25 Kohler on sthm really a bad motor.. or not?

Discussion in 'Mechanic and Repair' started by mowisme, Jul 22, 2007.

  1. mowisme

    mowisme LawnSite Senior Member
    Messages: 910

    I posted in wrong area earlyer..so I'll post here. I bought a sthm Scag with a 25hp kohler command. one coil is acting up- quite rusted so will clean and re-gap and see if that helps. cuts out only occasionally. here's my question though: I seem to recall that the 25hp Kohler command is a very troublesome motor? what do you guys say? any input appreciated. p.s. I got it with a blower on it instead of mower- so need to search for a deck. anybody delt with scag blower- never seen one before.
  2. tomo

    tomo LawnSite Senior Member
    Messages: 660

    hello 25,26efi ,27carburettor ,28efi .

    These are all the same engine ,differences are carby and of course the efi set up and some more small differences such as different camshafts etc .

    The main problems with the 25hp was it was the first large bore size kohler .
    This caused a few issues mainly due from the head gasket sealing area being reduced [The 20hp had greater surface area on the head gasket due to its smaller bore .] Together with this as horsepower goes up additional heat is generated placing greater strain on the seal .

    Many upgrades are now available such as a --head stud kit
    --a different head is required with the above but included is a revised oil drain gallery [it all helps]

    If u have the nickle seal bore [verify this with kohler customer service ] these can cause trouble .A specialised hone is required and even then the bores are most likely worn out .They pit badly as if the coating is coming of and there is nothing u can do .

    Most parts on a kohler have had an, upgrade ,but not all of them are as succesful as kohler might think

  3. thecrankshaft

    thecrankshaft LawnSite Member
    Messages: 172

    The same cylinder head is used on engines that use studs or bolts. However, even new 25 HP engines are built with bolts. Some are, some aren't. I really wouldn't consider it an upgrade.

    A big concern is the head gasket and the reduced sealing surface. The casting of the block has changed to put more material between the oil return from the head and the combustion chamber.

    I have never had any luck boring out nickel-silicon bores. Kohler doesn't recommend it.
  4. tomo

    tomo LawnSite Senior Member
    Messages: 660

    hello, i was made aware of the different heads by kohler customer service [ I also have some of the old type in work shop ].As this was 2-4 years ago the upgrade and a head casting which suits both bolts and studs maybe available now ??
    The stud kit is a better way of clamping head to block and according to kohler provides greater and more even clamping force to head gasket .

    The shape and position of the drain back hole was also upgraded .I do not no the year this changed suspect around :1999-- 2001

    Nickle seal is a coating and cannot be rebored ,but the block can be sleeved with a cast iron liner .As u would expect this is to be carried out by only an experienced machine shop.

  5. LawnBoy89

    LawnBoy89 LawnSite Senior Member
    Messages: 967

    I have one on my lazer head gasket blew last year I tried fixing it, made it worse, I ended up takin it to the dealer who I'm friendly with they had the same one a couple years old with only 200 hours on it they put it on it seems to be working fine except backfires when I turn if off if I don't let it idle down for more then 30 seconds.

    Tomo....can I put a different carb on and get more horsepower out of the motor without making any other changes?
  6. Bill Kapaun

    Bill Kapaun LawnSite Senior Member
    Messages: 956

    "except backfires when I turn if off if I don't let it idle down for more then 30 seconds."
    If it has the fuel shut off solenoid on the carb, you're not supposed to idle it down at all when shutting it off. The idea behind them is that it shuts off the fuel to the Main Jet and the rotating engine "purges" gas fumes out of the exhaust system so that there are no fumes to AFTER fire. (Back fire is through the carb)
  7. tomo

    tomo LawnSite Senior Member
    Messages: 660

    hello ,will require at least carby and cam shaft, hard to justify cost for 1--2 hp if using stock parts in a stock situation .Better off spending the money on better maintenance .


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