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  1. #1

    1200NPV compression relative to breakin questions

    my buddies Yamaha SUV had the middle cylinder go lean due to dirty carburetors and just started coming apart when he caught it at 90 psi.

    I flex hoaned all three cylinders to clean them up, installed new base and head gaskets with spray copper on the head gasket and Yama bond on the base gasket.

    carburetors were rebuilt with genuine Mikuni kits and were pop off and pressure tested for leaks.

    since he is still using the oil pump we mixed in 1.5 ounces off oill for every gallon of gas for the break in period.

    the break-in consisted of 20 minutes at 0 up to 1 quarter throttle 20 mph Max followed by seat removal and 15 minutes rest.

    then, 20 more minutes at 0 up to three quarter throttle 35 miles per hour max then removed and cool 20 minutes.

    last stage was 20 minutes at 0 up to full throttle.

    we did not go through 10 gallons of gas by any means. When we did the compression testwith my fairly new Snap on Tools gauge the results were perfect on the front and rear cylinders at 120 psi however on the cylinder that we replace pistons, rings and wrist pin we only had 104 after multiple tests.

    a couple questions on this.

    1. the cylinder head had some of those dings and dents from piston junk mashing into it. I took a dremel tool and smoothed everything out so there were no raised edges but a person could argue that the volume of the cylinder head has in fact change somewhat. Could the difference in 16 psi be due to the fact that there is more headspace in this particular cylinder?

    or, since our break in was only 60 minutes long and only 5 gallons of gas or so could it just be that the Rings have not completely set?

    most importantly. Say for example that the motor is still healthy but we have low compression in the middle cylinder because of a difference in actual volume. is there anything unhealthy about running 1 cylinder low like this or is this just the net result of lower horsepower now?

  2. #2

    Join Date
    Aug 2006
    The head cleanup should not have effected it unless you really took a lot off. Most likely you haven't seated the rings all the way, a flex hone is not really gonna prepare a nikisil wall that well. You did not mention changing the piston or rings, if you didn't that could be the problem. A piston could have collapsed a bit from the heat and still look good, which would allow it to rock and effect ring seal, same thing with the rings, they may have too much wear to seal right.

  3. #3
    Sorry, this is a NPV steel cylinder. I used the 220 grit flexhone and it left pretty good looking crossthatch marks. I've used it before on other npv cylinders and got real sweet 120psi numbers so I dont believe it was the fault of the hoaning. The other 2 non-damaged cylinders that I honed tested perfectly at 120 after this short breakin.

    The low piston is new from SBT, new rings and wrist pin. The old piston that was replaced was in pretty good shape. Rings were still free and no scratches in the cylinder wall that you could catch a fingernail on. Only minor aluminum deposits on the wall in a couple places. I honed a little bit, then used a razorblade at a steep angle to peel (chip) the deposits away...taking great caution to not scrape the wall. Then when the deposits were 95% or so gone I flex honed it till they were gone.

    When I dryfit the rings by setting them inthe top of the cylinder, there was a very very small gap that the sharp edge of a razorblade would fit into. Then I put the piston upside down on top of the ring and pushed the ring in about 1" to check the gap. It was ever so slightly larger. I did not test the gap with a feeler gauge unfortunately but dont think it was too much for sure...and highly doubt it was too little. The rings were instaled correct side up and the aarow to the exhaust for sure.

    My buddy is out as we speak running the rest of his 10 gallons or so of gas through. I told him to re-do the 3rd breakin stage till his low fuel alarm goes off. Then to run it up to max rpms for 30 seconds to 1 minute a few (5 times or so) and hold it there...alternating to low rpms to allow it to cool.

    I also told him if it flat dies on any of those high rpm shots that he needs to idle back to the trailer so we can pull the head.

    Thanks for the reply on the head work. No, I didnt remove enough to matter IMO. Just the raised spots and rounded the edges of the divots a bit.

    Hoping this is all worry for nothing and that he texts me some sweet numbers in an hour or so...but at least the early bad numbers post may help someone else not worry in the future LOL. I'll report back with the results or pics of a brand new fried piston I still cringe at the thought of my last SBT shortblock that I broke in according to the letter of the law and yet had it come apart at hour 4.

  4. #4
    Status update: raw compression on a hot (not warm but HOT) motor remains in the 100 to 104 catagory. On a cold motor with 1 tablespoon of 2 stroke oil added down the plug hole it is 125 on the same cylinder. Any intel as to what this means? My best guess is that the rings will continue to slowly wear down to final shape and that it will seal up nicely...or does this more so indicate loose rings (new rings overhoned cylinders) so the compression is likely to remain as-is?

    If it remains 120, 104, 120...obviously we lose some power but is this totally unhealthy to keep running like this? I cant imagine why it would be. We dont care about the loss of a couple miles per hour at all on this ski. It keeps right up with our other overloaded SUVs just fine.

  5. #5

    Join Date
    Aug 2006
    If you honed a lot with a flex hone, you may have cyl that I'd no longer perfectly rond and thus lose a bit of comp. if you add too much oil you will raise comp due to oil taking up space in chamber , same as smaller head chamber. If you crank with less oil that would indicate the oil was sealing up bad rings or loose clearances. Should have mic'ed piston one cyl so you would know for sure. I believe you will be ok running this way though but don't run if it gets a lot lower like 85-90. That much. Blowby would cause piston damage and contamination of the crank case with exhaust junk.

  6. #6
    Cool. Thanks for the input! We are running a full tank through monday afternoon. That should easily tell the tale of where this thing is at. Roger on the mic of the piston.

  7. #7
    That cylinder with 104psi has an underlying problem.......a new piston and new rings ,without any oil being dropped in the sparkplug hole ,should produce way above 130 psi......

  8. #8
    after breakin period should be 120 to 98 yami 1200xl as of this past weekend read 130-131-129....all 3 piston are 1.0 over......after every weekend ride I check the compression across...

  9. #9
    We rode hard yesterday and put another tank of gas through. It has great throttle response and top end speed of 45 on gps. Most of the SUavs I have worked on or have first hand knowledge of top out right in the 50 to 52mph area on gps if they are not loaded down and have no wind or chop to contend with. I could feel his hole shot was a bit sluggish but highly suspect the wear ring (likely a worn prop and bulged staiess ring) as the speed problem. Maybe some of the speed sluff also attributed to the 15% loss of power from cylinder 2 (about 3 mph or so). The compression has checked out identicle to the first breakin test so far.

    I understand that it isn't right b7t these are stock size pistons on a non-pv motor and I have yet to see more than 122psi on any 1200 npv with stock pistons to date. The 1 overs get into the 125 to 135 zone but not the stock bore in my experience. This is testing with a new snapon gauge and crosschecking with a new autozone gauge.

    That being said...the underlying problem doesnt seem to be so much of a problem as it is a condition of install. I believe I either flexhoned too wide or that the SBT piston that I installed was from their "A" pile. It is my understanding from talking to SBT that when they get their piles of pistons in from their manufactures that they individually mic them and then catagorize them as small, med and large A, B, C but still within that particular bore group. Then when they need 3 of them they grab 3 closer matching ones and bore to those specs. We r talking thousandths but it does make sense that they are not exactly the same when they get them. Anyway...that's why I say A pile.

    Regardless, my clearance is simply too much and the compression weak because of the bore size to piston girth (a problem I've been dealing with since I was 16. This particular SUV is set up for fishing so there is just soo little need for long WOT sessions anyway. I suspect it will outlast many of the other npv motors in our group but we'll check back in if she implodes to let y all know. We'll change it out for the right bore when we get to the mid 90s or sooner if it starts to drop this season.

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