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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 28 Dec 2006
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Second carb opening

My XT600 3AJ would missfire and hicup at 3000 rpm or higher when engine got hot. After alot of trial and error I think I have fixed this problem by changing the stator plate but still using oringal pick up,
Now the bike feels low on power when I acelarate it feels like you are riding with the choke on.I have disconected the choke cable but no different.
Today I took carbs off again and to measure when the second carb starts to open it was at about 9mm I have adjusted it to open at 5mm but ran out of time to put every thing back together.
What affect would you get if the second carb was opening not opening till 9mm?
Is there any thing else I can check before carbs get put back.
I cleaned carbs about 5 weeks ago but found what I think is very fine rust at the bottom of float bowl,my bike used to have inline fuel filters on petrol pipes but these were removed by a mechanic as he said they were very close to the exhaust and as I had fuel filters in the tank they were not needed has any one had problems fitting inline filters?
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Old 28 Dec 2006
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Possibly secondary carb might have a jet blockage or wrong float hight. Run the engine till warm, then slowly twist throttle till you can see second carb opening up. The sound of the engine should change a little (not just getting louder but a slightly more deeper note) as more fuel/air is burnt. If the engine just increases in volume its time for another clean and check up. Use aerosol carb cleaner. As for 5mm or 9mm before 2nd carb opens,( I think standard is 5.5mm so 5mm will be fine) a to early opening would mean air speed through two carbs (rather than one) would be too slow so fuel/air mix wouldn't be so good and so less power at low revs (below 3000rpm) but above that will be ok. While a late opening wouldn't flow enough fuel/air at higher revs so would be low on power above 3000rpm. Fit inline filters and cable-tie them away from exhaust
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Old 28 Dec 2006
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timing?

not sure about how the yammie ignition is set up, but a charectaristic of retarded static ignition timing is loss of torque, especially at lower RPM. retading by 10 degrees or so may give the effect you are mentioning.
Is it feasible that the pickup is in a different place?
only a guess tho, apologies if I am wide of the mark.
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Old 28 Dec 2006
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Im not sure if you can move the pick up on XT,s, seems fixed in place by two bolts. If the carb service don't work try contact cleaner and a bit of a polish with wet and dry on the cdi and ignition electrical connections, they might be corroded or dirty.
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Old 28 Dec 2006
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The only reason I thought it may be possible is due to the stator plate being swapped out. not sure if a different model or type may have been accidentally used.
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Old 28 Dec 2006
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Could be that different models have the bolt holes in different places, might be worth checking yamaha part numbers or comparing original part with replacement if its from a different model of XT.
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Old 29 Dec 2006
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Thanks for quick replys
I will put carbs back on bike tommrow and sort some inline filters.
When I replaced the stator I used a pattern one from Eletrex £95 as the Yamaha one is about £400.I had to use my old pick up as no one makes a pattern pick up and you can only get one from Yamaha if you buy there stator. The stator and pick up are not adjustable.
I have also tried a different CDI and coil and also cleaned all connections.
Are there any tests I can do to work out if its a carb or ignition problem?
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Old 29 Dec 2006
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The second carb is a CV with a diaphragm, have you checked to see if the diaphragm on the CV is OK?
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Old 29 Dec 2006
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The only way to check the ignition electrics would be to test the resistance of the various components, you will need a workshop book (do you have the Yamaha sevice manual) and a good mutimeter. As for carbs, just try whats been mentioned and make sure slides, jets, and diaphrams are good. Is the choke part of the carb moving freely? And the colour of the spark plug will tell you a lot.
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