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TW 125 trouble starting and restarting


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My first post here so please be gentle with me ;-)

Bought a tw125 a few weeks back and I'm really pleased with it. However it's just started mucking about. If anyone could give me a couple of pointers I'd appreciate it.

I'm suspecting a carburettor issue.

Once started it runs great. Starter motor turns over fine. Good compression, there's a good spark and I can bump start quite easily. It doesn't idle correctly - either running too fast or too slow and thens stalls. Once it stalls, it turns over but will only fire after a long struggle!

I have had the carburettor apart and the starter plunger was a bit stiff to get back in to the housing so I'm wondering if it's not seating correctly? I'm also wondering if the throttle is sticking open slightly? I've cleaned it out with carburettor cleaner but I don't have an airline to really blow it out properly.

Is it worth persevering with the carb or possibly look at something else?

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Hi jim,,,a mate has the trailway 125" and I noticed its a pig to start when hot, Now going on moto x [4 strokes] they were the same.

and they cured this by adding a Hot start" lever, this injects more fuel, As I believe when engine is hot the fuel evaporates as it entres the head, :eusa_think:

Now am not saying this is your problem, but if your idle is hunting- you may have a pilot jet or air leak,

Now pop up to new members and say hello... :yeah:


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Sounds fuel related.

How fresh is the fuel?

Have you cleaned out the petcock?

Have you tried starting it with the fuel cap open (if the tank vent is blocked bikes can be hard to start)?

How old is the air filter?

Has the spark plug been changed?

Have you tested how tight the HT lead cap is on the coil wire (they tend to be twist in's?

Any cracks in the rubber parts between the carb and cylinder or carb and airbox?

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Sorry, fuel tap. Bike tanks tend to run on gravity so all the bits of grime and stuff goes into the tap. Most have a mesh in them that acts like a filter.

The basic rule of bike fault finding - unless you are sure what is wrong - is cheapest and easiest first. If nothing else it eliminates possibles and you know when you changed items.

The tank vent being blocked is a classic hence trying to start it with the fuel cap off.

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Bugger. Looks like I'm back at square one. Went out for a long ride yesterday. Stopped a couple of times and had trouble restarting. This is beyond my meagre skills now. I've had the carb apart so many times. Blown it through, rechecked spark and compression (as much as I can without a compression tester). Cleaned the petcock through and filters, I'm gonna have to surrender it to a local bike man to see if he can get to the bottom of it. Really frustrating cos once started it runs like a dream. You can only do so much as a novice:-(

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Aye jim, if it intermittent fault, its hard to find, :eusa_doh: ,,its inlimination process , coil could be breaking up when hot,, av see brillo pads in fuel tanks, that swish about , causing intermittent faults , :yeah:

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I'm wondering now if it's possibly something more fundamental. In fact I'm wondering if it's a couple of issues. Poss inlet valve prob and battery.

When I get it running (quite easy to bump it), it's backfiring every once in a while

Here's what I've checked:

Carb (cleaned thoroughly by me and a local mechanic), reset carb to spec, carb join to the cylinder, air filter, spark, cut end off ht lead, plug gap, valve clearance, timing and a load of other bits and pieces. I've got a new battery on charge which I suspect may aid starting but not tick over.

Can anyone suggest what I try next? I'm quite up for dropping the engine if necessary.

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