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The Hill Boys

HELP! WR450F header pipe

11 posts in this topic

Ok, having some problems on decel. Instead of "coasting" when letting of the throttle, the bike seems to "engine break" causing me to lunge forward a bit. Lots of popping now on decel too. I've done a lot of research indicating an air leak in the header pipe as the probable cause. Question. the flange used to bolt the header to the engine sits about 1/2 away from the surface when the bolts have been tightening. Every other bike I have owned, this flange sat flush to the engine surface. Is this normal? Also, one of the bolts is a regular 10mm bolt while the other is a hex head requiring an allen wrench. Why is this? I can't get the other one as tight as the regular bolt. Is that my problem? I bought the bike used and this is the way it came.

I bought a YZ muffler a while back and slipped it on to the stock header. Backfire on decel to too much to bear. Should I invest in a new header pipe? If so, should I get the YZ and match it up with the stock YZ exhaust or stick with stock WR header/muffler?

Thanks in advance!

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Yes, excessive decel popping is often caused by an exhaust leak.

Yes, the flange sits away from the head. When both bolts are tightened equally (use anti-sieze) the flange should be parralell.

Yes, a open muffler like the YZ will make popping worse, As will a poor fit.

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Ok, having some problems on decel. Instead of "coasting" when letting of the throttle, the bike seems to "engine break" causing me to lunge forward a bit

Erm, are you coming off from 2 stroke? all 4 strokers engine break when you cut the throttle. Excessive popping on deceleration can also be a sign of lean pilot circuit. Try to unscrew your pilot screw on carburetor a bit. You can also check whether your pilot circuit is lean by looking at your throttle response right off idle. On warm bike, I repeat, fully warm bike, hold your clutch in and open throttle by smallest amount you can and listen to revs. If bike responds immediately, then it is not lean, but if revs remain constant and then, after some time jump up, it's sure indication of lean running bike. This effect is more evident when you apply smaller throttle, I'd say in range of 1 milimeter.

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Thanks all! Yes, I am coming from 2 strokes. However, the engine braking has increased dramatically since I bought the bike last year.

I did all the mods, AIS removed, JD jet kit, gray wire, etc. After I did all of that, I had gone on several rides and it ran flawlessly. I did pick up a YZ exhaust and tried to tune the bike with it on, but the popping was ridiculous. So, I put things back the way I had it and ran it a few more time and all was good.

Didn't ride for about 6 months, but occasionally started it up and rode it around the block. Just recently, it has been difficult to start. Once started, wouldn't idle very well and I started noticing the engine braking and popping more.

Yesterday morning, I noticed the adjustable fuel screw that I bought was totally unscrewed and perched on the cylinder. Thankfully, I didn't loose it. Not remembering what setting I had it at, I screwed it all the way in and then backed it out 1 3/4 turns. After the bike was warm, I started playing with it. I found that it revved higher around 2 1/2 to 3 turns out. Interesting, at 3 turns, the screw was so loose, that it would vibrate out.

I left it set at the 2 1/4 mark. Bike ran great up through the gears, but still had the decel pop and engine break.

Thanks for confirmation on the flange. Does your have the two different style bolts too?

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There's something wrong with your screw then, it shouldn't fall out after 3 turns out definitely, but anyways, 3 turns out seems way too much for me. If I were you, I'd be looking for next size pilot jet B)

Tuning by highest rev is much more inaccurate(at least for me), I found that using throttle response I can feel difference of turning screw by 1/8 of a turn. Jetting the bike is all about running it best way, and having best response is in my case that sweet spot :smirk:

Also check for vacuum leaks, it could cause lean situations.

And yeah, about exhaust header bolts. I've got 2001 wr426, mine has different bolts too. I think its all about tools, you couldn't get to inner bolt if it was allen type.

Keep us posted about your jetting issue.

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You let the bike sit too long, the pilot jet is beginning to clog. Replace the jet, reset the fuel screw. You should never open it more than 2.5 turns.

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:thumbsup::banana::lol:

Ok, I think I'm finally headed in the right direction!!! I rejetted (went to JD Blue #4) and a 170 main. I also checked the GYTR AIS caps. Removed/tightened the cap on the front of the engine. I bought a tighter clamp for the little rubber cap near the carb. I put the YZ exhaust back on, bought some gasket seal and put a little on the head pipe. Set the MSR fuel screw at 1 3/4.

Bike started right up but was still backfiring/popping on decel. Turned the screw in a bit (need to go figure out where it ended up) and the pop went away except for chopping WOT.

Went for a ride around the neighborhood. First thing I noticed is the severe "engine braking " that I was encountering (almost sent me over the handlebars before) was gone:banana::ride::banana::(:banana:. Smooth excel down the street follow by smooth decel produced no pops!!!! Again, opened it up on a straight and then chopped the throttle and did have some popping. I can totally live with this. I think I may need to play around with my pilot circuit a little more, but am VERY please with the results thus far!

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:ride::banana::(

Ok, I think I'm finally headed in the right direction!!! I rejetted (went to JD Blue #4) and a 170 main. I also checked the GYTR AIS caps. Removed/tightened the cap on the front of the engine. I bought a tighter clamp for the little rubber cap near the carb. I put the YZ exhaust back on, bought some gasket seal and put a little on the head pipe. Set the MSR fuel screw at 1 3/4.

Bike started right up but was still backfiring/popping on decel. Turned the screw in a bit (need to go figure out where it ended up) and the pop went away except for chopping WOT.

Went for a ride around the neighborhood. First thing I noticed is the severe "engine braking " that I was encountering (almost sent me over the handlebars before) was gone:banana::lame::banana::smirk::banana:. Smooth excel down the street follow by smooth decel produced no pops!!!! Again, opened it up on a straight and then chopped the throttle and did have some popping. I can totally live with this. I think I may need to play around with my pilot circuit a little more, but am VERY please with the results thus far!

Popping on deceleration has nothing to do with main jet or needle. The only active circuit in the carb when throttle is closed is your pilot circuit.

Popping on deceleration is caused by lean pilot circuit and it seams a bit weird to me that turning screw in i.e. leaning it more made popping go away :lol: according to mods you've described you should have had to open the pilot screw by more than 2 turns, if you're running stock pilot jet ofcourse.

And thoughts about engine braking. engine braking depends only on rpm of motor and idle setting of your bike. Lean or rich mixture has nothing to do with it. In fact, there isn't happening any combustion in your engine when you do hard engine braking because carburetor cant mix sufficient amount of gas with air, and A/F ratio goes up wildly. If you don't believe me, try this on your bike: get some rpms and cut the throttle, and while you're bike is engine braking, hit killswitch. You wont feel any difference on the stopping force, even the engine sound won't change :) the only thing that will change is that when you release killswitch, most likely you will get one big pop

instead of several little ones.

Try to jet your bike for maximum performance, not minimum popping. It's all about having fun :lol::ride::thumbsup:

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Popping on deceleration has nothing to do with main jet or needle. The only active circuit in the carb when throttle is closed is your pilot circuit.

Popping on deceleration is caused by lean pilot circuit and it seams a bit weird to me that turning screw in i.e. leaning it more made popping go away :p according to mods you've described you should have had to open the pilot screw by more than 2 turns, if you're running stock pilot jet ofcourse.

And thoughts about engine braking. engine braking depends only on rpm of motor and idle setting of your bike. Lean or rich mixture has nothing to do with it. In fact, there isn't happening any combustion in your engine when you do hard engine braking because carburetor cant mix sufficient amount of gas with air, and A/F ratio goes up wildly. If you don't believe me, try this on your bike: get some rpms and cut the throttle, and while you're bike is engine braking, hit killswitch. You wont feel any difference on the stopping force, even the engine sound won't change :bonk: the only thing that will change is that when you release killswitch, most likely you will get one big pop

instead of several little ones.

Try to jet your bike for maximum performance, not minimum popping. It's all about having fun :bonk::ride::cheers:

Ended up taking it to a local shop. Bad seal on carb slide. Bike runs awesome now. No pop, normal engine braking. 45 pilot, 1 1/4. :ride:

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