Went from 20 below zero to looking like we will get 18 inches of snow tomorrow up here in Wisconsin so I guess I will spend some time in my little shop working on some projects. I got a hold of a 94 400L that the owner no longer wanted. He had it at a shop and they put new starter, battery and head gasket in it because he couldn't get it started. They slobbered the recoil and starter up with red rtv when they put it back together and gave it back to the guy. He said he could now get it started but it started hard. He ran it for a bit and it started making noises and after awhile it died completely. He gave up on it so I ended up with it for a deal I made on a sportsman with a plow that I fixed up. Thought since I didn't have anything into it, this would be a good test mule for a total rebuild on old 400 since I never done much on these other than top ends and rebuilding the water pump. I figured with 5000 miles on the odometer, the bottom end should be loose and counterbalance should need attention. Pulled the plastic and the clutch and amazingly the crank was tight and spins freely. I pulled the head and jug and found out why she gave up. The notorious fragile 400 piston. Rod was good with no play but for the life of me, I can't find any pieces in the bottom end. Where the heck did all this go and shouldn't it have beaten up the cylinder? I should have took a picture of the cylinder but I left it at the machine shop yesterday. It was smooth with no gouges or ridge on the top. The guy at the machine shop measured it and said it was still in tolerance for the factory piston but would still recommend a bore and new piston. I also got a new cage bearing for the rod while I was at it even though it was tight. I spun the crank a hundred times and looked all over but can't find any pieces in the bottom end. I blew it out and rinsed it with the plug out but nothing but a little oily crud and two stroke oil came out. Is this odd? Shouldn't the piston pieces be somewhere? Since the rotating assembly rolls smoothly and is tight and the counterbalance is good, I guess it's going to just get a top end. I was kinda hoping to finally do the lower end on one of these just to actually get the experience of doing it. I thought with that many miles, it should be more worn than it is. The machine is a 94, but the tag on the recoil says 95. I just chalked it up to someone replacing the recoil at some point.

Is this common when a skirt breaks that the pieces pass through or does it normally trash the bottom end? I have done quite a few V8 engines in cars and normally found small pieces of piston material in the crud in the oil pan when tearing down for a rebuild but I am puzzled on it happening on a two stroke and not busting the thing in half, lol

 

Probably spit a few pieces straight out the exhaust little by little before it has a chance to pepper the head or trash out the bottom end completely.I've found the same thing before. I'd still flush that bottom out throughly and if in doubt you know the routine by know.. A new rod kit,crank bearings and seals,water pump seals,etc. Those cast pistons were a joke. Plus 20 Below Zero... I complained when we had a couple days of around 18 degrees at night..

 

Thanks OPT,

20 below wasn't bad as long as there wasn't any wind. I got stuck responding to a head on crash one morning at 32 below and the hydraulics on my wrecker darned near stalled the big CAT diesel in my wrecker when I engaged the PTO, lol.

Maybe this thing just broke away in little pieces over time. Kind of a lot of piston to be missing to not cause any signs of damage. I looked and looked but there are no marks on the crank at all. I have a small tube I can attach to a vacuum to suck things out into a bag and nothing was there either. Flushed the bottom end out with some diesel fuel and only the normal crud came out. I sprayed it afterwards with some wd40 to get the diesel out and I will check it again for any other signs of material. Maybe I just got lucky on this one, lol

Thanks,
Mike

 

Funny thing too on some off these pistons that shatter the skirts is that many can still have a good compression reading if the rings haven't stuck. I've found a few that would show decent compression,idle ok,but would blubber and wouldn't take throttle. With part of the skirts missing they couldn't compress the fuel,air,oil mixture good enough under a broken piston.Plus that screws up port timing,etc without a skirt.This would drive some people nuts as they would head for the carb,change jets,etc and still couldn't figure out the problem. I learned when they brought machines like this in for me to check out, I'd pull the reed block and exhaust first. That's one way to find the problem.

 

I got my cylinder back from the machine shop along with a new piston that had the rings set up for it. I lubed everything up good and put the piston in and slid the jug and head on. Filled up the coolant and bled the head. Filled the tank with a couple gallons of premix gas. Rebuilt the carb and put it all together. Cranked it over and she popped right off. Ran good and no leaks. Very responsive. The question I have now is I went to check the oil pump. I pulled the line off at the intake and there is oil in the line but not really coming out at idle. I tried to rev it a little but it dribbled a bit but not steady, mainly because I wasn't opening it up much. How do I make the pump think it's WOT without actually pinning the throttle? On the 250's and 300's I can actually lift the pump lever up, not really on this 400. I have to pull the plate anyways to make sure it's set right anyways.

Thanks
Mike

 

Have to remove the pump cover plate and turn the wheel up all the way and hold it to have the pump at it's highest rate. You should tell fairly quick if it's pumping ok. Make sure the cable ends stays in the slot when you release it. You can see the mark on the pump wheel and the mark on the pump body. As is it's a tad rich,but wouldn't hurt anything.