08-19-2012, 05:23 PM #1
Strike one. MSX 150 - new pistons - bad compression
Well... just when I thought my MSX 150 was good-to-go... and man was it running awesome... I got thrown a curve ball.
Something is wrong with my pistons (I believe)... my compression is only 75#. Doom on me.
Read the back story of how I got the ski and it's original problems here... http://www.greenhulk.net/forums/showthread.php?t=176246
The engine came with low compression (110#) which was likely the result of the overheating it suffered which the dealer did fix. It was a stripped plastic waterpump drive gear. So I pulled the engine, pulled the head and pulled the pistons. I wanted to make sure the cylinders weren't damaged and to put new rings in to get compression back where it should be (145# +/- 15#). The cylinders were great looking... no flakes, no scratches, no scoring... so I just lightly de-glazed them. I did NOT measure the cylinders for out-of-round. Doom on me... I was hoping to get lucky. I got 2 new pistons (cause you can't buy just rings)... standard size (cause you can't overbore Nikasil coated cylinders) and put them in. I did NOT check my ring gaps. Doom on me again. Pistons were installed with a ring compressor and ring gaps were staggered per service manual. I had no troubles putting the pistons back in. They seemed to me to fit perfectly. Engine turned over with new pistons as I would expect. Everything was lubed well with engine oil (pistons/rings/bearings/rods/wristpins) upon reassembly. Used all new gaskets and put the engine back together. Also put in a 160 degree t-stat, the new upgraded oil tanks and a Precision stage2 reflashed ECU.
Only other hurdle I had was getting the cam sprocket timed correctly. Since I didn't tear the engine down the whole way... I didn't get to gears on the front of the engine that have the alignment dots on them as per the service manual. The service manual did have a good diagram of the overlap cam phase at TDC. Using this diagram... and careful measurements, I believe I got the cam sprocket set correctly.
So showtime. The engine ran great! It idled smooth at 1550 rpms and reved great. I spend a day and close to 2 hours breaking in my new pistons. I varied my revs all day and took it easy the first 30-45 minutes. Later I did do a number of WOT runs. It ran awesome! Boost was great. No troubles at all, no lights, no check engines, no overheating. Basically I thought it was good-to-go. Except it was hard to restart. I thought this was due to heat soak combined with a weak battery. It was a hot day and I was running the ski pretty good.
I put the ski back in the garage to changed out the break-in oil... put in new Mobil1 15w-50. And I drained the distilled water I was running for coolant and put in fresh 50/50 pink stuff. As I did this, I decided to check my compression (both plugs out). It was only 95# each. I was bummed. It ran SO good... how could that be right?
I topped off the charge on my new AGM battery and put it back in the lake. I couldn't even get it to start in the water... and it was cold... right off the trailer. Good thing I live next to the boat launch... I waded it over to my dock. I checked battery voltage with my meter. 12.6vdc resting (after a LOT of cranking at the launch trying to get it to start). 11.8vdc cranking (and not starting). So it's not the battery. So I'm aware of this low compression... and I'm pretty sure it's part of (if not all) the reason it won't restart (in the water). The compression is too low and it can't get itself started when the jet pump is in the water (starts instantly on the trailer). I prop up the rear of the ski on a log and hit the starter. Starts instantly. Dang it. Proceeded to run it for another hour (maybe not the best choice... rings just need a little more seat-in time, right?). It still ran great... but in hindsight... I think it was down just a tad on power from that first break-in day.
Back in the garage again. All fluids look great. Still zero check engine lights, indicators or trouble codes at all. Still starts instantly on the trailer. Checked compression again today and it's at 75#. Doom on me. It's getting worse. So it's officially "broke" and parked back in my shop. Guess I'll get the joy of pulling my engine once more.
If you're still with me (reading) to this point... thanks! Here is where I'd love all suggestions for troubleshooting and things to check.
- if the cam sprocket was 1 tooth off... would the engine still run and idle so good? (I'm thinking no)
- if the cam sprocket was 1 tooth off... would that cause my compression issues? (I'm thinking it could... but comp #'s going down point to something else)
- what happens if your ring gaps are too tight?
- what happens if you put the pistons in backwards? (It didn't have a clear marking on either intake or exhaust side of the piston like the service manual said. Instead it had a big white dot which I put towards the front of the engine)
Thanks in advance.
08-19-2012, 08:32 PM #2
The pistons in mine had the white dots facing the PTO. (rear)
Did you check to see if the head warped when it overheated?
08-19-2012, 10:43 PM #3
Why would you put markings to the rear? Seems like they would always point forward. I was annoyed that I couldn't find any "mark" on the intake or exhaust side of the piston... just the white dot... which would face either forward or rear. The service manual says the "piston mark will face the intake side of the engine". If there was one I guess.
I did check the head and block for warpage in all different angles. They both were nice and flat. And it doesn't seem like I have any head gasket issues since my coolant and oil do not appear contaminated nor am I loosing any of either.
Would a backward piston cause my compression issues? I wonder why they wouldn't be symmetrical...
08-20-2012, 06:17 AM #4
its a little strange that both cyls. are down an even amount. I would like to think both pistons wouldn't suffer the same amount of dammage & end up at the same #. I'd look the head over real good & check your valve timing before you tear it all out.
08-20-2012, 09:38 AM #5
Yeah... good suggestion. I'm going to pull the valve cover and inspect really close. If I have the valve timing off... I guess I could be "burning up a valve" which might explain why my compression has gone from 95# to 75#. And yeah... they both are the same #s too.
08-22-2012, 08:32 AM #6
Looking at the service manual today & came up with another thought. Have you checked valve clearances? if there too tight as everything heats & expands it will go further out of spec. maybe holding a valve open? perhaps you swaped 2 shims on re assembly? might explain compression drop as it gets hot... just a thought
11-24-2012, 10:19 PM #7
Well... I've torn back into my MSX150. I have to say, pulling the engine is way easier the 2nd time around.
So I have the engine out and the head off once again. Everything looks really good as it did when I put it all together. The engine has less than 2 hours on it since last overhaul. No headgasket issues detected. Cylinders still look clean with hint of deglazed cross-hatch I did last time along with the new pistons. If the big dots on the pistons were supposed to go to the rear (not how service manual described it), then my pistons are in backwards. But being symmetrical side-to-side, I wonder if that would cause my issues.
I did check my valve clearances this time. Intakes were all in spec range. Spec range for the exhaust valves are (.007 -.010 in) and mine were ([.009, .006][.009, .007]). So one valve on the MAG cylinder was just out-of-spec tight... and one valve on the PTO was barely in-spec tight. I checked all the shims and I can't see where this is the result of a mixup. Plus I was very careful last time... going one-by-one and labeling everything. Could these two tight valves be the cause? Seems to me they would need to be way tighter off the spec range for trouble like I'm seeing. All 4 exhaust valves and stems look the same... color-wise.
On a tip, I also flipped the head, inserted the plugs and filled the domes with water... to ensure the valve seats were good and not leaking. It held all the water for over 15 minutes, not a drop leaked or detected in the intake or exhaust ports. So looks like the valve seats are good. I also checked the head mating surface for warping. Perfectly flat from every angle using my machinists straight-edge. So that's still good.
Next up is the cylinder bores. I will be borrowing a bore gauge from a friend and will be thoroughly examining the roundness of my cylinders. This was the cause I was leaning towards. Being an open block... I could see the cylinders warping on a bad overheat. Wish I had checked this the first go-round. Guess we'll see.
Appreciate the feedback.
12-14-2012, 11:02 PM #8
Well... got a chance to check my cylinders with a bore gauge today.
The taper at the top of my cylinders are beyond spec. They are .0005" fatter than at the bottom of the cylinder. Manual says taper limit is .008mm (.0003") so I am beyond that. Hard to believe less than a thousandth of an inch could cause this issue... but I'm really hoping it has. I have a good second block I'm planning to use so I checked it too. It was perfectly even top to bottom and all the way around the bores. So I'm crossing my fingers.
Here's a pic of the old block. The plastic sleeves on the 4 head studs are all melted and pitted and looking bad. That should have been my first clue back during my first engine disassembly to how hot this engine had previously gotten. The cylinders being all aluminum with Nikasil plating in an open deck block, it's easy to see how they could be adversely affected by a bad overheating.
So I'm stripping this old block down and will be porting over all the components into the new block. It's coming apart pretty well. But the driveshaft coupler and PTO stubshaft was a serious PITA! I accidentally broke a finger off the coupler. Dang it! Are these couplers the same across all MSX? They don't look like they'd be a unique 110/150 item. Anyone got a good extra one laying around they want to sell me?
Next up is getting the flywheel off and splitting the block. Gotta save my main crank bearings. I have the full set that came on my new block... so between those and these from the old block... I'm hoping to be able to get my crank set properly in the new block. I'm going to use plastigauge to ensure my crank bearing clearances in this new block. Same for the counter balancer shaft too. Should be interesting. Here's my pile o' parts forming in my shop.
06-25-2015, 08:07 AM #9
Is this the Msx you currently run? As it pertains to engine overhaul, can the cylinders be bored and replated with Nikasil? Just curious as to what I have to look forward to.
06-25-2015, 10:48 AM #10
Old thread back from the dead! Ahh... the memories.
Yes, this is my current MSX 150... but not the current engine block. For a quick recap... I bought mine with a severe overheat (broke plastic waterpump drive gear) that warped the block... tanking compression. I had hoped a pair of new pistons/rings (back then, could only buy them as a set) would fix my problem... but nope. New pistons can't fix a warped block.
I ended up getting another used block (supposedly good... right Marto? with a crack in the Nikasil)... which I then used as a core to have sleeved for stock 750cc pistons. This is my current block.
So to your question... if you have Nikasil failure in your current block... what are your options. You can send your block off to a re-plater like Millenium and have it replated back to stock specs. Or you can buy a set of sleeves from Randy and have them installed in your block. Prices for both are not far off each other. I, and most others, went the sleeve route... for our rec skis.
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