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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 30 Dec 2011
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XT550 HELP ! all back together but running poor

Yes ... unfortunately more help needed.

Finally got my xt all back together after installing a new top end. new piston and rings 1st os - rebore on the cyl - and recut valve and valve seats.

At first it started up just perfectly - second kick - idled smooth and ran really well.

I was taking it easy for the first run being cautious not to exert too much force on the new parts but making sure to run it up and down the rpms in low gears. after about 5 or 6 min or running i noticed it was backfiring a bit on decel, just a small pop once in a while.

I then put it in the garage for the night and the next day I took it out and it started up just fine - first kick this time and idled fine.

after it warmed up, I started to run the motor up a bit higher in the rps to about 3/4 throttle and after about 10 minutes of riding it started to backfire a lot more on decel and then it was running really rough at idle and it stalled out on me a couple of times.

it became difficult to start but did fire it up again and I managed to get it home by keeping the throttle on.

it is pulling fine and there are no flat spots but the backfiring is quite bad and the bike will not idle. it runs but skips, sputters and it is hard to tell but sort of sounds like it is popping out the exhaust and maybe a little gasp from the intake as well.

If the choke is open it will idle at about 2000 rpm but it falters, sputters and is generally unhappy.

I checked the plug and it looks perfect.

checked the valve clearance and the exhaust was still good, the intake had opened up a bit so I readjusted it. still no running improvement.

I have tried adjusting the air screw and it doesn't make any noticeable improvements.

I sprayed wd40 around the intake boots and noticed no increase in rpms, I check the exhaust manifold tightness and that all seems ok too, it doesn't appear to be leaking anywhere although I believe a leak would diagnose the running problem.

Also, while the bike was apart I did a very thorough carb clean so I am pretty sure that jets are ok. everything was spotless and cleaned in an ultra sonic bath - all jets removed and looked clean.

Any ideas? My next step is to pull the carb again and inspect the pilot jet and its circuit, pull the exhaust and check my seals. also will do a compression test and see where that is at - hope i didn't blow something up in there.

any other tests i can do to help diagnose the running issue?

Thanks in advance.

ps. the bottom end knock i was experiencing (http://www.horizonsunlimited.com/hub...-have-go-60120 ) was indeed solved by my new balancer parts - at least one good thing came of the rebuild so far.
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Old 30 Dec 2011
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Possible fuel starvation if not drawing in air.
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Old 30 Dec 2011
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Do you get a good flow if you pull the fuel line off the carb?
Float valve sticking, float level?
Breathers pinched or blocked?
Fuel quality?
Air filter?
As you say, crap found its way in to the carb?
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Old 31 Dec 2011
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Valves?

You got the valve timing right? Easy to get one tooth out on the cam when re-building.
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Old 31 Dec 2011
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.....would start to check carb again:

floater level

needle valve (check underneath valve seat! Some carbs have a net filter there - but make sure anyway that fuel supply channel works fine)

.....and as I had to learn this the hard way also:

forgotten rags (intake)

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Old 31 Dec 2011
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Did you check/clean the filter on the float needle valve?

This is what it looks like on your 550.



Mezo.
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  #7  
Old 3 Jan 2012
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"gotta check the jets" - quote from Two-Lane Blacktop

thanks guys, sometimes all it takes is a once over.
I pulled the air filter - gave it some gas, and whatever was stuck in there - probably something in the pilot circuit - came dislodged and now she runs just dandy. - i am running some seafoam through it to clear any left over debris but i'll pull the carb when i have a chance and give it another once over - for now it seems like whatever was clogging is now gone.

happy to have my baby back. now i just have to wait for better weather and then the hot springs are back within reach again.

thanks for all the help, glad there is all this support out there,

i'm still learning but hopefully i can pass on all things learned to others out there.

cheers
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Old 3 Jan 2012
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Glad to hear.
What's seafoam?
I'd be tempted to pull the carb apart and give it a good dose of looking at, and carb cleaner.
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Old 3 Jan 2012
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Quote:
Originally Posted by oothef View Post
Glad to hear.
What's seafoam?
I'd be tempted to pull the carb apart and give it a good dose of looking at, and carb cleaner.

oothef Could have been one of these products

Sea Foam | Products

Bug cleaner! ?

Lets see that bike running to the
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Old 3 Jan 2012
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Mmm, snake oil......
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Old 12 Jan 2012
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XT550 Service Manual - Index of /josip/manuali/yamaha/xt550 (Long download be patient).
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