High Compression?
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  1. #1
    Senior Member Boarder255's Avatar
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    High Compression?

    So I've been searching for a solution for quite a while for my underpowered TW. It's got almost 11,000 miles on it. I thought for the longest time it was something carb related. I rejeted, replaced almost every part on the carb, and even got a used good carb body (Thanks TWBrian!!). Still not the solution. Against my will, I took it to the stealership and they told me "it must be a carb problem", a terrible way to waste $50. Well I did some researching of my own and wanted to test compression, bought a fancy new compression tester and what do you know. The TW was running around 160psi with the throttle open. Closed it was around 125psi. Spec is 129 and maximum of 149psi. So I was obviously high compression. After some calls around to my friends who are more mechanically apt than I, the consensus was it was carbon deposits in the combustion chamber. I took the head off the cylinder and what do you know!! Yuck, there is my problem!





    After 2 cans of Chem B-12 and a bunch of elbow grease...





    Note the fresh gasket (I learned the hard way you can't re-use the old one).

    I made sure to spray cleaner in the intake and exhaust valve openings and got them looking as clean as I could. I also noted that there was no cleaner leaking through, so my valves are sealed well right? I button it back up (again), and it seems to be running better. If before the clean it was at 60% of it's original power, I'm now at approximately 80% (butt dyno of course). Still not like it used to be. I can't pull the front wheel up in first and 5th gear is useless unless I'm on a flat or downhill run. Its running on a diet of seafoam and 87 octane and I tuned the carb on the lean side (I had a 116 main jet, and now put in a 114) hoping it will burn up any additional deposits that I missed. I test the compression today and I'm still at 165/170psi. Still high... Does anyone have any ideas what else it could be? What else could cause this high compression?

    Love the forum and having all this great info at my finger tips. I'm willing to do whatever it takes to get this thing back up and running at 100%.

    Dale

  2. #2
    Senior Member Toyanvil's Avatar
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    I would run high octane fuel with No seafoam and check the spark plug, also have you ever cleaned the spark arrestor at the end of the exhaust pipe?
    09 TW200

    04 BMW 1150GS

    61 Sunbeam Alpine

  3. #3
    Senior Member Boarder255's Avatar
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    I hadn't thought of the spark arrestor. I will try to pull it tomorrow.

    It has run on premium (93/91) fuel since I've owned it (9000 miles). I think that is what contributed to that terrible combustion chamber. If I run higher octane, it's harder to combust the fuel, and if I have any un burned fuel (easier to do with high octane) it would turn into carbon deposits correct? I'm not a professional mechanic, so this theory is only based on reading about octane fuel.

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  5. #4
    Senior Member tinman tim's Avatar
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    yes, low octane fuel is more volatile, and burns faster. The reason for high octane fuel is so you can compress it more without it pre-detonating. I always run 87octane fuel.
    clean your exhaust and you should be good. TIM

  6. #5
    Mel
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    Senior Member Mel's Avatar
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    My best guess some where along the line the free head space has been reduced on that engine. Either one end or the other of the cylinder has been turned down for truing the surface, or the head has been shaved. It does take some effort, but a gob of clay on top of the piston, fully install the head, rotate the piston thru top dead center disassemble and measure the combustion space. That is about the only way compression can be increased unless a different piston has been installed.
    Mel

  7. #6
    Junior Member TW669's Avatar
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    Just another possibility, maybe the compression tester reads to high, check with another compression tester. From my expierience if you take 3 different (el cheapo) testers you will get 3 slightly different results. Unless you use a calibrated tester (which is expensive) the result is more like a guideline.

  8. #7
    Senior Member fishguy's Avatar
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    All the TW pistons I have worked with (1995+) are flat on top. Is that a special piston? Check to make sure the exhaust value is opening correctly. One time, after having my head off, the exhaust valve tappet adjustment screw came off and the exhaust valve was not working at all. The bike ran with about 50% power, amazingly enough.
    Last edited by fishguy; 08-18-2013 at 09:06 AM. Reason: typo

  9. #8
    Senior Member Boarder255's Avatar
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    Thanks everyone for all the responses, very helpful. I tried to pull the spark arrestor and the screw of course was nearly stripped to begin with. Even after 24 hours of PB Blaster soaking, it still refused to budge. SOB! I'll either drill it out or potentially grab a new OEM exhaust that is on CL near me.

    Another symptom that is troubling is my terrible gas mileage. I'm getting right around 40mpg! Before the incident that caused my significant power decrease, I was easily getting 65-70mpg. I'm not sure if this is due to me having to hold the throttle open more to get up to speed or what.

    I would like to attempt everything else before removing the cylinder head again (I don't want to have to buy the 30 dollar gasket and wait another week for it to get here).

    Thanks again and I will let you know when/if I get it sorted out.

  10. #9
    Senior Member elime's Avatar
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    A lot of good advice here so let me throw in my 2 cents worth.

    My spark arrestor was stuck solid and I ended up breaking it trying to get it out. Instead I suggest removing the muffler and running "open pipes" for a butt dyno test. It is not that loud. If you do break it you can buy a new insert but it is $40 or $50.

    When I re ringed my TW (along with base gasket, valve lapping) my compression was 160 psi after I seated the rings. About 3K or 4K miles later it is about 140 or 145. If I do 10 compression checks in a row I get different results and I have no idea why. I am using a Snap On gauge.

    My butt dyno says my TW runs best on Super Shell, but only in the summer and it is on the second tank that I start to feel the difference. I openly and freely admit it could be my imagination but give it a try. Even if Super Shell is 50 cents per gallon more, and I doubt it is that much, two fill ups is about 3 gallons, that works out to only $1.50 more and if it solves your problem it is worth it.

    When you replaced the 116 jet with the 114 did you use a brand new jet? The hole in the center erodes and gets larger resulting in a rich mixture. A long story in how I found this out. If you did not use a brand new 114 jet this is where I would start.

    BTW, I too wasted $75 at the dealer for a diagnosis. He said it was my muffler but a brand new 114 jet solved the problem.

    Good luck and keep the updates coming.
    Last edited by elime; 08-19-2013 at 08:43 AM.
    Long live the internal combustion engine!

  11. #10
    Senior Member Toyanvil's Avatar
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    Have you checked the rubber mount that holds the carb to the head. if it is old or loose at the carb you may be leaking air past it. You can check it with WD-40 with a cold motor, just start the motor and spray around the mount and see if the RPMs change.
    09 TW200

    04 BMW 1150GS

    61 Sunbeam Alpine

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