Picked up a 99 YZ400F - Questions (yes, I've been searching)

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(obligatory bike picture included)

 

 

Yup, I've been searching, I found a lot of good links and info on here already.

 

A few days ago I picked up a 99 YZ400F, decent bike overall, runs strong.

I've been doing some maintenance on it, chain and sprockets, brake pads, new air filter.

The previous owner already had the front forks/seals done, oil change and a new spark plug.

The engine itself sounds really nice, no valve chatter or other odd noises.

 

It pops a lot and has a hard time idling... I pulled off the hot start connector tube to the intake manifold and had a friend plug it with his finger.

Seemed to help a ton with the idling problem, so I bought some vacuum port block off plugs and plugged it up for now.

Upon looking at the hot start hose that goes to the intake manifold it had 2 small rips in it which was most likely causing a it to run a bit lean.

 

1. Previous owner said he cleaned the carb but from what I've read on here its very common for the pilot jet to get plugged up and cause popping and idling problems. I was planning on maybe bumping up the pilot jet to a bigger size... What do you guys recommend? (I'm 500-1000 ft above sea level)

Also should I just leave the main jet stock or should that be bumped up a tad too?

 

2. Looking for input on a long lasting rear tire that hooks up. I plan on doing mostly trail riding, with a little bit of riding on the dirt roads to get to the trails.

A friend recommended a Michelin Starcross, and I've read a few things about the Dunlop MX52... not looking to spend a fortune on a tire but I know it's usually you get what you pay for.

 

3. I'm planning on adding lights to the bike. So far I have gathered I should be looking to purchase a Stator and Flywheel from a WR426.

That seems like the best bang for the buck right? Again I'm looking to do this on somewhat of a budget. 

What else do I need besides for the Stator and Flywheel from a WR426? I have seen multiple threads on this subject but I have yet to find a list of everything that is needed to do this conversion. Any help or links to a list would be greatly appreciated. 

 

4. Another thing is the bike is too loud, I don't want to get a ticket for loud exhaust so I think I'd just like to go back to the Stock yz400f silencer. Right now it has some kind of FMF silencer on it. The badge on it is worn away so I can't really tell what model it is, I'm guessing a Powercore but it has a USFS approved screen type spark arrestor end cap on it.

 

From what I have read so far is that a silencer from a 1998 yz400f will work, but I'm also reading that the silencer from the 2000-2003 yz426f will work also? It just bolts right up then? Just looking for confirmation on this before I buy one. If you have one for sale, let me know, thanks.

Nice bike and a nicely worded,coherent post!

I have a 2002 YZ426 and it would pop a lot on deceleration and had a bad hanging idle. I replaced the jets with new ones and problem solved. I would recommend trying that. They're cheap so you could get a couple different sizes and play around with it...I would start with stock jetting and go from there.

My YZ426 stock exhaust is loud as hell. Not sure if a stock YZ400 pipe would be much quieter. The full exhaust from a 426 will bolt on to a 400 but I'm not sure if you can simply swap just the rear slip on portion. Also a 426 header I believe is higher up and out of the way so you don't have to remove it to change the oil filter...bonus

I'm curious to see what responses you get for the WR flywheel and stator swap as I'm in the market for that as well but a little unclear...

Also the 426 was made from 2000-2002. 2003 and up is 450 cc. Most stuff from a 400 and 426 is interchangeable from what I can tell.

Enjoy the bike! Its the OG of big bore 4 stroke MXers.

Thanks Baldwin.

 

I bought a new rear tire and had it mounted today, so question #2 is not really a big deal anymore.

Went with the Michelin Starcross 110/90-19

 

Was going to edit my original post but it looks like after a certain time period you can no longer edit your post(s).

 

So if I do end up getting a 426 silencer, I'll make sure to get the header too.

I might start looking for that setup to make oil changes easier, thanks for the info.

That's a awesome tire I would recommend when your front wears out put one on the front they hook up great

Great looking '99, I also picked one recently and have been dealing with the same issues as you here is what I have found:

Carb, get one from a '06 - '09YZ450f, it is a much better design. I installed an '07 on mine and it was a remarkable improvement in starting, idling, and running.

 

I have read that you can mount any exhaust '98 - '05 on the '99.

 

Lighting

 

I know that I will need to modify the stator or wiring harness to float the 12v ground and make the WR stator work with a YZ CDI, but I have not gotten there yet. I'll put together a post on the subject when I get further along.

Import Silvia, thank you for that post,

Lots of good info and links in there... you the man!

Ok so I took the bike apart and removed the carb.

Also pulled the spark plug.

 

178 Main jet

45 Pilot jet

65 Starter jet

 

I didn't look at what clip the needle was set at, forgot

The carb looks pretty darn clean, I will blow compressed air thru the passages before reassembling.

 

The spark plug was barely hand tight, the crush washer was not crushed on the spark plug.

The plug tip was white... not sure if it was from jetting or from the plug being barely hand tight and it was sucking in air thru the spark plug threads?

 

What should I do with the jetting?

Photo of the plug

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The plug tip was white... not sure if it was from jetting or from the plug being barely hand tight and it was sucking in air thru the spark plug threads?

 

What should I do with the jetting?

 

The white is from the plug being almost new.

 

Replace the main jet with a 170.

By far the most popular tires for Michigan trails are the Michelin S-12s.  They can chunk a bit if you ride the rockier trails, but are great in the soft stuff.  They have served me (and just about everyone I ride with) well for over 10 years. (Just re-read that you got the Starcross.)

 

The stock WR426 silencer will only be legal in Michigan if you have the stock pea-shooter insert, or a tip like the White Brothers Vortip, which is not manufactured anymore.  The GYTR tip will only get you to 96 dB which is not technically legal for trails.  If you ride with the CCC, the GYTR tip in a stock can will not pass sound check.  

 

Get an FMF Q pipe, it will, typically pass the sound check at CCC events and be USFS approved for spark arrestor.  I sometimes go a bit louder than 94 dB if the packing is getting worn, or if I've packed it poorly.  The FMF Q can get you below 94 dB and at the same time be both lighter and better performance than the WR426 exhaust even with the GYTR tip.

Edited by Pooley

Ok... What is the stock main air jet and stock air pilot jet sizes?

I can't this info in my manual.

Right now my main air jet is 200

And the pilot air jet is 100

Edited by blee03

Ahhh... found it.

 

jetting99yz400f_zpstirqxkuv.jpg

Ok... What is the stock main air jet and stock air pilot jet sizes?

I can't this info in my manual.

Right now my main air jet is 200

And the pilot air jet is 100

 

Don't change them.

Don't change them.

 

Should I replace them though?

I mean they look fine but everyone was all adamant about buying a new fuel pilot jet, I figured maybe these ones need to be replaced as well?

Edited by blee03

hey everyone, i just signed up. i'm fairly experienced since 12 with bikes (off road), and Yamaha has always had weak points, like starters, and chokes. I have a yz400f, and have been reading your posts, and have the same issues. Did all, everything, and still popps and dies with choke in. THEN it dawned on me, None of the posts ever mentioned getting a new choke plunger...lol  if you pull out the plunger and look at the rubber thingy inside the plunger, it gets "squashed" and cant line up with the hole inside, and will let small amounts of ENRICHENING  GAS in the mix, EVEN WHEN  PUSHES IN, And if it is bent, like most get, it will not completely cover the 1/8 inch hole in the choke port.  I KNOW THIS, because, i owned a 920 virago, and the chokes were notorious for allowing gas to pass through, and fowling up the plugs quickly, and even hard starting to popping and dying. With that being the case, you could try and tune your bike's jets for 10 years, and will never get it tuned, until you replace that bugger.

hey everyone, i just signed up. i'm fairly experienced since 12 with bikes (off road), and Yamaha has always had weak points, like starters, and chokes. I have a yz400f, and have been reading your posts, and have the same issues. Did all, everything, and still popps and dies with choke in. THEN it dawned on me, None of the posts ever mentioned getting a new choke plunger...lol  if you pull out the plunger and look at the rubber thingy inside the plunger, it gets "squashed" and cant line up with the hole inside, and will let small amounts of ENRICHENING  GAS in the mix, EVEN WHEN  PUSHES IN, And if it is bent, like most get, it will not completely cover the 1/8 inch hole in the choke port.  I KNOW THIS, because, i owned a 920 virago, and the chokes were notorious for allowing gas to pass through, and fowling up the plugs quickly, and even hard starting to popping and dying. With that being the case, you could try and tune your bike's jets for 10 years, and will never get it tuned, until you replace that bugger.

Thanks man for this post,

My bike runs good but sometimes it doesn't like to idle, I can adjust the idle 10 times and it still gives me issues

I will definitely be replacing the choke plunger asap.

I doubt that will help.  The fact that it runs with the choke open and then acts up when you push it off tells me that it is both opening and closing as is.  If it wasn't,  there would not be the change in fuel delivery that is causing it to run poorly.

 

One unfortunate fact about the original Flat FCR carbs is that they can wear and internally gunk themselves up to the point where they simply can't be fixed in any practical way.  If you can find a carb body gasket (goes between the upper and lower halves of the main carb body that are separated by removing the 4 Torx screws; JD Jetting has them), then you might solve your problem by taking the body apart and scooping the goop out.  Just don't disassemble it if you don't actually have the gasket.

 

A badly worn or cracked/chipped vacuum release plate on the slide is another potential trouble area.  Also, try disconnecting the TPS.

How can you tell whether you have an early FCR or the newer ones that work better? I'm curious as to what each of my bikes has, the YZ is set up very well.

The original carb had no built in hot start, and was used on all YZ400F's.  The hot start on the 426 models is bored into the carb body next to the choke on the left side. Both of these are early FCR carbs.  On all of the later FCR-MX carbs ('03 onward), the hot start is cable operated and runs in a vertical bore in front of the slide.

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