Hi

Went out the other night and the bike was running perfect. We did a long high speed on the road link for maybe 4 or 5 mins and when we slowed down my engine just died. It was like the kill button was pushed in.

It didn't start back up until it had cooled down and then ran fine again.

Later on we did a similar length run on the road and the same happened again.

I have read that when the guys are doing the long links between sections on the SSDT that they run with the choke on to richen it up enough for the sustained wide open runs.

Is it likely that it's just that or should I be looking for something else?

As well as a new set of rings....

You nicked the piston. Pull the exhaust header off and check the piston.

Just out of interest, why were you thrashing a trials bike up the road for 4-5 minutes?

Edited January 24, 201511 yr by the addict

I had those symptoms on my KTM running too big of a main jet, fouled the plug. Same thing could happen with a nicked piston too like lineaway said. So check the piston first and if it looks ok run it briefly at wot, hit the kill button, and pull the plug to see if it's fouling.

@the addict. We were moving from one section to the other without loading into a van.

I'll pull the pipe off in the morning and have a look at the piston. It's gonn need a top end anyway now I think.

It still runs for now so it can wait till I get back from California!

Dellorto carb? If so it's fuel starvation due to the floats sticking 'up' on the two guide pins. Won't happen off road due to the bumpy ride shaking the floats about.

OK there is a lot to discuss here. The reason the guys in the SSDT will throw the choke on for a sustained run is two fold. The additional fuel will help cool the engine but also provide extra oil as the ratios run by most riders won't support such wide open sustained operation. Think of it this way, at typical trials speed residual oil pools in the bottom of the crankcase lovingly coating your bearings. When you do a short blast of high RPM that oil gets splashed up into the fuel/air charge to give a little bit more of the good stuff to your cylinder walls. That's why after a good long hill climb most bikes are smoking like chimneys. Even the guys running 100:1. Extra oil has been dumped through the engine into the exhaust where it smokes as it is burned off. Now take that hill climb and do it for fifteen minutes and you will eventually run out of "extra" oil. Add to that the high temperatures that result from pushing an engine hard and you can easily scuff a piston with a hot spot in the cylinder and lack of oil. Engines are designed to operate within a heat envelope and if you push it outside that envelope odd and sometimes expensive things can happen.

As for your problem it may be something entirely different. Heat does funny things to electrics and it's possible you heated them to the point where they could run but couldn't provide the energy needed to fire a cold mixture. Betas are famous for this as a sign of imminent stator failure. Runs OK cold, dead when hot. It's also possible you have insufficient fuel system venting where the engine sucks fuel out of the tank before it can suck enough air in to replace it. I think this unlikely as that usually resolves itself within minutes of stopping unless you have a damaged fuel cap vent.

Partial seizure which is what the other guys are concerned about usually scores the piston and often pushes the aluminum from the piston into the ring grooves sticking the rings and lowering the compression. Usually a bike will run lousy after such an event or at least not at its best.

If there is poor airflow you can boil fuel in the carb if it gets too hot. Rare but possible. Same for boiling fuel in the tank and fuel system. Check fuel line routing, nothing near the pipe without a heat shield.

The key piece of information we're missing is, does it run good after it finally starts? Does it run good now?

Because they spend a lot of time running at low speed / low fuel flow the float valve on trials bikes tends to be sized a bit on the low side, sustained wide throttle openings tend to lower the fuel level in the float bowl until its too low to properly fuel and lubricate the engine.

When you slowed down at the end of your run the piston / cylinder would be very hot with clearances in critical areas at a minimum. Closing the throttle to slow down shuts off the fuel flow completely starving these areas of lubrication so there is a rapid increase in friction. If you are unlucky the piston seizes completely but it sounds as if you are not quite at that stage, but pretty near.

Use the cold start enrichment for frequent short bursts and blip the throttle frequently when slowing down.

You don;t give age of your bike but the sprung loaded pin the the float valve plunger can become worn and notchy in operation. Check yours moves completely smoothly over is full travel. If it has rough spots this can cause the symptoms you describe.

Edited January 24, 201511 yr by dadof2

When you slowed down at the end of your run the piston / cylinder would be very hot with clearances in critical areas at a minimum. Closing the throttle to slow down shuts off the fuel flow completely starving these areas of lubrication so there is a rapid increase in friction.

Forgot about that. Good point.

Because they spend a lot of time running at low speed / low fuel flow the float valve on trials bikes tends to be sized a bit on the low side, sustained wide throttle openings tend to lower the fuel level in the float bowl until its too low to properly fuel and lubricate the engine.

When you slowed down at the end of your run the piston / cylinder would be very hot with clearances in critical areas at a minimum. Closing the throttle to slow down shuts off the fuel flow completely starving these areas of lubrication so there is a rapid increase in friction. If you are unlucky the piston seizes completely but it sounds as if you are not quite at that stage, but pretty near.

Use the cold start enrichment for frequent short bursts and blip the throttle frequently when slowing down.

We run several different Ski Resorts during the summer months. We`ve lost many a top end from running downhill for long periods of time. It`s hard to get the riders to stay on the Gas on some of the steep runs!

I had similar symptoms a few years ago with a failing stator, would generally stop after a run between sections & take 10-15 mins to cool down before it would start up again & run perfectly until the next time it got hot & slowed down again

Hi all.

Thanks for the replies. It seems I have one of two possible issues.

Either I have a failing stator or it's too lean to sustain long wide open throttle runs.

I'm out on it today and hopefully it will become more obvious which issue it is. If it happens without the fast work then I guess the stator is on its way out!

I haven't had chance to look at the piston yet, I think I'll just use it today anyway as I'm only marshalling at fast eddys extreme enduro so it's not going to get abused pootling round the woods.

Also insure the fuel line FROM TANK TO CARB IS NOT ROUTED AGAINST THE MUFFLER! This heats the fuel in the line and added to the heat radiation from the exhaust can cause the fuel in the carb bowl to boil.

The line should come out of the tank and go forward toward the clutch line on the left of the intake then loop back wards towards the carb inlet staying away from muff.

I have seen this area insulated on back of muff as well.

Edited January 25, 201511 yr by copemech

Mann MUFF!

Well it ran fine all day today, I didn't get a chance to do a WFO run due to too many trees!

Pretty sure it's electric related now. I'll investigate more when I'm back off holiday.

If normal running was found all day, makes me think it is not an electrical issue.