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Discussion Starter #1 (Edited)
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.

 

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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 ……

Welcome to the boad
 

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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.
 

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Discussion Starter #4
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.
 

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Discussion Starter #5
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?
 

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Discussion Starter #6
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.
 

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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/detail/yamaha/2jx-85540-m0-00/b1167111?m=146828&sch=226669
 

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Discussion Starter #8
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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Discussion Starter #9
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/technical-write-ups/6821-using-timing-light-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.
 

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Nice that it was mentioned that the original carb kit was an aftermarket one rather than OEM Yamaha parts. Many have had issues with the non-OEM parts.
While the full throttle issue could still be from lean main jetting I am guessing that based on statistical occurrences the '87's CDI is faulty.
 

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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/technical-write-ups/6821-using-timing-light-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.
It's tough to get a decent read on spark plugs with just a couple of runs up the road.
You have to ride under load at a set throttle position for a bit, before the plugs will read accurately.
In other words, find a gentle slope...
Use a fresh plug and travel up this slope under 1/4, 1/2, 3/4 and finally full throttle....checking the plug at each step of the way.
This will tell you which of your carb circuits is too rich or too lean...or just right.
Some guys will use a new plug for each step...your call.

http://www.hondachopper.com/garage/carb_info/plug_chops/Plug_Chops.pdf

Plug Chop how-to

How to read spark plugs

These "Plug chop" methods help indicate if the MAIN is good at full throttle, but by using different throttle positions and holding them while under load, will help to troubleshoot other carb circuits as well.

This page identifies Mikuni carb circuits and where they come into play in the throttle position: http://www.iwt.com.au/mikunicarb.htm


OR, you can do the "burble" test that I mentioned in another post:
Get your bike fully warmed up...
Find a LONG straight stretch of road.
Start off slowly and shift through all gears to 4th.
You shouldn't be going any faster than 20 mph at this point...then....
Crack the throttle wide open.
You should accelerate smoothly all the way to top speed, without any burbling, sputtering or missing.
The take off might be slow, but as long as the acceleration is S-M-O-O-T-H...it indicates that your jetting is pretty much bang-on and you are good to go.
 

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Discussion Starter #13
Thanks guys - lots of good info here. I just ordered one of the Tom Bauer CDIs, so I hope to have an answer soon.

Maybe once I have the CDI fixed I can do a better spark plug test.
 

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Discussion Starter #14
CDI came in, and it idles great now with no missing or stumbling. But it still won't run right under full throttle. Dammit.

I guess I'll pull the carb and soak it in carb cleaner overnight (I just used the spray before), and check the coil and spark pkug wire. Probably should hook up the timing light again just to see, but it was really hard to tell anything at much over an idke as the light flashed so quickly.

Does anyone have a link to the description of the various ways the carb components work and interact? I know I saw it before, but can't find it again now that I'm looking for it. It would help me understand and troubleshoot much better - it waa something showing the range of RPM the pilot screw adjustments affected, then the pilot jet, and the main jet. I think.
 

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Discussion Starter #16
I got pissed off and tired of ordering a part, waiting a week or two for it to come in, installing the part, and still having the same problem and repeating the whole waiting game over and over. So I went through everything I could think of that wasn't crazy expensive and just ordered and replaced it all. The good news is that the bike now works great. The bad news is that I don't know exactly what the issue was and I likely replaced a bunch of parts I didn't need to. Oh well - it was an old bike and they'd wear out eventually anyway.

For anyone searching, in addition to all the stuff I did before I added the following:

- Full carb soak and clean (instead of the spray with carb cleaner method I did before) - I did see some gunk come out of it here, so I'm thinking this was likely my issue?
- Replaced all internal carb parts I could find with new OEM parts (main nozzle, coast diaphragm set, all the o-rings and gaskets), although I managed to break the new 114 main jet when installing it so reused my old main jet
- Replaced the ignition coil, spark plug boot, and spark plug
- Replaced the fuel stopcock assembly and all fuel hoses
 
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