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Discussion Starter #1
Hi everyone! I just bought a Honda 1999 TRX400. The following mods were already made, so I'm trying to fix black smoke coming out at high rpms. I'm new into ATV repairs, so even though I've been reading so much, I still don't get it...

It has a nice white brothers pipe, K&N filter, but it's giving black smoke at high rpms on the firsts shifts. I checked the carburetor and it has a pilot #38 and a main #170... I think the air/fuel is all the way in, because if I let it out, fuel will leak... What should I do?

I've been looking to re-jet it, but don't know what should I get.. Any help is greatly appreciated!
 

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I think you are turning something other than the fuel/air screw to cause a leak - I'm guessing you are turning the float bowl drain screw located on the LH side of the carb near the bottom of the float bowl. The fuel/air screw is centered underneath the carb between the carb and the intake boot.

Replace the pilot jet with a new jet (many old jets have been drilled out rendering them worthless) - adjust the fuel/air screw for the smoothest idle (when the engine is fully warm - about 15 minutes of run time). If it idles fine now, then leave it alone.

A std. main jet was a 148. A 170 is probably too big and causing the black smoke from unburned fuel during full throttle. I would suggest getting rid of the K&N filter (unless you are all out racing - it let's too much dirt pass through which will cause premature wear of the cylinder and rings. Jet back to about a 150, install a UNI, TWIN-AIR, or STOCK air filter and try that. Use Brand name 93 octane fuel. I have always used BP or SHELL and have had no problems.

Let me know how things work out.
 

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Discussion Starter #3
Thanks for your answer! It sounds like I have to get back to that carburetor tonight. The only thing is, that you are saying to go stock on everything again? 150 is almost stock, and putting stock air filter will lose the power I gained with the mods, don't you think? I'm not racing or anything, but I love speed and power, and will love to get more out of it... But you get a point, I don't want premature wear :mad: I'll start with the air/fuel screw tonight, and see if there is a difference. Then I'll re-jet and see how it reacts to that... I'll keep you posted.

Thanks again!
 

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Ok. I found the way to move the fuel screw while the engine was running. It felt better and faster response on acceleration, but now the muffler it's popping pretty loud... Does this means it's now running lean?

Should I keep adjusting the fuel screw?
 

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Sorry about a slow answer, but I was in Wisconsin for a few days. My daughter got married Friday and I gave her away to what I think is a deserving man.

Now, the fuel screw is for idle only. It shouldn't affect acceleration, but if set too lean may contribute to popping in the exhaust. Most popping is caused by the mixture being too lean when the throttle is closed suddenly. Adding back pressure to the exhaust in on way to get rid of it, increasing restriction in the intake is another and increasing the idle speed will help too.

I'm trying to keep in mind that you have black smoke at full throttle. That usually indicates that the jetting is too rich at full throttle.

If you like speed and power, remember that "lean is mean!". A lean mixture produces more power than a rich mixture. A rich mixture burns cool and is easier to ignite. i.e. choking a cold motor to make it start. A lean mixture burns hot and that is what produces the best power. The power comes from the heat. And a fast burning (lower octane) fuel produces more power than a slow burning (high octane) fuel. The engine builder determines what octane fuel is best for the design. Honda usually recommends about 92 octane fuel demanded by their short stroke engine design. Here in the U.S., our 92 octane pump gas is diluted with alcohol which is high octane, but burns cool and has less BTU's per pound than gasoline. What we buy out of the pump is about an 84 octane gas with about 10 to 15% of ethanol added which derives a 92 octane rating but burns more like 100 octane. If you use a racing fuel without alcohol you will instantly realize an increase in power. Ask the guy with a Flex Fuel engine why he isn't using E-85 (85% ethanol). It might be cheaper at the pump, but the engine develops less power and gets lower mpg. I have heard that it actually costs more per mile to burn E-85, so the folks with the flex engines just use gas like everybody else.

When you get the jetting figured out, you will have a well performing machine. But pick a fuel and stay with it. I tried some $24 per gallon alcohol free racing fuel and while the engine performed better, I figured out that for my type of riding, I could live with the 92 octane pump gas, but I only use BP gas in my bike, I will use any old shit in my car and truck.

Maybe I should move to Alaska - they don't have alcohol in their gas. They don't have any ethanol producers in their state. Engines that get 40 miles to a gallon here get 50 mpg there.
 

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Discussion Starter #6
Oh wow! That was very useful.. I didn't know most of that stuff man...

Well, I just changed the spark plug because apparently the previous owner never replaced it and it died on me.

Yeah, the black smoke was with the air/fuel screw all the way in. Right now, I'm 3 turns open and I haven't see the smoke. What I'm getting know is... At low speed, I feel the quad doing weird stuff... Like if it was dying... As soon as I push the throttle a little, it runs great... I will try to give 1 turn in to the screw. I will still run 2 turns out, which shouldn't be as rich as when I got it and hopefully that should fix the weird stuff that is doing right now.

Thanks a lot for all your help! You are helping me getting there and I feel like I'm almost there...

One more thing... Winter is coming and I know that different settings must be done to run on lower temps... Do you mind if you tell me what these changes would be?
 

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I don't make any changes for winter - just run premium fuel (alcohol free if you can get it) and allow longer warm up time - longer warm up does not mean letting it idle for a long period - it means ride it moderately till it is fully warm and performs as desired.
 

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Awesome! That's great to know... Hopefully this year I'll get to ride a quad on snow! Thanks a lot man!
As a fellow Texan, I'm assuming you live somewhere up north or pan handle because the odds of snow in south is about zero! Guess you never know though, I wouldn't mind sliding around in the snow some too. :D
 
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