Posts
17
Joined
5/23/2022
Location
Little Rock, AR, USA
Bought an 05 yz 250 that hadn't been taken care of. Bike ran fine enough. Pipe was barely hanging on. Splooge everywhere but it ran. Tore it down to the frame. Cleaned it up. Installed a pro circuit pipe and silencer. New cylinder. New top end. New OEM style reeds. Installed a JD jetting kit. Bike fired up and idled wonderfully 2nd kick after putting it all back together. But now I've got a problem and I can't figure it out. The bike had a lean bog to it at the first crack of the throttle until it finally got fuel and then it would scream all the way through the gears. I went richer on the needle (blue) and moved the clip down one position from 3 to 4. Got a little better. Main problem now is that the bike is a bit on the rich side at idle and 0-1/4 throttle but when you're wide open and on the pipe, if you let off the throttle and try to get back on it it's got a lean bog like it can't get fuel for a couple seconds until it finally kicks in. Makes for a very sketch ride on the track. So I've jetted the bike slightly rich but it's like the carb doesn't fill up fast enough after being full throttle. I adjusted the float height and it seemed like it was fixed when going down the road through the gears but I took it to the track this weekend and the issue came back. Haul ass, left off gas to turn or whatever, get back on it and there's no fuel for a couple seconds. I checked the fuel for debris. I've checked for trash in the jets. Disconnected the fuel line from carb and the tank is spitting fuel out just fine. Need some guidance here. I'm not an expert and I'm not sure what to check next.
Measuring the float height in the correct way, I know some of them can be tricky.
Take the petcock out and clean the filter in the tank.
Check/replace fuel cap for faulty vent (or if you’re running one of those dumb stubby vents)
Go over the wiring harness with a fine tooth comb to look for damaged or frayed wires.
Got a picture of the plug?
https://www.vitalmx.com/forums/Tech-Help-Race-Shop,42/YZ250-Lean-pilot-…
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There are a few common issues with the PWK that comes on the YZ250 that cannot be cleaned by a normal cleaning that can cause issues like you are having. They are also very difficult to troubleshoot and very expensive or impossible to fix. This is why I suggest trying a known good carb from another YZ250 to at least facilitate the troubleshooting process.
Issue 1: The jet block D-shaped o-ring. The oring between the jet block and the main body of the carb can and does go bad. If it goes bad it can suck air or extra gas and make the bike run like crap. This o-ring is not sold anywhere. The one that fits the aftermarket PWK's and is sold by places like Sudco and JD does not fit the OEM YZ carb. Also, the passages inside the jet block are very difficult to clean thoroughly without removing the tamper resistant torx screws and the jet block.
Issue 2: The small rubber tip on the power jet solenoid. This small rubber tip can easily get torn or rotted on older bikes and it will then clog the power jet passage or, if you remove the tip altogether, will not seal the passage when it is supposed to. This tip is a piece of rubber the size of a match head and is not sold. You have to buy the entire solenoid for over 100 bucks. Also, the power jet nozzle is very difficult to clean if it gets clogged.
Issue 3: The pressed in brass float valve seat. The seat is pressed in the carb body and is not sold separately. If it gets worn or corroded it can be difficult or impossible to repair. I think there was an obscure place selling replacements online at one point but they are hard to find.
Also there is the very small passage you see in the bottom of the throat near the front. I believe this is an idle circuit and works with the choke and air screw. It is hair thin and very hard to clean. Old bikes that have sat around get this clogged up pretty bad sometimes.
These are the reasons I say try a known good carb to at least make sure you are running down the right rabbit hole.
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Since, I posted that, I believe jet-block o-rings are now more available, mostly due to all the people swapping out the jet-block for the STIC block. Check with JD Jetting or STIC for those.
Honestly, the stock jetting is very good for these bikes and a good starting point for tuning. The JD kit is a waste as the stock Yamaha N3EJ needle is very good. I don't have any experience with the JD needles on the YZ250 so can't really offer any meaningful advice on which way to go with jetting.
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Put the stock jets back in just to make sure (50 pilot, 178 main, Stock needle 2nd clip). Same issue persists. Played with the air screw between 1 and 2 turns out and it doesn't seem to make a difference. Carb has been rebuilt recently. The needle (forget the name) that the floats sit on seem to be working properly and everything appears to be functioning. Can't think of what could be causing this. Wish I had another carb to try so I could rule it out but I don't unfortunately. Any ideas ?
If the needle jet (brass tube the needle is in) has wear it will let unmetered fuel into the engine. You can only replace it with a STIC or new carb. The tiniest wear will cause problems. I bit the bullet on my 94 and bought a new genuine carb.
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