symptom: sputtering at steady/part throttle

[TheHarp]

My '08 YZ450 runs great as long as throttle is being added or it is wide open. The problem is at part throttle when held steady...it bucks and sputters.

The bike has stock header and can with the Pro Moto Billet end cap. It has head work. I haven't messed with the jetting since I've had it, but not sure what's in there as I bought it used.

I need some advice on what I should be looking for to solve this problem.

Thanks.

[ttr250dude]

Did you buy it with the PMB endcap and porting already done when you got it?

[TheHarp]

Did you buy it with the PMB endcap and porting already done when you got it?

There's a thread around here somewhere describing the porting that was done to it before I got it. I had it fixed by someone who knows what they're doing.

I installed the end cap.

[grayracer513]

Try disconnecting the TPS and see if the problem goes away.

[TheHarp]

Try disconnecting the TPS and see if the problem goes away.

Will do. If the problem goes away does that mean the TPS is bad?

Thanks.

[ttr250dude]

You might be running too lean.

[grayracer513]

Will do. If the problem goes away does that mean the TPS is bad?

Thanks.

That's one possibility. It could also be out of adjustment if it's been disturbed, or it could be a jetting issue involving the major diameter of the needle (even if the TPS clears it up).

The bike can be run long term with it unhooked without a great deal of negative effect. It's not ideal, but not so bad, usually.

[TheHarp]

Thanks guys. I'll try it out as soon as my new lower oil filter cover bolt arrives...arrgg.

[gasnit]

I have a 2009 YZ250F and with low to medium steady throttle, mine also sputters. I have tried different jetting with no improvement....I have not tried unplugging the tps.

[TheHarp]

Hey guys...just following up. Disconnecting the TPS seems to have fixed the problem...the bike is dramatically smoother at steady/part throttle now.

The question I have now is -- what are the downsides of not having a TPS?

[BlindLOKI]

This is straight from grayracer he helped me to understand it a little while ago. its a lot like the fi bikes just not quite as dependent on it.

The TPS on a carbed bike does just what it did on a carbed car engine. Don't remember that? Then there's no use in my telling you it works like a vacuum advance, I suppose. The TPS provides load state info to the CDI so it can alter the timing map at part throttle. It's designed so that a high TPS output voltage indicates closed throttle and low voltage indicates full. That way, if it fails open, as it most commonly would, the CDI simply fixes the timing on a 2-D map (engine speed only), and assumes a full load. Thus, disconnecting it has no effect on full throttle performance, and only a little on part throttle

also, if you have and ohm meter and the service manual you can check/adjust the tps and be back to running with it as long as its not out of spec. reading from black to blue should be 4-6 k ohms i believe

[grayracer513]

The downsides are minimal most of the time. In theory, with a well designed 3D timing map, the part throttle performance should be a little crisper, and fuel economy at part throttle/cruising should be better with the TPS fully functional. It has no effect on full throttle at all.

[Wiz636]

also, if you have and ohm meter and the service manual you can check/adjust the tps and be back to running with it as long as its not out of spec. reading from black to blue should be 4-6 k ohms i believe

Just be sure to use the new cover letter for the TPS report. Do you need a copy of the memo?

* Sorry, couldn't resist a little 'Office Space' humor.

[BlindLOKI]

Just be sure to use the new cover letter for the TPS report. Do you need a copy of the memo?

* Sorry, couldn't resist a little 'Office Space' humor.

yes i am in need of the cover letter... does this involve that fax machine cause i swear i will take a sledge hammer to it....

[grayracer513]

... if you have and ohm meter and the service manual you can check/adjust the tps and be back to running with it as long as its not out of spec. reading from black to blue should be 4-6 k ohms i believe

The correct adjustment procedure is to back probe the TPS voltage while the engine runs. The resistance thing is just a basic test. Check the manual.

Checking resistance can be useful in another way, also. You can rotate the TPS slowly through a full swing and watch to see if the resistance swings up and down smoothly, or if there are dips and spikes, which would indicate faulty spots in the unit.