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Old Faithful brewing in Mag Cylinder

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  • Old Faithful brewing in Mag Cylinder

    Greetings everyone, and thank you for having me as a member. I can't tell you how many times I've used this forum on my quest to revive a couple of fun machines and share those good times with my kids. For all the information I've use so far I say thank you.

    Short background, I'm currently working a 2 -1996 Polaris SL780 machines. The main machine was actually purchased new by my family in 1996 and the other one was acquired about 2001. Both of the machines were handed over fully to me and my kids just this year.

    I have been the main rider and mechanic of the original machine it's entire life cycle and it has always been a great running machine with zero issues (except for the grips, MFD, and fuel fill neck way back in the day). Around 2004 I did install an OceanPro head, a TDR water box and a Skat-Trac impeller (can't remember the pitch at this time) and I ran those mods for a very long time with zero issues, and fantastic results in performance. The other machine was picked up by my father around 2001 and it had some carb issues with the midrange bog, but otherwise ran good. "Dad's" machine is stock except for the impeller (same one as used on the "original" machine).

    Both machines were sitting and had not run for about 6 years. This year I decided to take them out of retirement and share them with my younger kids as something to do during this hot summer.

    Before they even saw the water I went through them pretty extensively. I pulled the head, carbs, and reed cages off of mine to check the pistons, cylinders, and crank for rust and wear. Everything looked band new. so put it all back together. I took the carbs and cages off of "dad's" to look at the crank and put a scope down the heads to see the pistons and cylinders (Note I did not remove the heads, This is important later). Again everything looked great. I took the PTO covers off and gave the crank a spin and both machines moved freely and smoothly. Next I removed the exhaust pipe (not the manifold) from both machines and left the carbs off and did a leak down test. Both machines passes 3 separate leak down tests with 2 different gauges with only loosing 0.5 inHg per test. So seals and gaskets seem to be good. Next all the fuel and oil line were replaced (oil pump primed) and a full carb job for both machines, with new everything, pop off pressures tested (passed with correct black spring installed) and dry land setting set.

    After about 2 months of odd nights working on them it was time to test. I put my machine in the lake and did a 10 minute low speed pass to check the low speed carb setting on the spark plugs. When I started to pull the spark plugs out to check everything looked "ok" (little lean) on the PTO and the Center cylinders. When I got to the Mag side, there was a little geyser of steam coming out and you could hear droplets dropping onto a hot surface and vaporizing into steam. About 1 drop every 2-3 seconds. Figuring that I tried to cheat and reuse the head gaskets (after cleaned and coppered) the heads on that side did not seal. So I loaded it up and headed back to town without testing the other ski. (I did fog the cylinders and crank case incase there was water) Ordered up new gaskets and installed. Took both machines out to the lake again, tested my machine at low pass, and the same geyser again. Thinking the head gasket material hadn't gotten hot enough to seal, took it out for a high speed test. The machine ran great, except I dry fouled a plug on the run, as confirmed when I pulled the plugs when I got in and the center plug was black and dry (a little too rich), but I had the same geyser again where you could hear droplet hitting something hot and flashing off. Frustrated again I took dad's machine out for a test run, and low and behold after the low speed test, the MAG cylinder had the same water droplets vaporizing inside at pretty much the same rate. Now remember One machine has brand new head gaskets and the other was not opened at the heads.

    Does any one have any theory? I put a scope down the heads last night and there are no holes in the pistons. I checked compression, and on dad's all 3 are at 140 psi and on mine 165 psi (OceanPro Head).

    My only thought at this time is there is an internal exhaust gasket leak between the exhaust port and the water jacket of the exhaust manifold. Anybody have any other words of wisdom or experience with this issue? Both machines ran great and you couldn't tell by running them. As for seeing steam in the exhaust, it would be hard to tell because I'm pre-mixed 40:1 in the tank as well as oil injected for the first tank because they sat for so long and I did carb work.

    Sorry for the lengthy post but I wanted to get all the history out of the way and skip over all the stuff I already have done in the first post and to get down to the brass tax of the issue. Thanks everyone.

  • #2
    Welcome to the Hulk.

    My first impression when you say you hear sizzling, I assume exhaust. There is a small orifice inside the brass fitting at the top of the exhaust pipe. That inject water into the exhaust stream to keep the rubber parts cool. Check your expansion pipe to water box rubber connection for blistering, and the water box to hull exit hose for squishyness. When they get hot, they got soft.

    Many times when you remove the spark plugs, you will see steam/condensate coming out of the cylinder that has the intake port exposed. It's possible you saw that, or maybe it was steam coming from the exhaust. But I would think if it was water dripping inside the combustion chamber and sizzling on the cylinder or piston, you would know something was wrong with that cylinder being so hot. A lot of times the paint will start to discolor when it gets that hot.

    Another sign of water intrusion into the combustion chamber is a completely clean piston. The water actually steam cleans the piston domes. Are your MAG pistons clean while the others are carbon coated?

    Please report back
    My ski says made in the U.S.A...... Can yours say that???


    • #3
      Thank you for the injection suggestion. I'll definitely take a look at it and the hoses.

      My observations still make me wonder if it's in the cylinder. The sizzle comes from inside the cylinder and a corresponding "puff" of steam is released out the plug hole with each drop. So the drop has to be forming at or below the plug hole. Any thing above would go up the pipe? If it is coming out the exhaust where is it coming frome? Aren't the exhaust gas and water cooling chambers separated? The other baffling thing is both machines are doing the same thing, so if both engines stopped in the exact same location to have an open exhaust port on the same cylinder, I need to go buy a lottery ticket.

      As for the pistons 5 of the 6 are Carbonell coated. There is one mag piston that has a clean "spot" about 3/16 inch wide and 1 inch long on the exhaust port side. It starts right at the piston edge. The rest of the piston top is carbon coated. If I get a chance I can try to tank a picture with my bore scope.

      Thank you again for any help it is much appreciated.


      • #4
        If the sizzle is coming from inside the combustion chamber, it would have to be a head gasket leak, or a cracked cylinder. I still would find it hard to believe the piston or cylinder is so hot that water would sizzle off of it. Unless you have a cooling problem. If the cooling system is working properly inside the engine, it should not get that hot. Every time I've ever seen water sizzle it was on the water box due to a cooling issue.

        You have a "wet pipe" exhaust. The brass fitting at the top of it, has a hose that injects water into the middle of the pipe. The water cools the exhaust gas and alters the speed of the returning shock/sound wave. Plus it also cools the rubber parts.

        On a "dry pipe", water and exhaust gas are separate until the end of the expansion pipe. I believe that part is called the "stinger". This is where the water mixes with the exhaust gases and helps cool the rubber parts.

        It's not really uncommon for the engines to stop at the same spot. You only have 3 cylinders, so it's a 1 in 3 chance of the MAG cyl exhaust port to be open.

        Regardless, if you have something that is sizzling, you have a cooling problem somewhere. Check the filter screen at the jet pump, check the openings inside the jet pump venturi, and check your fitting and hose at the top of the expansion pipe for obstruction.

        If the skis sat for a long time, it's not unusual for spiders or mud dobbers to make nests inside openings and cause blockages.

        Pictures would help. Usually the whole piston dome is clean when water is introduced to the combustion process
        My ski says made in the U.S.A...... Can yours say that???


        • #5
          Looking at the Polaris parts breakdown and looking at my machines it looks like there is a water inlet into the exhaust manifold right at the exhaust outlet on the jugs that is the main cooling supply for the engine if you follow the arrows on the image below, and looking at the exhaust manifold there appears to be a water jacket around the exhaust gas chamber. That this why I was going down the path of the exhaust gasket between the manifold and the jug. (Sorry the one image is from ebay but it's the same part number as what I have).

          As for the internal temps, we run a lot of Polaris Snowmobiles (some domestic twins and a couple of Fuji triples), and we are tuning exhaust temps 4 inches after the piston skirt to be around 1150 F at WOT with EGT probes, so I was assuming these being a 2 stroke of the same nature they would have similar exhaust temps out the exhaust port. Again this was why I was thinking water jacket around the exhaust manifold, leaking water into exhaust chamber was causing the flash off out the cylinder plug hole.

          As always, I more than grateful for the information and I always love to learn new things. Especially when it involves internal combustion and the art of going fast.
          Attached Files


          • #6
            That is a small water feed going into the exhaust manifold. (It's a 3/8 fitting "T'd" off the 3/4 connection) I do honestly believe that has more to do with the "self draining" part of the system. The main water feed actually goes into the lower part of the expansion pipe. Then into the exhaust manifold, through the cylinders, in the cylinder heads and out the water bar.

            I'm sure the exhaust temps would be the same temp as any other 2 stroke engine. But as you noted, everything is liquid cooled. Nothing but the exhaust gases should be that hot.

            Were any parts of the engine hot to the touch? Nothing should be.
            My ski says made in the U.S.A...... Can yours say that???