TPS(throttle position sensor) help on yz400 '98

While cleaning the carb, I disconnected the TPS sensor off the carb before I read in the manual that you arn't suppose to, did I screw anything up?

How do I re-calibrate it to maximum effiecency? Could that be a cause for a slight bog during rapid throttle twists?

Thank you very much for your help,

Brian Bills

Did you disconnect it from the carb or did you just disconnect it at the coupler?

If you did it at the carb you should be ok as long as you were very careful. If you notice a difference then check the manual.

I disconnected it from the carb. Am I going to damage anything if I start the bike up and it isn't working correctly?


Brian Bills

No, I did that once too. It didnt do anything. Like I said as long you you just put it back on and nothing got moved you'll be fine.

You will not damege anything, but it will never work right again. Time to get a new set-up. Oh and be carefull with these carbs, threre pretty spendy.

The slight bog you feel is the accelerator pump pumping too much or not enough gas on twist of the throttle.


Thats what you need to do to the rod, this is shown without the black boot covering it.

Sometime while doing this you will want to take off the accelerator pump cover and clean the diaphragm. You will also want to clean the carb while your at it.

Before you do this you should pull the carb completely off the bike. Pulling the subfame off with everything still attached to it is easy, and clean.

And here are the directions...

Ok, now you are going to need a little plastic collar, size 3/32. It needs to be between 1/4-1/3 inch long. If you decide to get a collar, they are available at hobby shops as wheel collars. This wil be the stopper on the actuater rod.

Now, you are going to slide that stopper over the rod, almost all the way up to the bend, and put it back into the plastic arm. It is a bit tricky, but stay calm.

Now, from the right hand side, you are going to slide the float bowl somewhat back to where it is supposed to be positioned. The reason you need to do it from the right side is because you need to make sure that little rod slides back into the brass fitting. (See pic)

Once you have the rod in the brass fitting, push the bowl all the way to where it is supposed to be, and turn the carb back the left side, making sure that rod don't fall out. It may be easier to do this with 2 people, but if I can do it...

Tighten them allen bolts back up, securing the float bowl, and double check to make sure the rod is in the brass fitting. Now rotate the carb. to it's normal position, and go back over to the right hand side.

Remember that stopper you put on the rod??? The distance between that and the brass fitting should be roughly .024-.025 inch.

That is close to the width of a dime, if no feeler gauges are at hand.

Before you move that stopper, however, fill the carb with gas (attach tank hose and turn petcock on for a sec, then take it away) and give it a few (2-3) full throttle twists.

This primes the AC pump. If you don't do this, no gas at all will be pumped. Just a warning!!!

OK, got the pump primed, slide that stopper down the rod to .025 inch distance between stopper and brass fitting the rod goes into.

And you are done!!!

And then you have done the Taffy mod.

If it ever starts to bog again, you may need to prime the pump by backing the stopper up all the way and giving full throttle twist, and make sure there is gas in the carb. Then adjust the stopper to where it was before...

If you ever run the bike completly dry of gas, you may need to prime the pump as well.

I have employed this mod with great success already.

I actually got a chance to use the bike over the weekend and have noted a few key differences that maybe you guys could help me with.

First, after the bike is warmed up and I've ridden it around a bit, when I pull into the pits and just let it idle, the RPM takes approx. 30 seconds to drop to an approprate idle speed. It was not like this before.

Second, The idle isn't very reliable at all and dies quite frequently, and is much harder to start. Before it was very easy to start and didn't require the use of the hot start as it does now.

Thirdly, when I let off the throttle the bike pops like mad! This pisses me off as I had this problem a while ago, but it sort of when away, until I decide to clean the carb.

ANy specific help with these problems would be greatly appreciated, I'm very frustrated.


Brian Bills

It seems too lean on the pilot jet and too rich somewhere else.

I don't understand how these settings could be changed just from a cleaner carb. The only thing that might be at fault or that isn't exactly how I had it is the tps sensor.

You will not damege anything, but it will never work right again. Time to get a new set-up. Oh and be carefull with these carbs, threre pretty spendy.

Maybe he was right. :)

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