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Discussion Starter #1
Hi guys. Looking for some input maybe. I have an 03 and an 07 250EX. The 03 is faster and I don't know why. The 07 seems to lack power. It runs fine, doesn't sputter and no dead spots in throttle. But side by side the 03 is faster. This is whether you use the sport clutch on the 07 or not. I have had many 250EXs over the years and never had any issues. It doesn't feel like the carb is dirty, but I'm thinking of a rebuild anyway. I thought maybe I'd check compression. I figure these two quads should be about even or maybe the 07 would be a bit quicker since the 03 is beat on. It just feels a bit sluggish for some reason. Runs fine otherwise and it takes off when you hit the throttle, just doesn't seem to get up and go. Anyone have an idea? Plug was changed not long ago, but I will be changing it soon. Oil will be changed as well and I only use Honda GN4. I also use 93 octane in all my quads. My main quad is an 06 400EX and it calls for 93 so I put it in all of them. Thanks for any ideas you might have.



Rich
 

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Fatten it up

Hi guys. Looking for some input maybe. I have an 03 and an 07 250EX. The 03 is faster and I don't know why. The 07 seems to lack power. It runs fine, doesn't sputter and no dead spots in throttle. But side by side the 03 is faster. This is whether you use the sport clutch on the 07 or not. I have had many 250EXs over the years and never had any issues. It doesn't feel like the carb is dirty, but I'm thinking of a rebuild anyway. I thought maybe I'd check compression. I figure these two quads should be about even or maybe the 07 would be a bit quicker since the 03 is beat on. It just feels a bit sluggish for some reason. Runs fine otherwise and it takes off when you hit the throttle, just doesn't seem to get up and go. Anyone have an idea? Plug was changed not long ago, but I will be changing it soon. Oil will be changed as well and I only use Honda GN4. I also use 93 octane in all my quads. My main quad is an 06 400EX and it calls for 93 so I put it in all of them. Thanks for any ideas you might have.



Rich



Rich, the 01-05 TRX250's are too lean from the factory and the 07 and later are even leaner. I'd richen the main and pilot jets on both 2-4 sizes. On my stock 05 I richened pilot until it would "cold" start on a warm day with little no choke. Unfortunately I don't remember the numbers but I think I went 2 sizes over stock on pilot and main jets. I'd recommend jetting the 03 until its running best, then jetting the 07 the same. You'll probably also need an 01-05 needle for the 07.



-Dan
 

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Discussion Starter #3
Hi guys. Looking for some input maybe. I have an 03 and an 07 250EX. The 03 is faster and I don't know why. The 07 seems to lack power. It runs fine, doesn't sputter and no dead spots in throttle. But side by side the 03 is faster. This is whether you use the sport clutch on the 07 or not. I have had many 250EXs over the years and never had any issues. It doesn't feel like the carb is dirty, but I'm thinking of a rebuild anyway. I thought maybe I'd check compression. I figure these two quads should be about even or maybe the 07 would be a bit quicker since the 03 is beat on. It just feels a bit sluggish for some reason. Runs fine otherwise and it takes off when you hit the throttle, just doesn't seem to get up and go. Anyone have an idea? Plug was changed not long ago, but I will be changing it soon. Oil will be changed as well and I only use Honda GN4. I also use 93 octane in all my quads. My main quad is an 06 400EX and it calls for 93 so I put it in all of them. Thanks for any ideas you might have.



Rich



Rich, the 01-05 TRX250's are too lean from the factory and the 07 and later are even leaner. I'd richen the main and pilot jets on both 2-4 sizes. On my stock 05 I richened pilot until it would "cold" start on a warm day with little no choke. Unfortunately I don't remember the numbers but I think I went 2 sizes over stock on pilot and main jets. I'd recommend jetting the 03 until its running best, then jetting the 07 the same. You'll probably also need an 01-05 needle for the 07.



-Dan

Thanks Dan. The 03 runs fine. Still running strong! I have rebuilt carbs before, but always with stock jets. I don't really know where to find different sizes for my quad. Any site you can recommend that might tell me my stock sizes and sell different sizes? Thanks
 

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Discussion Starter #4
Hi guys. Looking for some input maybe. I have an 03 and an 07 250EX. The 03 is faster and I don't know why. The 07 seems to lack power. It runs fine, doesn't sputter and no dead spots in throttle. But side by side the 03 is faster. This is whether you use the sport clutch on the 07 or not. I have had many 250EXs over the years and never had any issues. It doesn't feel like the carb is dirty, but I'm thinking of a rebuild anyway. I thought maybe I'd check compression. I figure these two quads should be about even or maybe the 07 would be a bit quicker since the 03 is beat on. It just feels a bit sluggish for some reason. Runs fine otherwise and it takes off when you hit the throttle, just doesn't seem to get up and go. Anyone have an idea? Plug was changed not long ago, but I will be changing it soon. Oil will be changed as well and I only use Honda GN4. I also use 93 octane in all my quads. My main quad is an 06 400EX and it calls for 93 so I put it in all of them. Thanks for any ideas you might have.



Rich



Rich, the 01-05 TRX250's are too lean from the factory and the 07 and later are even leaner. I'd richen the main and pilot jets on both 2-4 sizes. On my stock 05 I richened pilot until it would "cold" start on a warm day with little no choke. Unfortunately I don't remember the numbers but I think I went 2 sizes over stock on pilot and main jets. I'd recommend jetting the 03 until its running best, then jetting the 07 the same. You'll probably also need an 01-05 needle for the 07.



-Dan

Thanks Dan. The 03 runs fine. Still running strong! I have rebuilt carbs before, but always with stock jets. I don't really know where to find different sizes for my quad. Any site you can recommend that might tell me my stock sizes and sell different sizes? Thanks

I had a great idea, I checked the repair manual for jet sizes...DUH! So I have that, just need a good site to find bigger jets. :)
 

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jetting specs

Thanks Dan. The 03 runs fine. Still running strong! I have rebuilt carbs before, but always with stock jets. I don't really know where to find different sizes for my quad. Any site you can recommend that might tell me my stock sizes and sell different sizes? Thanks[/QUOTE]


I had a great idea, I checked the repair manual for jet .sizes...DUH! So I have that, just need a good site to find bigger jets. :)[/QUOTE]

I pulled the bowl off my stock 05 TRX250EX this afternoon and I'm running a 45 pilot and a 95 main jet. I've run this jetting successfully from 2300-7800 feet elevation, you might have to go one step richer if you are at sea level. I believe stock 03 is a 42 pilot/92 main. Your 07 has a tamper resistant idle screw that is covered by a brass plug I've been able to remove the plug by finding a flat screwdriver that will just wedge into the hole and twisting until the plug came out. The Idle mixture screw had a "D" shaped head that you'll need a motion pro 08-242 socket to adjust. Alternately, the 01-05 mixture screw will fit in the 07 and it adjusts with just a screwdriver. I'd re-jet the 07 to 45/95 and get an 01-05 slide needle (16012-HN6-003) and see how she runs. Good luck.
 

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When I was a kid, I had the misconception that 'more fuel meant more power'. I could not buy bigger jets for my Sears mo-ped, so I drilled the jets with the smallest drills in my dad's drill index. The engine would start, but it had no power and would not run above half throttle. This was my first lesson in carburation and air/fuel ratios. My dad had to buy a new carb for me. $35 isn't much today, but in 1963 it would buy 150 gallons of gasoline.

As my education in engines progressed, I learned that more fuel was not more power, but just the opposite - more air meant more power! Power comes from the heat and speed of the burn. Low octane fuels burn faster and the heat produced is higher, so the lower the octane the more power the engine would produce, but too much heat will damage the engine and too fast of a burn will damage the engine also, but it takes longer.

High octane fuels were formulated for long stroke high compression engines. Ideally the fuel is ignited by the spark plug, the expanding gas pushes against the piston and the flame goes out when the piston reaches the end of it's stroke. If the fuel ignites before the spark (pre-ignition), it may be caused by too low of an octane fuel, something in the combustion chamber that is hot enough to ignite the fuel vapor (glowing carbon, a hot electrode on a spark plug or a sharp, slightly jagged edge at the top of the cylinder) or a too lean mixture coupled with high compression.

An over rich mixture ignites easier when the engine is cold because only the vapor burns. When fuel is cold, it doesn't vaporize as easily as it does when it is warm - therefore a cold engine needs a richer mixture, but a warm engine performs better with a leaner mixture.

The power generated by a fuel is dependent upon the BTU's of the fuel in question. Ethanol has a higher octane (burns more slowly) than gasoline and when added to gasoline, will raise the overall octane rating of the gasoline it is added to, but alcohol does not vaporize as readily as gasoline and therefore is harder to ignite resulting in a lower temperature flame and less BTU's per pound. Since you have to burn more alcohol to get the same amount of BTU's, fuel consumption increases (less miles per gallon) as the % of alcohol increases so does consumption and mileage decreases. E100 has 76,100 Btu per gallon, compared to 114,000 Btu per gallon of gasoline, therefore it takes approximately 1/3 more pure ethanol power a vehicle the same distance as gasoline.

Both the 2003 & 2007 TRX250EX, for all intents and purposes, are the same vehicle. That being said; take 10 2003 or 2007 models and put the same driver on each one and test them. There is always one fastest and one slowest vehicle of the lot. Attribute that to manufacturing variations of assembly.
 

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Wrong again. The CORRECT air/fuel mixture ( around 12.5-13.0) makes the most power, neither lean nor rich. If your theory were true then the 07, which even you pointed out was leaner, would be faster than the well worn 03. This is the first forum I've been on where the Mod is the Troll.
 

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Who said anything about ratios? 12 to 14 to one is about the correct ratio depending on the quality and type of fuel. Fuel that releases oxygen in the process of combustion need less atmospheric air and use of supplemental oxygen requires more fuel. i.e. nitrous oxide injection: When you heat nitrous oxide to about 570 degrees F (~300 C), it splits into oxygen and nitrogen. So the injection of nitrous oxide into an engine means that more oxygen is available during combustion. Because you have more oxygen, you can also inject more fuel, allowing the same engine to produce more power. Nitrous oxide is one of the simplest ways to provide a significant horsepower boost to any gasoline engine. Nitrous oxide has another effect that improves performance even more. When it vaporizes, nitrous oxide provides a significant cooling effect on the intake air. When you reduce the intake air temperature, you increase the air's density, and this provides even more oxygen inside the cylinder. MORE AIR = MORE POWER, but with increased air, increased fuel is needed too to maintain the 12 to 14 to one ratio and more power will be realized when run closer to the 12 to one side of the equation. The hotter the engine the leaner it can be run. Lean is mean; ask a drag racer.
 

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Discussion Starter #9
Thanks for the help. I bought a rebuild kit that will just do stock jets, but I wanted all the right parts. I will get bigger jets though and mess around with it. I have to find the right site to buy the jets. I have seen kits, but nothing for my year. I'm not sure if the year matters much, I don't know how much that carb changed over the years.
 

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The hotter the engine the leaner it can be run. Lean is mean; ask a drag racer.



Yeah the ones with holes melted in their pistons. More theory that has no bearing on the actual topic of discussion. Assuming you actually have a bike or quad, you should prove your theory to everyone and drop your mainjet about 5 sizes and go run it real hard for a couple hours. Make sure you get a good top speed run in and really wind her out. Let us all know how it works out.
 

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Jets

Thanks for the help. I bought a rebuild kit that will just do stock jets, but I wanted all the right parts. I will get bigger jets though and mess around with it. I have to find the right site to buy the jets. I have seen kits, but nothing for my year. I'm not sure if the year matters much, I don't know how much that carb changed over the years.

Sorry for the distraction.



Pilot jets (85 works well):



https://www.rockymountainatvmc.com/parts/pro-x-n424-21-series-keihin-pilot-jet-p


Main Jets (Shouldn't need to go over 95 unless you are close to sea level or have aftermarket pipe):


https://www.rockymountainatvmc.com/parts/keihin-99101-393-series-main-jet-p


Check your stock jets to see what your starting point is before ordering. Go up 1 size at a time, test ride, pull spark plug. If plug is black it's too rich, white too lean, mushroom brown just right. Pilot is idle to part throttle and can be fine tuned with idle mixture screw, main jet is high rpm. Needle controls mid range, you can move clip up or down to adjust, but I didn't have to mess with mine. These jets will work in both your 03 and 07. Good luck.
 

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I have a 2004 CRF250X run 91 to 94 octane non-ethanol fuel, stock jetting, stock air filter, stock exhaust with no modifications other than a dent in the head pipe, Keihin carb, appx 700 ft above sea level and the engine performs better than my friends who have modified and rejetted for more power. My plug comes out brown on the metal and white on the porcelain. First spark plug was used for 8 years before acting up. Piston comes out light tan with little to no carbon build up.

I also have a 2005 TRX400EX - was a POS when I got it - hard to start, no acceleration, backfired from the exhaust on acceleration and worse on deceleration - had a almost complete loss of power about 2500 RPM - blew black smoke at all speeds and stopped pulling about 4000 RPM - didn't have to worry about hitting the rev limiter. After removing the HMF exhaust and installing a stock muffler, scrapping the Performance Aluminum air box with K&N filter and installing a new stock air box and filter, rebuilding the carb and installing stock jets (tossed the Dyno jets in the garbage) - adjusted the valves, changed the oil, new spark plug and it is a dependable trouble free ride, but it is used as a quad and not a racing vehicle - it's fun to ride in the fields, but the bike is for the woods.

Piston on the left is out of my bike - piston on the right is out of a bike modified for more performance - the bike the black piston came out of never beat me in a race, but he obviously had more power - it's hard to understand.

Go ahead, put in an 85 pilot jet and let me know how that works for you. I would stick with the 42 pilot jet that it came with unless you are using gas with a high ethanol content, then go to a 45 and adjust the pilot screw for a smooth idle. Main jet - 95 is stock but with junk fuel you could go to 100 without a problem for better top end performance. Slide needle position is rarely changed, but street bikes that run at steady highway speeds for years can benefit by having their needles dropped one notch to compensate for wear on the needle and in the needle jet - it smooths the engine and increases fuel economy.

This is fun
 

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Discussion Starter #13
The hotter the engine the leaner it can be run. Lean is mean; ask a drag racer.



Yeah the ones with holes melted in their pistons. More theory that has no bearing on the actual topic of discussion. Assuming you actually have a bike or quad, you should prove your theory to everyone and drop your mainjet about 5 sizes and go run it real hard for a couple hours. Make sure you get a good top speed run in and really wind her out. Let us all know how it works out.



I'm not a pro with carbs, but I know a bit about engines and the leaner it is the worse it is for the engine. Lean means hot and that will destroy your piston. I have a 400EX that had a pipe and jetting done and it was running so lean smoke was coming off the pipe and you could smell hot metal. very bad! Maybe with a V8 you can do a bit lean, but with an air cooled quad you're asking for trouble. Just saying. I do appreciate the input though



Rich
 

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400ex

Hey, I have a 1999 Honda 400ex, I was wondering if anyone is running a 426 bore with a stage 2 hot cam? I’d like to run the same set up, I’d like to know if I will feel any Difference with those mods compared to stock thanks
 
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