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Carburetor tuning pointers

35K views 24 replies 13 participants last post by  glmsas1 
#1 ·
I saw this the other day and thought it would be a good thing to add to our site. The tuning techniques will help with any style carb although they are talking mx pumper style carburetors. Sure beats typing it all out:laugh:

Four-Stroke Carburetor Jetting Tricks on 4Strokes.com
 
#3 ·
Thanks Shadetree. I don't have to tell you how many jetting questions we get asked and answer,,we probably miss half of them,,lol! It was well written,,better than I could have covered everything on one post,,so it warranted a copy for members to read with carb tuning issues...FI, well thank the manufacturer for the poor EPA mapping,lol!
 
#4 ·
#5 ·
At my wits end!!

Hello,
I am a new member. I have a 2000 Honda Foreman 450. 27" swamplites. Snorkeled. I currently have had a problem with it cutting out really bad. I have tried every size jet possible. Have had the carb of more than I. An count. It will fire up and idle. Bit as soon as I give it a little throttle it starts cutting out. If I hold the throttle in halfway it will Rev up and down on its own. Someone please help. I am ready to just put a for sale sign on it.
Thankyou.
 
#6 ·
When you snorkel a quad it restricts the airflow depending on snorkel inlet size. What that does is cause a rich condition on fuel so you would need to go smaller on the pilot jet, and lower the needle,,raise the clip,,to get it to run right. Find the right pilot jet first by adjusting the fuel air screw to the best response and idle where the screw is between 1 to 3 turns max out. If you keep turning it out and it doesn't change it would need to be bigger. If you turn it in and it still won't idle at its best and respond the best with throttle input it is too big and you need to go down a size and try again. same with the needle, raise it one clip after you do the pilot and find the best position where it transitions to the mid range throttle position smoothly. Then onto the main jet, and go down until it revs out the best from 1/2 throttle up,,,don't go too lean where it can damage your piston..its trial and error..Someone on here has probably snorkled a atv like yours and can maybe give you a baseline to start with. It also depends on if you have a stock or aftermarket muffler, and if you have a 1 1/2" snorkel,,less air flow,,or a 2" snorkel. I've never snorkled an atv personally,,but that is basically what is happening.
 
#7 ·
Thank you. I will try everything y oh said. I have the smallest jet I have in it now. It is a 130. The snorkel starts at 1 1/2 but is all 2" all the way. I got an hmf jet kit but it didn't have all the jets in it. Didn't even have the main jet. As for my muffler..it is the stock one. But the dumbass that I bought it from cut it open and took everything out of it. Then bubble gum welded it together again. But thanks again I will try these things.
 
#8 ·
Your welcome. Make sure you have pilot jets because it needa changed also. When it won't throttle up off idle tells me your pilot and needle height need work. Not just the main. Good luck and let us know what you find.
 
#9 ·
I have been working on tricking FI machines to run their best lately without buying expensive controllers. In hot weather my atv runs flat. Its from a wrong afr ratio from the ecu. Im tricking it by wrapping the airbox with reflective tape and relocated the iat. It sits in such a place that traps engine heat and makes the air fuel ratio wrong. Cold air intake is next on my list for summer riding. Im making headway..lol! Old dog learning new tricks with modern machines.
 
#10 ·
Fed up!!!

Well did everything that you said. I actually bought a carburetor rebuild kit. Replaced everything in the card. The damn thing is still running rough and with the throttle held in halve way it revs up and down on its own. Also it is leaking gas like crazy now..ready to sell it. Does anyone have anything for me. It's a 2000 honda foreman 450s
 
#13 ·
Well did everything that you said. I actually bought a carburetor rebuild kit. Replaced everything in the card. The damn thing is still running rough and with the throttle held in halve way it revs up and down on its own. Also it is leaking gas like crazy now..ready to sell it. Does anyone have anything for me. It's a 2000 honda foreman 450s
If you don't have it already here is the link for your repair manual. Chapter 15 covers your carb.DepositFiles

Look at 15-4 for checking your float height. You can do it with a tape measure, make sure it is at 3/4". It is not adjustable on this carb so it would need replacement if it is off..It sounds like it is if it is overflowing fuel, that or you still have an issue with the float valve being stuck and not shutting off the fuel flow.

The up and down racing in the midrange does sound like an air leak. Check all vacuum lines going to the diaphram on the slide, and the diaphram itself for tears. You can also use starting fluid and spray it around the intake manifold and carb while idling to find an intake leak..it is the only time i would use starting fluid on an engine.
 
#11 ·
^^^you have an air leak somewhere make sure there are no cracks in the intake boot!! don't sell it over one little problem!!
 
#12 ·
Could be a float sticking issue. Where is the gas leaking out of?
Have you checked the plug since installing the kit?
RE:Fi. I also have a sportbike & have gotten the O2 & cold air intake sensor low readings (when cold) from the manual, then put a resistor in there at its lowest point to trick the ECU into thinking its always cold out. I could buy a Power Comander but I spent $6 on a pack of resistors from Circuit City instead.
 
#14 ·
#15 ·
I recently purchased a 2002 Foreman 450 S and I've tuned it up and added Sea Foam to the fuel but it still has a popping sound into the intake. I was hoping it will clear up as time and fuel with additive go bye. When it gets to full running temperature is doesn't pop as much, only idle. Riding it around it doesn't have any problems. Are these 450's cold blooded and I should be patient waiting for the fuel additive to work or do I need to fear the worst with a burnt exhaust valve? It would seem that under a load the burn exhaust valve would be as poor on performance as it would idling. I was wondering if there are vacuum hoses that I can inspect? The air cleaner box has lost it's brackets and it was hanging on the carburetor. I raised it back and tied it off to take the weight off. The ATV has been sitting for quit some time. Thanks
 
#16 ·
Since it has been sitting for a long time, its time to tear the carburetor out and thoroughly clean it. i suggest soaking it in carburetor cleaner. Once done, set your air fuel screw at 2 1/4 turns out, once you have it running you can tune the idle mixture from there.
 
#17 ·
Restoring 88

Hi all. Currrently restoring a 1988 Fourtrax TRX300 that I bought fro my neighbor. Overall it's in very good condition and the only apparent issues seem to be cosmetic. I bought new plastics, tires and rims and and an exhaust. Changed the oil and oil filter and added a new air filter. It's been running fantastic and my neighbor explained that there was a bit of a gas leak before I bought it so I figured no big deal. Well lately there's been an even stronger gas smell than there had been before and a little investigating led me to the culprit; an open ended tube sticking out of the middle of the bike. it leaks gas pretty good sometimes, albeit just a steady drip. When I turn the petcock off, the leak stops. Could this be the carburetor overflow tube? I'm probably not even saying that right. I'm a newbie and I don't know much but I'm determined to learn. My wife and I now live on a big property so an ATV is a necessity and also a fun way to haul wood and plow snow. It will also double as a recreational vehicle on the weekends. Anyway, any help with the gas leak issue is greatly appreciated. Thanks!
 
#18 ·
Your carb float or the float needle / seat likely has a problem. Needs a thorough and aggressive cleaning and maybe a carb kit installed. I have also seen the brass overflow tube in the bowl cracked due to water freezing.

Your problem could also be as simple as a loose bowl drain knob or bad o-ring on the bowl drain.

Find a copy of the shop manual around here. Grab some ziploc storage bags and a digital camera to help you stay organized and clean it. Shindy makes a good carb kit or your local dealer can help you.

Rick
 
#19 · (Edited)
Newbie

Thanks man. Gonna give that a try this weekend. I don't have a high mechanical aptitude but seems as though I will need to develop one. ATV's seem to need more maintenance than a car does. You seem very knowledgeable so I have another question: oem size for my 300's tires are 23x8x11 in the front and 25x12x9 in the rear. I got some Kenda Bear Claws 23x8x11 front and 25x12.5x9 rear. They look ok but the difference in size from front to rear is massive. Not only does it look a little odd, I'm so raked that my trailer does not sit level when attached which becomes an issue when it's full. I was considering going up an inch in diamater to a 24x8x11 Bear Claw but Kenda does not make this tire in 24x8x11. Closest is 24x9x11 which is 1 inch wider. I like the idea, but the 23x8x11 are less then an inch from rubbing the shocks on the inside so I'm fearful that 24x9x11 will be too wide. Any thoughts?

Land vehicle All-terrain vehicle Vehicle Automotive tire Tire
 

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#20 ·
1986 Fourtrax 350 acceleration uneven

My 1986 Fourtrax 350 will start and idle great, when I push the thumb throttle a third to a half, it backs down, then increases. It does this both in neutral and in gear, although in 3rd+ gear, it doesn't seem as bad while there is a pull on the motor, but when you get closer to max speed of that gear, it gets worse.
Could this fuel pump malfunction or carb? This just started in the last 6 months and has gotten worse.
 
#21 ·
Fuel pump malfunction? I don't believe you have a fuel pump. Your fuel flow is gravity fed.

I'm thinking dirty carb though it could be a manifold vacuum leak.

You should start a new thread for this topic.

Rick

Sent from my SAMSUNG-SM-N900A using Tapatalk
 
#23 ·
i have a 1999 Foreman 450 ES and recently changed the fuel filter on /off/res and the fuel line with an 8mm fuel line which is a little bigger than the 7.55 OEM. and for some reason it will idle fine start fine, but starting about midway on the throttle it starts to sputter and run rich and spit some fuel droplets back to the air filter side.Ive screwed the external pilot screw all but about 3/4 turn in. i've taken the bowl off and sprayed out all the jets and have run fuel stabilizer through this carb for more than 2 yrs and the bowl was perfectly clean.
I wouldn't think this slightly larger fuel line would be my issue though.
Is there any other reasons why it would run overly rich like that.
thanks
gary
 
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