new to me 98 yz 400 f help... pop pop fizz

Hello all, first post here, been a lurker for awhile ;) I apologize that my first one has to be a problem, but I'm looking for a little helpf if possible.

I graduated from MMI so I have a small amount of knowledge/mechanical aptitude, with that said, here is what I'm experiencing.

First recieved this bike and got it to start but it ran really rough, hot and popping. I assumed a good carb clean and I'd be off with an awesome bike/steal. I pop the seat off and the aircleaner is destroyed. Replace it and clean the carby out real good, hook everything back up and start the 4 stroke shuffle. After kicking myself stupid, adjusting the pilot (leak on yamaha?... adj. screw anyways...) trying choke, no choke, hot start, adjust idle knob... nothing. If I do get it to light up its still the same pop pop fizz fizz oh shit its hot! lol

So I go to work do my research.... Never had a bike with a manual hotstart before so I replace the worn out tubing on it, test the button it holds all the pressure I can put to it... Put a freshy plug in it, gapped correctly.... kick kick kick same old song and dance. I got it to run ONCE and it sounded GOOOOD. only problem was I had the choke AND hotstart on hahaha, im assuming a freak accident as it died shortly after and returned to pop pop fizz fizz when I did get it going.

I decided to check the tps, as i havent had a carb bike with those either lol I test the resistance and I'm getting 6.11 k ohms fully closed at about 80 degrees F. spec is 4 k - 6 k at 20 degrees celcius, or 68 degrees F. could this be my problem? i'm really hoping not to have to throw 300 onto the bike right away as the wifey is already pissed I got a new "toy" hahaha I am going to try and kick it tomorrow (too late now) with the tps disconnected and see what happens. Can it run decent without a tps? any way to service/fix it? (i know manufacturer says no but I know you guys got some tricks up your sleeve, right? :eek: )

anywho... if no change with the tps disconnected I'll rip the valve cover off and see what my clearances are looking like. (i know i know i shoulda checked these first but I wanna ride... :lol: hahahaha)

Well, sorry its long as hell, but any help/insight would be greatly appreciated.

Thank you for your time,


P.S. Excellent site with what seems like a good bunch of people... rare find these days hahah :banana:

edit oooppsss.... sorry missed the yz forum by a couple inches to the left!

Edited by BlindLOKI

Check to make sure the plate in the carb slide is incorrectly. What you have stated the bike doing is what it will do if the plate was flipped upside down....This is very common when people put the car back together. If you pull the slide out of the carb there is a plate that is seperate from the slide that needs to go in the correct way or it will do exacly what you are saying. Many time the bike will not start unless it is giiven throttle or the idle is cranked way in. there is a flat end to the plate and a end with dog ears,I believe the ears go up and the flat side goes down....

Just a thought.

good thinkin... I will check that out as well. Can I trust the manual with the orientation? it shows the dog ears down, perhaps to let a little air through with the throttle closed? I will be pulling the carby because it seems I didnt get the float needle good enough it wants to pee gas all over my garage now :banana: such a nice bike from the time I brought it home hahaha

Well... with the TPS disconnected guess what, it starts right up and idles fine with the choke on then dies as soon as the choke gets pushed off (need to tune it as I havent been able to run it at all) but as soon as I connect the tps and try again, right back to square one... what all does tps control? timing/spark?

im thinking about giving her a try after playing with the fuel screw to get it to run correctly....

Your TPS may be out of adjustment or have failed. Follow the instructions in your manual exactly to test and adjust it.

TPS tells the CDI what ignition specs to use. Between throttle position and actual engine RPM, it tries to give you an optimum mapping. Disconnected, it runs in a retarded limp mode.

Get the TPS right before you mess any more with the carb. Once you are ready to hit the carb again, install a brand new, not just 'clean', pilot jet.

cant adjust the tps per manual specs... its shot. resistance between blue/black should be 4-6 k ohms and im getting 6.11 k ohms. i even tried to work out the math (reading times .13 to .15 i think) and tried to adjust it anyway but no go. that leaves me needing it to show 749 to 900 somethin between the yellow/black when adjusted correctly but i can only get anywhere from 149 to 319, not even close.

a friend of mine raced 250's and he said he always unplugged his after modding... :S

Unplugged, your bike will be slower. Sad to say, a new TPS is (over priced) pricey.

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