Jump to content

backfiring when letting off throttle?


Recommended Posts

I was told it is my needle setting as it happens at around 50% throttle.

so am I overall running too lean, or too rich?

elevation is 4000-8000 feet.

should I move the needle up? or down?

should I touch the fuel screw or main jet, which is stock 185 BTW. 2008 ktm 450.

thanks.

Link to comment
Share on other sites

Since closed throttle is controlled by the pilot screw and pilot/slow jet and decel. pop is a lean condition (although you will have decel pop with correct jetting) first try going out (richer) a little on the pilot screw.

Link to comment
Share on other sites

However............ Think about it for a minute. You own a race bike and are trying to make it run like a trail bike. Not going t happen. Put on a nice quiet muffler and the popping will go away. What you need to do to minimize it as much as you can is ensure there are no exhaust leaks. Try a new headed to cylinder head gasket. Make sure the exhaust is not twisted as a result of a fall.

Lastly, seet your fuel screw for a perfect idle with afully heated bike. Do not tweak it to compensate for something it is not supposed to do.

Link to comment
Share on other sites

However............ Think about it for a minute. You own a race bike and are trying to make it run like a trail bike. Not going t happen. Put on a nice quiet muffler and the popping will go away. What you need to do to minimize it as much as you can is ensure there are no exhaust leaks. Try a new headed to cylinder head gasket. Make sure the exhaust is not twisted as a result of a fall.

Lastly, seet your fuel screw for a perfect idle with afully heated bike. Do not tweak it to compensate for something it is not supposed to do.

Ive got TONS of friends with "race" (do you mean track?) bikes (yz450f, trx450 with 490 BB, cr500r, yz 450f bored and stroked to 515cc). Hell, I own a crf450r that is DIALED in for trail riding, it has all race engine high performance parts, yet all the guards and bells and whistles of a trail bike. why didnt I just get a 450x? I dont know. too late now though and I couldnt be happier with my R... after two years and tens of thousands of dollars I swear it is PERFECT (or im just getting used to/comfortable on it...)even now, I still wouldnt get an X. theyre bigger and heavier, and the engine are a tad more complicated (electric start, decompression, etc..) not cool.

but still though, I dont understand whats the big deal turning a "race bike" into a "trail bike", especially when were talking about the carbs here. a carb is a carb right? its just jetting, the simple concept of fuel + air.... all engines deal with it, whats the difference if im running a ktm track engine or a trail engine... ive got an engine that pops when I decelerate... it doesnt sound like a healthy pop, definitely a jetting issue. I can probably figure it out where, how, and why. I just need to know if the popping is happening because im running too lean or too rich?

I bought this bike for cheap and I intend on keeping the money thrown into it at a very minimum which is why I want to change my jetting for under 10$ versus a 1000$ full exhaust system.

I ordered a adjustable fuel screw... I am hoping maybe I can solve the problem with that.. if it doesnt help it, then I will be changing jets around.

Link to comment
Share on other sites

its an old 450, my main one, been rebuilt a few times. this ktm is actually pretty much my brothers bike and he rides with me every time I go out.

I think I am just going to start with the fuel screw and tinker with it at different elevations when warm.

Link to comment
Share on other sites

You can jet away a decel pop, no question. But it will be at the expense of run-ability. You'll be way rich on the idle circuit of the carb if you go that route of the fuel screw and pilot jet. This can cause hot starting to be a nightmare. You will be too rich on the straight diameter of the needle too (at least from idle to about 1/4 throttle.

You can instead use a slightly richer needle on the straight diameter. This may reduce some of the excessive richness of doing to the fuel screw/pilot jet method, perhaps improving hot starting but slow speed, steadfy throttle will still be terrible. But you will no longer pop.

You are going to find the fuel screw does not add enough fuel to eliminate the popping. You will need to go up a size or more on the pilot. Experiment, that is how you learn.

I have friends who trail ride YZ's. It is a lot of work, even with the AP optimized for slow speed, a heavier flywheel and lower gearing. I am lazy and therefore run a WR. I am way past my race days.

Link to comment
Share on other sites

I'd check the seal at the exhaust headers to the cylinder. A gap in that will cause what you're experiencing. And until you fix it, it's only going to get worse. It's not a big deal, mine's been doing that occasionally since I bought it.

Some day I'll pull the header off and throw some copper gasket sealer around there and it should fix the problem.

Link to comment
Share on other sites

I had the same problem with my 650r. It wasn't just the normal pop, but a loud crack. I went down one size on the pilot and now it is just the normal pop. I'm guessint that unless yours is just the normal, a smaller pilot would take care of it.

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

×
×
  • Create New...