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1990 XT600 Rough Running 1990 XT600 Rough Running
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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 10 Aug 2014
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1990 XT600 Rough Running
I acquired a 1990 (marked on the headstock) bike that was non-running a few weeks ago. I got it running today. It is difficult to start (electric starter) and runs rough with occasional mild backfires. The engine wasn't rebuilt, it doesn't smoke. The carbs were knackered with broken screws and bits missing, so I sourced a fresher set from eBay. I haven't made any adjustments on the carb, just installed it. The exhaust is stock, but the air filter is K&N. I haven't replaced the spark plug or coil or spark plug wires. The airbox seems to be free of leaks, I did replace the carb holder boots between the carb and the manifold.

I'm not sure about the fuel/vacuum lines and haven't found a diagram to show me where everything goes. That might help.

So I'm asking where I might begin to make this engine run smoother and not backfire.

Thanks to all!

David
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Old 10 Aug 2014
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I'd say try adjusting the valves first.
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Old 10 Aug 2014
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Hard to start, backfiring and running rough could be a lot of things including valve clearances.

But it does sound like my old 43F. Bringing that back to life from a scrapyard was a long process and for part of that time the pilot jet was blocked. It used to rev OK with a big handful but would pop and backfire on the over run and it didn't want to idle.

I finally stripped the carbs (for the third time I think) and found the blockage. I used the bristle from a nylon brush to poke the blockage out and it ran much better after I put it all back together.

As always on a bike of that age, the carb rubbers can look OK but actually be shot and leaking. But you have already done that so you can tick that off the list.
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Old 10 Aug 2014
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I echo cleaning the carbs. Best way to clean is to get a gallon Chem Dip and soak the carb parts after disassembly. Remove any rubber seals, o-rings etc from the parts and do not soak them. After a couple of days wash the cleaner out with spray carb cleaner and chase all the passageways with a small wire or as above. It takes more than just spraying them.

I recently cleaned a carb for a friend. It was a 2007 XR650L Honda. It had been sitting for a few years and was gummed up so bad the needle and seat were stuck as well as the choke assy. They would not even pry out. 2 days in the chem dip and they came right out.

Be sure to check the brass tube the main jet screws into. It unscrews from the carb body. Check the row of hole in the brass body. I take a straight pin or needle and put through the hole to clear them then spin the brass emulsion or Jet tube to finish cleaning the hole. Be careful to NOT make the series of holes larger.

Wish you the best.
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'84 XT600 is now bored to 2nd oversize and new OEM pistons and rings installed. No more smoking.
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Old 10 Aug 2014
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Dont forget to take out the filter inside the carb and clean, also check the diaphragm. Buy a new spark plug iridium is good, empty tank and take out the fuel cock and clean both filters and inside tank.
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Old 11 Aug 2014
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In case you did not know, there is a filter on top of the float valve. You'll have to remove the float and needle valve then a small screw. You will have to grab the brass with pliers and carefully wiggle it out. You will see a screen on top of it.

When you remove the float pin, do not drive the pin out as you can easily break off the stand off the pin goes thru. Better to get some flush cutting wire cutters or a knife blade and pry out the pin carefully.
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'84 XT600 is now bored to 2nd oversize and new OEM pistons and rings installed. No more smoking.
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  #7  
Old 14 Aug 2014
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Thank you everybody. I'll let you know how things go. I feel confident I can get it sorted.
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