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XT600E carburetor problem XT600E carburetor problem
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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 Jul 2018
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XT600E carburetor problem
Hi,

I have recently installed a Mikuni TM34-B65 and fuelfilter and I am having some issues with it which I can't figure out.

Issue #1: RPM increases when I turn the choke off(push the choke in).
I have heard that it is caused by an airleak but after spraying some WD40 around the rubbers the RPM didn't change.

Issue #2: Bike is running hotter than it should?
Could most likely be because the carburetor hasn't been properly adjusted yet.

I would love to know what you guys think might be the problem.

Current mods:
Dominator slip-on
GPR exhaust header
Wiseco high compression piston(11.5:1)
Mikuni TM34-B65
NGK Iridium IX sparkplug
K&N airfilter
K&N oilfilter
TrailTech Vapor
HEL brakelines
Oxford heated grips
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  #2  
Old 31 Jul 2018
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After a night sleep I could finally think straight and figured it was the jetting.
Still haven't gotten the jetting right but I am getting close.
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  #3  
Old 31 Jul 2018
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very hot it's because its running lean , look at the color of the spark plug
Last edited by bacardis; 1 Aug 2018 at 18:14.
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  #4  
Old 1 Aug 2018
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Left carb:
Pilot jet: 15.0
Main jet 12.5
Mixture screw: 6 or 7 turns out

Right carb:
Pilot jet: 17.5
Main jet 12.5
Mixture screw: 6 or 7 turns out

It's still backfiring on engine braking and when revving in neutral.
It is also still running hot. Will try a bigger pilot jet in the left carb and a bigger main jet in the right carb.

I have also replaced the k&n airfilter with the stock airfilter for less airflow.
Last edited by MrRedTube; 1 Aug 2018 at 19:03.
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  #5  
Old 1 Aug 2018
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Changed the jets a bit, still running lean. Saw a post on another forum where someone used #20 pilot jets with stock exhaust and stock airfilter. Ordered #22.5 and #25 will see how those turn out.

Current jets:

Left carb:
Pilot jet: 17.5
Main jet 12.5
Mixture screw: 6 or 7 turns out

Right carb:
Pilot jet: 17.5
Main jet 15
Mixture screw: 6 or 7 turns out
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  #6  
Old 2 Aug 2018
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I would think your pilot will need to be in the 22.5 to 25 range but you getting it . Backfiring coasting is a lean symptom along with a bad coasting enrichment diaphragm . Main jets seem way, way small , does it rev cleanly and feel like it has power all the way ?



.
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'86 XT600/3TB motor,,, '88 XT600, ,,'92 XT600e,,, '08 Husky TE510,,, '05 DRZ463,,, '95 KLX650 ,,,'96 KLX650
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  #7  
Old 2 Aug 2018
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Quote:
Originally Posted by jjrider View Post
I would think your pilot will need to be in the 22.5 to 25 range but you getting it . Backfiring coasting is a lean symptom along with a bad coasting enrichment diaphragm . Main jets seem way, way small , does it rev cleanly and feel like it has power all the way ?



.
I didn't rev it all the way out of fear of something going wrong. Might have taken it up till 3.5k-4k rpm and it didn't feel that bad. The guy on the other forum was using the 10.5 main jet on his XT600. Will see how it really revs when I get the larger pilot jets.
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  #8  
Old 4 Aug 2018
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Current jets:

Left carb:
Pilot jet: 25
Main jet 112.5
Mixture screw: 6 or 7 turns out

Right carb:
Pilot jet: 25
Main jet 115
Mixture screw: 6 or 7 turns out

Idle is running a bit rich now which is great, super happy with that.
But the main jet is too small. Went aggressive on it today and it is running lean when the throttle opened. I can notice power loss when having the throttle more than 50% open. It also heats up pretty well when the throttle is opened.

Will be ordering some new main jets(120, 122.5, 125, 127.5, 130) and see which one makes it run a bit rich.
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  #9  
Old 5 Aug 2018
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Current jets:

Left carb:
Pilot jet: 25
Main jet 125
Mixture screw: 6 or 7 turns out

Right carb:
Pilot jet: 25
Main jet 125
Mixture screw: 6 or 7 turns out

Drilled out the main jets because I couldn't wait for the other jets to arrive. It now feels like I don't get any power after opening the throttle about 50%. It runs decent enough for me to go to work. Will put the 130 in when I get it.
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  #10  
Old 5 Aug 2018
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I don't know XT600's or your new Mikuni carb but the mixture screw 6-7 turns out caught my eye. Is that right for that carb? In my experience mixture screws are usually 2 turns out (give or take a turn) and if the idle mixture isn't right you change the jet until it is. You then fine tune by turning the screw. Most idle screws I've seen would be on the point of falling out at 6 or 7 turns.

Your power loss over 50% throttle may well be a main jet problem but I've had the same issue with the fuel level too low in the float chamber. Might be worth a check if the new jet doesn't do it.
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  #11  
Old 5 Aug 2018
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The 6-7 turns currently is because the screws are in an annoying location and I can't really be bothered. Bad practice, I know.

The float should be good, so we'll see when I get the new main jets. If it still runs like shit I will take it to the tuning shop. Something that I should have actually done at the start.
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  #12  
Old 6 Aug 2018
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Had to use the bike for work today. It was a 20 minute drive. On the highway at low rpm the bike was around 115 degrees celcius. When I got home it was at 153 degrees celcius.

The new jets will arrive tomorrow. Will let you guys know how those work out.
Will also be calling dynojet tuning when they are open to have someone that actually knows what he is doing fix the mess I made.
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  #13  
Old 7 Aug 2018
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Unfortunately they will only be able to give you a guess as to what is needed . I don't think those carbs are a common swap so nobody will have good info a this combo . Even so each motor is different so if another had the same carbs ,theirs will be jetted a little different . At least they should get fairly close .


I like wideband A/F meters for tuning to keep this guessing down to a minimum , especially when it's a pain to change jets .
.
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  #14  
Old 7 Aug 2018
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Quote:
Originally Posted by MrRedTube View Post
Had to use the bike for work today. It was a 20 minute drive. On the highway at low rpm the bike was around 115 degrees celcius. When I got home it was at 153 degrees celcius.
I hope the new jets do improve things because 153 C must be getting towards the point where even the best oil is beyond its limit. How are you measuring this - IR thermometer? - and where? The exhaust headers / manifold should be way over that (maybe 300C) but for the rest of the engine even your 115 C is too high.
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  #15  
Old 7 Aug 2018
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Quote:
Originally Posted by jjrider View Post
Unfortunately they will only be able to give you a guess as to what is needed . I don't think those carbs are a common swap so nobody will have good info a this combo . Even so each motor is different so if another had the same carbs ,theirs will be jetted a little different . At least they should get fairly close .


I like wideband A/F meters for tuning to keep this guessing down to a minimum , especially when it's a pain to change jets .
.
The tuning shop does jetting, adjusting and dyno run, so they should be able to fix it.

Which wideband AF meter do you recommend?
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