SL900 running lean on one cylinder, trouble finding problem..

[javiert99]

I've got this posted over at pwctoday as well, but any additional help here would be appreciated.

Some may remember my 1997 SL900 blew a hole in the front piston a few months back only a couple weeks after purchasing it. After reassembly of the top end and installing the reed cage and intake manifold, a pressure test revealed that there were some major air leaks around the reed cages.. apparently the factory oring was no longer sealing very well. Used some anerobic, fuel resistant gasket maker (518 i believe) and sealed it up, no more air leaks. I believe the air leak is what caused the MAG piston to begin with.

The ski prior to the piston failure never could get past 5400 rpm or so, and had a mid range rpm stumble. Never got a chance to diagnose before she blew.

Got her back together and she runs pretty much the same as before, no go past 5400 and still has that stumble. After some break in runs I took her home, removed the carbs, adjusted them to 3/4 (just a tad rich over stock 5/8") turn on the low mixture needle (no adjustable high on this model) and checked out the carbs, all clean, parts appear to be new, and the needle seat springs all match that of the one in my genuine polaris carb rebuild kit, both in height and in coil windings.

Took her out again, same crap pretty much.

Took the carbs off again tonight and checked to make sure the correct 140 main jet was in place, correct jet is in there. Took the carb and cleaned it out pretty well after disassembly, but honestly, all the components appear clean, looks like they were rebuilt shortly before my purchase. I removed the fuel transfer lines b/w the carbs and those are clean as a whistle, next I'm goign to remove the accelerator pump lines to see if I can find anything in those.

I'm running out of things to look at in this, can anyone think of anything else I should take a look at?

I have spark in all 3 cylinders, I'm currently running a tank of 32:1 premix, once that's done I'll be going back to 40:1 (eliminated oil pump during the reassembly).

Here's my plug reading indicating how I found my lean condition after Sunday's ride:



Another photo:

[dank]

Re: SL900 running lean on one cylinder, trouble finding problem..

That center carb is for sure not working properly or there is a major leak on the manifold/reed cages or base gasket. Check all those areas for signs of leaking. A simple test would be to get some carb cleaner and while at idle spray some around the carb base/intake and the cylinder base. If the idle changes then you know you have a leak. Do it in a well ventalated area and don't spray a lot.

Those are CDK-II carbs and have a small screen on the bottom of the needle and seat. You will have to remove the jet block to get access to it. Make sure it is clean. I had a SLTH with the same carb give me trouble since it was only partialy blocked and would idle and accel OK but would not reach top speed and ran very lean. The jet block has one screw in the center that holds it onto the carb body. Make sure to get a rebuild kit for the CDK carbs since the old gaskets don't seal very well after they have been used.

BTW: not the accel pump.

[javiert99]

Re: SL900 running lean on one cylinder, trouble finding problem..

Well, I pressure tested the engine w/ the intake manifold attached, no leaks there as the engine held pressure for quite a long time w/o any drop whatsoever. I actually pressure tested it twice on two different days to make sure. A carb base leak is a possibility, but there are new gaskets in place and both mating surfaces are nice and clean (as clean as the other two). I'll put the carbs back on and spray the carb cleaner as you suggested.

I did have the jet block apart and removed the screen from the seat, all looked clean, although I did blast some carb cleaner through the whole jet block. I'll go ahead and replace that oring now as well.

If it's neither of the above since I've kinda already gone through those, what else could malfunction within the carb to go so lean? I'm lost!! Thanks for the suggestions so far..

[javiert99]

Re: SL900 running lean on one cylinder, trouble finding problem..

Things are pointing to a possibly out of index crankshaft. I'll be doing some measuring next week once I receive the tools I need (thanks to a kind board member on another forum) and I will know for sure.

If that's the case I'll probably be giving SBT a ring after all for a premium 900 engine.

Anyone know if SBT is open on Saturdays? Since I live relatively close by (about 2 hours) I wouldn't mind driving up there to pick up the motor on a nonwork day.

[javiert99]

Re: SL900 running lean on one cylinder, trouble finding problem..

Update!

It appears before I purchased this craft the crank was rebuilt by some clueless individual. There is grey gasket sealant/maker that oozed out b/w the crankcase halves at some point, from what I understand, a factory motor would not show those signs.

Anyways, it appears the crank was reassembled out of phase.

Front (MAG) cylinder at TDC, degree wheel set at 0*.

Turn motor counterclockwise 120* and the rear PTO cylinder comes to TDC instead of the center. Turn it another 120* and you end up with the center cylinder at TDC. It's backwards for some frickin' reason. From what I understand this is how a FUJI motor should be, not a domestic. Looks like someone thought they were reassembling a FUJI.

Anyways, just thought I'd give a heads up as to what this problem finally turned out to be, a misassembled motor. I'll be calling SBT this week probably :)

[Technical Support]

Re: SL900 running lean on one cylinder, trouble finding problem..

I don't recall the firing order off the top of my head - but that would be the first time I've heard of that happening! Glad you found the problem though, I wouldn't have even thought to look at that.

[javiert99]

Re: SL900 running lean on one cylinder, trouble finding problem..

I don't recall the firing order off the top of my head - but that would be the first time I've heard of that happening! Glad you found the problem though, I wouldn't have even thought to look at that.

Was wondering if you could find the firing order by chance. I'm 99% sure this is the problem. Polaris manual indicates turning the crank counterclockwise while doing index check and that there should be a 120* turn between the front and center between their TDC's. Right now it's a 240* turn to get from MAG TDC to center TDC.

Weird huh?

[Technical Support]

Re: SL900 running lean on one cylinder, trouble finding problem..

I can check Monday morning when I go back to work, but I'm sure someone else can verify between then and now.

[ShowMePro1200]

Re: SL900 running lean on one cylinder, trouble finding problem..

Pardon my ignorance here (and it will show probably), but if you have two pistons out of phase (reversed as in your case), wouldn't you then have two plugs reflecting the lean condition as you showed in the pics, as opposed to only the one? I'm just asking out of curiosity, no experience here.

Also, did the boat run fine (all three cylinders at least) before it blew the mag piston way early? If the crank wasn't messed with since then, it seems that it would be the same now. Can a crank slip out of index that far?

Interesting saga - keep us posted. Absorbing input here. :)

[javiert99]

Re: SL900 running lean on one cylinder, trouble finding problem..

Showmepro, I honestly don't have enough experience to say whether or not I would have two lean pistons.

When center should be firing and getting fuel, it's still halfway on the upstroke, while #3 is firing, it's on its downstroke instead of its upstroke, so I imagine that could cause a rich/lean condition on one piston or the other. In my case, center lean, PTO rich. All carbs tuned identical, minus jets which are the stock settings (staggered).

The motor is running the same now as it did when I first purchased it (right before the MAG piston blow). It's missing about 1000 rpm, and 10mph on the top end, has a horrible hesitation on the bottom end. The assumption is not that the crank slipped out of index that far, it's that the motor was rebuilt with an out of phase crank. Once I have confirmation of what the exact firing order is (impossible to find online it appears) I can confirm that this is the problem.

TS, if you could on Monday (assuming noone else posts) that'd be great. I'd like to place a motor order on Monday so I can get this thing back up and running by maybe next weekend, so that would help me confirm.

Much appreciated!

[javiert99]

Re: SL900 running lean on one cylinder, trouble finding problem..

So apparently the igntion fires all 3 cylinders at a time, so there isn't a firing "order" per say, but what I need to know is what order the cylinders hit TDC..

Right now mine is 1, 3, 2

I need to know if the correct order is 1, 2, 3....

If anyone has a domestic 900 or 1050 motor, if you could confirm that your pistons go to TDC in order 1, 2, 3 that'd be great. You'll be spinning the motor counterclockwise to find out.

[javiert99]

Re: SL900 running lean on one cylinder, trouble finding problem..

update #3241324312412341324

Sooo... apparently the crank is in the correct order, domestic motors do fire 1, 3, 2.

It looks like the plug wires for #2 and #3 may have been backwards the entire time (from the previous owner). The way they are now makes sense, center plug wire coming out of CDI goes to center cylinder, as its the correct lenght for that as well. Apparently that's not the case, center wire is the #3 coil and goes to PTO, other wire goes to center.

I've got some stuff to put back together (carbs and jet pump) and I'll try again with the wires switched around.

I'll get to the bottom of this I swear.

The crank, measuring 1, 3, then 2.. is within index. It's a pain there isn't more info about Polaris ski's out there, would have been alot easier to clear up the ?'s.

[javiert99]

Re: SL900 running lean on one cylinder, trouble finding problem..

Well, switching the plug wires didn't help.

TS do you think you could confirm for me the order at which the cylinders should hit TDC? If it is in fact 1, 3, 2, then the motor checks out.

If it's supposed to go 1, 2, 3 then something is wrong.

I can't go any further without knowing, I'm at a complete loss and about ready to give up on the thing and get rid of it (how I don't know).

[Technical Support]

Re: SL900 running lean on one cylinder, trouble finding problem..

1-3-2 is correct.

[javiert99]

Re: SL900 running lean on one cylinder, trouble finding problem..

Thanks TS, appreciate ya looking that up for me. That being the case, I think I give up.