Yet another problem with my 426. Carbs this time.

First it was the kinked breather hose blowing out my gaskets and then the decompression assembly went bad and now this! :)

So I had a bad surge and decelerate problem at just under full throttle in all gears. My Dad and I tried to fix it by changing the needle clip.

After doing this the bike sputters or misses real bad at about 1/3 throttle. Only by cracking the throttle real hard can you get past it. It seems to get better the higher gear you are in. First gear is very bad. By four it's almost gone. And fifth runs like normal (except for the surging). We put the needle back where it was and that changed nothing. Took the whole carb apart, cleaned it, and put it back on. Still have both the low range and high range problems. Any ideas to get me running? Please! :)

What is your elevation?

Sounds like you just need to get the jetting sorted, something that can make a huge difference on this bike.

Please reply w/ your elevation and any other jetting changes you have made, besides clip position, if any, vs. stock.

Hick's always got good advice so listen to what he says. Also if you're just under full throttle, the needle isn't going to have as much an affect as the main jet. As mentioned above, have you tried going back to stock settings as a starting point?

Here are my specs: Elevation-3800 feet. Main Jet-160. Pilot jet-48. Needle on #2 clip. All the other jets are stock. This is the way it was when I bought it. I have tried a 158 main, different clip positions, and turning the fuel screw all to no avail. The bike has a Dr. D exhaust.

The lower RPM problem did not exist at first. It appeared after we moved the needle clip. But putting the clip back to the original position didn't fix it.


Here are my specs: Elevation-3800 feet. Main Jet-160. Pilot jet-48. Needle on #2 clip. All the other jets are stock.

Good lord, that is a huge pilot jet. Stock is 42.

Try a 40 pilot (for warmer temps, for cooler the stock 42 may be better)

keep the 160 main.

Keep the clip on #2 for now, then try #3 after you change the pilot (stock is 4, but that should be too rich for you, but go ahead and try this also if you still have problems).

Also, verify that your accelerator pump is working properly (you never know, this can cause weird problems that are hard to pinpoint).

At some point in the future I would recommend you try a different needle for your elevation, it isn't a huge difference but will help roll-on from low rpms.

Here is the Yamaha PN:


Try it on the 3rd clip first, but it may work better on 2nd in warmer weather.

Hope this helps.

Those sound like good ideas. We thought the pilot was rather large, too. I'm going to order a 40 and a 42 off the thumpertalk store. We don't have a dealer nearby.

As for the accelerator pump... when I had the carb off I tried it a bunch to see how it worked. It is shooting about a 2 second burst of fuel that just hits the bottom of the carburetor.

I'll tell you how this works out.

Thank you very much for the help.

I thought I would give you all an update.

I got a #40 pilot jet and stuck it in. This seemed to change nothing. I tried turning the fuel screw anywhere from almost seated to 2 1/2 turns out. I also tried clip grooves 2-4. None of this changed the way it ran. It still has a bad miss somewhere around 1/4-1/3 throttle. And if you accelerate past that it still surges real bad. Almost like the kill switch is shorting out, but runs fine at idle and WOT.

Is this an electrical problem? Any ideas where to look first?

Well it sounds like you have ruled out jetting as a contributing factor.

I'm stumped.

Are you sure the problem is associated w/ just throttle position regardless of rpm?

Still stumped.

I dunno, check your spark plug cable connection to the boot.

Try unplugging your tps and gear indicator. You may try a search on tps and/or gear indicator.

If you could find a donor bike to swap CDIs with that would be nice.

I don't know wheather it is throttle position or RPMs. I have tried unhooking the TPS, no luck there. I think it's time to find a "good friend" with a bike for testing parts.

Thanks again.

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