Newbee on YZF400 - won't idle off choke

First post.. please go easy!

Got (what I think is) a great deal on a YZF400 for me, and a TTR125 for my (14yo) daughter. Mine won't idle without choke even when warmed up but rides fine. Previous owner didn't ride a lot, so based on search here it looks like the carb probably needs a good cleaning.

What priority should I give this? It's easy enough to restart. Probably going out to trails with some friends for the first time next weekend, do I need to get this resolved first?


I assume you've tried to adjust the idle speed?

Carb problem or bad gas

I would try to solve it. Its a pain to keep the bike alive all the time while riding, and it may drag to thru corners etc. waste of energy...

I assume that you tried to adjust the idle screw.. i didnt get mine to idle right till a got another carb on, see:

Maybe it will help to adjust the fuel screw but my guess is no, i tried cleaning mine, adjust fuel, locking the ACC pump, clean it again and so on nothing worked for me, other then the new carb :busted:

Yeah, got the idle adjuster all the way in, makes no difference (it does adjust idle speed with choke but bike still dies without choke)..

Also does pop/backfire on decel., or even just revving it in neutral.

I don't think I have a stock carb?




Edited by eflyguy

More research, looks like it might actually be stock for this bike:

Mine is 5BE1 01NC02

I did put some seafoam in when I bought the bike. Probably have close to an hour of running on it since, *BUT* on 87 gas. This bike needs 92, right? I'll replace that before the weekend for sure..



The carb is stock. Note the number 5BE1... on the bowl. More than likely a pilot jet problem, or possibly a damaged vacuum release plate on the slide.

So can I access the jet without pulling the whole carb off?


You can rotate the carb and replace the Pilot Jet by removing the bolt on the bottom of the float bowl.

I had similar problems on my ’98 WR400F. When I checked out how much it would cost to rebuild it, I decided to buy an ‘05+ carb. They bolt right up, the cables are much easier to mount and to adjust, and it runs a lot better. This is without even changing the jetting for the decreased CC. I would recommend taking the carb apart and cleaning it as best as possible and then replacing the jets. If that doesn’t solve the problem, avoid spending more money on an old worn out carb and get a ‘05+ on Ebay or Craigslist. I found mine for around $200 with cables. They can go for $290+ without the cables so just keep looking to find a good price. Good luck. The bikes are more than worth the work when they run correctly.

Given the low use this bike saw, and that he never used any additives, I'm guessing just the jet.

So I should be able to remove the jet from below to clean it. Just worry about the pilot?

I've got a service manual (Clymer) but it's focused on removing the whole assembly for a thorough cleaning. I'm pretty handy (do my own maintenance on two street bikes including valve clearance check) - how long would it take to just pull the carb? Looks like no more than an hour?

On a bike like this that's had little maintenance, is that really the best way to go?


I don't like the Clymer manuals. But then actually, the OEM manuals for the 400's use photos instead of drawings, and they don't come across well in PDF's, so I recommend downloading the manual for a 2000 YZ426.

It' important to use as wide a screwdriver as will fit the slot of the jet, which usually means grinding the diamond shape off a standard screw driver, or using a gunsmith's/precision screwdriver. As for cleaning, the problem is usually varnish deposits, and simply spraying and blowing won't remove them. If you have something you can soak the jet in the would dissolve such things, that may work, but read this post:

Well I pulled the carb. Cool how the rear frame pivots up (I did that just to make it easier to get it out..) Cylmer manual was spot-on in detail and steps so I can't complain there.

Pilot was visibly obstructed, could barely see thru it. I put two kinked strands from a brass brush (way smaller diameter than the hole) thru and just worked them back and forth, and I can already see a difference. I don't have carb cleaner, I'll pick some up later and clean everything. Guess I should soak the jets?

The bowl and most parts in it are coated with a haze, sort of white and greasy. I'm not sure if this is the 'varnish' that accumulates but something is in there. A few bits of debris in the bowl, but not bad.

Is it common to completely drain the carb (from the port on the bottom) for winter storage?


There are two ways to store a fuel system: wet or dry. Wet storage is good as long as the fuel remains fairly fresh in all parts of the system, but not good if the fuel goes stale.

Dry storage bypasses that problem, but potentially creates a new one; seals drying and shrinking. Over time, in any fuel system that spends a good amount of time wet, the fuel penetrates the seals. In many cases, certain chemical components of the seal are actually broken down, dissolved, and replaced by components of the fuel. This is OK, unless the fuel is removed for an extended period that is long enough for it to evaporate back out of the seals, which can cause them to first shrink, then crack.

I don't store my bikes long in the climate I live in, but I keep them wet. Something like my weed whip, which I have out for 2-3 hours every couple of weeks, never has fuel in it long enough to be absorbed into the seals in the first place, so I dump the fuel back in the can and run it dry.

Thx. Probably wet and make sure they run every now and then. Was told my daughter's TTR should be started up at least every week.

Update on the carb - cleaned and ready to go back on when it's light. The pilot was bad. Could hardly see thru it, even when I'd run the brass bristles through. After blasting with carb cleaner it have a very clean round hole. I've got to believe that did the trip!

92 gas, correct? I poured what was in it into my lawn tractor and will get fresh in the AM.


Ah crap.

Put it all back together and it won't start. It fires but then quits. I think it's flooding. I am using the 'proper starting technique' as per the video linked in the sticky - first time trying that and it certain does make it a lot easier!

Fuel seems to be coming out of an overflow. I did remove the float to blow cleaner thru the port, but did not touch the little flap that the float valve rests on - I know how sensitive those parts are!

That alone is probably the issue, right?


There are 4 common reasons for flooding:

  • The float needle is worn, dried out, damaged, or contaminated with dirt
  • The float itself sticks or binds
  • The O-ring between the needle seat and carb body leaks
  • Float level set too high (possible incorrect assembly)

With the bowl off, hold the float up against the needle gently and turn on the fuel. It should shut off the flow. If not, try to see where the fuel is coming from (past the needle of past the O-ring)

Thx. Will try that ASAP..


dont twist the throttle when you try and start it =) my old carb leaked too, i polished the float needle seat with a cotton swap on a drilling machine and it stopped.

The starting trouble i bet come from your pilot screw, either the washer or o-ring is missing.. or both. This is how it should and have to be:


I didn't touch the pilot screw when I disassembled the carb. I only cleaned the three jets, main needle and float valve. The bike started fine with choke (before I messed with it). It just wouldn't idle when warm with choke pushed back in.

The float needle seat doesn't look to be in great shape. Trying to reach a friend at a dealer now to see if they have the whole float needle valve set..


Edited by eflyguy

Ugh. Not in stock. Suggestions on how to smooth the needle seat in case I can't get the replacement by the weekend?


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