so what is it with the 06?

[jimfergie]

I just had the dealer and Yamaha let me take a carb of another new bike to put in mine, (the usual popping on decel) Rode it yesterday at the track, the bike actually runs great, fairly minimal pooping on the last bit of rpm's. I had another guy with an 06 and an expert rider ride it and he thought it was fine, the little bit of popping is OK. So this morning I put in a 168 MJ (we have a 48 PJ, stock needle 5th clip, 3 turns out FS and had a 165 MJ yesterday) and it pops pretty bad after riding it for 5 minutes. So obviously it's 15 degrees cooler this morning, maybe 60-65 and yesterday it was maybe 75-80. I sealed the exhaust, checked the hot start for slack, rejetted, reneedled several times, NEW carb, so the question is what is it about the YZ that requires this, the 06 CRF I had didn't do this at all, never touched it, no popping ever. The carbs are different on these two bikes, both FCR's but different. At the lowest RPM's on decel it seems the motor simply can't pull in the fuel it needs, or the air it needs to pull the fuel. Any thoughts?

[racegmc]

I have the same problem! I always have to adjust the fuel screw to make it stop popping. Really is a pain to deal with every time you ride.

[grayracer513]

Why exactly are we all so worried about popping on deceleration? And if it's a concern, have you checked your exhaust system for air leaks, like at the midpipe clamp?

[450 vet]

youll probably think im smoking but how are your valves?

[matt116]

I think it is the difference in the slide height or "throttle valve height" as Yamaha calls it. I can only compare the 03' to the 06' it's all I have specs on.

03' slide height 3.40mm

06' slide height 1.25mm

So when you chop the throttle on the 06' the slide closes more, less opening = less fuel and air getting to the motor causing the lean condition when decelerating with the throttle closed.

I know it's only 2.15mm difference but that could be enough when you factor in the shape of the slide.

This is just a thought I have yet to prove this to be the problem.

[MX1967]

You guys are suppossed to adjust you fuel screw daily as temps require. If you never touched the one on your CRF 450, it was probable set too rich in the first place so the bike never popped on decell. I've adjusted my fuel screw DAILY since I owned my 426 in 02...then on my 04 450 & now on my new 06 450....it's really easy to fine tune your carb with the fuel screw.

[02YZ426]

I don't worry about popping on decel... these bikes will do that a little and I think its pretty normal. Almost every one of these bikes I've seen pops on decel to a certain degree. Now if its backfiring obnoxiously than thats a different story, you probably need to adjust the air screw or fix an exhaust leak. How many hours are on your bike? Maybe you need to re-pack you muffler.

[Fastdaddy]

If its popping on the decel then just pin it and forget about it.

[Butta]

Yeah, adjusting the fuel screw EVERY day is such a pain!!!! Give me the valve concerns with the CRF's any day.......who wants to have to reach all the way down to the bottom of the carb and turn the little fuel screw every day....man, TOO MUCH!!!!

[jimfergie]

I checked the valves, one intake was just a bit tight so I reshimmed it, as I said I sealed both the exhaust joints with high temp silicone. I'll look into the slide, interesting. No, I've screwed around with the fuels screw from nothing to 3 turns out, really very little effect on popping or idle speed. I can hear what is considered "a little popping" on decel, but this is quite a bit more than that. Again the CRF and my 04' KTM 450SX were never touched, didn't matter as to altitude or temp, over three years. Maybe they were rich, I don't know, they were as they came from the factory, they started and ran with no fuss. I also see that the WR uses the Air Cut Off feature on the Keihin specifically to eliminate the popping on decel, why on the Yamaha?

[Ga426owner]

Yeah, adjusting the fuel screw EVERY day is such a pain!!!! Give me the valve concerns with the CRF's any day.......who wants to have to reach all the way down to the bottom of the carb and turn the little fuel screw every day....man, TOO MUCH!!!!

Valve concerns vs adjusting the fuel screw......please tell me you are kidding .........the very first thing any Yamaha 4stroke owner needs is a fuel screw and a rejetting. If you are serious about riding you will be adjusting the screw most every day for temp and humidity changes....if not get used to popping, simple as that or go back to a CRF.....

It is not that big of a deal -

BTW mine does not pop on decel

[Lemay427]

were you valves out of spec already? or did you just make a small adjustment to your liking?

[grayracer513]

I also see that the WR uses the Air Cut Off feature on the Keihin specifically to eliminate the popping on decel, why on the Yamaha?

The WR has an air injection system that runs air into the exhaust system. It is that air that the Air Cut-off cuts off (to prevent REAL exhaust backfiring), not anything that passes through the carb.

I set my fuel screw strictly and only based on how the bike runs, not by whether or not it pops. I also find that I don't very often have to tweak it much to get it to idle right, transition to off-idle smoothly, or run clean at light throttle, which is most of what the idle circuit does. Most of the time, the bike pops a bit, sometimes not, sometimes quite a lot. I frankly don't care one way or the other, unless it gets excessive, as long as it runs right.

[jimfergie]

The Air Cut Off is on the left side of the FCR, it's on the WR apparently not on the YZ and the CRF's Keihin doesn't even have the molding on the side to have it. Why not? For those who don't mind the popping, obviously it depends on how much and how loud is acceptable to your ears. Sometimes it's mild and OK, the next day it's more quantity and louder. That brings me back to original question, what is it about the YZ that causes it? the 3 intake valves, the velocity of air through the carb, the different slide, the different Keihin ? On the intake valve, it was just one, the left one and it was right on the edge so I went to the next shim down so it was more in the middle of the spec.

[jimfergie]

Matt, is the spec on the slide you're talking about the air space left open when the slide is at an idle position? I noticed the part number for the 06' slide is different from past years. Being that it's only $250 it unlikely I'll buy one to try it. I've been thinking of playing with clip position vs. idle screw setting....

[grayracer513]

The Air Cut Off is on the left side of the FCR, it's on the WR apparently not on the YZ and the CRF's Keihin doesn't even have the molding on the side to have it. Why not? ... On the intake valve, it was just one, the left one and it was right on the edge so I went to the next shim down so it was more in the middle of the spec.

The source of the air in the air injection system on a WR is the filtered air from the carb intake. It is ducted from the intake bell through an external steel tube along the right side of the WR head to the exhaust port, where it flows into the exhaust stream to reduce unburned hydrocarbon emissions in the exhaust. Because it has a carburetor, it is necessary to cut off or divert the air flowing to the exhaust in order to avoid serious exhaust backfires resulting from the added air mixing with the fuel passing through the engine in the heat of the exhaust pipe. Cars with AI systems have one form or other of the same thing (Bypass, Diverter, or Dump Valves). On the WR, the cut-off valve is located at the carb, but that still has nothing whatever to do with cutting off any air that flows from the carb through the intake port. You ask why CRF's and YZF's don't have this. Once again, they don't have the system that the cut-off valve was intended to control, simple as that. Besides, the Air-Cutoff only prevents backfires caused by the AIS, not those that occur due to lean idle or other air leaks, and WR450s running the system can and do sometimes pop through the exhaust.

Intake valves that are tighter than spec will create a lean condition by diluting the intake charge and lowering the strength of the vacuum impulse. To that extent, they could contribute to an intake pop. If you changed a shim size by a full .05mm and you are now in the middle of the range, it was below spec before. If you changed out an odd sized shim used at the factory (like a 188), that's different, but bear in mind that YZF's are intentionally built as near the minimum valve clearance as possible, and even at the minimum, in spec is still in spec, and the bike will run just fine at that setting.

Some engines "cackle" out the exhaust on decel, and some don't. Anyone who has spent much time around auto racing is certainly familiar with this. The YZ450 just happens to be one of those that does, and I really don't get why it makes people so darn nervous.

[KTMKarl]

You know gray racer brought up a point that I haven't thought about yet. usually in the stock car world a car will "flame-up" on decel due to a mixture that is too rich at idle or in the case of some odd-ball small carb classes it can be a sign of a power valve issue causing a too rich condition at the end of the straight. (many use the power valve as a high speed lean out system a valve that closes to late won't let the engine "clean out" at the end of the straight) It could be that your running mixtures too rich. Personally I haven't experienced this issue. My motor acted lean when new and the temps were below 40F. Once she broke in the motor started likeing leaner settings and then it started getting warm outside and I keep getting leaner as the mercury rises. I have went all the way back to stock jetting and have actually lowered the float level a scosh to keep her running clean in the chop. You may be chasing your tail, try the stock settings again. Regardless you'll need more than 5 minutes run time to get the motor up to temp ecspecially if riding at a track that allows for some speed. I have witnessed several folks starting up the bike and fiddeling with the fuel screw way before the motor warms up, they always end up with an idle mixture thats way to fat.

[jimfergie]

good info, I'll go back over that info on the Air Cut Off valve tonight, thanks. I returned the old carb to the dealer today, I didn't want him to be out, but Yamaha gave them the OK to replace it, the new one seemed to be able to have it's fuel screw be more effective at idle. I blew some money on the Power Now, I won't put it in though until I mess with jetting some more. I think my next step is to try the extremes of pilot jets and fuel screws (40 vs 50 or whatever) as well as the extremes of clip settings to see if I can come up with a solution. The shim I used, by the way, came out of my Hot Cams kit and it was about 1/2 a step less than the shim that came out of the bike.

[jimfergie]

GrayRacer when I look at the parts diagram for the WR I see the whole AIS tubing and such, but on the side of the carb is the Air Cut Off Valve, diaphragm cover, etc. So these are connected? if so, how? And for all this discussion the main reason the person new to YZ's like me is the fact that the KTM and CRF didn't do it! I'm just super curious and want to see if I can correct it, if in the end I can't, then I can make the decision then to live with it or sell it.

[matt116]

Matt, is the spec on the slide you're talking about the air space left open when the slide is at an idle position? I noticed the part number for the 06' slide is different from past years. Being that it's only $250 it unlikely I'll buy one to try it. I've been thinking of playing with clip position vs. idle screw setting....

Yes.

I know those specs are used to set the slide to a specific value so you can set the accel pump timing. I have found the idle speed I like, to set the slide height close to the spec-ed dimension.

When checking the accel pump timing on my 06' I had to back out the idle screw .75 turn to lower the slide to the 1.25mm spec.

I have tried the all the jetting combos prescribed on TT and they all felt to rich to me, I'm now back to stock.

Try holding the throttle open just a little instead of just letting it close all the way when entering a corner. This can even help on braking bumps as long as you don't pull the clutch in.