Motorcycle Repair: Engine Bogging Down, honda ct 70, oem honda parts

Question
Hello,

 I'm hoping you can help me with a situation I'm having with my bike, a 2004 Pagsta.  This bike apparently has a 97cc replica of a Honda CT 70 mini cycle engine.
 What happens is sometimes when I'm cruising along at sustained speeds of 40-45 mph, the engine bogs down, as if it isn't getting any fuel.  So, I pull in the clutch lever and start feathering the throttle until it "comes back to life."  And then it does it again and again, making for tedious riding at these speeds.  It seems to run fine at slower speeds.
 Thank you very much, and any information you can give me would truly be appreciated.

Steve

Answer
Steve, checking with the PAGSTA website, it seems the Chinese are making duplicates that are not DOT legal. Their quality control and engineering and materials are often not as good as OEM Honda parts, so you can expect most anything to go wrong in these applications.

When a bike engine "bogs down" it is either having some kind of electrical interruption, a fuel starvation and/or as a result of the mentioned items, a piston seizure.

If the bike wasn't broken in properly, not tuned well or has contamination of the fuel system, usually the piston will seize in the cylinder bore. Signs of this happening are metal particles on the spark plug tip and new noises coming from the middle of the top end of the engine.

If the bike was modified from the original specifications by changing air filter or exhaust systems, then the carb jetting can be too lean, which causes overheating and piston seizures again.

Start at the beginning...  Get the compression checked, adjust the valve clearances to .002" cold on the compression stroke. Verify that the fuel tank is venting properly (can cause vapor lock of the fuel when the vent system isn't working), Determine that the float level and the main jet size is correct for this application and try to verify the ignition timing. If the bike has a CDI the ignition timing is pretty much preset, though.

Check for air leaks at the intake manifold (gaskets and O-rings) and be sure that the bolts are snug.. not TOO TIGHT or they will strip.

If you see metal on the spark plug or can look down into the plug hole and see damage to the cylinder wall or piston crown, then it is time for a teardown and rebuild with an oversized piston. When you are doing all this, you need to figure out why it did what it did....  and then break it in slowly, per the owner's manual suggestions. Keep the oil FULL to the upper mark on the stick.

Get the service specs on all tuneup related items from the distributor or dealer.

Bill Silver