Hua Sheng lost power after engine came to a sudden stop

[abikerider]

Hi,
I am looking for answers to a problem with my Hua Sheng. I have it paired with a firebelly transmission. When I first installed it the idle was pretty smooth and the bike topped out at 35 mph on the flats. There were some initial issues with the firebelly transmission which caused it to break it's primary chain a few times. That issue has been resolved with better sprockets. The problem with the engine occurred the last time the transmission's primary chain broke and jammed, and caused the engine to stop abruptly. Now it seems to have less power, idles roughly and only makes it to 28 mph. I have since replaced the spark plug, adjusted the valves, inspected the carburetor, and changed the oil but with little improvement. There is also a new buzzing vibration that occurs around 21 mph which I don't remember being there before. My question is, what could cause this sudden drop in performance? Could the cam gears have skipped a tooth and changed the cam timing? Could the crank have gotten twisted and caused a shift in the cam timing while also causing the new vibration? Any other ideas?

Also, is there a service manual available in case I want to start disassembling the engine for a closer look? Maybe I should just buy a new engine? Any advice is appreciated. Thanks.

 

[tyrslider]

1st off, to be fair, I want to clarify a little. There was an issue w/ the sprocket that mounted to the output shaft of the motor. It was rated for the loads we put on these things but was clearly not up to the task, as it broke. As promised I fixed the issue at my expense. The second time was because the gaskets that you made were added w/o taking the spacing into consideration and adjusting for it. These are the incidents that I know of.

I don't think that your motor is damaged. There's something else going on. People stall there motors out all the time by dumping the clutch. If you "twisted" the crank o/p shaft it would not likely stay even remotely true. Cam gear shouldn't be capable of skipping a tooth. I suppose it could, though unlikely, break teeth. If it broke teeth and changed the timing then every time the gear rotated it would likely continue to skip more teeth and continue to go more out of time. I'm not positive on this motor but I don't think it would idle if it were a tooth off.

Hows your fuel filter? These gas tanks notoriously dump crap into the fuel line. Sometimes things seem to be connected and are not! I'm not saying they aren't, just saying consider that they might not be.

As always keep me posted and feel free to call and I'll help in any way I can!

 

[GearNut]

While I have not worked on these engines, every small engine I have ever worked on has the flywheel keyed to the crankshaft. This key (woodruff key) insures that the flywheel magnets used to operate the ignition system are properly timed to the ingition cycle. If the engine is suddenly stopped by means of the output shaft, the inertia of the flywheel causes it to continue turning even though crankshaft has stopped. This causes the key to shear. It is designed to shear in an effort to prevent the flywheel's inertia from twisting and thusly damaging the crankshaft.
After this occurs the fltwheel's magnets are no longer in time with the ignition cycle.
This must be corrected before the engine can operate properly. A new woodruff key is definately necessary.

 

[abikerider]

Let me say that I am not looking for blame here, I just want to try and figure out why the engine is behaving the way it is. The only event that seems to preceed the engine losing power is the chain breaking the second time after the new sprockets were installed. The reason the chain broke the second time seems to be because the spacer on the end of the crankshaft came loose, probably because I used blue locktite when assembling it even though red was recommended, my bad. This had nothing to do with the gasket I made and the recommended shims which I used. The loose crankshaft spacer seems to have caused vibration in the chain which rattled the masterlink enough to cause its locking clip to come off. Tyreslider was kind enough to send me a replacement spacer, bolt and bearing after I discovered that they were loose and worn. After assembling the new parts with red loctite there have been no further problems with the chain.

My guesses for how the chain breakage could cause the engine problem are just that, guesses. I am just asking for help from someone with more knowledge and experience with small engines because I have run out of ideas. As far as the fuel system goes, I was very meticulous in cleaning my tank. I even used electrolysis to remove rust deposits that were in the tank. I also have a large paper element see-through fuel filter that I installed initially and which does not show any dirt. When I turn on the fuel valve after the bike's been sitting a while I can see the fuel flow in the filter. I also took off the carburetor and checked the passageways for any cloggs with compressed air. I suppose I can drain the tank and remove the fuel shutoff valve to see if its screen is clogged just to be sure. I have tuned-up the engine with a new NGK spark plug and valve adjustment. The compression is very good when I pull start it and sometimes wants to pull out of my hand if I don't get it to the beginning of the compression stroke before yanking it.

I appreciate Gearnut's advise and will check to see if the key for the flywheel has been damaged when I get a chance. Considering the problems others have had with keys shearing on these chinese kits it doesn't seem far fetched that mine is damaged. If it is this I owe Gearnut a beer.

 

[thatsdax]

Hi.. The Cam Gear that drives the Cam may have been damaged. The Centripetal forces of a large crank being stopped suddenly could have damaged the teeth and the sync of the Cam. The Cam gear is nylon and may have been damaged. The only way to know is to visually inspect it. Since it is nylon, it may have spun, this could cause interference. If this is the case, then you will have bent push rods. This is easier to inspect. Remove the valve cover.. Push down on the rockers and remove the push rods. Roll them on a sheet of glass to see if they are bent. Enjoy the ride....

 

[tyrslider]

This is a non interference motor (6:1 compression). If the cam gear is nylon then it is less likely the problem! Good looking out Gearnut, I think your idea is most likely thus far.

Abikerider also not looking for blame but trying to be fair to my product and own any problems that are mine. Most of all I'd like to get you back on the road!. I'm following what's going on and trying to come up w/ any help I can. Thanks.

 

[abikerider]

WHOOHOOOOO! I found the problem! Gearnut you were right! I removed the flywheel and found the sheared woodruf key which I've enclosed a picture of. This has been bugging me a while now and it's great to know the problem is solved. Gearnut, I definitely owe you one and thanks tyrslider and thatsdax for your input as well. Now, where do I find a 3mmx5mmx13mm woodruf key? I guess I'll try my local small engine dealer first. I hope this helps others with similar problems in the future. Thank you , thank you, thank you!

 

[tyrslider]

That's Awesome! These motors really are well designed and tough little buggers!

 

[thatsdax]

Great !! you found the problem !! ACE Hardware. You can get woodruff keys there. They have a big selection. Enjoy the ride...

 

[abikerider]

Well it seems the local Honda small engine shop is reluctant to order a replacement key as he thinks it will not be the same size as the Honda. I went to the local hardware store and found a close match and after some light filing it fit in nice and snug. Now my next question, what should the magneto gap be set to? I want a nice powerful spark!

 

[abikerider]

One more question, anyone know what torque the nut that holds the flywheel needs to be tightened to?

 

[GearNut]

I am glad to have helped you.
Well done grasshopper! You are one step closer to being a master mechanic.
I usually slice up a piece of a 2 liter bottle and set it over the magnets on the flywheel. Then I set the magneto over it and let the magnetic force hold everything nice and tight as I tighten down the magneto screws. Doing it this way insures that the magneto is nice and close to the magnets as well as properly aligned, down"to a gnat's a$$", to the flywheel. Afterwords I give the piece of 2 liter bottle a good tug to get out of there and I am done.
As for the torque, I know I have seen a very comprehensive list for all the fastner sizes here. Do a search for them or wait. I am sure someone else here knows what it should be.

 

[wog383]

I found the honda GXH50 magneto gap setting. It's 0.3mm to 0.5mm or 0.012 inch to 0.020 inch.
Mine was making a backfire at high rpm and making heavy carbon build up on my spark plug no matter how lean or rich I set it.
With the magneto gap in spec, its way responsive and a far smoother idle.