New 99 yz400f, starting issues, advice please


20 replies to this topic
  • yz400f_noob

Posted August 07, 2012 - 12:17 AM

#1

Right so before i start this is my first post, i have used this site alot for reference and think its great but apologize in advance for any stupid comments/questions and thank you in advance for your time and advice.

So i have just bought my first thumper, have been a sports bike an 2 stroke man before this but here we are and i love this bike. It is a headache to start though, When i purchased the bike it didnt run, had majorly old fuel in it which smelt not of this world, the plug was wet, cylinder was wet, oil was murky, exhaust had water in it and a huge split. SO i flushed and replaced oil and filter, drained and cleaned tank, drained carb at float, cleared cylinder, left to dry, fixed weld in exhaust, and replaced the plug. Now after all of this and a fortnight of learning the starting technique i was getting the bike started but not after 50 kicks and being to exhausted to even ride!

im a shorter guy at 5'10 and 70kg its harder for me. I thought it was pretty awesome after riding for the last 2-3 weeks getting good air having good fun, but watching my uncle start his bike (01 426f) with but a ease of a stoke on the kick and me having to jump in the air and slam the kick down like a lion some 15-20 times before it starts i thought there had to be an issue. I tried the next day at 7am cold motor, and i started his 1st kick with no choke!! so i know i have the procedure down, so i try my bike, after an hour or so it wouldnt start so i gave up, plug still mint and clean, nice tan, carb has gas, good spark.....

After reading through here i have decided to clean my carb, take it right apart and reset according to factory, figured out quite a few jets were blocked up, also the accelerator pump diapraghm has fallen to pieces, so new one of them ordered, next is checking resistence in electrics i guess and valve clearance. Any other thoughts as to what it might be other then that? any recomendations or other fix's to the carb anyone can reccomend while i have it off (bearing in mind i am in new zealand, slightly cooler temps and more moisture in air)

Also i have been told replace oil every 4 hours so i have been, and filter every 3 changes, manual refers to "races" how long is a race? i ride about 3-4 hours everytime i go out so just always do one..

Honestly any tips or any info please do let me know, i have read through this site for the last two weeks looking for stuff but always seem to find myself in slightly the wrong catogory or cant relate.

Thanks Again in advance for any help everyone.

  • corndog

Posted August 07, 2012 - 07:51 PM

#2

Welcome noob!!

The biggest clue in there was that it started first kick, cold with no choke. This would indicate that your pilot circuit is too rich.

Start by setting your fuel screw per: http://www.thumperfa.../jetting.htm#PJ

Checking the valves is always a good idea with a hard starting bike: http://motoman393.th...ech/valves.html

As for when to clean the air filter, you can't really put a number on it, in my opinion. It should be cleaned when it's dirty, which usually happens faster in the summer and slower in the winter. Inspect it after each ride. It wouldn't surprise me if you had to clean it after each ride in the summer.

  • Trinitycounty

Posted August 07, 2012 - 10:45 PM

#3

Check valves as states above

  • yz400f_noob

Posted August 18, 2012 - 12:37 AM

#4

Right so, have pulled the carb to bits, found that it was blocked up, needed a new ac diaphragm so did that, relaced the throttle cable, and gave her a go today. Found it wouldnt start, tow starting was the only way, had the pilot jet screw 1 3/8 turns out as per manual, so i turned in a half turn, found it started easier but now it would only give me a couple minutes of good riding before it started to bog and die. after you leave it for a few minutes (or keep trying to start it for a few minutes as i did at first) it starts back up fine and runs mint, but then the same process happens again, checked the plug, no spark but clean and dry as a bone, electrode was white, not tan as it should be. replaced the spark plug started up fine and went good but same process yet again, and checking the new spark plug same storey... clean carb, clean air filter, new spark, good compression, new cables everything seams good but no go...? I checked the valve clearances and wrote them down, checked manual if in spec and they were but then discovered that the piston was not at tdc on compression through the pictures in the manual being different to where i had the cams. I suppose i should check them properly again, but please if anyone has had a similar issue after cleaning the carb please help!

  • grayracer513

Posted August 18, 2012 - 06:49 AM

#5

After it has died and still has no spark, test the stator per the manual.

  • Pooley

Posted August 18, 2012 - 10:07 AM

#6

I'd check your valve clearances again. If it starts cold, dies warm I would suspect that your clearances are too tight and once the valves warm up and "stretch" something is sticking open just enough.

  • yz400f_noob

Posted August 20, 2012 - 01:38 PM

#7

Grey im sorry i incorrectly wrote my last post, i should have said same storey again with new spark plug that it was clean and dry and electrode was white but still had good spark, must have just killed the old plug for some reason.But upon your advise I just checked all electrics last night as per manual and everything seems good except one test, being the source coil 2 was out of spec on the cdi magneto test, this is testing through the plug. It says the pink and black wire should put out between 44-66Ω resistence on Ωx10 but i only get a reading of .5Ω unless i change to Ωx100 which is obviously wrong but the it gives me 55Ωwhich is in the limit...?. do i need to replace the magneto? the other two tests were good, or is this enough to start bugging with my spark

Pooley you are onto it, Also checked the valves again yesterday, found that they were all much to tight, one in fact was so far out i couldnt even fit my .04mm gauge in it when supposed to be between .15-.20 and the other two were at least 1-2 sizes out, both exhaust were out also but 1 -2 sizes will get new shims today (freakin 12 dollers each in NZ!!) and put head back together, hopefully this will help with the issue of dying, however i believe that was caused by carb as never did that before i cleaned the carb. hopefully the good valve clearances will help make starting easier also. Though i am worried about that one very tight valve, do you think this may have damaged it?

Now after much more reading of this site and greys posts about 05 and up 450f carbs i have purchased a 06450f carb off ebay, it comes with hot start and accell cables an aftermarket throttle tube and fuel screw also boyeseen quickshot ac pump cover. With any luck this will make life easier or less troublesome, from all the posts ive read it will so i hope you guys are right! In the mean time i have re cleaned the stock slant body fcr carb and found the float was out of spec, re set the spring on the needle and seems to be in spec now, also left jets in carb cleaner overnight so an crap should hopefully be eaten off now, hopefully this will allow me to test after valve shims are done, carb will taje a couple weeks to get here

Man i hope after all this work this beast will actually be some fun to ride on, i love this bike its superquick an gets great air but man oh man if i cant get it starting right....

Sorry to drag but one last question, yamaha lowering link, has anyone bought one of these, do they really do a great job? or is it the horror storey people talk about that messes up the handling of the bike, even with the front being set to counter this. Im a small guy an this is a big bike so this sounds like it might be my friend.

  • grayracer513

Posted August 20, 2012 - 02:08 PM

#8

Grey im sorry i incorrectly wrote my last post, i should have said same storey again with new spark plug that it was clean and dry and electrode was white but still had good spark, must have just killed the old plug for some reason.But upon your advise I just checked all electrics last night as per manual and everything seems good except one test, being the source coil 2 was out of spec on the cdi magneto test, this is testing through the plug. It says the pink and black wire should put out between 44-66Ω resistence on Ωx10 but i only get a reading of .5Ω unless i change to Ωx100 which is obviously wrong but the it gives me 55Ωwhich is in the limit...?. do i need to replace the magneto? the other two tests were good, or is this enough to start bugging with my spark


For one thing, the actual resistance is supposed to be 44 - 66 ohms. The scale suggested by the manual apparently assumes the use of a 0-10 scale, but the resistance still has to be 44-66. When reading on an analog meter with a scale that swings 1-10, you would need to use the Ωx10 setting so that scale would show you between 4.4 and 6.6. A reading of 5.0 (50Ω) would be fine, but not .5, which would in fact be 5Ω. On a Ωx100 setting, 55Ω would read as .55 because the scale that looks like it says 0-10 is actually 0-1,000. Another reason autoscaling digital VOM's are so much easier to use. You might have weak batteries in the VOM if it's giving you two different readings on two different scales when both were zeroed before the reading was taken.

If in fact, your reading fails to fall in the specified range, then it's bad. That simple.

  • FinchFan194

Posted August 20, 2012 - 04:35 PM

#9

I'd check your valve clearances again. If it starts cold, dies warm I would suspect that your clearances are too tight and once the valves warm up and "stretch" something is sticking open just enough.

Grayracer is this accurate?

  • grayracer513

Posted August 20, 2012 - 07:36 PM

#10

With the stainless valves the 400's had, it might be, but probably not. It would actually be backward with Ti valves. In most cases, the aluminum head expands more than the valves do, and clearance increases as it gets hotter because the cam is "lifted" away from the valve seats.

This is why so many 250F's complain that they're hard to start and have no compression cold, but run fine when warmed up. It's also half the reason that the valves can actually wear into a condition in which they have negative clearance when cold; the valve shims have to be smaller before the valve will even be closed and the clearance reach zero.

One thing to understand about the phenomena is that the head expands more per degree, but the valves get hotter, especially the exhausts (1500℉ or so, normally). Because of this, valve clearance tends to stay surprisingly constant, even though it does change with temperature.

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  • yz400f_noob

Posted August 20, 2012 - 11:40 PM

#11

Right so Grayracer so far i have replaced the battery in my meter an retested and it has come up with a reading in spec this time! thank you.

I spent today redoing the valve clearences, they were well out i had to go down in shims about 4-6 sizes in some of them to get specified valve clearances, i have the inlets all at .15 as per manual but need to get two more shims to bring the exhaust up, closer but still only .20 each when both need to be .25.

However i decided to put the newly cleaned again carb on the bike and give it a go like this, now it took me a good 9-10 kicks but i got it alright, im not sure if it started easier or not yet though, is it normal to take quite a few kicks after working on the carb and cams? but the big big thing i noticed was that it actually idles!!! my god this bike has never idled like this, always had to have the revs right up to get it not to stall but it actually idles, i let it idle a few minutes to warm but it started lowering and lowering the idles so i kept raising but eventually it stalled, it hot started well, 2 maybe 3 kicks, but same thing. i tried to richen the idle a little and also raise the idle screw but kept stalling, any advice on this? or should i just keep the revs nice and high till my 06 450f carb gets here?

I might add that it will be 3 weeks so i kind of really do want this to run in the mean time. will update after trying another cold start tomorrow, any advice on the idling?

and before any more advice is given may i please thank the great minds of this site particularly Grayracer, its people with the knowledge like you that save idiots like me from going insane with a dead bike

  • yz400f_noob

Posted August 23, 2012 - 12:34 AM

#12

Grayracer,

After the weird idle i decided to double check valve clearances. This is driving me crazy now :banghead:

Everytime i check the clearance its to tight, i then get the shim number with the current clearance and use the chart from the manual to provide me with new shims, but some how the clearance stays the same.

Example, i was on a 170, 165 and a 170 shim on inlets and 170 and 160 exhausts, needed to move to 165, 160, and 155 inlet, 155 and 150 exhaust. This some how did not change the clearances, made them even mind you but still well under. i have done this process now about 5 times and have not even riden the bike, i follow the manual to the detail and everything seems fine, bike definately runs and starts a little easier but everytime i check the clearances are back to the same. I am now down to 155, 145 and 150 inlets, and 140 and 135 exhausts and they only go down to 120.

Is this part of the random valve issue you wer refering to Gray, do you or anybody have any advice, im starting to get a good collection of shims but bugger all else

  • marv02

Posted August 23, 2012 - 05:06 AM

#13

If I was you I would pull the head off and take a look at your vavle seats and the valves there self if every time you going to a smaller shim the vave could be suck up in to the heads due to bad valve seats or the bad valves there self.

The the weird part of your story normaly the valves dont go bad all at once.

And measure the vavle stem lenghts also you got the bike used who knows what has been done to it from the last ownners before you.

Once you get the valves out do a quick spark test and find out wicth vaves you do have in the bike so know whats you realy have.

Edited by marv02, August 24, 2012 - 05:22 AM.


  • grayracer513

Posted August 23, 2012 - 06:52 AM

#14

If you reshim a valve to the right clearance, then run the engine for an hour or so, recheck and find that the valve has closed up again, the valve is worn out. Pull the head and have a motorcycle machine shop take a look at the valves.

The likelihood is that the seats are OK, but if you continue to run the engine like that, they won't be, and you could also break a valve off it the engine as a result of the valves "scrubbing" the seat as they close.

  • yz400f_noob

Posted August 23, 2012 - 11:28 AM

#15

Wow looks like i really have bought a lemon. MMM looks like ill have a longe few weeks ahead of me.

Thanks Guys

  • yz400f_noob

Posted August 25, 2012 - 01:35 PM

#16

Well, turns out my clearance problems is not a potential rebuild. Its using the wrong tool for the wrong job. I hade a flat feel gauge set, when i needed an offset set of feeler gauges. The straight feeler gauges meant that a gauge was rubbing against the cam and cap whilst checking causing fricktion, giving me an incorrect reading. After getting an offset - set of feeler gauges i found i had made my clearance much to large, it was changing i just couldnt feel it acurately. But now it is bang on perfect, both inlet and outlet in spec and a much higher number on the shims. Smallest now is 145.

How ever after doing this and resetting the pilot screw again i still cant get that bad boy started nicely. Alsmost like its too rich, but its cold and night, and im at sea level, and its standard jetting? It starts kind of with a mother of a kick with choke in but when the choke is out itll not start and most likely kick bck on me.

Last couple nights trying to start it have been the same, then if i go to throttle to try to bring revs up it dies.

Any suggestions? ive been reading many threads but...

  • yz400f_noob

Posted August 25, 2012 - 11:21 PM

#17

is it possible that cam chain "stretch" could put my timing out making it more difficult to start?

  • frankie_b_jr

Posted August 26, 2012 - 08:40 AM

#18

Silly question but, when you tore the carb down did you pull the throttle plate out? If so, did you put the vacuum plate in upside down? Maybe someone else can explain it better than me.

  • grayracer513

Posted August 26, 2012 - 12:25 PM

#19

is it possible that cam chain "stretch" could put my timing out making it more difficult to start?


It can stretch, but not enough to keep the bike from starting.

  • yz400f_noob

Posted August 27, 2012 - 09:16 PM

#20

Silly question but, when you tore the carb down did you pull the throttle plate out? If so, did you put the vacuum plate in upside down? Maybe someone else can explain it better than me.


Hi i thought it might have been, though the silly question is the one that isnt asked when the answer is needed.. But to answer your question no, each time i have pulled the carb apart its been the bottom half, just to gain access to all the jets and do a good check an clean. The slide seemed good, the needle was set right and looked good.so no probably not what i did.

Im still waiting for my 450f carb hoping itll fix. ive been trying to think what else but running out of possibilities





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