2.ibsrv.net/gimg/www.off...545d93aff4.jpg

clean real good with brass brush and apply marinetex. Let cure and flat file. Reasemble with new gaskets


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