Yamah TW125 problems

[tris]

Morning all

For my sins I bought an old Yam TW125 (TW200 common in the US and essentially the same bike) single cylinder to go and play in the mud with my stepson

It is doing something I can't for the life of me work out

When I start it from cold (no choke) it revs its nuts off but if I ignore my mechanical sympathy genes and go for a ride to get it hot it then idles too low

So it feels lean when cold and rich when hot

What I have done so far:-

I've had the carb apart a dozen times and it's spent probably 2 hours hot in my ultrasonic cleaner so hopefully its clean. I put a new set of gaskets in the carb too

Tappets have been set to spec

Ignition is via a CDI and is non adjustable

It has new air filter and plug

I've checked and can't see any issues in the rubber connection between the carb and head that might give rise to an air leak

Pet cock has been off and checked and it flows freely

Things I have observed but don't see how it would affect anything:-

i) Winding the idle mixture screw in or out appears to do nothing - only found that last night
ii) Fuel level in carb might be a tad low, but its at an angle so difficult to be sure
iii) There is a hole in the exhaust between the head and the silencer

I've manged to beat a Breva 1100 into near submission but this simple little bike defies me

Any words of wisdom would be very much appreciated!

Cheers

Tris

PS, I'm on the TW forum too trying to work this out!

[Kiwi_Roy]

Sometimes putting a carb in an ultrasonic cleaner results in one of the plugs in a drilling getting rattled out,
If the idle mixture screw seems to be non effective perhaps it's a clue.
For it to rev its head off there must be too much air getting in, is the slide or butterfly sticking surely.
I know dick all about carbs, hopefully someone who knows those bikes will be along shortly.

[severely]

One question. Did you disassemble the carb when cleaning it? The conditions you describe are those of a plugged pilot jet. I recommend removing all jets from the carb and recleaning all, particularly the pilot jet {slow speed} and mixture screw. Making sure all O-rings, springs are in proper order on the mixture screw, if you can't get the pilot jet clear just buy a new one. Good luck.

[Mike Tashjian]

When I put the carbs on small bikes I back off the screw that holds the slide up for idle adjustment. I then make sure the throttle cable has just a bit of slack. The enrichener needs the throttle to be closed for proper operation.  If you start without it you are definitely going to be lean.  There should be at least a start full position and then a half warm up position.  Fully warm no enrichener after riding a bit, idle speed can be adjusted much easier.  A lot of the small bikes require the idle speed to be slightly adjusted at every weather change.  Mike

[Murray]

If you spray some WD40 or similar combustible around the intake manifold at idle and the idle speed picks up you have an intake leak either the rubber or the gasket between the head and casting. might be worth checking the manifold bolts are tight as well.

[tris]

Severely:- Yep everything was off and I could see through the jets. I can't be certain about the internal passage ways though

Mike T:- That's good info. The one thing I forgot to mention is that I fitted a NOC throttle cable as I thought the old one might be sticking. Maybe I didn't do that job as well as I might have!

If the slide was high, it seems to me that would render the idle circuit inoperative hence why the idle mixture screw appears ineffective? Does that make sense?

Thanks for your input everyone.

[Mike Tashjian]

Yes, the slide should be down to make both the enrichener and idle circuit work correctly.  I like to start with just a bit of slack in the throttle cable and move the bars side to side to insure the slack remains.  Then the low speed jet and slide can be adjusted for idle speed and quality.  The pin on the slide should start in the recommended position also.  It shouldn't mess with the idle but it can mess with off idle throttle response.  Mike 

[tris]

Cheers Mike

Since asking the question I had a google and found the Dellorto Tuning Manual (http://www.iwt.com.au/private/Dellorto%20manual.pdf) and had pretty much convinced myself on my hypothesis

I'll dodge around domestic management tonight if I can and go and have a play

Watch this space!

Tris

[tris]

Well I've made some progress

Did as Mike suggested and the bike started and idled reasonably. The sort of reasonable that makes you think "I'll dial that in once it gets hot"

However, as it warmed up the idle slowed down so much so that the only way to keep it running was to open the throttle or wind in the idle stop screw.

But that got me thinking

Its been the same story every time, idles ok cold when I've rebuilt the carb and put everything back to base setting. Slows down when hot which I've been managing by screwing the idle stop in which means it reves its nuts off next time I start it from cold.

This tends to suggest that the carb is OK, but I popped the carb off again and tore it apart (I'm getting good at that  ) to double check for blockages in the internal passages.
Spraying carb cleaner down every hole proved they're all clear

So I've been thinking - what else can change the idle speed when hot but not cold and come up with

1) Ignition circuit
2) Valve clearances
3) Exhaust blocking as it warms up

Valves I've set more than once and I'm sure they're OK and exhaust seems like a long reach, which leaves me with ignition

I plan to do the following in order of ease to do

1) Its a new plug but I'll get another and see what that does
2) Check the ignition timing. It allegedly had a re bore before I got it so its possible the cam timing is a tooth out or something. Easy to do as I can have a look at the timing marks
3) Check resistances across the coil/HT lead
4) Ignition is triggered via the stator coils somehow. Not sure how that works so I'll need to do a bit of reading
5) Spray and clean every contact between everything in the circuit

Any comments?

Cheers Tris

[Mike Tashjian]

I was just looking at the Mikunituning.com web page.  It gives a good explanation of the tuning for their carbs. I can't remember what brand the tw has but there are similarities between brands and that should give you some more insight.  It says the float level should be set and that the low speed circuit should be tunable after the engine warms.  It gives some remedies for your observed low idle.  One the float level and second the pilot jet may be too small.  And it states all the carb function from idle to about 1/4 throttle is working off the low speed circuit.  I had a tw200 years back for the kids to ride. They had a blast with it so I am sure your ready to have it running well.  Mike

[Moparnut72]

I would look at the advancer for the ignition timing. It might be greasy and sticks at full advance when cold and then works correctly when it gets hot and the dirty grease softens. Only a guess. I am also working on an old Yamaha 125. A 1964 YA6. The advancer on it was pretty sticky. I haven't run it yet but I am getting closer.
kk

[tris]

Thanks Mike

My baby TW is fitted with a Teikei carb and I'm not overloaded with info on it. Can you let me have a link to the Mikuni site you referred to as they might be similar?

Moparnut
No distributor on this bike. Just some weird "source coil" that triggers the CDI. Not worked how that works yet!

Re riding it, well the times I've been out on test runs trying to sort this out, well, it just makes me laugh as I power up to 52MPH 

Its so light and chuckable compared with the Big Breva and I look forward to riding it properly

[Moparnut72]

Yours is more advanced than mine. Mine is a points and coil ignition.
kk

[Mike Tashjian]

The web page I found with some good information which should help with tuning a carb is www.mikuni.com › pdf › vmmanual.  Many of the service manuals should have similar information.  Yes, top speed for even the 200 is pretty low.  My kids learned clutch operation quite quickly with that bike and could easily start off in second gear if they forgot to downshift in the yard. Amazing that you can have so much fun with those at such low speeds.  Mike