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Discussion Starter #1
Hey all, i have a 1990 tw200 that has been giving me fits. hopefully, im getting close to getting it lined out but i think i have a lingering spark issue. when i try to start the bike, it pops a bit and cranks longer than i think it should but it will start on its own after 5-10 seconds of cranking. once it starts, it idles fine, whether its cold or up to temp. however, it will basically take no throttle. if it revs up at all, it almost immediately starts to miss and tries to die. if i apply throttle for longer than 2 or 3 seconds, it pops (i think from the intake) and dies. it will start right back up on its own but again will take no throttle. also, when i give it just the slightest throttle to get it off idle to where it misses but is just on the cusp of popping and dying, the exhaust smells very rich.
the tw actually ran last year and i was able to ride it around the property several times and had probably an hour of run time on it. on the final ride, it started to sputter like it was running out of fuel so i limped it back to the garage and thats when it began this issue of not taking throttle. i *thought* it was a carb issue (the stock carb was a pooch so i had been running a chinese 01+ clone when the bike was operable) so i tried another new chinese carb and the symptoms did not change. at that point, i gave up on the 200 engine since it was noisy and i wanted to do a 225 swap anyways so i figured spending anymore coin on the 200 was wasted money.
fast forward a few months and i bought an 02 ttr225 with no spark to swap its drive train into my tw. i fought diagnosing the ttr for about a month before just swapping the stator, cdi, and pulser coil from the tw onto the ttr and suddenly, it had spark. i had already replaced the stator on the ttr and the pulser coil passed the test in the manual so i think its cdi had died. i cleaned the ttr carb, got everything tidied up as best i could, and sure enough, the ttr started. it sounded good and i let it idle for a minute then went to give it throttle and it popped out the intake and died. fired it back up and found out that it now has the exact same symptoms as the tw did when i mothballed it a few months ago. the ttr still wore its factory carb so i swapped the new chinese tw carb onto it and it made absolutely no difference; it idles fine but just misses, pops, and dies when any throttle is applied.
at this point, two different bikes with two different engines and three different carbs have had the exact same issue with the same symptoms. based on some reading, i have a theory that the factory tw cdi is advancing the timing way to much and causing these hassles. i think it may be cutting spark or may be overadvanced enough that it doesnt ignite the fuel and it gets send out the exhaust which would account for the rich smell. unfortunately, i dont have a way to actually test the cdi and im also fairly certain that the factory ttr cdi died its own death and simply wont produce spark at all. the only components to stay constant through all of these symptoms are the factory tw stator, pulser coil, and cdi box so i believe that one of them is the culprit.
has anyone had these symptoms before or has anyone heard of a cdi box being able to cause these symptoms? i hate to throw more money at it without being able to diagnose the part or swap it with a known-good one but id like to have this pooch percolating in the next two weeks and im running short on time. thanks!
 

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Other than smelling rich, your symptoms point to a lean mixture, such as from a vacuum leak.
Is boot from carb to intake intact?
When carb was inserted into boot was it lubed with water, diluted liquid soap or even spit?

BTW, it could smell rich from fuel present without combustion.

Is spark plug getting wet?
 

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MY TW CDI did exactly the same thing as yours. Since there are no tests for the CDI, I tried everything else first. Spark plug, China carb, ignition coil, all stator coil tests, fuel filter, etc. Still ran EXACTLY the same with everything I swapped out. Would fire up and run fine at idle but crack the throttle and it would sputter and die. Replaced CDI from Tom Bauer and it fired right up and ran perfectly. Tom says the advance-curve circuit goes bad on the original CDIs. Install one of his and crack on!
 

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I think you're right. Sounds like spark timing is way advanced.



Other than smelling rich, your symptoms point to a lean mixture,
If it's lean enough, the spark won't ignite the mixture every cycle, and raw unburnt fuel would be sent out the exhaust.

^which would explain the on-throttle lack of power, but that would cause popping in the exhaust, not the backfiring through the airbox.
 

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Discussion Starter #6
i appreciate the responses! i had been in contact with Tom for a week or so while intermittently trying to diagnose the bikes. based on your responses, a few responses from yamaha pages on facebook, and the little parts swapping and testing that i could do, i just went ahead and ordered a new cdi from Tom. im not happy with spending the money on a 'maybe bad' part that i cant accurately fault test, especially considering that i gave $80 for the tw and $450 for the ttr, but letting them sit in the corner torn apart wont get them running and local shop rates will eat me up quickly if i have to go that route. thanks for the responses and ill post back in a few days when the new cdi lands and i get a chance to install it.
 

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Discussion Starter #7
got one of Toms cdi's installed tonight and the bike fired right up and took throttle for the first time since ive owned it. thanks for the responses, cant wait to ride this pile when i get it put back together!
 

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Thanks for the details on your symptoms. That will help others diagnose if they should have issues. Have fun riding it now.
 
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