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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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Old 21 Feb 2014
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Join Date: Nov 2011
Location: vancouver bc canada
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Xt550 carb issues / carb swap vm28's

Hi all.
Having trouble with my carbs. I have had the bike for years and have had troubles with the carbs from the get go but have always been able to trouble shoot the problem and fix it. Until recently.
The carbs have always caused a stumble between 1/8 and 1/4 throttle just as the secondary comes in and I was able to live with it if I was patient and gave it the right amount of pilot tuning. In October I took the bike out into the sand dunes and ripped around for the afternoon. It ran fine all day but Once I got back it was running like a POS.
I figured there was some shit in the carb or air filter so I disassembled it and spotlessly cleaned everything. (I have an ultrasonic, and am very meticulous, I know from experience that this carb is picky). I put it back together and it would barely run. It was super super lean, backfiring almost everywhere through the throttle range and getting so lean at idle that it would quickly die if you didn't have your hand on the throttle. I could partly remedy this situation by opening the pilot screw as far as it would go, 4-5 turns out rich. But then you had to constantly blip the throttle to keep it from rich stalling at idle. And the plug was still reading lean.
Sounds like air leak right? Well I checked everywhere with a propane torch and nothing no noticeable increase in rpms. I had an extra set of intake boots from my parts bike, mounted them, no change. Ordered a new air cut valve diaphragm and parts, installed, no change. Everything was set up and dialed perfectly, float level was bang on jets were super clean, all air passages had good flow through them. POS carb. !!@&$@!?
So, as a thought, maybe there is some problem with the internal passages of the air cut valve, some leak internally in the body that I can't test. So I disabled the air cut valve by simply removing the spring that holds the diaphragm open thus making the diaphragm act as if it is always engaged allowing the richer mixture to flow to the intake manifold. Well this actually improved things a lot. Now she will idle , I have the pilot screw almost all the way in, .5 out, and the plug is looking a lot better, but now on the rich side. However the stumble as the secondary comes in is horrible now and if you let it idle for a few minutes the mixture gets too rich and it will die eventually. POS carbs!!
I am starting to think that maybe it is something else like a cylinder gasket leak or something but I am not sure.

Anyone have some similar Experiences? Any advice?

So I am stumped and want to ditch these things, but it's so hard to find a good condition xt600 set out there for an affordable price, if you do the 2 into 1 banshee manifold you have to move the rear shock, has anyone tried running 2 vm28's maybe with a custom intake manifold that gently moves them away from the shock. I am not sure they would fit but just looking for carb alternatives and have searched the web all over to little avail. Hoping someone here will shed some light on a xt550 or xt600 carb swap.

Thanks in advance.
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