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Old 07-07-2013, 10:52 AM   #9031
alonzo
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Carb issues?

Here's my problem:

2006 Yamaha XT225, 1600 miles. Snorkel removed from airbox, muffler opened up (Z1 mod), 42.5 pilot jet, 130 main jet and 0.058" shim under the needle. Carb is Mikuni BST34.

Low and mid-range are fine. Top-range is too but I get an occasional stumble/spit/miss-fire/whatever. Both when accelerating and at steady high speed (High speed on an XT is anything > 65mph)

I originally shimmed the needle to .030" which definitely improved mid and upper range but there was stumble (actually, there was stumble before I shimmed.) Then shimmed to .058" which further improved mid and upper range and still stumbled. Then I shimmed to 086". Power was good but stumble seemed worse so I backed down to a .058" shim. Power still good and stumble not so bad.

I have checked and adjusted the float (per the 1999 pdf service manual.) The spark plug looks good (medium grey) and is gapped within spec.

I think my jets are pretty clean as I run only ethanol-free fuel and usually toss in a couple of ounces of TechRon.

This may not be a carb problem at all but somehow it just seems the likely one.
If you have any suggestions as to where I might look for the problem I'd really appreciate it.

Thanks.

Also, If anyone here has a late model service manual (later than the 1999) and could look up the float setting for me I'd really dig it.

-- alonzo
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Old 07-07-2013, 10:54 AM   #9032
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Originally Posted by alonzo View Post
Also, If anyone here has a late model service manual (later than the 1999) and could look up the float setting for me I'd really dig it.

-- alonzo
14.1-15.1 mm
"Below from the float chamber mating surface (front)" <-- thier words not mine
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Old 07-07-2013, 12:33 PM   #9033
alonzo
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Originally Posted by nuggets View Post
14.1-15.1 mm
"Below from the float chamber mating surface (front)" <-- thier words not mine
Thanks, nuggets.
Same as in the 1999 manual. What does it say for stock jets?
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Old 07-07-2013, 01:10 PM   #9034
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Quote:
Originally Posted by alonzo View Post
Top-range is too but I get an occasional stumble/spit/miss-fire/whatever. Both when accelerating and at steady high speed (High speed on an XT is anything > 65mph)
Put some tape on the throttle housing and the edge of the grip. Mark zero throttle with a sharpie. This is best done with the engine idling, so you can tell when the slack in the cable has just been taken up. Turn off the engine and mark wide open. Now take a tape measure (metric works best in my opinion) and measure the length of the arc. Put a mark at the mid point. Duplicate this procedure to mark the mid-point between here and zero throttle opening to get 1/4 open. Repeat for 1/8 and 1/16 openings. With the help of the marks, determine precisely during what throttle opening(s) it misbehaves. Don't have an accident trying to look at the marks while riding. If you do, I'm not responsible! Report back with results. This will help us determine what circuit(s) are responsible.

Regards,

Derek
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Old 07-07-2013, 02:31 PM   #9035
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I got an occasional (very occasional) high rpm miss under load today on mine too. Z1 mod, snorkle out, 130 main stock pilot. Since it was under load an at high rpm I assumed it was a lean condition. I was considering putting my snorkle back in with a uni filter to see if it goes away. Ultimately, I want to shim the needle .097 inch.
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Old 07-07-2013, 02:52 PM   #9036
Tom S
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Originally Posted by jspringator View Post
Ultimately, I want to shim the needle .097 inch.
.097?! WTF? Thats would be 5 needle clip positions & be way too rich. Where did you come up with that number?
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Old 07-07-2013, 03:57 PM   #9037
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Quote:
Originally Posted by alonzo View Post
Here's my problem:

2006 Yamaha XT225, 1600 miles. Snorkel removed from airbox, muffler opened up (Z1 mod), 42.5 pilot jet, 130 main jet and 0.058" shim under the needle. Carb is Mikuni BST34.

Low and mid-range are fine. Top-range is too but I get an occasional stumble/spit/miss-fire/whatever. Both when accelerating and at steady high speed (High speed on an XT is anything > 65mph)

I originally shimmed the needle to .030" which definitely improved mid and upper range but there was stumble (actually, there was stumble before I shimmed.) Then shimmed to .058" which further improved mid and upper range and still stumbled. Then I shimmed to 086". Power was good but stumble seemed worse so I backed down to a .058" shim. Power still good and stumble not so bad.

I have checked and adjusted the float (per the 1999 pdf service manual.) The spark plug looks good (medium grey) and is gapped within spec.

I think my jets are pretty clean as I run only ethanol-free fuel and usually toss in a couple of ounces of TechRon.

This may not be a carb problem at all but somehow it just seems the likely one.
If you have any suggestions as to where I might look for the problem I'd really appreciate it.

Thanks.

Also, If anyone here has a late model service manual (later than the 1999) and could look up the float setting for me I'd really dig it.

-- alonzo
pull the airbox side plate/cover off and run the bike up the rpm on the road (under load) again. If it's worse, then its lean bog up top; if its better then it's a rich bog on top. It only takes a couple minutes to do this and it tells you for sure which direction to go in. I wouldn't be surprised if you were rich on top and needed to drill a few holes in the top of your side cover. Could be way off base but sometimes even I get one right.
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Old 07-07-2013, 04:07 PM   #9038
Tom S
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I have seen it suggested to apply a bit of choke to add more fuel. If it gets better its lean. worse too rich.
Wish Id thought of that.
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Old 07-07-2013, 04:59 PM   #9039
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Quote:
Originally Posted by Tom S View Post
.097?! WTF? Thats would be 5 needle clip positions & be way too rich. Where did you come up with that number?
http://www.xt225.com/forums/ubbthrea...rue#Post113334

Go down to the Mikuni carburator section, and after that, the snorkle section. It was .092. I'm thinking the snorkle helps the carb achieve vacuum that is necessary for these carbs to work right. It also says that most people that replaced the pilot jet ended up going back to the original pilot jet. That is why I left mine alone. It doesn't say why.
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Old 07-07-2013, 05:45 PM   #9040
dav_dman
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Originally Posted by Tom S View Post
I have seen it suggested to apply a bit of choke to add more fuel. If it gets better its lean. worse too rich.
Wish Id thought of that.
yeah you can add fuel that way, if its lean.

removing the side cover adds air , if its rich.

Mine did the same thing and it was a rich stumble on top. Drilled a few holes in the side cover and no issue since. My fuel economy is only 65mpg tho. I like everything but the snorkel removal....its a little louder than i like.
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Old 07-07-2013, 05:50 PM   #9041
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Quote:
Originally Posted by dav_dman View Post
yeah you can add fuel that way, if its lean.

removing the side cover adds air , if its rich.

Mine did the same thing and it was a rich stumble on top. Drilled a few holes in the side cover and no issue since. My fuel economy is only 65mpg tho. I like everything but the snorkel removal....its a little louder than i like.
Could you be a little more specific about the size holes you drilled and how many?
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Old 07-07-2013, 06:48 PM   #9042
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Originally Posted by jspringator View Post
Could you be a little more specific about the size holes you drilled and how many?

I don't know about the other guy but...


We have two 1" holes in our air box cover that we use 1" conduit plugs in while at low elevations and remove plugs at higher elevations.
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Old 07-08-2013, 07:11 AM   #9043
alonzo
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Quote:
Originally Posted by motolab View Post
Put some tape on the throttle housing and the edge of the grip. Mark zero throttle with a sharpie. This is best done with the engine idling, so you can tell when the slack in the cable has just been taken up. Turn off the engine and mark wide open. Now take a tape measure (metric works best in my opinion) and measure the length of the arc. Put a mark at the mid point. Duplicate this procedure to mark the mid-point between here and zero throttle opening to get 1/4 open. Repeat for 1/8 and 1/16 openings. With the help of the marks, determine precisely during what throttle opening(s) it misbehaves. Don't have an accident trying to look at the marks while riding. If you do, I'm not responsible! Report back with results. This will help us determine what circuit(s) are responsible.

Regards,

Derek
OK. The results are highly subjective but:
Altitude: ~1600'. Temperature: upper 70's. Humidity: yes.

I begin to get stumble at 1/2 throttle but only at higher engine rpm.
At an indicated 65 mph in 5th gear and 3/4 throttle: stumble.
At an indicated 65 mph in 6th gear and 3/4 throttle: no stumble.
At anything above 3/4 throttle in 4th, 5th land 6th it's pretty much going to stumble.
Interestingly, at high enough rpm (like a slight downhill at ~65 mph) it will stumble a bit at as little as 3/8 throttle.

With the hills around here it's hard to have steady enough conditions to determine just what is going on. Load on engine and engine speed are changing all the time.

But, maybe there will be some clues in the above data.

EDIT: I think I'm beginning to see where this is going. Just because it was so easy to do I pulled the airbox cover off and went back out on the freeway.

WOT at an indicated 80 mph -- no stumble. And, no stumble at all (well, maybe a rare 'hick' but nothing I can't live with.)
So, looks like a smaller main jet and/or more holes in the top of the airbox.

-- alonzo

alonzo screwed with this post 07-08-2013 at 07:45 AM
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Old 07-08-2013, 08:25 AM   #9044
dav_dman
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Quote:
Originally Posted by alonzo View Post
OK. The results are highly subjective but:
Altitude: ~1600'. Temperature: upper 70's. Humidity: yes.

I begin to get stumble at 1/2 throttle but only at higher engine rpm.
At an indicated 65 mph in 5th gear and 3/4 throttle: stumble.
At an indicated 65 mph in 6th gear and 3/4 throttle: no stumble.
At anything above 3/4 throttle in 4th, 5th land 6th it's pretty much going to stumble.
Interestingly, at high enough rpm (like a slight downhill at ~65 mph) it will stumble a bit at as little as 3/8 throttle.

With the hills around here it's hard to have steady enough conditions to determine just what is going on. Load on engine and engine speed are changing all the time.

But, maybe there will be some clues in the above data.

EDIT: I think I'm beginning to see where this is going. Just because it was so easy to do I pulled the airbox cover off and went back out on the freeway.

WOT at an indicated 80 mph -- no stumble. And, no stumble at all (well, maybe a rare 'hick' but nothing I can't live with.)
So, looks like a smaller main jet and/or more holes in the top of the airbox.

-- alonzo
i wish i had just gone back down 2.5 on the main jet instead of drilling holes in the side of the airbox. Now when i go thru water I have to really watch to make sure get any inside the airbox. I might just go ahead and buy some waterproof mat'l we used on jetski's to keep water out of the intake and put that in there.
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Old 07-08-2013, 09:39 AM   #9045
alonzo
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Originally Posted by dav_dman View Post
i wish i had just gone back down 2.5 on the main jet instead of drilling holes in the side of the airbox. Now when i go thru water I have to really watch to make sure get any inside the airbox. I might just go ahead and buy some waterproof mat'l we used on jetski's to keep water out of the intake and put that in there.
I looked at the airbox this morning and since there's no room on top for extra holes (unless you enlarge the snorkel hole) I've decided that I won't be drilling it. Like you, I don't want holes in the side cover.

-- alonzo
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