Help me troubleshoot a rough idle please!
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Thread: Help me troubleshoot a rough idle please!

  1. #1
    Junior Member Matt in TN's Avatar
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    Help me troubleshoot a rough idle please!

    I just bought a 1987 TW200. It had been sitting awhile and would start but not run, so I replaced the battery (wouldn't hold a charge), air filter, spark plug, changed the oil, and cleaned and rebuilt the carburetor. I started with the idle mixture screw 2.5 turns out and it needed the choke to run well, so I adjusted it out another turn and a half before it would run with the choke all the way in (after warm). I can't get the idle speed set to 1400 as the idle won't stay consistent. Between that and a pretty bad miss/lope I'm worried I may have the dreaded CDI issues. There's a chance I missed some trash in the carb, but I was pretty thorough and all the jets appear to be open and clean. Pilot jet is brand new, but I re-used my old main jet once I cleaned it out. Boots around the carb look good with no obvious air leaks on either side, and the idle speed doesn't change when I spray carb cleaner all around it while running.

    Can anyone watch and listen to this video and give me any tips on what to check? To save time, here are where I think the informative points are:

    0:00 - typical rough idle
    0:06 - full throttle, and obvious missing/hesitation. Tach says 7,950-ish
    0:20 - realease throttle completely. RPMs drop and hold around 1,860 for about 40 seconds, then finally drop to 1,400 at about the 1:00 mark. Idle then slowly continues to drop until it dies.

    Last edited by Matt in TN; 08-11-2019 at 02:44 PM.

  2. #2
    Super Moderator Purple's Avatar
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    Sounds like mine when I turn the fuel off and run the float bowl dry

    Check the tank is breathing, and take a look at the float levels – probably something simple, but the trick is to find it ……

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    Senior Member elime's Avatar
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    What size pilot jet? Should be #40. I think #42 works a little better than the #40.

    Replace the main jet with a BRAND NEW #114. No matter how good it looks replace it.

    Check the fuel line going into the carb is flowing freely.

    Keep the possibility of a bad CDI in mind. I have two '87's and had Tom Bauer fix one CDI and I bought a new one.
    Long live the internal combustion engine!

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  5. #4
    Junior Member Matt in TN's Avatar
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    Quote Originally Posted by Purple View Post
    Sounds like mine when I turn the fuel off and run the float bowl dry

    Check the tank is breathing, and take a look at the float levels – probably something simple, but the trick is to find it ……

    Welcome to the boad
    Thanks! I should have added - it's a Clarke tank so no rust. I did ditch the old gas of course, and check/clean the stopcock. Gas flows freely through it to the carb, and it runs the same whether the gas cap is on or off (checking for a clogged breather) and whether the stopcock is turned to on or reserve.

    I adjusted the float to the spec in the book when I rebuilt the carb, but I did NOT replace it or check the floats themselves for leaks.

  6. #5
    Junior Member Matt in TN's Avatar
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    Quote Originally Posted by elime View Post
    What size pilot jet? Should be #40. I think #42 works a little better than the #40.

    Replace the main jet with a BRAND NEW #114. No matter how good it looks replace it.

    Check the fuel line going into the carb is flowing freely.

    Keep the possibility of a bad CDI in mind. I have two '87's and had Tom Bauer fix one CDI and I bought a new one.
    The kit came with a brand new main jet marked 114, but it wouldn't thread cleanly into the receptacle. I didn't want to force it and mangle my threads, so I used the old one. I tried to compare the two jet sizes, but I didn't have an accurate way to measure a hole so small - best I could do was find a wire that just fit through and it seemed to have a similar gap on both jets. Not very accurate, but at least they're close in size. How much different in actual size is a #40 from a #42?

    Where did you find your new CDI?

  7. #6
    Junior Member Matt in TN's Avatar
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    Is there anything to the fact that it won't drop to idle right away when I release the throttle? It seems like a big clue that it holds at 1860-sh for 40 seconds, THEN drops to low idle. I'm just not sure what that clue is telling me.

  8. #7
    Senior Member elime's Avatar
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    Quote Originally Posted by Matt in TN View Post
    The kit came with a brand new main jet marked 114, but it wouldn't thread cleanly into the receptacle. I didn't want to force it and mangle my threads, so I used the old one. I tried to compare the two jet sizes, but I didn't have an accurate way to measure a hole so small - best I could do was find a wire that just fit through and it seemed to have a similar gap on both jets. Not very accurate, but at least they're close in size. How much different in actual size is a #40 from a #42?

    Where did you find your new CDI?
    There is a very real possibility your high speed miss is being caused by a worn out 114 jet. I have been party to fixing a high speed miss on 3 TW's by replacing an old 114 jet with a new one. No guarantees it will fix yours but at least eliminate the possibility it is the problem.

    I don't know the size difference between a 40 and 42 pilot jet but when I upped mine to a 42 the use of the choke became much, much less. Any chance yours is a 38? I got a 38 in a crappy carb kit I bought years ago.

    I think I bought the 87 CDI from Bike Bandit. I also had one repaired by Tom Bauer. And I bought one from some company in Poland or Czechoslovakia. That CDI had has adjustable timing.

    See if this link works: https://www.bikebandit.com/oem-parts...828&sch=226669
    Last edited by elime; 08-11-2019 at 09:24 PM.
    Long live the internal combustion engine!

  9. #8
    Junior Member Matt in TN's Avatar
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    Good to know - thanks. My carb kit was an aftermarket thing and had a few parts that didn't fit. So to be safe, I just ordered a new OEM main and pilot jet (114 and 40) from the link you shared. Might as well eliminate the cheap possibilites first - thanks for the tip!
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  10. #9
    Junior Member Matt in TN's Avatar
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    Well - I finally got the OEM main jet, pilot jet, and went ahead and bought the pilot screw set while I was at it. It runs much better with the pilot screw 2.5 turns out, but after a couple runs up the road the spark plug told me it was too rich. At 2.0 turns out the spark plug says it's about right, but I can't really run it at full throttle to be sure because it's still having the missing problem. So I hooked up a timing light via this thread: https://www.tw200forum.com/forum/tec...check-cdi.html

    At high throttle the sparks are going too fast for me to REALLY tell anything, but it seems like the light is less constant when it's missing. At idle when there's a miss there's a most definite blank spot in the flashing that corresponds exactly with the miss. So I guess that means I need a new CDI. Dangit. Oh well - at least I know now.
    Fred likes this.

  11. #10
    GOF
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    Senior Member GOF's Avatar
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    My '87 was having issues. It would fire and quit. When I could get it to start, rarely, it didn't like to take throttle. Got one of Tom Bauer's CDIs. Problem solved! He was helpful with information and shipped so fast I was surprised it was even possible.
    https://www.tw200forum.com/forum/tec...200-bw200.html
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