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Discussion Starter #1
Help please I'm tearing my hair out. I'm in the UK and bought a (US Import) project 91 Tw from the internet as a non runner last year. I knew the bike had bits missing + broken but when it was delivered it was also missing the CDI and the pilot jet mounting was broken on the carb body.
I gritted my teeth and bought an Ignitech CDI and chinese BW Mikuni. Although the bike starts fine it wont rev cleanly or pull when you try to ride it. If you slowly open the throttle it usually revs but any quick action makes the bike pop and splutter. The plug is very sooty but this is maybe because its not got hot enough to burn off. I've set the valves normal and slightly loose no difference, new plug, new fuel, with and without air filter, sprayed around the carb mount for leaks, dropped the needle, checked for fuel flow and vacuum at the filler cap. Tried the carb with and without the vacuum vent blocked. Tried with the crankcase breather pipe off. I've checked the carb several times and put fine wire and sprayed through all the holes. Float level looks fine I tried it normal and slightly lower.
Any ideas would be appreciated, i don't want to be beaten by it but am getting close.
 

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You need to determine whether it is an ignition or carb (fueling) problem.

Use an automotive inductive timing light to find out if there is a consistent spark through out the rpm range.

jb
 

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So what this boils down to is – “it won’t rev cleanly or pull when you try to ride it. If you slowly open the throttle it usually revs but any quick action makes the bike pop and splutter. The plug is very sooty”

The words “Ignitech CDI and chinese BW Mikuni” do not exactly inspire confidence either ….

Can you confirm that the Ignitech CDI is the one that is pre-programmed for the TW200 – and that the Chinese carb (presumable meant for the BW), has the correct jets in for the TW200.

Sounds like she’s running terminally rich …….
 

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Discussion Starter #4
Thanks for the replies. Yes the Ignitech is the unit for the TW200. The carb was from ebay I followed a link suggested on an earlier thread. Does anyone know what jet sizes have been successful in these carbs? i assumed that it would be suitable out of the box as other people seemed to be pretty happy with it. i will need to borrow a timing light to check the spark. I will let you know how I get on with it. unfortunately in the UK there aren't that many TW200's (loads of 125's) so spares like CDI's and carbs are few and far between.
 

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One would hope the aftermarket unit would be set correctly. The soot observed would indicate an excessive rich mixture.
I assume you checked compression? You'll need to run through a jetting. Following someone else's lead might work but why bother?
Jetting is easy and you'll get a better product in the end. Be sure your float level is correct and needle functioning proper. Check out
part numbers on TW125s on CDIs wouldn't be unheard of to have same system as TW200 Perhaps even carb bodies interchange?
 

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Discussion Starter #6
Compression tested and up at 140psi which I think is ok. Carb stripped again to check jets. Sadly the chinese carb has no numbers on the jets. You get what you pay for!
 

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Maybe it's possible to repair the old TW carb with liquid metal and to rebuild the pilot jet mounting.
You can send me the carb, if you don't want to try it.

An other option is to buy a TW200 carb from eBay USA.

In the most cases it's better to rebuild/repair OEM parts as to buy cheap Chinese copies. And one of the biggest problems is to get spare parts for this stuff.
 

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Compression tested and up at 140psi which I think is ok. Carb stripped again to check jets. Sadly the chinese carb has no numbers on the jets. You get what you pay for!
The China carb will take standard Mikuni jets....readily available on line, or your local motorcycle parts shop.

You will need to experiment, but I think the main jet is somewhere between a 115 to a 120.

The guys on the ChinaRiders forum have been successfully using those carbs, and have the jet info.

jb

Edit: I checked the China Riders Forum...

China Mikuni (VM26) clone comes with: 100 main jet and 20 pilot jet.

Most change to a 120 main and a 30 pilot jet...you will still need to experiment to see which works best for your application.
 

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Discussion Starter #9
Thanks for the info on the jets and the possibility of repairing the original. It could be tricky to repair as I think there must be threads in the tube (broken off and missing) and also threads in the body so if I fasten a new tube on the two sets of threads would need to line up for the pilot to screw in cleanly. Think its maybe time to look for one in the US.
I took the exhaust off today to check there were no blockages. All good including baffles. There had been sooty fuel coming out of the joint between front and back parts. Tried with air filter not connected, plug still sooty and bike just dies when you set off.
 

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Discussion Starter #12
Yes flow ok on normal and reserve, checked for vacuum in tanks as well. Valve timing checked and ok. Tightened exhaust valve gap as it was chattering a bit. Managed to get up into 3rd gear but still no power and prone to dying, this was with the carb open at the intake. I then tried with the filter connected again and the bike ran worse. Plug sooty. I think I need to try some smaller jets in the chinese carb unless anyone has an original carb that they would like to sell to me?
 

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Discussion Starter #13
I'm thinking of buying a genuine Mikuni VM26 to try instead of the chinese version. Anyone any views on what jetsI should ask for. The company who supply (Motocarb) say I have options on main, pilot, needle, needle jet, throttle valve and main air jet.
Any views would be welcome.
 

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Discussion Starter #14
I went ahead with the genuine mikuni VM26........Amazingly it works, the bike starts and pulls like I presume it should do. Today I have sent off the registration info which will hopefully mean that fingers crossed I can ride it on the road in the uk in a few weeks time.
 
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