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Discussion Starter #1
Motocycle gear heads:
1…..I have a small dual sport with a Mikuni BST31ss CV carb. It has the EPA smog charcoal cannister on the side in a cradle. Half throttle to full throttle was ok. It would not idle. I took the carb off and cleaned it out with a new rebuild kit. The float needle jet was showing signs of corrosion, leaking fuel overflowing into the air breather and heavy fumes around where it was parked. After reinstalling the carb it started good. It did idle but would wind down then stall. Increasing the idle screw the idle would take off wild. It seems to be ok now after running it a little. Did it need a purge of the air in the lines?
2…..the factory pilot is 35 and 125 on the main jet for half throttle to full throttle.. There are no adjustables on the C-clip for the main jet needle. Only one setting for the C-clip. It does seem a little flat at about ¾ to full throttle in the the higher gears. I will probably experiment with a bigger main at 127.5 to 130 or maybe higher and leave the pilot the same at 35 because the pilot is only good for idle to about ¼ to ½ throttle. The change will be in the main jet for ½ to full throttle to get the flat spot out of the higher throttle position from ¾ to full throttle. What do you recommend?
3….maybe later I might start looking for a newer bike with EFI so I dont need to mess with kits and jetting. …..How much is a new EFI throttle body and a computer on the newer EFI bikes for maybe a 250 dual sport?
5….Right now my location is maybe 500-1000ft. If I do go to higher elevation with this carb what do you recommend for the pilot and the main at maybe 2500-5000ft for the thinning air?
 

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Discussion Starter #2
Gear heads,
The bike is factory stock. No modications. Factory stock carb BST31ss and exhaust.

I went to my local OHV in the mountains at about 2500ft. Pilot and main jet seemed ok at the 2500ft. Started and ran ok. Followed a trail ride up to about 5000ft.

Had some idle problems at the higher elevation. When I shut it off and tried to restart it at 5000ft it would not start and had a little smoke coming out of the air breather. I had to wait about 5-10 minutes then it started. I noticed the stock factory exhaust header pipe after the cylinder head was turning blue. I did not get into the main jet during the trail ride. Most of the time it was ¼ to maybe half throttle 2nd and 3rd gear climb up the trail to 5000ft
I can do a plug check by getting into 2nd or 3rd gear then floor it to full throttle then shut it off and pull the spark to see how the main is burning . Then put the plug back in and let it idle to see how the pilot is burning. I will need to run it on my neighborhood street to do the plug check for the pilot and the main jets. The neighbors wont like it running up and down the street and making noise and probably wont understand what I am doing. The only other option I have is to take it somewhere else where I can do the plug check.

Before I load the bike into my truck and find another location to do the plug check does anybody have any suggestions?.

Looks like it is running too hot. Is it too rich or too lean on the pilot or the main to make the exhaust pipe after the cylinder head turn blue?
Come on gear heads, how about some suggestions on the jetting and the original factory exhaust header turning blue.
 

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Headers turn blue due to heat, which is generally caused by a lean AFR.

And generally speaking, a flat spot at 1/2 to WOT would be caused more likely by a timing issue and not a jetting problem. If you have an adjustable ignition or can play around with different advance curves that might be a good place to start. It could also be a cam issue.

I work with Mikuni HSR's but I'm not familiar at all with your carburetor, so I don't know how it's set up and can't offer any specific advice. Mikuni does have free tuning manuals for many of their carburetors that you can get off their website, so you might try that first. If they don't have one for your exact model, take a look at one for another model, such as an HSR 42 or a VM series, and hopefully you can find something that looks close to yours and adjust from there.
 

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Oh, and if you pull the plugs you have in there now, just as she sits, and they're leaning towards the whitish range, then you'll confirm your suspicions about it running hot and lean without having to annoy your neighbors. It's not as accurate as doing an actual plug read, but it should tell you what you want to know for now.

Also, if your plugs aren't all fouled, oily, or burned to chit, look for a noticeable color change along the arm. If you see a small band of color change that is roughly where the arm of the plug makes a 90 degree bend, then you know the ignition timing is fairly close to being right. If the band continues on towards the plug body threaded portion, then you know the ignition timing is probably more advanced at some point in the curve then it should be, and that could be causing some hot running issues. (And maybe a flat spot.)

Hey, it's not a dyno run, but at least it can give you some more clues, right?
 

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Discussion Starter #5 (Edited)
Oh, and if you pull the plugs you have in there now, just as she sits, and they're leaning towards the whitish range, then you'll confirm your suspicions about it running hot and lean without having to annoy your neighbors. It's not as accurate as doing an actual plug read, but it should tell you what you want to know for now.

Also, if your plugs aren't all fouled, oily, or burned to chit, look for a noticeable color change along the arm. If you see a small band of color change that is roughly where the arm of the plug makes a 90 degree bend, then you know the ignition timing is fairly close to being right. If the band continues on towards the plug body threaded portion, then you know the ignition timing is probably more advanced at some point in the curve then it should be, and that could be causing some hot running issues. (And maybe a flat spot.)

Hey, it's not a dyno run, but at least it can give you some more clues, right?
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Oh, and if you pull the plugs you have in there now, just as she sits, and they're leaning towards the whitish range, then you'll confirm your suspicions about it running hot and lean without having to annoy your neighbors. It's not as accurate as doing an actual plug read, but it should tell you what you want to know for now.

Also, if your plugs aren't all fouled, oily, or burned to chit, look for a noticeable color change along the arm. If you see a small band of color change that is roughly where the arm of the plug makes a 90 degree bend, then you know the ignition timing is fairly close to being right. If the band continues on towards the plug body threaded portion, then you know the ignition timing is probably more advanced at some point in the curve then it should be, and that could be causing some hot running issues. (And maybe a flat spot.)

Hey, it's not a dyno run, but at least it can give you some more clues, right?
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thanks for some input ANGEL.
the bike is a 2007 Suzuki DR200se Dual sport in nice condition with 2500miles.
The carb is BST31ss CV. Somebody was in there at some time. The stock factory pilot jet from the factory was a 35 and the main was 125. i opened it up andthe pilot was the factory 35. The main was changed to a 127.5. The 4 phillips screws for the bowl were a little messed up. They really messed it up. So maybe they did mess with the timing.
>>>>>Oh, and if you pull the plugs you have in there now, just as she sits, and they're leaning towards the whitish range, then you'll confirm your suspicions about it running hot and lean without having to annoy your neighbors. It's not as accurate as doing an actual plug<<<<< thanks, I found an aftermarket kit. The float needle valve was finished. I plan to pull the plugs cold and get a look. If they white it's too lean on the pilot. I guess the only way to get a good reading on the main is warm it up, floor it and rev it up in 3rd then shut it down then pull the plug.

Just need to re-jet. Whoever was in there really messed it up.
>>>>>Headers turn blue due to heat, which is generally caused by a lean AFR.<<<<< yeah, it seemed to running hot climbing up the trail. hard starting at the higher levels. Sometimes it would idle ok, sometimes not so good. I had chat with some riders on the trial. One had a Suzuki DRZ400 with a CV carb. His DRZ had a carb and started fine at 5000ft. The carb needs more work......ill look at the timing.
The AFR(air fuel ratio) is definitly messed up.
I put a kit in it with the new factory installed pilot and main. Still has a problem. I thought it would be ok if i put back to factory specs....thanks again.....
 

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Discussion Starter #6
angel....ill start a new thread in the forum.
One more thing i did not mention. When i disassembled the carb i used carb cleaner in a can with the straw.
I wanted to use a Chem-Dip in the large can and soak it for about an hour in the Chem-dip. Could not find any Chem-dip carb and parts cleaner. It has been discontinued here in CA.
I might take it apart again and do a clean again.
Do you feel a Chem-dip is best or is the carb cleaner in the can ok to clean out the small passages in the motorcycle carb or is the cleaner in small can ok? What do you think?
 
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