Lost power at 55mph

[Redleader]

So I went for a ride last Sunday I had been riding for about 30 mins so everything was nice and warm. I was doing about 55mph the throttle was at about 90%. Then all of the sudden it was like it was slowly losing power, I dropped the clutch rolled off then on the throttle came back to life. It did it again about 2 mins later so I turned around went home. Everything seemed ok it had about 75% of the tank of fuel. But earlier that week I had to turn up the idle to about 1250 or it felt like it was barley running. I did the valve lash about 900 miles ago and aside from this it had been running pretty well. According to the manual you only adjust every 3000km or 1800 miles i German spec’d

[JerryHawk250]

Recheck the valve clearance.

[TxTaoRider]

Might check your spark plug. Actually I'd replace it either way.

[clash 168cc]

Fuel cap not venting?

[severely]

Fuel contamination?

[Redleader]

So I redid my valves, the spark plug isn’t more than 1500miles old but looked good I replaced it after 500 miles, had a good burn color. Fuel cap gasket looked good. It might have been the valves but I have had a moment to open it up for a long period to see if it does it again. Thank you for the help

[BrokeAss]

As a motorcycle mechanic, that symptom is usually one of three things...a leaking vacuum petcock diaphragm, a failed tank cap vent, or a pinched fuel line starving the carb of fuel at WOT.

[Megadan]

idle should be 1500rpm since nobody else stated so.

As for your issue - if it does the same thing again open the fuel cap immediately and see what happens. The fuel cap vent may be sticking shut and not preventing a vacuum buildup in the tank.

[zero_dgz]

Originally Posted by BrokeAss

As a motorcycle mechanic, that symptom is usually one of three things...a leaking vacuum petcock diaphragm, a failed tank cap vent, or a pinched fuel line starving the carb of fuel at WOT.

We can rule out one big one on most of these Chinese bikes, as they have mechanical slide carbs and thus no diaphragm. I had a similar-ish issue on my Bashan that turned out to be the float height being set hella low from the factory, though. So that's something to check.


Genuine imported Chinese sediment in the fuel tank blocking the screen on the pickups on the petcock is a possibility, as well. Typically these are pretty easy to remove and hose off with carb cleaner or similar. With the petcock removed and its mounting hole open, you can swish out any crud in there with a little gasoline or, if you're less brave, kerosene. Preferably directly into your fire pit or burn barrel.

[BrokeAss]

Originally Posted by zero_dgz

We can rule out one big one on most of these Chinese bikes, as they have mechanical slide carbs and thus no diaphragm.

Wasn't referring to the carb, I know they're not CV carbs...I was referring to a vacuum petcock...if the diaphragm inside is torn or otherwise leaking you'll have fuel starvation at WOT when vacuum is weakest. I'm not the world's foremost expert on Chinese bikes so I don't know if any are equipped with vacuum petcocks or not but it's worth a mention.

[zero_dgz]

Gotcha. Yeah, none of the Chinese bikes I've seen have a vacuum petcock, either. Manual only. (It's cheaper that way, I'm sure.)