Engine Bogging Down

Wondering if anybody can help me with my engine bogging down at top engine RPM. The problem just started this past week and I have not been able to figure this one out. On WOT the engine screams to top RPM, then the engine bogs down to about half the top RPM, then cycles back up to top RPMs. This will continue as long as I keep the throttle wide open.

My top speed has gone from about 22mph to 15mph and I have very little torque

I have gone through forums to find anything I may be missing. Here is what I've found, checked and replaced

Engine: 49cc China engine
Gas Mixture: 32:1
Engine Mileage: ~300miles
Spark plug: NGK B6HS (Brand New), gapped .025
Spark Plug wire: Upgraded to auto coil wire
Old Spark Plug Color: Black (running rich)
Fuel Petcock: None, threaded fitting --> external filter --> carb
Carb: Stock (not NT), C clip moved from 2nd notch to 1st notch


I haven't been able to check compression due to lack of equipment and I have not been able to verify CDI but I never have trouble starting, keeping the engine running.

Two things I notice
1. Dark fuel has been dripping from the exhaust after stopping the engine
2. Fuel is dripping from the air filter after a run

So yes I know the engine has been running rich. But I'm wondering if the engine is being starved of fuel at top speed. My intake/gasket have been ported to allow more fuel at high speed.

Any help is appreciated

Other information that may be helpful

Problem occurs both while bike is stationary with engine on and while the bike is moving.


I'm walking through different scenarios in my head. If anyone is willing to help me do a sanity check or add more scenarios to check that would be great.

Too much fuel being supplied by Carb: If there was too much fuel being supplied the engine would sputter and bog down. But the problem would be more likely to occur at all RPM ranges.

Not enough fuel being supplied: Engine would run fine at idle but would cut power at the high end of RPM range.

Air leak: Engine would run rough at idle but would be even more pronounced at WOT. Would affect all RPM ranges

Latest update

- Tried opening gas cap, no improvement but not worse either

- Carb is stock with a Round Black Filter housing (not NT, or CNS). I have tried running without a filter for a few minutes to see if improved airflow solves the problem. No luck, but I've drilled some holes in the cover to increase airflow anyways.

- Wiped all areas with gaskets, will check for any leaks/residue

- I have an new extra carb lying around. I'm thinking of tossing that on to see how that changes the performance.

Update:

- I had a extra carb from another build lying around. I popped that on and the engine screams now. However I can still feel it surging. I may have two different problems and the new carb fixed one of them. Haven't checked the plug yet to see if its running less rich. My torques and top end power seem to be where they should be now.

- The surging is better but not gone

- Good idea with the fuel filter. I haven't changed my fuel filter for a very long time Shameful I know but the fuel flows perfect every time I drain the tank and I don't see any debris because its a clear filter. I guess there could be dust clogging it that I can't see.

- The foaming issue something I haven't checked yet. I don't think the engine vibrates more than normal although it vibrates the bike fuel filter enough that the fuel jumps all around at WOT. It seems reasonable that the same is happening in the carb. I'll do a check on the mounts this weekend.

http://motorbicycling.com/archive/index.php/t-5417.html

- The prime tickler on the old carb was leaking everywhere. Im thinking this didn't help anything and I'm suspicious that the gasket wasn't doing a great job.

Over the weekend I did discovered that my mounting bracket sheered in a location that was barely visible and the location of the break was in a place that still supported the engine enough to keep it from appearing loose but probably vibrating like crazy at WOT.

I replaced my bracket with one double the thickness. I've run into a chain length issue but hopefully I'll be up and running to give a report on if a good motor mount fixes the problems.

Oh and thanks for the info on NT carbs. I was under the impression the NT carb was the carb with the square air filter

Hey all, thanks for all the help.

I replaced the broken motor mount and the engine runs great without any bogging down.