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Thread: Low Compression & No Start

  1. #1
    Senior Member Fred's Avatar
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    Low Compression & No Start

    Am unable to start a TTR225 motor just installed in the Betty Boop TW. Motor has never run for me and came with no pedigree on history or why it was literally in the garbage.
    Valves were adjusted when new donor motor was on the bench. They were pretty close to being in spec. Engine turned over nicely on bench test both by hand and by electric starter. Runs through gears nicely, clutch works, plug gets spark, carb seems to be delivering fuel.
    Compression was tested with cold motor & throttle held open and observed to be ~ 50psi @ 5,100 ft elevation. Extrapolating correction for elevation indicates this would have read ~80psi at sea level. Still significantly low, right?
    Going to re-test now after putting some oil in spark plug hole to see it that identify a ring issue. Am assuming the 50 psi is indicating a real problem.
    Opinions?
    Suppose I can pull head and look for issues, swap TW head and valve train for TTR head and valves.
    But would I be wise to plan on piston and rings too just to be safe?
    And as long as I am looking at enhanced jugs for Betty Boop should I not consider not just going from 196 to 223cc, but maybe a full blown 250cc cylinder, piston & head? ( we guys like em bigger)
    Ken and plumbstraight like this.
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    Senior Member jtstdub's Avatar
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    Do a wet test... Table spoon of any weight oil over 30... and see if the compression comes up.

    Then do a cam to crank timing check.

    Jim
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    Super Moderator Purple's Avatar
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    Bent valve stem, or deformed valve stem where it meets the cam, shot rings - could be any of them. The only way to find out is to drop the head and take a look at what's going on. Even if the rings are shot, you're still going to need to go in there after them

    IMO, you'd be better off trying to fix the TTR motor than you would boring Betty's out. Very few "big bore" kits match the customers expectations, and a 10% over bore is unlikely to match that with a 10% performance increase

    The TTR swap however I can vouch for - more torque and top speed (15 / 45 is standard on the TW225) ....
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  5. #4
    GOF
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    JTSTDUB's instructions will probably find the problem. But you could also do a cylinder leak-down test. Put the piston at TDC on the compression stroke and pressurize the cylinder. See where the air is escaping. Out the exhaust suspect the exhaust valve. Out the intake suspect that valve.Out the oil fill, rings or piston. Pull the oil filler cap when you do this.
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    Super Moderator Purple's Avatar
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    Quote Originally Posted by GOF View Post
    Out the oil fill, rings or piston. Pull the oil filler cap when you do this.
    For the benefit of future readers, you'll need to clamp the crankcase breather pipe to detect any excess pressure at the oil filler cap ......
    JerseyJeeper likes this.
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  7. #6
    Senior Member Fred's Avatar
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    At worst case I could get replace the top end with this but re-cycle Betty Boop's 196CC head. NEW LISTINGYamaha XT 225 Top End Rebuild Kit Cylinder Piston Refurbished 1992-2004 #99

    Will not the Serow's case accept a XT250 cylinder & piston? I wasn't thinking of boring Betty Boop's engine, it shall sit on a shelf. Instead install compatble parts for the new donor 223cc motor.

    Question: Anyone have a link, or info, on how to research a engine's pedigree with just an engine serial number? Wish to confirm correct part compatibility before ordering.
    TW-Brian likes this.
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    Senior Member TW-Brian's Avatar
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    Fred, I am so sorry that you are having problems with this engine . Please accept my sincere apologies for sending you down this rabbit hole!

    As mentioned already, if low compression is confirmed then the head needs to come off to see if you have valve problems and/or cylinder/piston/ring problems. If the head is okay (fingers crossed!), then that Ebay offer may not be a bad way to go, since a cylinder bore and piston/rings would probably cost you close to that. I would also definitely consider using a Wiseco high compression piston (I can get you the part number of the one that I used when I get home tonight).

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    Senior Member Fred's Avatar
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    Scholars and gentlemen all!
    Nice helpful group to hang out with here. Thanks.

    As far as another threads mention of no neutral light I have jumped across the side stand safety interlock grounding that circuit so as to restore starter motor function.
    Still no neutral light. While I believe I have good spark measures via a timing light like device is there some other ignition cut-out beyond the starter motor cut-outs? i.e. could I have an undiagnosed ignition error? Spark plug, cdi, stator etc are all from the working Betty Boop chassis.
    TW-Brian likes this.
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    Senior Member TW-Brian's Avatar
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    Quote Originally Posted by Fred View Post
    As far as another threads mention of no neutral light I have jumped across the side stand safety interlock grounding that circuit so as to restore starter motor function.
    Still no neutral light.
    Fred, I am not an electrical type of guy but can I assume that you have confirmed that the neutral switch in the transplanted TTR engine is functioning and that neutral bulb in the control is not burned out?
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  11. #10
    Senior Member Fred's Avatar
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    No, 'cause I'm a Neanderthal.
    I suppose a test for both would be to jump across neutral light's terminals at connection under seat grounding out the circuit. If switch is bad then neutral light should now come on when bike is in neutral. If starter then can engage when bike is in gear with clutch out then that would indicate a bad bulb in dash.
    Any faults in my logic?

    EDIT: I did connect the TTR neutral switch to the lead from Betty Boop before buttoning up left side cover. Did not test the switch nor take any special efforts to clean switch's terminal .
    TW-Brian likes this.
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