Help with YZ426!

[Rot Box]

I think there are enough Yamaha/Keihin FCR guys here now that maybe I could pick your brain on a little problem I'm having

Long story short: A friend bought a YZ426 recently and he said it rarely starts and runs like crap when it does start I offered to help tune it in for him and man I've been busy fixing little things that just weren't right. After installing new wheel bearings (it helps if they have balls in them ), cables, levers--and fixing a lot of other neglect now I'm sorta scratching my head with the carb.... It has the power now intake mod, boyesen quick shot, and an aftermarket fuel screw to make tuning easier. A slip on spark arrested muffler and NO air box lid seems to be the only other modifications.

After starting it I found the ilde to be erratic and what seemed to be too much fuel on bottom. Not responding well to the fuel screw I pulled the carb down and found a 48 pilot which I swapped out for a 42 (stock jet btw). The bike starts decent now but it has a nasty bog when the throttle is wicked. A slight bog seems acceptable to me but this is flat out fall on your face bog while riding with a warm motor. Here's what I'm thinking please let me know if I'm on the right track here

1. Too much fuel or too quick of a shot from the accelerator pump? Is the timing on the pump adjustable? If so how do I go about doing this?
2. Pilot jet? Seems to run and idle fine on bottom as long as I have the idle set fairly high.
3. Needle too high? Mid to top seems great though....


I'm convinced there isn't a 2T out there that I don't feel comfortable tuning, but since I've been living under a rock for half my life this new 4T stuff is a little out of my league. Anyway we're sitting right on 4500ft. here in Cache Valley. Thanks for any tips or advice!

Andrew

 

[rholbrook]

First of all, start with the valves. Make sure those are within tolerances. If you have done that, then start with the carb. Next make sure that your not sucking any air. It may be the Leak Jet os too big or its squirting fuel too soon. There is an o-ring mod that you can do as well.
http://www.thumpertalk.com/forum/showthread.php?t=604077
What other jets do you have? Where is the clip and what needle is it running. You still need to give us some information.

 

[Rot Box]

Great thanks! I'll try the O-ring mod. I'm not seeing any slop in the linkage though or is there more to this than I'm seeing Needle clip is one down from the top on a OBEJQ.

Here's what I'm seeing with jetting--bigger pilot makes matters worse :check:. 42 pilot and ~1.5 turns of the fuel screw seem to work in harmony. Easy starts and smooth idle.

Next up this bike does not have the BK mod, but there is a nearly instant squirt of fuel from the accelerator pump once the throttle is snapped. From what I'm reading the quicker the accelerator delivers fuel the better for eliminating the bog. My heart laughing tells me too much fuel too soon is bogging the motor... Ughh....

Anyway this bike just had the valves adjusted by the local Yamaha shop. That said maybe I'd better re-check them


EDIT: Excuse my ignorance, but whats a leak jet?! LOL

 

[YOUKNOW]

I agree with rholbrook, check the valves. They get over looked so easily on a 4-stroke. Also might want to check to see if there is any cracks on the header pipe, I had a small crack near the cylinder and it ran horrible. Hope that helps.

 

[rholbrook]

There is a starting technique for the 426. A buddy of mine has one and if he follows it, it starts right up.

1. if it hasn’t been started in a couple of days twist the throttle a couple of times
2. Grab the decompression lever
3. Push the kick-starter about an inch and a half down, let it come back up to the top
4. Kick it....hard and through the whole stroke. Make sure you have the gas valve open and the choke on if your cold starting.

One of the main issues new 4 stroke owners find is they twist the throttle when kicking. This is an absolute no-no (never). The slightest twist and you may add minutes into the time to get it started.

For this altitude 42 should be close. I would start the needle in the 3rd clip. FS start at 1.5 turns. What is the main jet? I know it doesnt make a lot of difference with starting and idle. All bikes will bog depending on how fast your ripping the throttle open. I would do the o-ring mod. What your doing is slowing that initial squirt.

 

[Caleb]

Here's a very helpful chart that shows what jetting affects the different points in the throttle.



I agree with Russ, adding the o-ring mod (even on my CRF) made a world of difference in off idle bog. The bigger the o-ring (thicker diameter I should say) the more snap it will have. When you buy a jd jetting kit, it comes with two o-rings, I put the smaller of the two on my crf. Almost, if you have to open your fuel screw more than 1.5 turns, it means your pilot jet is too small, if your fuel screw is in all the way or almost all the way, it means your pilot is too large. Ideal for the fuel screw should be around 1 1/8th turn out.

 

[Rot Box]

There is a starting technique for the 426.

I would start the needle in the 3rd clip.

What is the main jet?

There sure is! And if it isn't done right it could be kicked all day without starting! Although there are some problems with the bike I don't think my friend knew the technique so I'll have to teach him once the carb is sorted out. Once you roll over TDC starting isn't too difficult (compared to a 2T 500 anyway ) but the auto decompression is a really nice feature on the newer bikes that's for sure.

Third down from the top right?

And it has a 155 main which seems a bit lean compared to what most people are running, but everything past the initial bog acts great.


Anyway great help thanks you guy's! I have absorbed more information today than I ever thought possible from endless reading. Seems like the timing of the AP squirt and the o-ring (or stiffer diaphragm spring) are pretty much mandatory. I'm convinced that my pilot circuit is set pretty good and I'll raise the needle a notch, but I'm 90% sure I have an issue with the AP. Hopefully tomorrow I can watch the spray and see if it meets specs and try out the o-ring :greg:

I usually work on friends bikes for free because It ultimately makes me smarter and I don't give them back until I feel they are running top notch--and I'm very meticulous so I feel good about it in the long run. That said I've learned three things here. 1. I'm too nice and time is money. 2. There's a reason I still ride a two stroke. 3. There's a reason EFI took off like wild fire! LOL Thanks again you guy's!

Andrew

 

[Rot Box]

Success!!! I have a really bad habit of talking myself into things... In the long run it was the pilot jet and needle clip placement that made it run so terrible. Now there is a slight bog once in a while if I really hammer down but nothing I'd worry about even if it were my own bike.

 

[rholbrook]

Nice, now lets go riding. Where did you end up on the pilot? 42? and what clip position?

 

[Rot Box]

Nice, now lets go riding. Where did you end up on the pilot? 42? and what clip position?

You say when! I'm chomping at the bit--I haven't even rode my new CR yet

Pilot is 42 and clip is in the 4th position down.