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86 xt600 stumbles off idle 86 xt600 stumbles off idle
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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.
Photo by George Guille, It's going to be a long 300km... Bolivian Amazon

I haven't been everywhere...
but it's on my list!


Photo by George Guille
It's going to be a long 300km...
Bolivian Amazon



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  #1  
Old 4 Jun 2017
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86 xt600 stumbles off idle
I've got an 86 xt600 I bought non running. The po said it ran great but studdered on acceleration. I cleaned carb full of powder including float valve filter replaced float valve seat and valve, replaced diaphragm thoughrly cleaned carb 3 times through the process of trying to sort this. Adjusted secondary so butterfly is horizontal at wot because 5-7mm was way past horizontal. The bike starts 2nd kick every time idles great revs great but when ridding it Boggs just off idle and eventually clears up. It's a great 2nd gear trail bike at the moment because it'll lug along great and accelerate well but on the road it's sketchey due to the hesitation. I checked for air leaks and found none, new air filter and plug. Tested everything electrical I could with multimeter and everything was within spec. I have a parts bike I swapped cdis with and got the same results. What am I missing?
Also plastic Clarke tank and new petcock
After a little more digging I'm seeing the ignition is supposed to fire at 2 different places. Does anyone know what controls this? I've seen it mentioned in a couple places but no one has explained how this works and I'm not seeing it in my manuals.
Last edited by Live42day; 5 Jun 2017 at 02:45. Reason: Adding info
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  #2  
Old 5 Jun 2017
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Quote:
Originally Posted by Live42day View Post
After a little more digging I'm seeing the ignition is supposed to fire at 2 different places. Does anyone know what controls this? I've seen it mentioned in a couple places but no one has explained how this works and I'm not seeing it in my manuals.
this is interesting... explain what you mean or at least where you found this info... surely you didnt hear it from us here in HU
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  #3  
Old 6 Jun 2017
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its on the internet it must be true lol
I've been digging around so much I don't remember which forum it was. I did find in the Clymer and Yamaha manual after much digging, a photo, under engine disassembly, that showed 2 square attachments on the flywheel and the only mention in the manual was "inspect ignition pickup tabs".
I have a parts bike with a seized motor that I just pulled the flywheel side cover off of. It has 2 small metal pieces bonded to the flywheel. There is also what appears to be a sensor attached to the cover.
What I had previously read was that the plug fires at 2 different degrees in the rotation. the early fire was to make start up and idle easier and the second spark was for higher revs.
I will attach the photos off my bike when I get my phone charged up again.
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  #4  
Old 6 Jun 2017
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the pic of the flywheel shows the 2 pieces bonded to the flywheel. The other shows what I am guessing is the sensor that triggers the spark on the left of the photo.
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86 xt600 stumbles off idle-img_0879.jpg  

86 xt600 stumbles off idle-img_0878.jpg  

Last edited by Live42day; 6 Jun 2017 at 01:27. Reason: Update
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  #5  
Old 6 Jun 2017
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Sounds like a problem with either the low speed jet or needle jet, especially as you say you found the carb full of muck. There could still be some left in the very small jets. Failing that if you raise the needle a notch it will richen up the low to mid speed running and may help it. The usual cause of hesitation on throttle opening is the air responds immediately but the fuel doesn't.
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Old 6 Jun 2017
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A very very long time ago I had an XT125 - horrible slow heavy learner bike, but at 17 I didnt care much.
I remember when it had an issue with starting, the local bike shop told me the low speed coil in the stator had failed. This is the one that generates the spark at low engine revs - ie when you kick it over to start it. For ages it wouldnt start off the kicker, but if I bumped it at a slightly higher speed it would start perfectly and run normally. When I eventually got round to changing the stator it cured it immediatley.
I know this doesnt help with your problem, but might go some way to explain your other question about the sparking.
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  #7  
Old 7 Jun 2017
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86 xt600 stumbles off idle
Hi Live42day,

The 2 triggers on the flywheel with the pick-up give low and high speed ignition timing. This provides ignition timing advance from about 2 degrees static to 5 at idle, going up to 12 degrees at 1200 rpm. That is low speed timing.
The 2nd trigger takes over and with CDI provides high speed timing from 1200 to 4600rpm giving total advance of 36 degrees.

Agree with Des Senior, it is unlikely to be your causing your problem.

You can try using a little choke at the speed you experience the problem to 'diagnose' if it is lean mixture causing the stumble. If that improves performance Des Senior is correct. Suspect dirt in the low speed area of primary carb. Consider PO may have made jet and / or needle position changes.

Hope you get it sorted, you have a great bike.
p
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  #8  
Old 7 Jun 2017
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Thanks for the info info
I tried the choke with no change so I guess it's back into the carb I go. Thanks again.
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  #9  
Old 8 Jun 2017
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Quote:
Originally Posted by Live42day View Post
I tried the choke with no change so I guess it's back into the carb I go. Thanks again.
Based on the two old XT600s I have owned I would replace the inlet rubbers while you are in there. They are not cheap but can leak unseen and that causes major hassle as you suspect and try to fix carb issues when the rubbers are actually the problem.

I have also read that the pilot jet still supplies a large amount of the mixture even with a wide open throttle so take care to make sure that is clear. I used a nylon brush bristle to poke one of mine through.

Both of them also needed the pilot jet to be further out than spec. I think one bike was almost fully out and the other was 4 turns out.
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  #10  
Old 8 Jun 2017
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Rich
I thought about the inlets but the plug is always black so it seems to be rich. Tried out to 4 turns with no effect on the primary jet. I guess I know where I'll be all weekend.
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  #11  
Old 2 Jul 2017
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Sorted it sort of
Well, with a little help from eBay I sourced another factory carb. Cleaned it out and she runs like a champ.
I had corrected float levels on the original carb, which made mid to top end way better. It was however still stumbling off the bottom end. I guess if I ever get the parts bike motor rebuilt I'll have a carb for it now that likely needs a sonic cleaning and new jets throughout. I figure the pilot circuit must be plugged still. The pilot needle was replaced but the pilot jet must be buggered still.
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