Upper Rebuilt

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I had to rebuilt the upper portion of the engine on the vmax 600.  It is common to have to rebuild a 2 cycle engine after an extended amount of use, unfortunately the rebuild for this one came prematurely.  After a bit of investigation the reason for the engine burning up was 2 fold.  First and foremost it wasn’t getting coolant.  After 8 years of sitting…I should have dropped the fluids and refilled them.  Clearly my fuck up.  Also, I do believe it was a bit starved for oil.  That alone wouldn’t have caused it to burn up as fast as it did…but both together and BLAMO busted sled.

I checked things out and I would need a piston kit, gaskets and a replacement jug.  I could have “honed” the jug maybe, but there was a pretty good gouge under the exhaust port.  I’m not sure how honing would have affected things…so a new jug was the safest bet.  I also needed to order the rod bearings from another company because they were not included in the piston kit.  All in, $220 for the piston kit and gaskets, $150 for the jug, $45 for the bearings.  I estimated about a day worth of time to get things tore down and slapped back together.

The wife and kids took off for Fargo for the weekend on Thursday, so I did what any guy would…took the day off Friday to enjoy the quiet.  I spent that day off in the garage working on the sled, where I would have been alone even if the women were home…whatever.


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Earlier I had take the head off to see what I was working with.  That’s how I saw the gouge mark in the center cylinder.  I should have tore it down enough to inspect the crank…but I didn’t want to even know if that was bad.  A new engine would have been about 700 or so.  I can’t justify dropping that kind of cash on something that is purely a toy.

The first order of business was to pull the jugs and pistons off.  Getting the jugs off was a little difficult due to the exhaust manifold.  There are 3 bolts on the bottom side of the manifold that need to be pulled out.  Unfortunately, with the angle the engine sits you can’t just crank them out.  I had to unbolt the engine and lift it a little bit to get at those bolts.  Kind of a pain in the ass.

Popping the pistons out wasn’t near as bad as I expected.  Normally you’d have 5k miles or so on the engine before a rebuild (barring some oil/cooling fuck up) so things might be a little more stuck together.  For mine, it wasn’t bad at all.  You pop out one of the surclips, then find a socket that fits in the hole and just apply some pressure.  They came out a lot easier than I expected.


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Once you have the piston out, you can easily see the damage that was causing our problem


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Below is what a piston should look like


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This is where I ran in to a bit of a problem.  The rod bearings that I’d ordered from dennis kirk, the ones that are “guaranteed fit”!  Where too god damn wide for the pistons I got.  They fit in to the rods just fine…but would not fit inside the pistons.  Comparing them to the ones I took out, they are a couple mm wider.  I lucked out in that the old rod bearings that I had were in really good shape, so  oiled them up and reused them.  Not the best practice, but I would have had to wait at least 2 more days and not be “guaranteed’ they would fit.  I could have tried to run to town and get some, but that would have been a pain and cost me at least an hour.  I coated the bearings and the pins with a good amount of oil and, surprisingly enough, everything slipped together like it should.


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After the pistons are clipped in place, I need to slip the jugs over top of them but keep the rings in the proper spot so they can expand as necessary.  It took me a few min to figure out how to do this with the first one, but I saw that you can squeeze the rings together and then just work the jug down on the piston.  I did put a coat of oil on the inside of the jug and with some patients…it did slide together as it was suppose to.

After all 3 jugs were on, it came time to bolt the manifold back together.  The 3 bolts that proved to be a pain coming off were virtually impossible to put back on.   I decided to skip them.  The top bolts are holding it together for now, and it will finish out the season.  At some point I might have to put them back together like it’s suppose to be..we’ll see.  As always, I’m sure if there is no evident problem I won’t worry about it.



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Now the moment of truth.  I was ready to fire it up.  I pulled…and pulled…and pulled.  I didn’t think the god damn thing was ever going to fire.  I put some gas down the cylinder, and she fired…but it sounded like shit and idled really high…then she backfired and died.  I pulled my guts out again, and she finally fired but sounded like it was only running on 2 cylinders.  After pissing around with this for a bit, she finally ran and sounded fairly good.  I swung it around and ran it around the yard for awhile.  She sounds good, but the idle is so high, around 3k-3.5k.  That’s about 2x what the idle should be.  I ran it around a bit to make sure things functioned properly…it seems like it does but man…that high idle.

I shut things down for the night and decided I would try to get a tank of gas through it the next day.  That will tell me if it’s using oil, and it will give the new pistons and rings time to “break in”.  I did a little bit of research and found that the high idle problem can be caused by the carbs not being seated properly and some air leaking by.  This can be a huge problem because it can thin things out and burn up the engine.  I’ll look at it in the morning and see if I can find an air leak.  I’m hoping so…because that would be nice and easy.

The next day…

I pulled and pulled, but the god damn thing didn’t want to start.  I finally had to pour some gas down the cylinders to get it to go.  This isn’t a good sign.  Once it did go, blamo 3.5k RPMs.  I checked the carbs, everything seemed seated properly so I figured it had to be the exhaust manifold.  I created a few more joints in my arm and somehow got the lower bolts in and tightened everything down.  Fired it up, same.  I took the carbs off, adjusted the mix screws to their 2 turns, they were about 2.5.  I made sure all the boots were screwed down tight, no air gaps.  Fire it up…same.

Finally I took a flier and disconnected the throttle cable.  Now it wouldn’t start…hrm.  I used the idle screw to open up the throttle a little, tried it again and she took off.  I used the idle to dial it in to about where it should be.  Clearly the throttle cable was the problem.  I checked and it’s extended as far as I possibly can.  The only option I could see was to bend the bracket that holds the cable.  I did a bit and fired it up, now everything seems to be right in sync.  I did some fine tuning with the idle screw and she’s right where I’d like her to be.  Spec is 1600, I set it about 1900.

I rode her around a little bit and everything seemed to work alright.  I only went about 1/2 throttle or so for the ride, I want to give it some time to break in the new pistons.  After my ride I checked the oil reservoir and to my surprise some was going.  It must be using some.  I’ll still run oiled gas for awhile just to be sure.

At this point of the year, the snow is going to disappear very quickly.  I’ll be lucky to get 1 decent ride in.  This was more about a battle of wills.  The sled said it wasn’t going to run…I said it was!

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