Motorcycle randomly loses power

Question

I have a single cylinder, 200cc, carbureted, 5 speed, 12V, 2006 Lifan LF 200 motorcycle that typically runs fine. It will sometimes randomly lose power and I cannot get it to go above 25 mph. When this happens, my bike appears to lose all torque above 4000 rpm and I cannot get the engine to spin any faster in any gear unless I am going down hill or down shifting. When this happens, even in first gear, I can feel my bike weakly accelerating then once I hit 4000 rpm it loses all power. When I open the throttle and it is maxed out at 4000 rpm, the bike shakes as if it regains power for a split second and then loses it again. It feels like it is trying to pull away. This happens randomly both on cold starts and in the middle of a ride. After 5-20 minutes of being bogged down, it will suddenly regain power and pull away (I will hold the throttle open for 10 minutes and suddenly the revs go up again and I have full power).

If I disengage the clutch, I can fully rev the engine, therefore I don't think it is a problem with fuel delivery. My fuel tank cannot have negative pressure so that cannot be the problem. I checked my brakes and they are not engaging by themselves so that is not the cause. If it were a slipping clutch I should still be able to rev higher, just not transmit the power. I replaced my spark plug. I cleaned the air filter but I may try just removing it and see how it runs. I checked my oil and it is fine (I am considering loose shavings or something jamming the transmission as a possible cause though).

Any ideas would be greatly appreciated. I haven't been able to find any answers online and I asked two mechanics and they could not give me an answer (although they did not do any in-depth checks on the bike).

Answer 1

There are a few things for you to look for

• First - A low battery can initiate misfires as your ignition system will require 12.6 volts or under load the spark will not have the required electrical power to jump the gap of the sparkplugs under high-load/resistance situations. Your battery could be getting a lift while charging at certain RPM's which is why the symptoms will go away after a period of time.

• Second - Test your charging system. You may not be charging well. I have seen stators that will only charge at a certain RPM. It's very odd but you may this issue or a bad leg on your rectifier allowing a bit of AC into your system OR only charging at a higher RPM off the two remain legs/phases of your charging system.

• Third - It's not your transmission. If you had aluminium debris in your crankcase the transmission would grind it up nicely, put it in your oil, where it would get filtered out with your oil filter. Two things, change and disembowel your oil filter, check it for debris. Drain your oil and filter it to see if you have debris. Either way, positive or negative, a bad transmission would not reflect the symptoms you are experiencing.

• Fourth - More than likely not your carbs unless you have an automatic choke. Your choke could be staying on and making your AFR too rich. Check to ensure your manual choke is getting all the way turned off. Take off your gas tank and operate the choke cable, if you have one. Validate that there are not wires or other cables interfering with the full closure of your choke. If it's an automatic choke, again, check to ensure it's closing all the way when the vehicle heats up. Be sure to rev it a tiny bit to make sure the automatic portion of the choke slips to a leaner setting. You will see what I am talking about when you inspect this portion of your carbs.

• Fifth - Fuel tank, after riding and the tank is hot on low fuel so gasses can really build up in it and it can get hot from the engine running, open the gas tank. Do you here a sucking or positive pressure release sound? If you do, that could be the issue.

• Sixth - Be patient with the troubleshooting. Do the work when you are not under pressure to have it running again the next day for a work commute. Start on Friday night if you have the weekend off. You have the whole weekend to get it right then.

Helpful Hints

• Test your stator and rectifier

• Test your battery under load

Answer 2

Check the float chamber and inlet filter on the carb fuel inlet, and have a look at the manifold to carb connection for an air leak.

Answer 3

If it starts lovely and doesn't stop, then it'll be too rich air/fuel mix, which means choke or air filter issue. If it is difficult to start, it overheats, or just stops for no reason, it'll be a dirty carb (main jet) or an air leak between a carb and head. Check any rubber cracks, gaskets, any 'oily' areas.. If not, I'd change a spark plug lead. But it would be the last thing I'd do.