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  1. #1
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    loss of compression 300Promax

    Rebilt and now lost compression after 150 hours . 100-105psi. 95RON gas and QS Premium Plus oil used only. starting ok, but low idle (500-550 no stable) and only 5000-5200wot.


    does the reason for trolling? 25-30 hours on trolling.

  2. #2
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    no explain?

  3. #3
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    What all was done on 1st rebuild

  4. #4
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    new 6 pistons (0,02ower), rings, 1 rod (all WSM), all head gaskets. Oil eating good (oil pump had no problem in my opinion, eat 1 gallon on 1 50gall tank of gas)
    first 6-7 hours break in with oil 1/25 in fuel tank
    Last edited by Tranq; 08-25-2019 at 09:00 AM.

  5. #5
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    What's was comp after 10hrs break in?

  6. #6
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    didnt check compression after break in, but worked good and strong. but recently (10-15hours) was starting troubles, loose speed and rpm and i checked compression.
    i think because I caught pikes trolling on a racing motor long, my master said that this engine is not intended to work idle for a long time. in Russia, no one has the wholesale operation of these motors, only 250EFI was a lot

  7. #7
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    Well I dont see idle time takin from compression, what was done to block to accept new pistons? All bored over size or a hone with stock size slugs?

  8. #8
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    all were hone of course. may be pro max must have another rebild sleeve sizes than 250?

  9. #9
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    Well sumtime the hone will work , but most times ,once you do a hone the clearances r at the max end of the specs ( to much clearance) , I did a wsm rebuild bout 45ish hrs on it with same compression,, (knock on wood) gonna need to measure all the bores and c what u got

  10. #10
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    What will be the new rebuild offer? resleeve everything on Wsm and Wsm piston std, or put the original merc std pistons, and put new wsm std sleeves, or anything other? neen hone?
    and i want to replace all crankshaft bearings

  11. #11
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    Bore all 6 to .015 over with .015 wsm kit

  12. #12
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    why not to standart resleeve and std pistons?

  13. #13
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    U can ton of $ imo waste $ if block/cyl ain't messed up bad , if in good shape just little to much clearance I'd bore and rehone, tear down and send it to known good machinist, get plenty references ,,he will tell u what needs b done after he measures bores
    Last edited by Merc 2.5; 08-25-2019 at 04:15 PM.

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