2013 TXC 250 starts cold, stalls hot, then won't start.

yeah, I got thoughts.

In an old-school OHV motor, with a cam down by the crank, and a long valve train with pushrods, (ratio'd) rockers, maybe followers with rollers etc- components grow with heat. different metals grow at different rates. Valve lash was huge to account for this... at least until somebody invented hydraulic lifters (1940s?) which allowed engines to run at basically zero lash when oil pressure was adequate.

when I was a kid, I was into air-cooled VWs (no hydraulic lifters) and valve adjustments were constant- and best to run 'em a bit loose too (this allows for the valve to cool on the seat at TDC). Steel cam, pushrods, rockers, valves; in a motor with a magnesium alloy case, iron cylinders, and aluminum heads, with an 18" valve train (guessing)- and the exhaust valve lash was 0.006-0.008" IIRC; same as your husky. None of this is a factor in your husky either- hence my question of why OHC motors are spec'd with so much lash. It seems to me that 0.002-3" would be fine.

But I am not a valve train expert by any means (just an ex-aerospace engineer with a lot of questions).

On an overhead cam (OHC) engine, the valve train is short, the metals are similar, and there is very little asymmetrical growth due to heat. And valve lash usually increases with time; although there are sometimes factors which will reduce lash (sunken seats, valve stem stretch, etc). Basically, with heat, the cam lobe may grow, the shim & bucket may grow, the valve stem might too... but I'd doubt that this would add up to 0.002" in this short distance.

The ADC operates at low (LOW) RPMs only, momentarily holding one exhaust valve a bit off the seat at BTDC, allowing some compression to be relieved. The distance is small to begin with, and any increase in lash reduces the effect of this.

A KX250F valves are 0.005-0.009" on the exhaust (intakes= 0.004-0.007") per the manual. If you stay on the tight side of these specs on your husky you'll be fine. This hasn't been said, but valves are adjusted with the engine cold. A very few are set hot, but not yours.

Again, if anyone can educate me on OHC and valve lash, I'd appreciate it.

I'm still thinking you need to clean your injector, btw.

good luck.

Great discussion. Thank you for the feedback

The injector was sent off on Monday to Injector RX in Houston for ultrasonic clean and flow testing. I'll post the report when I get it. I just got an email that it arrived at their facility yesterday. I hope to get it back within the week. I'll clean the throttle body while I'm waiting.

kewl. slight overkill, because you could've done it yourself but what the hell. I'm a little surprised there was nothing in Denver or CS tho.

you're on the right track. I'm hoping this fixes it.

RIP Chris Cornell.

Is the throttle body seriously "gunked" up? Internally?

...and are we talking dirt, gas residue, or oil?

I haven't pulled the TB off yet. Stupid job keeps me from fixing the bike.

I'll post pics.

I'm not sure that you need to go that far- maybe though.

You are probably right. I do not believe it is contributing to my idle issue, but I think it may explain why the choke was not working. It was almost completely clogged. I cleaned it with q tips.

Actually, that could be your whole problem. This circuit bypasses air around the butterfly valve both during normal idling AND more so when it's pulled during cold start. The ECU sees a certain volume pressure of air (MAP sensor) and a certain throttle (actually zero, via the TPS) and goes to the injection map/table to see what pulse/duration of fuel you should receive.

When you adjust the idle "clicks" you're moving the plunger in or out minutely. When you pull it for cold start, you're pulling the plunger out a lot.

It's a bit dirtier than I would expect upstream of the butterfly. Make sure your air filter boot and the filter itself are not leaking.

Did you pull the plunger/cold start assembly out to give it and the air passages a good cleaning? Always good to see images, too- thanks. For some reason, I thought you had a Mikuni throttle body (2011-12).

Hey, while you have the TB out... could I get you to measure the resistance of the TPS? This is for my edification- nothing to do with your issue.

Well, the injector got a clean bill of health. Throttle body is clean and I set the tps to 0.680 with a homemade harness. No change. It still starts cold and dies when hot. There is a fair amount of exhaust backfiring if I let of the throttle quickly too.

The idle resistance is 0.668 K ohms.

I didn't adjust the valves yet. I'll do that next. I also want to test the fuel pump. That may take some rigging.

what was your baseline TPS setting before adjusting?

did you clean the cold start/air bypass passages? don't forget to set the idle adjust to 40+ clicks.

That resistance reading sounds like a voltage reading while the bike is running (ie, 668 mV). Resistance is measured on a circuit that is not live- which is why I wanted you to measure it while the TB was off the bike. I was looking for a number like 2000 ohms between the outer wires. no worries. OTOH, if that is the resistance (at idle position: 668 ohms) what is the total resistance at full throttle?

You might look around here and see what people are setting their TPS idle voltage at (.740v sounds familiar maybe?). Our Keihin throttle bodys and ECUs will be different than Mikuni's.

I messed up by not taking a baseline voltage. I thought this tps was not manually adjustable. That said, I proceeded to remove it when I took the to off, only to discover it is adjustable.

The throttle body is clean as a whistle, bypass included. It made a noticeable difference when I pull the lever out, but the bike still dies when it warms up.

The resistance value is as I reported. Check the vid. It shows the whole range.


View: https://m.youtube.com/watch?v=h6TB-BOhoDQ

You sound pretty close with the TPS voltage as is. On your last pic of the TB, you can see the gap in the adjustment slot for the TPS- but I doubt you're gonna eyeball it any closer.

That's a very nice meter.

...and thanks for the vid. I get the resistance as 668ohms to 1467ohms. So I am guessing it's a 0-1500ohm potentiometer. (also I'm thinking you were measuring between the orange and yellow wire, terminals 1 & 2, maybe; any combo of the 3 wires is good). much appreciated.

Might be measuring resistance to ground? Naw, guess not... but I got numbers that differed substantially at open throttle. here'e a cut 'n paste:
May 2018 edit: also, the resistance range of the stock TPS turned out to be 130 ohms to 5050 ohms (probably 5000ohms- meter error) unmounted on the TB. when mounted, the no-throttle/idle resistance was 730ohms, and full throttle 3920ohms. With an accurate current source (like DM reported) of 1mA this also would equal the the voltage reading in mVolts (or in volts: 0.730v and 3.920v). The throttle body I used was off a low-hour '14 TXC 250R. All readings above were taken between the yellow and black wires. When mounted on the TB, black to blue was steady at 4875ohms (why not 5050??) at all throttle settings. Yellow to blue was 4460 ohms idle and 1195 ohms full throttle. I gotta think about these last 2 readings, which I confirmed a couple of times.

I would say to inspect your intake manifold for loose bolts or cracks, but with the TB being R&R'd- I doubt that's an issue.

We're pretty sure you have a new WTS- I'm wondering if a wire is broken (ATS also)?

A coil could also act weird after heating up- however the pulse coil (CPS) on the stator assembly is a bigger flaky suspect with heat problems. [edit: naw, if you can keep the bike running at higher rpm's, it's probably not either one of these]

To test your fuel pressure, you can get a male FI quick disconnect (from china) for $2-$4 and make a plug-in test rig. But really, just slip a 1/4" flexible fuel line over the male fittings (and wire or hose clamps maybe), with a tee in the middle to put a pressure gauge on. This problem doesn't quite feel fuel pressure related, but maybe. I would suspect the internal fuel lines or filter more than the pump but the "starts fine after 5 min" thing is a mystery- so maybe not. You could probably run the fuel pump (in gas) for some time to see what happens- running a hose back into the tank so the pump does maximum work.

lets know what the results are when you adjust that exhaust valve.

let's step back a bit and define the problem:

• the bike starts easily (?) and runs fine (?) cold
• cold restarts are not a problem (?)
• runs fine (?) warm at higher rpms?
• will not idle once warm
• backfires heavily once warm, when the throttle is shutoff (?), rolling? or not moving too?
• will not restart once warm
• will restart 5 mins later warm/semi-warm

do you use the cold start at all? and if so, when?
what is the cold idle speed?
does starting fluid help on the warm restarts? (if not, may be an ignition problem)

if this is still an accurate description, the bike sounds like it's running very lean at idle still.

Just a thought. ..what about reflashing the ecu?

Did the TXC come with the Oxygen sensor? (I see it's plugged on teh head pipe) Depending on map maybe the ecu is looking for a signal. If Map 3 it shouldn't but might need to ascertain that. Prior owner.... Maybe they didn't reflash but removed O2 sensor and installed the JD and still had mixed results.

You are dead on, Trenchcoat85.

I can't find my tach, so I'm not sure what the idle is.

I don't need the cold air bypass to start the bike. Once warm, it will not run with it open.

I'll add one more thing that that I didn't notice earlier. The idle, even cold tends to hunt a bit, though this seems to occur randomly.

Also, the seller indicated the following:

• It would flat out die when ridden hot on the trails. The seller (mechanic) indicated this was resolved by replacing the water temp sensor on the head. I suspect that may not have fixed the owner's issue.
• The parts that came with the bike indicate that they replaced the temp switch as well.
• The bike had not been registered since 2015 and had old yellow gas in it.

I'm with you on the lean hypothesis. I'm beginning to suspect the pump dying a slow death. I've read several posts from riders with similar symptoms where they found a failing pump as the culprit. Also found this clever video of a KTM pump (below) that has basically the same pump. I'm going to swap the pump out. The bench tests seem like a PITA and at this point. It's dangerous with gas, messy with water.
I asked the owner if the ecu had ever been re-flashed. He said no. The TXC did not have the O2 sensor.
I'm going to be up all night wondering where my tach is now. Just moved 3 years ago and it's probably in a box.

View: https://www.youtube.com/watch?v=vwmKU3LRcDU

I think you can get a perfectly adequate fuel pump on ebay for $20 (I have no experience). It certainly wouldn't be a bad investment at that cost; but the stock husky (or ktm) pump is hundred$ for our bikes IIRC (unless you have an aluminum plate fuel pump mount w/ 6 bolts).

when you're taking the tank off, lift it up and remove the fuel hose. Now slip a 1/4" line over the fitting and run it back into the tank. hit the starter (a couple of times?) to see a good, STRONG, 2 second flow into the tank. if so, your filter is probably good.

great video. I've never seen the problem so clearly demo'd. ...and that's the flow you're looking for too, btw.

I have successfully "de-varnished" a non-working fuel pump using carb cleaner sprayed in both ends (there's a check ball in top IIRC) and also a small port near the bottom turbine also.

Running naphtha through it for 10 minutes might help. It might help your fuel pressure regulator too.

Arrrg! New pump is installed. No change in the problem. I guess I'll experiment with the valve clearances. I can't believe he bike will not run if they are within one shim of spec, but I'm running out of things to try.



View: https://m.youtube.com/watch?v=7RfHOEBJV8U&persist_app=1&app=m