YZ426 Carb Woes...

3 replies to this topic
  • corndog

Posted July 20, 2012 - 11:52 PM


I bought a low hour YZ426 from a friend, the bike has been sitting for 1-2 years. My friend was going to get the bike running before I bought it and he split the carb in half, like the way you're not supposed to. Since Yamaha doesn't make a gasket for that seal, I figured I'd try some sealant. Well, it started and ran, but wasn't quite right. Rather than chase my tail, I figured i pick up a used carb and save myself the hassle. I found a good price on a clean '04 450 card and read this thread: http://www.thumperta...on-on-my-yz426/ - so I figured I was good to go.

The carb shows up and I should have cleaned it, but I slapped it on and the bike wouldn't start. I figured the pilot was gummed, so I cleaned the pilot and she fired up. At this point, I had the '01 plunger screwed into the top of the carb, instead of the '04 plunger cable. The bike ran okay, not quite like it should, plus it had a little bit of a hanging idle. It was also popping on decel, but the bike sounds like there is no packing left. So, I figured I'd take it all apart and clean it thoroughly, plus install the proper plunger cable even though the clutch perch+hot start lever has not shown up yet. Then the thing won't start at all and fuel was dumping out of the bowl. I figured the float was stuck so I pulled the drain plug and jiggled the float with a tie-wrap, no more leaky. Still won't start, but no more fuel dumping out the carb.

The bike also has a weird issue after I tried to bump start it. I pushed it for a few seconds with the decompression lever pulled into try and clear it out in case it were flooded. I figured it wasn't a big deal since the manual states you can kick it with the lever in to clear a flooded motor. It never fired or ran with the decomp lever pulled in. Once I was thoroughly exhausted after pushing that bish up and down the street, I went back to kicking on the stand. Well, you could kick and kick and it would never hit that hard compression stroke. I figured I must have wrecked the exhaust valve by pushing it with the decomp lever in, but after kicking it ten time, it's now kicking as normal, rock solid on the compression stroke. So, maybe the cable was just sticky..??

Also note that I have never touched the TPS on either carb. Unplugging the sensor doesn't seem to have an affect on starting.

Sooooo, I just went back and pulled the bowl thinking maybe a piece of debris is in the pilot again, but nope, all jets clear. Plus, I put the old '01 plunger back in because I felt like I was having better luck as in the beginning on the second paragraph above. I can get the bike pop and sometimes run for a few seconds, but it dies on its own, or it dies the moment I touch the throttle. Here's the strange part, this is very repeatable - if I pull the choke and make it pop, I can then get it to run a few second with the hot start in. If it won't pop or fire with the hot start on, I just pull the choke until it pops, then go back to the hot start to make it run a few seconds. The fact that the hot start needs to be on would make you think it's super rich, but why does it want the chock on briefly? I was thinking that maybe there was some weird vacuum thing going on, the gas cap valve is working correctly. There seems to be good flow from the fuel line, so I don't think the filter in the tank is plugged.

I adjusted the fuel screw a bit, it won't do anything with it all the way in. The above is with it at 1.5 out, I haven't tried backing it out more than that yet. The pilot on both carbs is a 42, not sure of the needle position, I'll check that tomorrow morning. The bike has spark and good compression, again it's a really low hour machine, I'd guess less than 50hrs. I haven't checked the valve clearance, but given the fact that the bike was running pretty decent (ripping wheelies through the neighborhood like a 16yo from Baltimore) and considering all the carb oddities, I really feel it's carb related. I'm spent for today, any tips for tomorrow would be much appreciated!

  • corndog

Posted July 21, 2012 - 11:16 AM


Update! Cleaned the carb again, no change. Needle in the middle position. I discovered that the bike will start with the fuel screw backed waaaaaaaaaaaaaaay out. So, if the fuel screw affects the amount of fuel at idle and I have it backed all the way out, I suppose that means it's letting in the max amount of fuel and I should go up a size (or three) on the pilot? I read in the YZ426 jetting thread there was one dude with a 450 carb who was running a 50 pilot, though his bike wasn't running great. I'll see if I have a 50 and try that...

Edit: Been working my way up on the pilot. Started at a 48, then 52, now a 58, which is the biggest I have. The bike will start and run okay with the fuel screw three turns out. I'm stumped at what's going on.

Edit x 2: Just for giggles, I figured I'd drill out the 42 pilot to something stupid big. Smallest drill bit I had was .0635 (#52), which I think is around the equivalent of a 136 pilot, referencing this info: http://www.thumperta...95#entry6879695 Well what do you know, seems to run pretty good. Snappy, idles down quickly, easy to start, etc, all with the fuel screw 1.5 turns out. Clearly something else is wrong and this is just compensation for whatever that is...

Edit x 3: Rut roh, just read this: http://www.thumperta...r-stock-wr450f/ - back outside! lol,,,

Edit x 4: Yeaaaahhh, so...how 'bout them Giants? Yep, I put the slide plate on upside down. Rats, so much self inflicted agony! Put the slide plate back in correctly and put in a 45 pilot, she seems pretty happy at about 2 turns out. Need to either repack the muffler, or buy Bud Light graphics and leave it noisy and do a bunch of one handers or something. :)

Edited by corndog, July 21, 2012 - 02:39 PM.

Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • tl1000rlt

Posted July 21, 2012 - 06:53 PM


Even if the jets are quite aways off it should still run. Does it smell? What does the spark plug look like?

  • corndog

Posted July 21, 2012 - 07:24 PM


She runs like a champ now. Ran it up and down the street like a punk and it is smooth with excellent power, even with a 42t rear (sumo). I let her sit a while and washed it, started second kick with the choke. I'm calling it dialed until I get a chance to run it. The plug looked good, maybe a little lean. I'm going to get a new one in there before I take it out, then take a reading after the ride. I didn't really notice anything strange about the odor.

Related Content

WR Camshaft Swap Info - last post by jamesm113

WR Camshaft Swap Info

  • 0 replies

Is it worth it? (Cam and High compression piston) by macgi77

Dirt Bike   Make / Model Specific   Honda   CRF 150/230 F/L
  • Hot  36 replies

2016 YZ450 by CaptainKnobby

Dirt Bike   Dirt Bike Technical Forums   Suspension
  • Hot  59 replies

Chubby dad, looking at bikes , First trip to the orange/black forum ! by Slow_ride

Dirt Bike   Make / Model Specific   KTM   250-530 EXC/MXC/SXC/XC-W/XCR-W (4-Strokes)
  • Hot  28 replies

Help with 2000 yz426 by weekend woods racer

Dirt Bike   Make / Model Specific   Yamaha   YZ 400/426/450
  • Hot  36 replies

Join Our Community!

Even if you don't want to post, registered members get access to tools that make finding & following the good stuff easier.

If you enjoyed reading about "" here in the ThumperTalk archive, you'll LOVE our community. Come join ThumperTalk today!

The views and opinions expressed on this page are strictly those of the author, and have not been reviewed or approved by ThumperTalk.