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  1. #1

    Default Tohatsu MFS 20C Compression

    Anyone knows what the compression should be in psi for a tohatsu 20 hp 4 stroke? Model MFS 20C, THANKS

  2. #2
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    Default Re: Tohatsu MFS 20C Compression

    With or without decompression?
    Paul
    Certified Tohatsu TLDI Technician
    Hamburg, NY USA

  3. #3

    Default Re: Tohatsu MFS 20C Compression

    Quote Originally Posted by pvanv View Post
    With or without decompression?
    I don't know what you mean, I tested mine, (two cylinders), with gauge instead of sparkplug, cold engine, and electric ignition, carburetor empty from fuel.

  4. #4
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    Default Re: Tohatsu MFS 20C Compression

    Tohatsu 4-strokes have decompression that reduces the cranking effort; Once running at say 1000 RPM, the decompression is off.
    What numbers did you get? Was the carb wide open (It should be), and if using electric starter, crank until you get the highest number.
    Paul
    Certified Tohatsu TLDI Technician
    Hamburg, NY USA

  5. #5

    Default Re: Tohatsu MFS 20C Compression

    Hi pvanv, finally managed to find the thread... sorry.
    No carb was not open, spark plugs were both out & tested individually, one test resulted in both cylinders at 70 psi, another recent test using different dial resulted in around 64 / 66 psi with manual start,; pulling the rope thingy...

  6. #6
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    Default Re: Tohatsu MFS 20C Compression

    OK, so you need to open the carb and pull several times to get the highest reading. With the decompressor (pull start), those numbers might be OK.

    Does it start and run? What is the symptom? Are you getting spark? Are the plugs wet or dry?
    Paul
    Certified Tohatsu TLDI Technician
    Hamburg, NY USA

  7. #7

    Default Re: Tohatsu MFS 20C Compression

    Quote Originally Posted by pvanv View Post
    OK, so you need to open the carb and pull several times to get the highest reading. With the decompressor (pull start), those numbers might be OK.

    Does it start and run? What is the symptom? Are you getting spark? Are the plugs wet or dry?
    Hi, it starts with the slightest pull, no problem. Plugs are both dry, but one plug is blackish, whilst the other one (below the 1st plug) has the central ceramic under the spark arm almost completely white! I now switched them round so next time i use the boat at sea, i see the outcome.

  8. #8
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    Default Re: Tohatsu MFS 20C Compression

    So, why are you testing compression?
    Paul
    Certified Tohatsu TLDI Technician
    Hamburg, NY USA

  9. #9

    Default Re: Tohatsu MFS 20C Compression

    Quote Originally Posted by pvanv View Post
    So, why are you testing compression?
    A 20HP 4 stroke on a 13.5 foot boat should give better performance / speed but with just 2 persons & boat on plane i am only seeing 16 knotts. Some time ago i changed the prop to a smaller pitch otherwise the boat would not plane. I also bought a rev tester, the one you turn a few turns of the wire onto a plug & it gives the revolutions of engine, at full throttle it showing 3500 revs... according to engine specs it should be around 5500. One spark plug had the central ceramic white, doesn't look good to me, its supposed to be brown or black, so this plug might not be firing properly? I now switched them round, next time i use the boat I check again.
    Engine does not feel 'right' and I might be wrong.. but its a gut feeling...

  10. #10
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    Default Re: Tohatsu MFS 20C Compression

    Verify your tach. Should approach 6000. What pitch do you run now?
    Paul
    Certified Tohatsu TLDI Technician
    Hamburg, NY USA

  11. #11

    Default Re: Tohatsu MFS 20C Compression

    Quote Originally Posted by pvanv View Post
    Verify your tach. Should approach 6000. What pitch do you run now?
    I've got a 2008 20 HP Tahatsu w/electric start. with engine at operating temp, both spark plugs out, and throttle open I have 40 PSI Compression in top cylinder and 38 PSI in lower. This seems very low?? I looked in with endoscope pretty small view but I could see some carbon build up on edge of valve
    Engine revs in neutral quite well but when you open throttle under load it will push the boat but not throttle up. I've redone the fuel system from the tank to the cylinders.

  12. #12
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    Default Re: Tohatsu MFS 20C Compression

    Did you clean the carb? What method did you use?
    Paul
    Certified Tohatsu TLDI Technician
    Hamburg, NY USA

  13. #13

    Default Re: Tohatsu MFS 20C Compression

    Disassembled and cleaned carb with carb cleaner and air preasure. 3 times

  14. #14

    Default Re: Tohatsu MFS 20C Compression

    I don't know anything about decompression, whether it is on or off i don't know how to tell

  15. #15
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    Default Re: Tohatsu MFS 20C Compression

    Quote Originally Posted by DENoyes View Post
    Disassembled and cleaned carb with carb cleaner and air preasure. 3 times
    The acetone type spray cleaner is inadequate to soften the organic deposits. Submerge all non-rubber parts in real carb dip overnight, and then blow out with a whole can of spray carb cleaner to clean it out.
    Paul
    Certified Tohatsu TLDI Technician
    Hamburg, NY USA

  16. #16
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    Default Re: Tohatsu MFS 20C Compression

    Quote Originally Posted by DENoyes View Post
    I don't know anything about decompression, whether it is on or off i don't know how to tell
    That is why you want a Factory service manual. Not an aftermarket manual. The decompressor is a weight on the camshaft that prevents full closure of the exhaust valve below maybe 1000 RPM.
    Paul
    Certified Tohatsu TLDI Technician
    Hamburg, NY USA

  17. #17

    Default Re: Tohatsu MFS 20C Compression

    So given that , and the info in my first question is 40 psi low.

  18. #18
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    Default Re: Tohatsu MFS 20C Compression

    Not necessarily. Dies it go up much with a shot of oil?
    Paul
    Certified Tohatsu TLDI Technician
    Hamburg, NY USA

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