99 yz400f hard to start

I've got a 99 yz400f that I bought a little over a year ago and its really hard to start. I just rebuilt the bottom end and put a new piston and rings disassembled and cleaned the carb new spark plug. The valve clearances were within specs and the cams and such looked to be in good shape so I didnt do anything with the head. The service manual suggested I put a clean solvent in the intake and exhaust ports and there shoud be no leakage at the valves I used gas and there was some slight seapage but not like it was dripping just could see some moisture coming through the edge of the valve. The bike runs great "once its started" and sometimes when its hot I get lucky and it will start up with a couple kicks but then sometimes it acts lifeless as I kick till my foot gets sore and swollen. Its always hard to start when its cold usually need a truck or big hill. As far as I know everythings stock or at least stock replacement stock exhaust. Now moving on to the carb...I dont know if the previous owners messed with the jetting at all or not and I dont know much about jetting but I can tell you that the bike runs clean and crisp from idle to full throttle free of bog and it doesnt smoke...and just to mention the spark plug has a sort of tan color and from what I understand this is correct color. The carb has a aftermarket fuel air mixture screw on the bottom of the carb that I had to back out 3 1/8 turns to get correct tuning and I heard if you have to go more than 2 turns you have the wrong size I just dont know about this. Now forgive me for my terminology im new to working on these bikes I'll try to get it right theres one more problem I want to try and explain. Theres a fuel leak coming from a ripped rubber boot on a sort of plunger that moves up and down as you operate the throttle I think its called the accelerator pump. Now when I had the carb apart I thought the rubber boot was to keep dirt and moisture out but I didnt see any kind of seal or anything like that you could replace on the inside so im wondering if maybe theres suppost to be gas go into this rubber boot and is pushed out as the plunger goes up and down. I hope you can understand what Im trying to explain any advice or knowledge on this is much appreciated

The fuel leak is a ruptured accelerator pump diaphragm.

The fact that your fuel screw needs to be backed out that far is an indication that your pilot jet is either the wrong size, or that it's partially obstructed by a film of dried varnish, etc.

The hard cold starting is probably due mostly to operator error, but both the blocked pilot and the weak accel pump will contribute. Cold starting requires extra fuel, so you need to use the choke (black pull knob on the carb). You may also find that it helps to twist the throttle once or twice to prime the engine with a shot or two from the accel pump.



You also need to check the hot start valve and hoses to be sure that the valve closes and the hoses are not the source of an air leak.

Before the 400 I had the 01 426 and am well versed in the starting method of these bikes ofcourse I use the choke when its cold its the only way it will start and I couldnt find any air leaks using the old starting fluid method the pump diaghfram looked to be in nearly new condition along with any o rings in the carb

If fuel is coming up out around the AP rod, it doesn't really matter what the diaphragm looks like, it's leaking.

ok cool Ill replace this immediately I didnt know there was only fuel on the bottom side. any suggestions on were I can find this part and do you think this is my starting problem?

Yamaha or Sudco.com will have it (sudco has a minimum order amount, though)

Other than reducing the strength of the pump shot, it probably isn't a big part of your starting troubles.

well darn thought I was on to something I have to correct myself the fuel screw is at about 2 1/8 turns out. Does it take more than some carb cleaner to remove this film you talked about and maybe this fuel screw is causing the hard starting?

well darn thought I was on to something I have to correct myself the fuel screw is at about 2 1/8 turns out. Does it take more than some carb cleaner to remove this film you talked about and maybe this fuel screw is causing the hard starting?
The pilot jet is not the same as the fuel screw, and yes, it can take some small amount of physical force to remove the film, now that no one really has any old school carb dip around any more. Some shops in some places do have some of the good old stuff that will dissolve anything except metal overnight, but you can't but any. Read:


You might wanyt to check to see that the O-ring on the fuel screw is in place, and is not cracked.

ok I got the ap diaghfram installed it fixed the fuel leak but didnt help with the starting issue. any ideas of what to check or try? guy told me sometimes thats just "how it is" could this be true?

Check the pilot circuit like gray said

do you know somewhere i can ggo to learn more about the pilot circuit?

just do a quick search on thumpertalk, and read the link that was posted

ok I took the carb apart the pilot jet wont come out it wont tighten or loosen it just spins but not freely it takes some force to spin any ideas?

Well then it seems you have found your problem. Im not trying to be an ass, but are you sure its the pilot jet?

according to the book its the pilot jet

Better take that down to the shop and have it looked at. Could be time for a new carb.

ya im taken it to the yamaha shop today I really hope this is related to my starting problem

ebay time

I was just looking at the jetting database and the jets I have are stock except the pilot jet which is a 52 and its suppost to be a 45... im in the california valley about 500 ft 40 to 100 degrees farenheit not to much humidity. is the 45 what I want?

I think you have bigger problems right now than to worry about what size pilot you need. But 52 is way too big

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