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Discussion Starter #1
Hello everyone

I will try and make it as short as possible!!

I have a honda trx 400 ex 2005 for a year now and it runs perfect , every gear runs smoothly from closed to full throttle , no sputtering , boging or anything and she is stock , I did nothing to her except take the original exhaust spark arrestor out

But my problem is that my spark plug is so damn black !
Carbon fouled powdery black, the atv does smell heavy on Gas while idling also , and very rarely while trying to hold a wheelie at high rpm a shot of black smoke would come out sometimes !!

I think I need to rejet , my atv does not have an adjustable needle So I ordered a Dynojet kit which arrives next week ,

I was reading the instructions online and it says for my model to bump the main jet about 5 sizes up ,won’t this make my bike even richer , is this really necessary or should I just lean the needle ? Oh and to mention I have never touch the air/fuel mixture screw , could it be as simple as adjusting it that fixes my problem ?

I use the correct plug specified in the manual , I also read that riding at low speeds can carbon foul your plug , it’s that where I live atvs are road legal and I use it a lot around the village I live and I’m mostly at 0 to 1/2 throttle or a bit more , hardly at WOT

Like I said the atv runs perfect , no stuttering or bogs I just want the spark plug to look normal !

heres a pic of my spark plug , when I clean it it’s not as black

4340
 

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It could be as simple as your fuel or altitude. If your are 4000 or more feet above sea level, you need to reduce the main jet from a 148 to a 145 and turn the fuel screw in 1/2 turn - if your fuel is over 93 octane or has a certain additive, it can cause the carbon - low speed operation, long periods of idling, frequent cold starts and short periods of operation will also cause the carbon build up. Each time the engine is started, it should be run at approximately half throttle for no less than about 20 minutes to get the spark plug hot enough to burn the carbon off. You could also use a hotter DPR7Z spark plug. When jetting the carb, the pilot jet is for idle - the slide needle is for mid-range (1/4 to 3/4 throttle) and the main jet if for full throttle - the main jet is the only control for how much fuel is mixed with the air at wide open throttle. A very big factor of how black the spark plug gets is the air filter. If it's foam and over oiled or dirty, it will cause rich running at all throttle settings especially at full throttle. An obstructed air intake snorkel, a high fuel level in the float bowl due to a damaged float and crankcase oil diluted with fuel caused by not shutting off the petcock when the engine is not running can also cause sooty spark plugs.
 

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Discussion Starter #3
It could be as simple as your fuel or altitude. If your are 4000 or more feet above sea level, you need to reduce the main jet from a 148 to a 145 and turn the fuel screw in 1/2 turn - if your fuel is over 93 octane or has a certain additive, it can cause the carbon - low speed operation, long periods of idling, frequent cold starts and short periods of operation will also cause the carbon build up. Each time the engine is started, it should be run at approximately half throttle for no less than about 20 minutes to get the spark plug hot enough to burn the carbon off. You could also use a hotter DPR7Z spark plug. When jetting the carb, the pilot jet is for idle - the slide needle is for mid-range (1/4 to 3/4 throttle) and the main jet if for full throttle - the main jet is the only control for how much fuel is mixed with the air at wide open throttle. A very big factor of how black the spark plug gets is the air filter. If it's foam and over oiled or dirty, it will cause rich running at all throttle settings especially at full throttle. An obstructed air intake snorkel, a high fuel level in the float bowl due to a damaged float and crankcase oil diluted with fuel caused by not shutting off the petcock when the engine is not running can also cause sooty spark plugs.
First off thank you for the reply !! You really seem to know what your talking about !!

Im at sea level here in Europe where I live , under 5000 feet and the area I ride is 0 to 500feet

The Gas we use over here is 95 and 98 , I use the 98, the good stuff lol

my air filter is never dirty or dipped in oil (picture below , and yes I run my atv with the air lid on ) and air snorkel intake is

Tuesday when my Dynojet kit arrives first thing I will do is check the pilot screw and see how many turns out it is , since I’ve never touched it since I bought the bike

So your saying a hotter plug , the dpr7z is better for my slow speed operation? I’m usually riding 30km to 60km since I’m mostly in the village and not off road !!

I just want a normal looking spark plug lol
Once again thanks A lot for the reply

4341
 

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The octane rating is different for different methods of measurement:

In most countries in Europe (also in Australia, Pakistan and New Zealand) the "headline" octane rating shown on the pump is the RON, but in Canada, the United States, Brazil, and some other countries, the headline number is the simple mean or average of the RON and the MON, called the Anti-Knock Index (AKI), and often written on pumps as (R+M)/2. - here is the US, our highest pump gas is 91

I use VP Racing SEF (small engine fuel) with a 94 octane rating

Too high of an octane will produce a slow, cool burn and increased carbon deposits. I have found that my CRF250X (which specifies 93 octane) begins to lose power at 95 octane - our 91 octane pump gas is blended with ethanol, so I use the VP fuel which is non-ethanol. Here the higher the octane rating the higher the ethanol content. More ethanol means higher maintenance cost and lower fuel mileage. My Mercury Grand Marquis get 4 miles to the gallon better fuel economy with non ethanol 87 octane than any other grade of fuel. I buy my fuel from the bulk farm supplier and buy 200 to 250 gallons at a time. I get better gas at a lower cost than buying at the pump.

You air filter needs to be oiled to capture fine particles of dirt - I use UNI aerosol filter oil, spray the filter as evenly as possible, then work the oil into the foam (I wear rubber gloves while massaging the oil into the foam - a friend of mine does it in a big zip lock bag) - then I use paper towels to blot out all excess oil (I lay a couple paper towels on the floor, place the filter on the towels, cover with more paper towels and step on it to press the filter between the towels - NO TWISTING - pressing/squeezing will not hurt the filter - twisting/wringing the filter will stretch and tear the foam, but it really needs to be oiled.

Keep me posted on the results after jetting and for slow speed riding, just run in a lower gear to get the engine speed up to about 4000 rpm when cruising.
 

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Discussion Starter #5
The octane rating is different for different methods of measurement:

In most countries in Europe (also in Australia, Pakistan and New Zealand) the "headline" octane rating shown on the pump is the RON, but in Canada, the United States, Brazil, and some other countries, the headline number is the simple mean or average of the RON and the MON, called the Anti-Knock Index (AKI), and often written on pumps as (R+M)/2. - here is the US, our highest pump gas is 91

I use VP Racing SEF (small engine fuel) with a 94 octane rating

Too high of an octane will produce a slow, cool burn and increased carbon deposits. I have found that my CRF250X (which specifies 93 octane) begins to lose power at 95 octane - our 91 octane pump gas is blended with ethanol, so I use the VP fuel which is non-ethanol. Here the higher the octane rating the higher the ethanol content. More ethanol means higher maintenance cost and lower fuel mileage. My Mercury Grand Marquis get 4 miles to the gallon better fuel economy with non ethanol 87 octane than any other grade of fuel. I buy my fuel from the bulk farm supplier and buy 200 to 250 gallons at a time. I get better gas at a lower cost than buying at the pump.

You air filter needs to be oiled to capture fine particles of dirt - I use UNI aerosol filter oil, spray the filter as evenly as possible, then work the oil into the foam (I wear rubber gloves while massaging the oil into the foam - a friend of mine does it in a big zip lock bag) - then I use paper towels to blot out all excess oil (I lay a couple paper towels on the floor, place the filter on the towels, cover with more paper towels and step on it to press the filter between the towels - NO TWISTING - pressing/squeezing will not hurt the filter - twisting/wringing the filter will stretch and tear the foam, but it really needs to be oiled.

Keep me posted on the results after jetting and for slow speed riding, just run in a lower gear to get the engine speed up to about 4000 rpm when cruising.
Still waiting on my dyno jet kit & carb repair kit ,,, should come in next week ,,, so I will let know you how it goes !! Don’t think I forgot about you !!

today being bored I cleaned my spark plug very good put it in my quad and let it idle and gave it a few soft revs for no longer than 3 minutes,,,when I checked the plug it was dry carbon black ,,, do u think a hotter plug will be better ? From a 8 to a 7 ??
 

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No - a hotter plug will not help - the heat rang of a spark plug does not become a factor until the engine has heated to full operating temperature (15 to 20 minutes of riding) - then to check the spark plug you have to operate at a constant speed for about 3 minutes at the speed you want to check at - without changing the throttle setting, cut the ignition, pull in the clutch, come to a stop and pull the plug to see how it looks at that speed.
 

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Discussion Starter #7
No - a hotter plug will not help - the heat rang of a spark plug does not become a factor until the engine has heated to full operating temperature (15 to 20 minutes of riding) - then to check the spark plug you have to operate at a constant speed for about 3 minutes at the speed you want to check at - without changing the throttle setting, cut the ignition, pull in the clutch, come to a stop and pull the plug to see how it looks at that speed.

Ok so finally got my dyno jet kit , and carb rebuild kit , I put everything together , quad fires up on first try and idles and runs perfect like before !

Spark plug still black when I take her out lol , I might just have to live with it(will do a plug test at WOT soon) . Every jet is stock and fuel mixture screw is out 2.5 turns

What I do notice is slight poping upon deceleration going down hill , can I fix that by turning the fuel screw ?
 

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Maybe - backfiring in the exhaust during deceleration will not hurt anything and will occur if the exhaust has been modified or changed to a low restriction system.

It is the job of the AIR CUT OFF VALVE to shut off air to the idle circuit during deceleration to enrich the idle mixture to prevent backfiring in the exhaust during deceleration. If the o-rings are not sealing or if the valve is defective, it will let air into the idle circuit and promote back firing.

Two things you can do to alleviate the back fire without changing parts is #1 - increase the idle speed and #2 - adjust the fuel screw out 1/2 to 1 turn (as long as the setting does not adversely affect idle).

4352
 
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