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Discussion Starter #1
I’ve made a mild custom xt200. The engine is nearly identical to the TW. I’ve been chasing a few issues and think I have it narrowed to one.

The bike idles perfect when hot, and at Full Throttle it is shockingly fast. No misfires, just smooth.

But, at 1/8-1/4 throttle, the RPMs jump quickly up and down +/- 1500 with occasional misfires.

Recently replaced primary coil. The stator and pickup coil resistance is within spec. I replace the CDI with a new one and it got slightly worse(possibly more aggressive advance?).

Every connection has been cleaned and dielectric applied. Tried different plugs. Valves are good. Triple checked for intake leaks.

This problem has developed over a week of riding. Before this, I was extremely happy with my set up: K&N filter, vm26 with 17.5 pilot and 190 main, stainless xt 225 header, I cut up a brand new FMF And added a “power bomb” 30” down the pipe from the exhaust valve. When it runs, It feels like my old xr250! Smooth aggressive torque with occasional popping on deceleration.

I’m ready to chuck it off a cliff. Help.
 

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Hopefully Kris (Admiral) chimes in as he has the same bike.
 

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My guess is its the carburetor. Take it apart and clean it. If you gets the smallest speck of dust stuck in the pilot jet the engine runs very poorly. Open the idle mixture screw a turn or two and see what happens. Perhaps if you have a backup carb give it a try. Good luck. Remember to report back what it was. We all learn from the experiences of others.
 

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This problem has developed over a week of riding. Before this, I was extremely happy with my set up: K&N filter, vm26 with 17.5 pilot and 190 main, stainless xt 225 header, I cut up a brand new FMF And added a “power bomb” 30” down the pipe from the exhaust valve. When it runs, It feels like my old xr250! Smooth aggressive torque with occasional popping on deceleration.
Popping on decel usually means the engine is running lean. With a less restrictive air filter and less restrictive exhaust, the air (mostly) is flowing through the system faster possibly causing a lean condition. By changing the air filter and exhaust you most likely have to still tune your jetting. I have no idea what a "power bomb" is. What does your sparkplug color look like?

To clarify, the XT ran fine before you made the above changes? Did it have a stock air cleaner and exhaust before the above changes? Is the carb different than stock? As elime mentioned, most likely a carb issue (jetting). I'm not very familiar with carburetor sizes but you must have a different size carb than my '83 XT as your pilot and main jet numbers are way different than stock XT and my XT. Stock is something close to 40 pilot and 122. My XT with the Super Trapp is at 40-112.

When I got my 83 XT it came with an aftermarket Super Trapp muffler. The engine was running very rich with the stock air cleaner and aftermarket exhaust. I had to drop the main jet down before it started running correctly. I had to experiment with the jetting and it took several changes before it got dialed in. I took small steps to make sure I didn't go too far and lean it out. My issue was engine stumble at half-throttle and more pronounced in 4th and 5th gears. Started and idled ok and ran fine at slower speeds.

I see you've also gotten rid of the electricals (or most of them anyway). Just an observation not related to the issue.
 

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He did say he "triple checked for air leaks" so I discounted that but worthy of the mentions again. That would then make it a quadruple or more check for air leaks? :p
 

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Discussion Starter #9
Update! This is a great example of electric issues presenting as Carburetion problems.

When I check the timing with a timing light, the advance is bouncing around erratically at lower rpm. It’s consistent with no advance at low idle, and advance at high RPM, but 1000-2000 it is chaos.

I have tried a new CDI, different coil, different coil caps, different plugs...

The only thing I can’t replace is the pickup coil, as they are not made any longer. It has the proper resistance and it’s a simple Hall effect sensor and they “never” go bad.

I’m still stumped. Next step is re wire the whole bike I guess? Or chuck it in the lake?
 

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He did say he "triple checked for air leaks" so I discounted that but worthy of the mentions again. That would then make it a quadruple or more check for air leaks? :p
What is a good way to check for air leaks on the TW?
 

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Usually, spray a combustible gaseous substance around the boot area with the engine running – if it has any effect, you have a leak. Of course, this may cause the entire bike to go up in flames, but with the correct insurance, that could simplify things a lot

Far be it for me to propose simply wrapping both boots with a couple of feet of gaffer tape – but that’s the wimps way out

Nope – you want to go for the “spray option” – imagine the tales you will have to tell of how you jumped back from the bike – “almost burnt the whole place down” etc

Honestly, the tape method pales by comparison – forget I even mentioned it ……
 

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Thanks, haha. I guess first I'll get a replacement for the needle spring I lost the first time I took the carb apart (the tiny one that seats the needle) ... didn't realize there was a tiny little spring in there when I took the carb apart, and it's gonzo ... so I'll make sure and get the carb back to 100% (the bike does run) before I attempt setting anything on fire ;)
 

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Seriously though – chasing an air leak from the boots is fairly straightforward. You just have a spray something potentially inflammable onto the boots that will either cause a momentary increase in revs, or in the case of something like WD40, a decrease (perhaps). If your boots are intact, neither will have any effect whatsoever

I only mention gaffer tape as a quick simple (non-combustible) solution that many of us have lying around, that is risk free and effective

Small cracks in the rubber (like that spring you’re looking for) are almost invisible to the eye ……
 

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Discussion Starter #15
Problem solved!

I retarded the timing by about 1 degree.

The timing is not adjustable, so I had to cut slots in the pickup coil bracket. I might get a new used pick up coil, and cut 3/16” off the magnet.

Apparently the exhaust I made is scavenging so well at low Rpm, that when the advance kicks in around 1200, it knocks. I wish I could just delay the advance to 2000 RPM. Seems like it’s advancing really early.

Anyway, thanks for the answers. #NotaVacuumLeak.

Also, the FMF pipe out of the box will make any bike run worse! Total scam. It’s 50” of straight pipe! Exhaust systems need a pressure drop or change at 28”-30” from the exhaust valve. 50” of straight exhaust pipe will NEVER work well.
 

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