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Discussion Starter · #1 ·
I have an 01 TRX 90 with stock bore/stroke and stock carb and jets. Its sputtering on anything past 50% throttle, until I COMPLETELY remove the air fuel screw. The farther I back out the throttle screw, the better it continues to run until it's completely removed. Too rich?
 

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removing the air screw is not a basic tuning function and may have nothing to do with whats going on
Please go back to basics and check a few things first before getting into carb tuning
[h=1]ngine and Carb quick check list - TBolt USA Tech Database![/h]
Install Intake manifold to head and carb, oil gaskets and tighten bolts lightly at first then torque then down gradually in a cross pattern to provide a good seal and avoid air leaks

1)Throttle cable adjustment check:
a)Carb slide MUST hit bottom
b)Test it , You MUST hear the sound of the slide hitting the bottom of the carb
c)You must have free play or slack in the throttle cable
d)You must be able to turn the throttle a small amount WITHOUT moving the carb slide

2)Make sure the carb is 100% clean, free of debris and you can see light thru both main and pilot jets
You must start with a known good and properly oiled air filter

3)Double check you valve tappet adjustment .003intake and .004exh.

4)Make sure your exhaust gasket is installed properly.

5)Make sure you have a good intake to head and intake to carb seal and NO intake leaks

6)Make sure all electrical connections are tight and you have a solid ground from the engine to the frame.
and make sure you have spark


7)Check for good fuel flow and fresh fuel.

8)See our jetting guide and do a plug read.
http://www.tboltusa.com/store/tech.php?pid=61


Flooded Engine:
IF your flooded , pull the plug , Kick it over like 10 time WITH THE PLUG OUT
Put in a NEW PLUG

One more thing
I find at times it helps to just crack the throttle a tiny bit to let air in , just a tiny bit
 

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I have an 01 TRX 90 with stock bore/stroke and stock carb and jets. Its sputtering on anything past 50% throttle, until I COMPLETELY remove the air fuel screw. The farther I back out the throttle screw, the better it continues to run until it's completely removed. Too rich?
Sounds like you have a dirty or missing main jet or a float problem. Connect a clear hose to the carb drain barb fitting. Run the hose up to the top of the carb. Open the drain line and the gas will flow into the hose until it reaches the actual fuel level in the carb. The fuel should stop about 2mm below the blow seam. If the fuel level is ok I would start at the main jet and fuel needle. Check and clean as necessary.
in the pic below you can remove #19 that is your main jet and remove #13 that is your main jet adaptor but keep an eye on #2 that will fall out when #13 is removed.


Auto part Diagram Line art Technical drawing
 

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Discussion Starter · #4 ·
Sounds like you have a dirty or missing main jet or a float problem. Connect a clear hose to the carb drain barb fitting. Run the hose up to the top of the carb. Open the drain line and the gas will flow into the hose until it reaches the actual fuel level in the carb. The fuel should stop about 2mm below the blow seam. If the fuel level is ok I would start at the main jet and fuel needle. Check and clean as necessary.
in the pic below you can remove #19 that is your main jet and remove #13 that is your main jet adaptor but keep an eye on #2 that will fall out when #13 is removed.


View attachment 133917

I have cleaned the carb out several times and have some richer main and pilot which seemed to make no difference. I am still tracking down some smaller jets. I still think the bike is running way too rich, but seems to run well with the air fuel screw out. I would assume with the air fuel screw completely removed allows the carb to run at its leanest potential. Am I correct?
 

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Is the engine stock? Stock intake/air filter/airbox/exhaust?

If it is, buy a $20 rebuild kit and put everything back to stock then follow the suggestions given.

If I'm not mistaken, the "air screw" is actually a fuel screw.
- Screw it all the way in and no fuel flows on the idle circuit.
- As you back the screw out, you get more fuel at idle up to about 4 turns, then the nipple on the screw is all the way out of the way and it quits making a difference.
- Taking it all the way out? I have to give that some thought whether that's just a massive air leak or air & fuel or what but it doesn't matter because if it runs "right" with the screw all the way out you have other issues to address as suggested above.

Clean air filter?
Is the exhaust plugged?
Are the valves set right?
Is the float set right?
Are you getting good fuel flow?
What does "cleaned the carb out" mean? Did you take all the jets and needles out and clean all the holes in the carb and jets and verify they all flow and/or you can see light through them?
New plug?
Timing right?

The "air screw" is not your problem. Quit focusing on it and check the other things.
 

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Discussion Starter · #6 ·
engine is stock except a dg exhaust and uni foam filter. I have completely cleaned out the carb from top to bottom and all looks well (main jet, pilot jet, needle, needle seat, ect). I read somewhere that the air fuel screw richens up as you turn the screw in, and leans out as you back it off. It has something to do with air entering the venturi, but I don't recall all that I read. Valves are set properly, everything else looks fine. Since the only thing that gets me in the right direction is the air fuel screw being backed out, I'm working off of that leading to finding out what the issue is. So it seems the issue is carburetor related.


Is the engine stock? Stock intake/air filter/airbox/exhaust?

If it is, buy a $20 rebuild kit and put everything back to stock then follow the suggestions given.

If I'm not mistaken, the "air screw" is actually a fuel screw.
- Screw it all the way in and no fuel flows on the idle circuit.
- As you back the screw out, you get more fuel at idle up to about 4 turns, then the nipple on the screw is all the way out of the way and it quits making a difference.
- Taking it all the way out? I have to give that some thought whether that's just a massive air leak or air & fuel or what but it doesn't matter because if it runs "right" with the screw all the way out you have other issues to address as suggested above.

Clean air filter?
Is the exhaust plugged?
Are the valves set right?
Is the float set right?
Are you getting good fuel flow?
What does "cleaned the carb out" mean? Did you take all the jets and needles out and clean all the holes in the carb and jets and verify they all flow and/or you can see light through them?
New plug?
Timing right?

The "air screw" is not your problem. Quit focusing on it and check the other things.
 

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engine is stock except a dg exhaust and uni foam filter. I have completely cleaned out the carb from top to bottom and all looks well (main jet, pilot jet, needle, needle seat, ect). I read somewhere that the air fuel screw richens up as you turn the screw in, and leans out as you back it off. It has something to do with air entering the venturi, but I don't recall all that I read. Valves are set properly, everything else looks fine. Since the only thing that gets me in the right direction is the air fuel screw being backed out, I'm working off of that leading to finding out what the issue is. So it seems the issue is carburetor related.
In this and other forums there has been confusion about fuel screws and air screws. Just so we all are on the same page the air screw is located on the side of the carb. The fuel screw is located on the bottom of the carb.
ok with that removing the air screw 3- turns from bottomed out will increase the amount of air leaning out the mixture.

Not to second guess your work but the air filter is clean. Have you checked the float height using the hose on the drain barb trick? If the float is high it will cause a rich condition most apparent at 50% throttle and higher.
 

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Discussion Starter · #8 ·
In this and other forums there has been confusion about fuel screws and air screws. Just so we all are on the same page the air screw is located on the side of the carb. The fuel screw is located on the bottom of the carb.
ok with that removing the air screw 3- turns from bottomed out will increase the amount of air leaning out the mixture.

Not to second guess your work but the air filter is clean. Have you checked the float height using the hose on the drain barb trick? If the float is high it will cause a rich condition most apparent at 50% throttle and higher.
please se question me more! I second guess myself all the time... It helps me find the solutions more often than not.

I have run this bike with and without air filter, the results are the same. I have not checked the float height, but will get some clear tubing to test. Although if the level is off, I assume the only course of action is to replace the float since the float is plastic and cannot be adjusted?
 

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If the level is off it may be the needle or seat also. The plastic floats in my history have been ok. The seats and needle have been the problems. I don't remember if the seats are replaceable.
From there I would replace the pilot jet because the air screw works hand and hand with the pilot jet. What clip hight is on the needle should be middle. I may adjust it to the top if you are having problems at 50% throttle.
 

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Sometimes the more air you move through the carb and the more vacuum you create ( aftermarket air filter and exhaust ) the carb can become richer because the Venturi becomes stronger. When that occurred you will need to adjust the carb leaner.
 

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Discussion Starter · #12 ·
Not to be critical but in the exploded diagram I posted. is #2, that is the seat for the jet adaptor in place. And not upside down?
I cannot be certain, but I can check. I assume one side will be narrower? This part wasn't completely removed, I used carb cleaner and compressed air through there and there were no restrictions.
 

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I cannot be certain, but I can check. I assume one side will be narrower? This part wasn't completely removed, I used carb cleaner and compressed air through there and there were no restrictions.
You need to remove the main jet and the jet adaptor. Carb cleaner doesn't do what the can said. You need to remove the jets and carefully inspect them. When you look through them the holes need to be round. The jet adaptor had holes on the side that can not be cleaned with compressed air.
remove all the jets and blow out the Carb and all the passageways.
 

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Discussion Starter · #14 ·
You need to remove the main jet and the jet adaptor. Carb cleaner doesn't do what the can said. You need to remove the jets and carefully inspect them. When you look through them the holes need to be round. The jet adaptor had holes on the side that can not be cleaned with compressed air.
remove all the jets and blow out the Carb and all the passageways.
Thanks for all of your help. I was able to get it apart and realized I did clean the main jet holder, but not the needle adapter. I got a temporary donor 62 main jet/needle at the lowest position and got it running perfectly, albeit probably a little lean. I ordered a range of main jets between 68-75 to test and will report back what worked
 

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You are right, but in some instances I gave found that the jetting will need to go leaner. A lot of it depends upon the amount of vacuum and cfm through the carb. It's a lot like blowing across a straw in milk. The milk will rise in the straw. The harder you blow the higher the milk goes.
A carb works in the same manner. Air running across venturies and engine vacuum.
 

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Glad you got it fixed. Jetting for pipe/filter changes can be a pain - much worse when combined with carb issues.

You are right, but in some instances I gave found that the jetting will need to go leaner. A lot of it depends upon the amount of vacuum and cfm through the carb. It's a lot like blowing across a straw in milk. The milk will rise in the straw. The harder you blow the higher the milk goes.
A carb works in the same manner. Air running across venturies and engine vacuum.
x2 - I've seen it twice.

Once on an old Honda 750-4 with what had to be the best sounding 4 into 1 ever on it. It had to be jetted leaner after installing the pipe.
The other time on my 185s in the 80s - I put a Bill's pipe (the loudest pipe I've ever used) on it and had to jet leaner.

The back story is, the old Honda was my dad's. When I was having jetting problems on the 185s, he told me to go leaner based on his experience but I, as the know-it-all teenager at the time, would not do it because new pipes always need bigger jets. Until about the 10th time I had the carb apart. He was cool about it - just watched me toil away...

I listen to him now...
 

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Discussion Starter · #19 ·
Just came back from a desert trip and dialed the jetting in at 68 main jet which solved all top end problems. The main is the stock 40 and the needle set at full lean. Starts and runs like a champ.
 

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I realize this is an old post, but I have similar issues and know the float is allowing the bowl to overfill. It is running out the overflow. I have replaced the float and float needle, the seat is not replaceable. Float is plastic and not adjustable. How do you correct this issue?
 
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