HELP!!!! '99 yz400f

got some major issues and Im very stumped!!!!!

need some help please...

The bike is hard to start, when it finally gets started, it wont idle. When I revv the bike, the rpm's stay up, then come down slowly. Carb has been cleaned, and float height is correct. its running a 165 main, and a 65 slow i believe. i sprayed carb cleaner around the intake boot, and found a leak. so i siliconed the inside lips to seal it up. the bike also has a white bros supertrapp style slip on with the stock header. The fuel valve is also leaking a little bit when its in the closed position. the needle and seat is new. there is no hot start on the carb either. it just has a decompression lever on the clutch side of the handlebar. the spec for the piolet screw is 1.75 turns out. i had it set to that, and still ran like crap. ive had it anywhere from 1.0 turns out to 1.75 turns out, and have not noticed a difference. any input would be greatly appreciated. thank yall.

Check the hot start valve and hose that runs from the air boot to the intake port. The valve must be closed when cold starting and during normal operation. Otherwise, it's basically an air leak, and will cause the predictable set of problems.

Are you talking about the red plunger that is on the left side of the carb? because if so, this bike is not equipped with a "hot start". it only has a decompression lever. Also, I have adjusted the valves. They are all in spec per yamaha manual. I went into the carb earlier today to make sure everything was indeed clean. The jets that are installed in the carb rite now are a 170 main and a 65 slow. yamaha spec is 175 and 45.

on the 99's the hot start was separate from the carb. it mounted to a tab on the frame and a hose went from the hot start valve to a port on the intake manifold in front of the carb. It should have had one factory. My 99 did.

the decomp lever has nothing to do with the hot start valve.

what part of texas ?

Are you talking about the red plunger that is on the left side of the carb?
Yes, and the red knob is the hot start. It's mounted as jhender3702 describes it. If it's not there, look for the nipple on the intake stub, that should be capped off. If the red knob is actually on the carb, then you have a later carb from a 426, and possibly a 426, and not a 400.

Your bike does NOT have a 65 pilot (slow) jet. They don't come that large. That is the starter jet that feeds the choke circuit.

i do have an air nipple on the cylinder head that is capped off. and there is another rubber nipple that is under the carb boot on the airbox side. that too is capped off. are they supposed to be joined together with a piece of hose? and i did not find any tabs on the frame.

I am 100% certain that the bike is indeed a YZ400F. I read off the VIN number to a yamaha dealer, and they told me the year and model. because it had no stickers on it to designate an engine size. Today, I have rechecked the float height, and had to adjust it again. the bike did not take as many kicks to start this time, but still ran like crap, and did not idle. any suggestions? as jhendr3702 said, about the hot start being seperate from the carb. I do indeed have a capped off air nipple infront of the carb on the head, and a blocked off section under the boot for the airbox that connects the carb to the airbox. do those two nipples need to be connected by a hose, or some other kind of assembly?

With regard to the original hot start assembly, if you don't have it, the nipples must be capped. Leave them like that. If the carb has a red pull knob on it, read the stamped numbers located on the left side above the float bowl joint and post them. They most likely star with "5JG" if the red knob is there.

There is no red plunger on the carb at all, but i did get the stamped numbers. they do not start with 5JG. The number is 5BE1 01ML01. I have a question for you. For the most part when a bike has a DOHC set up, when you go to do the vavle adjustment, the engine must be on TDCC, and the cams will face at 10 o'clock and 2 o'clock. I figured that would be the norm for any make or modle. But the point im getting at here is, when i have all the timing marks lined up on the gears, and they are running in a horizontal line with the gasket mating surface, if i look at the right side of the bike, the cams are not at 10 and 2. The exhaust cam looks like it has shifted. It appears to be sitting at 3 oclock. Do these bikes have a history of the cam gears shifting on the cams, and to correct it, instead of removing the gear, and repositioning it, can i just retard or advance the gear to give the cam the correct orientation?

for jhndr3702, im located in Wichita Falls

that's what i figured... when i bought my 400 it came from there.

pretty sure it was 175 main 42 pilot. Make sure the pilot jet and circuit are way clean .

I have read the cams sprocket can move. Mostly from cam siezure in the head . IIRC the manual shows a good picture of the cam position in it. If it has moved... I would pull that cam back out and look at the head/cam holder. I also would not reuse that cam.. I think mine with the stock cam, they were pretty much like you said at 10 and 2. post a picture of it(you have to use like photobucket then copy in to here) grayracer, or certain others, will be able to tell you in seconds if it has moved.

According to the carb numbers, that's still the original carb for the 400. That means your hot start is missing.

The YZ400/426 is not normally hard to start, unless you don't do everything exactly right.

The bike is also very fussy about the idle adjustment as far as starting goes. It makes a huge difference.

As far as the cam timing goes, start by verifying TDC. Probe for the piston crown and see if the "I" mark (there's an "I" for TDC, and an "H" to the left of it) on the flywheel is in fact in agreement with real TDC. The Woodruff key could be sheared.

Next, with the engine at TDC, the only two timing marks you should pay attention to are the "E" on the exhaust, and the "I" on the intake. These should align with the top gasket surface of the head. This will place the cams in a slightly flatter than 10/2 arrangement on this engine. The cams can move inside the sprockets if they seize to the head, but this will rotate them backward relative to the sprockets. That would cause the intake to move toward 9:00, but the exhaust would move toward 1:00, not toward 3:00.

Going back to the carb, there is a floating plate on the back of the slide. This plate should be oriented so that the square edge with the small hole ear it is down. Also check to see that both corners of it are still there.

There is a neutral switch below the shift lever. Follow the light blue lead from the switch up under the tank and make sure it's not shorted on anything. A short in this will not affect idle in neutral, but would cut power when in gear.

is there a measurement that i can use that will tell me if i have a streatched out cam chain? and as i stated earlier above, the intake cam did not fall in the 10 o'clock position, so i went ahead, and advanced the cam on the chain 1 tooth, and the bike started a little easier. still would not idle though, but im assuming that i can tune out by messin with the carb?

Typically, the symptom of a "stretched" timing chain would be that the cam timing marks don't line up. They look a half a tooth off regardless of where they are put. If the bike has never had a chain replaced, or you don't know when the last one was, just get another one.

The only timing marks that did not line up were the marks on the intake cam. The exhaust marks lined up perfectly, and as i posted earlier, I advanced the intake cam an incriment of 1 tooth. the cam still does not sit at the correct position, but its been easier to start than before, and I have noticed that when I get the bike running, the header pipe does not glow orange within the first few min its running, as it used to do. I replaced the piolet jet with a 65. Thats all we had. it had a 42 in it, and the book called for a 45. I thought with the 65, that I could just not go so many turns out on the idle mix screw, but I was wrong, so I ordered a 45 piolet jet to put in it. also, I ordered a set of "Hot Cams" for it, both intake and exhaust. The exhaust cam has the option to install the automatic decompress or not. any suggestions? And also, something funny, when I took the piolet jet to one of the bike shops, they told me that yamaha was experimenting with different things on that year model bike, they didnt have any. So, I went to a mom and pop shop, come to find out, the piolet jets that these bikes have will fit into a Harley Davidson carb lol. they are the same depth, thread pitch, diameter everything lol. So if anyone is lookin for piolt jets for these carbs, go to your local Harley dealer. Also, I think the spec book I have is wrong, because I keep getting fuel out of the overflow tube with the float set at 0.35in. The book does not give metric measurements, so if anyone has the float height spec for this bike, that would be greatly appreciated. thank yall.

Also, I think the spec book I have is wrong, because I keep getting fuel out of the overflow tube with the float set at 0.35in. The book does not give metric measurements, so if anyone has the float height spec for this bike, that would be greatly appreciated. thank yall.
The Yamaha manual gives the spec in both English and Metric units on page 2-8 (0.35", 9mm).

It's important to note that this is measured between the lowermost edge of the float and the bowl gasket flange as the float needle JUST TOUCHES the seat. Do not compress the spring that is built into the needle at all.

If it floods at this setting, the needle/seat needs replacement.

i have already replaced the needle and seat, due to them leaking. i turned the carb usside down on my carb stand, and hooked up my iv bottle to it, and there was fuel leaking from the needle and seat. I replaced them naturally, and redid the float height to spec. I have also put in a set of Hot Cams. This has fixed the hard to start condition. From a dead cold engine, it started up on the second kick. Now, Im fine tuning the carb. My buddy was tellin me that the bike should idle just like anyother bike, and I said it should not. This is a racing machine, it will only need to idle for a very brief amount of time. Who is correct? Now that I am fine tuning the carb, I have put in a 175 main, and it had a 170 in it with the stock cams. I have also replaced the 42 piolet with a 45. That has helped alot. I can get the bike to idle for a bit, but its a high idle, and kind of rough. It will go from steady to high, to a little rough. Ive got my meter with a tach attatchment hooked up to it. The book says that the spec idle is 2000 RPM. When I lower the idle to around 2100 to 2200 it acts like it wants to die. I have the idle mix screw turned out to abot 3.00 to 3.25 turns out. When i "goose" the throttle, the RPM will raise up a little, then come down, but they dont come down as fast as it needs to. Then sometimes, Ill "googse" the throttle, and it will hesitate a little, then shoot up. What approach do I need to take on this bike, because its starting to get me down. Everything I do to it, really does not help that much. Any help is greatly appreciated.

Your friend is right. You're correct in that in practice, you may not find yourself at idle very often, but it should have a good, stable idle.

Your pilot screw is too far out, yet you still have symptoms of lean mixture ("hanging" idle, or slow to return to idle). Possible causes:

  • Too little clearance at the intake valves
  • air leaks between the carb and engine
  • air leaks in the hot start valve or tubing
  • partly obstructed pilot jet or idle discharge port
  • Pilot screw O-ring missing or damaged

the hot start assembly has been removed entirley. The system this bike uses, has a knob on the frame, and tubing from the carb to the air box. the nipple on the carb has been blocked off. The intake boot that connects the carb to the head, is brand new. The carb I have cleaned many times, with carb cleaner, and compressed air, and the O-ring on the piolet screw is in good condition, but if that is a possability that can be contributing to the running isses, I have an O-ring kit, I will just replace it. The instructions that came with the cams, explained the break in procedure. They are broken in now, and I am about ready to re-check the valve clearances. What and where is the idle discharge port, and what does it do?

What and where is the idle discharge port, and what does it do?

The idle discharge port is the hole in the carb throat where the idle fuel emulsion enters the intake air stream. It's on the bottom of the carb bore, on the engine side of the slide.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now