1989 XT600, Just bought it 8 months ago, has 7000 miles on it.
The engine will miss and sometimes even die at speeds around and over 50 mph, but will usually start right back up after coming to a stop. It is not an intermittent problem, it happens every time I ride it. I have had the carburetor overhauled by a local shop. I took the fuel tank off and flushed it and cleaned the fuel valve. Fuel seems to flow good to the carburetor. Bike looks great and low miles.
Could this be an ignition or wiring problem?
Any help would be greatly appreciated.

Is it only after a certain rate of revs?

Sounds like an ignition or CDI problem.

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Tom
I'm a country member...

Coil, ht, plug dying at higher rpm? Or the little fuel filter in the carb above the float bowl valve blocked? Or clogged jets? That milage and age means alot of sitting. Side stand cutout switch, if there? Kill switch?

It always seems to cut out or die at around that 50+ mph, or 3500 to 4500 rpm range in high gear. And it does so more under acceleration, not steady cruising. Although it wont allow me to get over this speed under any acceleration.

I have tried new a spark plug, and the kick stand switch is already bypassed. I may have to explore the carburetor and filters again, even though they were just recently cleaned.
Is there a way to diagnose the ignition without just replacing the CDI and coil etc. until you find the problem? These parts seem quite expensive.

When I read your first post I thought 'fuel starvation' and even more so after reading this. The conditions you describe are exactly the conditions where there is the greatest demand for fuel. Under 50 mph, or gently cruising, the fuel system can supply enough, but if you demand more (higher speeds or acceleration), it can't. I'd check every last bit of the fuel supply, including all the little inline filters and jets, before looking at the electrics. As Gawain said up there ^^, it's had a lot of standing about, precisely the conditions for fuel systems to clog up.

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2006 XT660R daily ride, 1994 XT600E about to be reborn, Blog: http://goingfastgettingnowhere.blogspot.com/

As other people have said,it sounds like some kind of fuel starvation.If I were you I would fit a clear inline fuel filter and when it next cuts out look at the filter and see if it's full of fuel.If it's not full of fuel then check the vent on the fuel cap,try riding the bike with the fuel cap open(obviously not with a full tank,but with enough fuel in there to replicate the conditions that cause the problem).If this cures the problem then the vent needs clearing.
If the filter is full of fuel then you know the carbs getting fuel,next step would be to remove the small filter on the float and clean all jets.Hope this helps keep us posted.

hi there if its cutting out like someone has flicked a switch it sounds electrical . if it slowly dies like you have lost power i would say fuel , have you tryed unsqrewing the filler cap a bit as the engine dies if its safe to do so ,you may have a blocked vent hole ie no air in no fuel out . do,s it always restart after dieing out if so fuel or air or a filter sounds a good bet .

Have you checked the diafragm on the second carb?
If that isn't working you will only get half the fuel you are due at higher revs.

__________________
Tom
I'm a country member...

Thanks for all the reply's and suggestions.
Very positive feedback, much better than I get at any Yamaha service department in my area.
Glad I found this forum.
I will start with these and get back, kinda cold and not real conducive to testing motorcycles here in Colorado in January.

Next time it happens take off the fuel filler cap. If it clears or makes a big sucking sound the air bleed holes in the filler cap are blocked. (a common fault)

I had a similar problem and posted about it on here if you do a search it was about 2 years ago you will find alot of advice suggestions.

On my TTR600 same as xt engine
I have had trouble with the small filter in the carb. If the throttle is held open for a while bike will splutter as if you are running out of petrol then cut out, you stop by which time fuel has passed through the blocked filter and bike starts again stright away and is fine untill you hold the throttle open and engine needs more fuel then the blocked filter will allow through I have since removed the filter in the carb no more problems.

Om my 3AJ XT it would be fine untill it was ridden hard for 10 miles or so when the engine got hot. The bike would start to surge and hesitate some times it would just pick up again and it would go great for a couple of miles then start surging again. If I stopped for 20 minutes or so engine cools down and is ok again untill it gets hot. I checked fuel, tappets, carb, air filter changed the plug, coil,lead HT cap checked lose wires swapped the CDI still the same I then bought a spare engine swapped the generator and it sorted the problem I am sure it was the pick up coil that was breaking down when hot.
I could not get a pattern generator with the same pick up as my bike and Yamaha wanted about £400 so bought a blown engine.
Where abouts are you in the country?
Good Luck

Check your manifold rubbers,sounds like it could be drawing air.
These tend to fall apart.
Mine would drive fine for 20 miles then would run like s*** and not rev out.
Took the manifolds apart,cleaned them and stuck them together with silicone.
Perfect.

I had a problem like that on a XT550.

I replaced the CDI, coil, carb, everything and the problem would not go away. The two things bandied about so far have been fuel starvation or electrical which it may still well be. But also have a look at the final option...airflow.

On the XT550, but was not on my bike, the airbox had a snorkel to the CDI which was missing. The previous owner being a bright spark had covered the airbox intake then with some gauze mesh to stop crud from the back wheel being flipped into the airbox. This was all well and fine until the guaze corroded slightly restricting airflow.

Anyway, the upshot was the bike would accelerate nicely to 80KM/h (50mph) and then start to backfire and miss. Handfuls of throttle caused the engine to cut, cough and splutter and treat my chain like it was bubblegum.

Check that your air flow to the carb is not impeded in any way.

I had the same exact problem a few months ago. It would cut, sputter, and die if I went above 55 for more than 5-10 minutes. Then it would start right back up again after stopping. I tore down the carb and found brown flaky crap clogging the little white filter above the needle all the way to the fuel inlet. I was actually sort of surprised it ran it all.

So, I scrapped the little white screen and fitted one of those clear metallic ones, no problems since.