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Yamaha Tech Originally the Yamaha XT600 Tech Forum, due to demand it now includes all Yamaha's technical / mechanical / repair / preparation questions.
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  #1  
Old 26 Feb 2013
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Spare a thought for my XT350??

Hello all! New to the forums, but was wondering if you could help me with a baffling issue.

I have a 1994 XT350 and have done a bit of work to it as it was a bit of a pig when I bought it and need some tlc.

I have stripped down the carb, replaced the jets with original ones and replaced the diaphragms as the bike wasn't running great. I've also replaced the coil as I suspect the original was on its way out.

The bike now runs great, accelerates nicely with plenty or torque and is no longer running hot. Starts easily and there is no signs or spluttering or struggling through all gears and speeds accept when I hit 50mph and this is where things get baffling.

You will accelerate up to 50 then bam nothing. The bike will not go any faster. There is no struggling or spluttering, just a constant speed of 50.
I have tried dropping it down a gear and giving it some welly, or leaving it in 6th and cranking the throttle, but nothing happens. The rev counter doesn't go up along with the speed.

I have checked the rubber boots as well as the chain and sprockets to see if they have been altered but they are all fine. The bike doesn't struggle at lower speeds and accelerates nicely through all gears otherwise.

I have looked endlessly for a sign of a restrictor to no joy.

I'm lost.

The only thing I have left to do on the bike now is to check the valves, but I can't see that fixing this problem. I could be wrong though.
Any help would be great as it would be nice to do more than 50 down the motorway!

Cheers
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  #2  
Old 27 Feb 2013
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Sorry can't help you really.
I had an XT350, of about the same year.
They are well capable bikes.
On the motorway, they were not their best atmosphere.
Sure they do more than 50 mph, I used to get about 70 out of mine, but it didn't feel good.
The front forks were just to "spindly" at that speed for my liking.
I would say a comfortable cruising speed on motorway would be below 70, at around 65 mph.
Now, mine was used and abused, so there wasn't the tightest of bushings etc.

It does start to sound like yours is a fuel related problem.
If you pull the fuel pipe off the carb. inlet, does the petrol flow out fast and free?
No kinked fuel pipes?
How about taking it for a hard ride, with the fuel cap left of the fuel tank?
Maybe a fuel vacuum problem?

Cracking bikes !!

vette
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  #3  
Old 27 Feb 2013
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A restrictor will limit engine speed, rate of acceleration.. I don't think it will limit top speed in any gear to a set speed.

A badly slipping clutch will also limit top speed, but it will also limit acceleration - the engine speed will increase faster than the road speed - usually found on steep hills, the speed will drop and the engine speed increase!

Do you have a wiring diagram for the electrics? Assuming you do then

Look at the ignition system - there will be a 'black box' that fires the ignition coil/spark plug. There will be many connections too it. You want to find the ones that come from

Carby - throttle position and/or manifold pressure
Bike speed - possibly from the speedo?

If you find one related to 'Bike speed' that could be the one limiting your top speed.

If you find 'Carby' then it (the bit on the carby) could be faulty. They can be used to stop the bike exceeding the pollution requirements at low speeds...

------ I too don't think it is the valves. But if they have not been done for a while .. you should do them.

------- Good luck.
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  #4  
Old 27 Feb 2013
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Yes, make sure the valves are in spec.

Will the bike red line smoothly in lower gears?

Blocked main jet?

Air filter?
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  #5  
Old 27 Feb 2013
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As vette says, check the petrol tank vent is OK, not blocked.

Also, get to your 'max speed', in top, and kill the engine.
Check the state of the plug as described in this recent thread

http://www.horizonsunlimited.com/hub...935#post412258
It may give a clue.

Is there a possibility, when you re-assembled the carb, something didn't go back as it should? Maybe check against a good workshop manual.

As suggested above, if there's some wiring link between speedo input and the 'black box', (I don't know if there would be, 1994 sounds a bit early for that sort of thing) there may be an issue there.

Good luck!
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  #6  
Old 27 Feb 2013
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Sounds like your secondary is not opening, that uses the vacuum of the engine to open, remove the air filter pipes & look down the right carb (one with the diaphragm) and rev it up & see if its opening properly.

Any leaks/splits in the pipe that makes the vacuum operate will cause problems, and if the diaphragm itself has any tears or not seated correctly will also make the secondary not operate.

Mezo.
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  #7  
Old 1 Mar 2013
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Okay I have checked the petrol tank vent and fuel lines and they are flowing freely, no problems there.
The clutch doesn't seem to be slipping as the tacho increases consistently with the speed and there seems to be plenty of pull with out that clutch slipping feeling.
I haven't got my manual on me at the moment so can't check the electric diagram. this bike has had a rough life and the electrics are not great on it requiring me to rewire some of it and replace connections. I have cleaned them all up and checked them all over, but I will check the black box to see what the wiring situation is there.

Quote:
If you find one related to 'Bike speed' that could be the one limiting your top speed.

If you find 'Carby' then it (the bit on the carby) could be faulty. They can be used to stop the bike exceeding the pollution requirements at low speeds...


I didn't realise that this could limit the top speed so thanks for that!

I've checked the carb over since and all seems fine, checked for blockages etc and the bike has never gone over 50mph even before I stripped and rebuilt the carb.
The secondary stage does seem to be opening now as there is a lot more acceleration and power than previously experienced. I'll double check though as suggested.
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  #8  
Old 2 Mar 2013
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Did you check to see if the float needle valve filter is clean? any debris from your tank is stopped here, if its blocked (common) it can starve your carb of fuel.



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  #9  
Old 2 Mar 2013
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A check for over all fuel flow.

With a full or near full tank of fuel (full tanks will show vacuum blockage faster than a near empty tank);

Turn the fuel off.
Remove the bottom of the carby float bowel. Or at least a drain bolt.
Place a clean container under the float bowel (an old 4l ice cream container is good).
Turn the fuel on.
Watch the fuel flow into the container.
Watch for a while - it needs some time for the vacuum to build up. If you get to over 2 liters and the fuel is still flowing at the same rate I'd call it good.

This only checks fuel to the float bowel (includes float needle, fuel tank, petcock, lines etc), not the main jet etc.
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  #10  
Old 2 Mar 2013
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Quote:
Originally Posted by McCrankpin View Post
As vette says, check the petrol tank vent is OK, not blocked.


As suggested above, if there's some wiring link between speedo input and the 'black box', (I don't know if there would be, 1994 sounds a bit early for that sort of thing) there may be an issue there.

Good luck!
.
There is no electric or electronic restriction on these bikes of this age, that I know for sure.

No need strip the speedo, or start looking for interconnecting wires.

If the fuel is OK, which it is starting to look like, then we have to look at the other two inputs.

1/ Getting enough air?

2/ Getting enough spark, at the right time.


For the first thing, I would ask

a) is the air filter clean, ?
b) is it unrestricted air getting into the filter, ?
c) and once past the filter, is the rubber interconnecting pipe, not being sucked flat, with the engine trying to pull air through the air filter, and on the open (dirty) side of the air filter.?

For my second point.

Use a new piece of copper spark plug HT lead.
If the current lead is carbon string based, then throw it away.

Get a new NGK plug, and gap it to 25 to 30 thou' Dr8 ES-L, spend a bit extra and get a platinum tip one. DONT OVER-TIGHTEN IT, JUST THE SLIGHTEST OF "NIPS)
As a guide, a new 12 inch hacksaw blade width is generally 26 though, so if you don't have a feeler gauge, then this might help you.

Now give it a try, come back with the results please,

Helps the next guy also.


vette
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  #11  
Old 4 Mar 2013
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once you get up to your 50mph,can you pull in the clutch and get the engine to rev any faster?
if for example 50mph=5k revs,does dropping a gear make the bike rev higher?
any chance the choke isn't fully off? check at the carb end,not just the lever
sure you are getting full throttle travel? something may well be limiting the cable or possibly in the carb stopping the slide from moving
Andy
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  #12  
Old 4 Mar 2013
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Many many years ago i had a dt that had no top end power but pulled well through the gears then just seemed flat n gutless, turned out it'd been gear down but as you've checked sprocket sizes/teeth my next suggestion would be similar to ten660 reply as when i rebuilt my wifes bike i routed the throttle cable wrong n therefore wouldn't fully open carb, also had end of a cable tie restricting opening causing same prob. Hope its something simple like this. Good luck
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  #13  
Old 5 Mar 2013
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I once had an XT550 that had the same symptoms, struggled to go over 100 KPH.
After stripping the carb & cleaning it out making no difference, I had the ignition CDI unit repaired. Turned out the ignition advance circuit was failing in the CDI unit when it warmed up, and was a common problem with them.
Problem solved.
It's easy to try another unit.
Bob
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  #14  
Old 6 Mar 2013
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I've had two of these - a s/h one from 1985 and a new one bought in 1992. Fine bikes. However, the second one was a massive disappointment in performance compared to the first. I later learned that the older bikes were (I think) 33 bhp, but the post 91 bikes were restricted to - wait for it - 19 bhp*. The feeling of riding it certainly backed that up. The newer bike would barely crack 60. There were two restrictors: one in the inlet, which was simply a collar sandwiched between stubs and carb, easy to file to flush or just throw it away. The second one was inside the silencer, and I never did work out how to get rid of that. But the bike felt numb at higher speeds, although it would accelerate fairly well at lower speeds. This may not be the issue with yours, but it might be relevant. If you have that collar in the inlet, then it has been factory restricted.

*Apparently for the Swiss market, although it affected all UK bikes. Or so a dealer at the time told me. This was before the internet, where such information was less easy to find.
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