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Discussion Starter · #1 ·
Ok i just upgraded from a cut way short dg to a full size hmf slip-on and now my jetting is off. Again:( lol. But any ways it back fires and pops and shoots about a 3 foot flame, im pretty sure its like rediculously rich. Havent pulled the plug yet. Lucky me i have like every jet for my carb ever made so i wont have to buy any but yeah im running a 145 main and a 40 pilot. I think the shorty pipe had alot to do with my jetting being so high. It was cut down stupid far and was stupid loud.

Oh yeah i also added a stage 3 cdi, monster coil dunno if that does anything with jetting

i have a 148,145(in it),142,140,138,136,134,132,130,128(stock)
those are my main jet choices

38,40
those are my pilot choices

Any Help is greatly appreciated!:D
 

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Discussion Starter · #2 ·
Anyone? Moose get on. I need your help bro!
 

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I'd say you should be running about a 140 main and a 40 pilot.
 

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Other than the piping what other mods are done? I see a cam in your sig. how about piston? drop the slow back to the 38 and turn the air screw 2 1/2 turn out from lightly seated.
 

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Discussion Starter · #5 ·
Piston stock rings stock. Upgrading this winter but as of now they are good so im running it. Leterally just a cam rockers stock crank. Then a full pipe [email protected] airbox lid removed and a cdi monster coil and thats about it.
 

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check the plug!!!!!! haha.

and well also just fyi, mine back fires and pops when im running out of gas if im on it hard (lean condition)
 

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Discussion Starter · #7 ·
Ok i put the 38 pilot back in and i steped down the a 142main and it starts a whole lot easier and seems to have way more power. It doged before now second gear the front end is off the ground and it hops the front off ever gear. Why would that be im so lost on why changing the pilot and the main down a bit would make such a huge difference? Anyone know?
 

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Why would that be im so lost on why changing the pilot and the main down a bit would make such a huge difference? Anyone know?
Because you're getting closer to the correct air/fuel mixture. The closer you get to the correct air/fuel mixture the better it will run. The ideal mixture is 14.7 to 1.
 

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Discussion Starter · #9 ·
Ok i understand now. But why would everyone else i know have to run a 40pilot and me run a 38pilot? And why would the pilot jet make such a huge difference? Cuz i ran this same main with a 40pilot and it was nothing like this. Believe me i would have left it its running the fastest it ever has.
 

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Ok i understand now. But why would everyone else i know have to run a 40pilot and me run a 38pilot? And why would the pilot jet make such a huge difference? Cuz i ran this same main with a 40pilot and it was nothing like this. Believe me i would have left it its running the fastest it ever has.
Maybe everyone else hasn't gotten as close to the correct mixture as you have. It depends on the amount of flow through your engine. All parts, such as pipes, air filters, carbs and engines are probably going to flow a little differently, even if they are the same brands and type as someone else is using. There's a lot of variables. They may be close, but it's not going to be exact every time.
 

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BTW, you better do a plug test and make sure it's not running too lean. They'll run a lot better when they're running lean.
 

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Discussion Starter · #12 ·
Yeah i gotta go buy a new plug to get a clean slate to start with then ill do a plug test. But it wasnt running hot so i dont think its all that lean. I rode it around the yard for a while and let it idle and stuff but then again i do have the oil cooler and a fan so i guess thet may be making it run cooler even if it were lean. But one more question why would it still be back fireing. I put a new exhaust gasket in and there are no holes in my pipe anywhere. But it still pops and throws a flame alot less often now but still about every say 3 or 4 minutes of riding. and really when i rev it or use the engine to stop to make a corner.
 

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The reason you can run a 38 jet is because you are still running stock piston and the compression isn't as high as an aftermarket piston. the backfiring is probably because your timing is off a tooth or two. I would step down a size on the main 140 is a bit much for just a cam mod. before I put my 400 carb on I ran a 134 main and a 42 pilot with my mods and it was running great then.
 

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Discussion Starter · #14 ·
But my timing is perfect. I checked it like 15 times. And it hasnt always been back fireing it started when i put the hmf on?
 

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Drop your main jet it's way too big, my opinion. My friends HMF on his 250EX backfires some too.
 

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Discussion Starter · #16 ·
Yeah im thinking billy was right on with the 140 main. I got the 142 in and it still seems rich. I dont think i would need to go any lower than a 140 but i guess we will see i bought 5 spark plugs today so i can keep trying from a clean slate so i get it right. It only back fires every once in a while now though. I think it may have been from when i was running way rich, it is re-burning fuel that is in the pipe yet. Either way the back fire is in the pipe not up in the motor. Its detinating way back in the end of the pipe.
 

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Discussion Starter · #17 ·
Now i have another related question. Is there any way of checking the pilot jet. Can i just leave it idle for a while then pull the plug to see if that is running where i want it rich lean wise? Or is it kinda a guessing game with the pilot jet? I am just so tired of jetting this damn thing i want to get it right and let it go already lol.
 

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To check the pilot put the one in that starts the easiest.As for mains you might have to go below 140.I have my homemade pipe and it runs a 42 pilot and a 140 main.But like helmut said every pipe is different.
 

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The way I always did the slow jet was, if I had to turn the mixture screw out more than about 3 or 3 1/2 turns to get a good idle, then I would go up a size on the slow jet. If I had to turn the mixture screw in to 1 turn out or less, then I would go down a size.
 

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Discussion Starter · #20 ·
How do you get to the mixture screw while its mounted on the quad? Or do you pull it every change on the mixture screw? And with the 38 it seems to start just about the same as when i had the 40. I have the mixture screw at 3 turns because that is what dynojet says i should have it for my elevation? Dunno if thats correct but i just got that off there site.
 
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