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WR 426 I Cleaned Carb now Pilot circuit HOT!

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This bike ran fairly well with only minor backfiring on closed throttle deceleration. Started easily when cold with choke. Hot starting usually did not require hotstart as ambient air temperatures where usually fairly cool and I probably never rode it hard enough to get it that hot. Usually only 1 out of 3 hot starts required using the hotstart valve.

Then the trouble started.

The previous winter I removed the carb and disassembled it. All parts where cleaned in Simple Green cleaner. I blew out all passages with compressed air. The carb was reassembled and reinstalled.

After reinstalling the carb it was almost impossible to get the bike started. I took the carb to a local ATV/Sled shop and had the mechanic test it. The mechanic said that the needle valve and needle valve seat

where worn and needed to be replaced. I ordered the said parts from Yamaha and installed them myself. I adjusted the float height to 9mm as per Yamaha settings.

The bike is still very hard to start.

THe attached pictures show the exhaust header pipes and mid pipes used on the bike. The dark, carbon coated pipes came off the bike before my carb cleaning escapade. The white heat residue coated pipes came off the bike after the carb was "cleaned".

The header pipe was so hot that heat from the pipe melted the right side fuel line on my Acerbis tank. Note in picture how close right side petcock is to exhaust header. Heat from the pipe was not an issue and did not melt the fuel line before I "cleaned the carb".

Obviously something changed with the carb.

My limited knowledge points to the pilot circuit being too lean. I have a Sudco adjustable slow air jet that is not installed. I am thinking I will install the adjustable air jet and try to tune out this issue with less air on the pilot circuit.





Model :wr426

Cam Timing (WR/YZ):yz

Main jet: 170

Pilot Jet: 48 (stock is 42(?))

PAJ:75 stock

MAJ:170 (stock is 200)

Leak Jet: NA

Starter Jet: 65 stock

Fuel screw (turns out):1 ¾ turns out

Needle Model/Clip position: OBDQ R 4th clip from top (stock WR needle I think...)

O-Ring Mod - this greatly reduced off idle bog

Grey Wire Mod (Y/N):n

TPS Connected (Y/N):y

Airbox Snorkel/Lid (on or off) off

AIS Removed (Y/N):y

Airfilter Brand: twin air

Exhaust Brand: White Bros. E2 mid pipe and muffler

Average Altitude: 1000m above sea level

Average Temperature:10 - 20c

Average Humidity: 50-70%

Degree of Satisfaction (0-10): 4

Motor mechanically sound. Valves set within spec.


fuel screw set at 1 3/4 - hard to start, lots of popping / backfiring

turn fuel screw out to 2 1/2 and fast idle increases still popping / backfiring

Has burnt fuel line on exhaust side - exhaust too hot.

replaced Pilot Jet (#45) with a #48. still hard to start and have backfiring / popping but less than with #45. Fuel screw set at 2 1/4 turns out. (This means I need a bigger pilot jet. Should order next 3 or 4 sizes up from #48).



Left Header Pipe showing lack of carbon deposits after carb 'cleaning'. Right header pipe showing 'normal' carbon build up before carb work.


Exhaust port after carb cleaning showing lack of carbon deposits and white ' high heat' deposits


Note proximity of exhaust header to fuel petcock and fuel line. The fuel line melted from header heat after carb 'cleaning'. Header heat was never an issue before I cleaned the carb....

Edited by Gurbax
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I think the slide plate is in right-side up. I'll check though....

I followed the site's instruction for posting pictures - even checked html code of others who sucessfully posted pictures. Same code. Now I'm stumped on two things...:thumbsup:

Thanks for your reply!

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I had a similar problem once with my yz 450 the carb slide was wrong way around. Took ages to find.

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Well, now I'm thinking I could have the slide backwards in the carb....

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Any solution on this? I just brought a 2001 WR426, went through the carb and same thing happened. Hard to start, pipe glows red hot within a minute, rough idle. Never did take the slide out. I use to be pretty good with carbs but this one has got me. Let me know if you got it running well. Thanks, HP

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Glowing header is normal whe the bike sits there. It must be rdden to keep it cool.

Hard starting is usually rider technique though it can be a improperly setup pilot circuit as well as numerous other issues (like valves too tight).

You have to think after 7 months, the OP resolved his problem too......

Fuel Screw/Pilot Jet

Fuel screw settings in the 'book' are recommended starting points. Every bike is different, as is the temp and altitude. Set the screw according to this method. Do it with the bike fully heated up.

Gently turn the screw all the way in. Now back it out two turns. Start the bike and fully warm it up, go for a 10 minute ride. Set the idle to speed to 1,500~1,800 RPM as best you can (I know, without a tach this is tough, just set it to were it idles relatively smoothly). Once warmed, slow the idle to the lowest possible speed.

*** When turning the fuel screw, keep an accurate 'count' of the amount you are turning it and record it in case you have to reset it for some reason. Makes life easier when you can just set it from notes Vs. going through the procedure again.***

Turn the screw in until the idle becomes rough or the bike stalls.

if it stalled, open the screw about 1/4 more turn. Restart it and slowly screw it in till you can just perceive a change.

If the screw can be turned all the way in and the bike still idles perfectly and does not stall, then you need to go down a size in pilot jet.

Now very slowly, open the fuel screw till the idle is smooth. Blip the throttle, let the bike return to an idle, wait say ten seconds. Confirm it is the same smooth idle.

If the screw has to be opened more than 3 turns to get a smooth idle, you need to go up a size in pilot jet.

If you find it does not stall with the larger jet but has to be open more than three turns with the smaller pilot jet, put the larger one in and set the fuel screw at 1/2 turn.

If the idle speed increased, adjust the idle speed knob to return the bike to a real slow idle speed. You must then re-visit the fuel screw. Keep doing this till the fuel screw is opened just enough to provide a nice steady idle at the lowest possible RPM. Once this is done, increase the idle speed to the normal one for your bike, typically about 1,800 rpm, but go by the spec in your manual.

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