YZ400F stumble at 5K RPM

[jb_dallas]

I have a 99 YZ400F. The bike screams at 6K RPM+ and idles fine, but I am noticing it cutting out a bit around 4500-5000 rpm (it has a Trailtech Computer). It kind of stumbles..not bad, but enough to notice. I try to avoid that range, but I trail ride and use that range quite often. I have a #50 pilot and the bike starts as good as can be expected for a YZ400. I believe the main jet is stock, but I dont think that could be the issue. Should I richen my carb needle position? I have an aftermarket mixture screw that I can adjust on the fly, but I cant seem to correct the issue. Im not certain that is the answer anyway.

Thanks

 

[Matt90GT]

50 pilot, that sounds really rich. What is stock? The 450s use a 40-42 stock and most around here at sealevel run a 45 pilot since the stock is a tad lean.

I would go back to stock on the pilot then adjust the fuel screw. If you still have problems, then adjust the needle clip position. YOu need to find what throttle opening you have the issue at, not the RPMs to narrow down what part of the carb you need to tune better.

 

[jb_dallas]

I think stock is 45 pilot. I have a 48 (not installed) that I bought a few months back, but that may be too rich also. My only concern with going back to a 45 is starting...those bikes are kind of tricky to start when tuned perfectly. Another thing is that Im not sure what mods may have been done, if any...which may effect jetting.

 

[_JOE_]

Where is the fuel screw set at? No need to play musical jets with the pilot as long a the F/S is within the normal range. A midrange condition could be needle position, ignition, fuel or silencer packing to name a few.

 

[jb_dallas]

I will need to check it again...I think I am about 1.5 turns out. I can richen the mixture by turning it out to 2, but that would cost me throttle response. Can you explain how silencer packing could be the issue...I dont get that one.

 

[_JOE_]

The packing allows a smooth path for exhaust to get out. It should be wrapped tightly to the core to keep the flow going through the core. If the packing is blown out the flow gets impeded by turbulance in the can which can cause some performance issues.

Your pilot seems good, not that it would affect the midrange much anyway. As long as you don't have any mechanical issues I would try moving the needle clip.

 

[jb_dallas]

Wouldnt the packing issue cause turbulance throughout the entire power band? The muffler is probably due to be repacked anyway. I may adjust the needle position. I could always change it back.

 

[_JOE_]

Usually blown packing will effect top end as well. If it's been a long time it's a good idea to do it before jetting.

 

[Rich Rohrich]

Is the rpm that causes the issue the same at different throttle openings, or is it specific to X rpm at Y throttle position?

 

[jb_dallas]

Rich

I only notice it at 5K rpm when I hold it in that range for a few seconds, usually 2nd or 3rd gear, then it clears up as I slow down or rev through the powerband. If I was hammering down on a track, I would never notice it, but I am riding trails. I think the throttle opening is usually the same or very close every time I notice the issue. I dont think it will miss at that rpm in neutral, although I havent actually held it there to find out. It seems like it has to be under load, thats why I thought it was fuel mid range related.

 

[Rich Rohrich]

Try unplugging the TPS and see if the behavior changes.

 

[jb_dallas]

I started the bike up last night and specifically listened to the engine and watched the tach. The bike really doesnt idle smoothly or hold any specific rpm at any specific throttle opening, it jumps around a bit...as much as 200 rpm. I had always thought that the idle was somewhat rough because it is a racing motor, as this is my first MX 4 stroke. Is this common in early generation...or all...MX 4 strokes?

 

[Rich Rohrich]

The cams have a lot of overlap, and the idle speed is about 1800rpm so it's not going to idle like a Toyota Camry. ;)

 

[jb_dallas]

Yeah, the bike idle jumps around from 1700-1900. Could aggressive cams cause the issues I described? I have no idea what has been done mechanically... it just ran strong and passed all of the tests I could think of when I bought it.

Rich, it seems like I have read about a modified YZ400 that you built. Would it run rough at low to mid range? Was it difficult to fine tune?

 

[Rich Rohrich]

Yeah, the bike idle jumps around from 1700-1900. Could aggressive cams cause the issues I described? I have no idea what has been done mechanically... it just ran strong and passed all of the tests I could think of when I bought it.

Rich, it seems like I have read about a modified YZ400 that you built. Would it run rough at low to mid range? Was it difficult to fine tune?

I never found the YZ4xxF engines difficult to tune, even when the power was well north of 50hp. I've had more practice at it than most, so I might not be a good point of reference.

Without riding your bike I couldn't say for sure what your problem is, but it's pretty common for guys to put a pipe on a big 4t that causes a huge dip in the torque curve where the pipe is out of tune. This dip in the torque curve can really upset the fuel curve and flat spots are not an uncommon side effect.

The stock Yamaha pipe and exhaust can are really good on these bikes.

Just something to consider.

 

[T_Bois]

Joe's got a good point.

Look at it this way, if the bike was bought used, it may be that someone has removed all the packing or not installed the new packing correctly. Who knows?

And I know what you mean about mucking with the carb when the bike already is easy to start and actually runs pretty good. All the more reason to look at the exhaust first.