Engine Knocking Sound After Sprocket Change – Need Help!

bikeman1212

bikeman1212

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Hey everyone, I’m dealing with an issue and could really use some help figuring out what’s causing it!

Here’s what’s going on: I was running a Huasheng motor with a belt-driven transmission and a 44-tooth rear sprocket, and everything was working great. I recently swapped to a smaller 33-tooth sprocket (or something close to that size). Since the switch, I’m hearing a strange knocking sound coming from the engine when I accelerate. I’ve put around 20 miles on it since making the change, but it doesn’t feel right, and I’m worried that if I keep running it like this, it could end up damaging the engine. I’m not sure if the problem is with the engine itself or the transmission.

I suspect the issue might be due to the extra strain on the drivetrain, since a smaller sprocket would mean less torque, right? My goal was to get a higher top speed without having the engine constantly revving at 6500 RPM.

What’s strange is that the knocking sound is worse when going uphill. But when the bike is on the center stand with no load, the noise doesn’t happen. I’m really hoping this isn’t a piston-related issue or something serious with the engine. Any ideas? 😕
 
I can not imagine the sound is coming from inside your engine. It's hard to diagnose without hearing the pitch of the sound and the frequency. Like does it knock with every revolution of the crankshaft or with every revolution of the transmission output shaft? or with the rear wheel. I would check and see if the chain is hitting any part of the frame seeing as it is now in a different location. Something is hitting something and you should be able to see the contact point. With the bike off the floor can you feel a bind or hear a noise when you turn the rear wheel by hand ?
 
Clutch rattle and/or valve rattle. Your engine is designed to be most efficient running around 6500~6800 rpm. Think of it like this if you're on a pedal only bicycle you should be in a gear that you can comfortably maintain a cadence (crank rpm) of 70~90. If a gear is too easy or too hard to comfortably maintain a cadence of 70-90, it means you're in the wrong gear. If you were pedaling in a gear that was hard for you to comfortably maintain a cadence of 70-90 your heart rate would go way up.

On my shifter bike the max hp is at 7000 rpm. If I'm in a gear and doing 7000 rpm at full throttle shifting to a higher gear won't make me go any faster. It'll cause the rpm to drop and slow me down slightly because I'm no longer running at max hp rpm.
 
Jerry's explanation is exactly correct. Simply put: installintg a 36T sprocket is like shifting your car into high gear before you have sufficient speed. The engine will struggle to increase RPMs. You might eventually go faster on flat ground but your hill climbing ability will drastically go away.

But back to the issue. Why did changing the sprocket create a knock? and where is it coming from?
Clutch rattle and/or valve rattle. There is no why the valves are going to make a knocking sound by changing the sprocket. The clutch though is a definite possibility. My 4 stroke Predator with a MaxTorq clutch vibrated and rattled horiffically when it had a spring that would not allow it to engage and lock up at a lower RPM. It did not knock, but made a rattling sound in tune with the vibrations. My Rat Bike had the same Hausheng engine and belt drive as yours. Unfortunately there is no way to change anything it that clutch. My gut feeling is.........this isn't the problem.

My question again is........are you hearing a definite knock.....knock..... knock......knock, or more of a gerneral rattling noise?
 
Bikeman had other issues over the last month - clutch bushing and spring tension IDK how that was resolved
I think he also shimmed the valve springs with a washer ? possible coil bind - Could be other mods made to the engine? IDK

Knock can also come from piston slap - Timing off ? Carbon build up in combustion chamber ?
Piston sliding to the side and slapping the wall at the beginning of power stroke - because of excessive piston pin/bushing tolerance?
Worn piston rings - piston hitting cyl sidewall
Warped cyl - piston slap
Cyl boar worn alot - piston slap
Bad crank bearing or excessive crank end play - cam bearing ? Cam ?

Yeah, there could be alot of reasons for an engine knock. I just have bunch of possibilities , No answers

To narrow things down put the 44t sprocket back on and see if the engine still knocks
 
Wrench said:
Bikeman had other issues over the last month - clutch bushing and spring tension IDK how that was resolved
I think he also shimmed the valve springs with a washer ? possible coil bind - Could be other mods made to the engine? IDK

Knock can also come from piston slap - Timing off ? Carbon build up in combustion chamber ?
Piston sliding to the side and slapping the wall at the beginning of power stroke - because of excessive piston pin/bushing tolerance?
Worn piston rings - piston hitting cyl sidewall
Warped cyl - piston slap
Cyl boar worn alot - piston slap
Bad crank bearing or excessive crank end play - cam bearing ? Cam ?

Yeah, there could be alot of reasons for an engine knock. I just have bunch of possibilities , No answers

To narrow things down put the 44t sprocket back on and see if the engine still knocks
Click to expand...
He's also using a 49cc 4 stroke engine which only has 1.6 hp and 1.475 ft/lbs of torque. When a small engine is put under a high strain it can cause valve rattle. His only real hope is to build a low speed bike, get a more powerful engine, or build a shifter bike.
 
In all my years of working in an automotive machine shop and building race car engines, Jerry, I have never heard of such a thing as valve rattle. The only noise a valve will make is a clicking sound if the clearance is way too loose. Or if it falls into the cylinder and goes "BOOM!" Maybe you can further explain this. You might want to edit low speed bike to high speed.
 
michael whiteman said:
In all my years of working in an automotive machine shop and building race car engines, Jerry, I have never heard of such a thing as valve rattle. The only noise a valve will make is a clicking sound if the clearance is way too loose. Or if it falls into the cylinder and goes "BOOM!" Maybe you can further explain this. You might want to edit low speed bike to high speed.
Click to expand...
Clicking, rattling it can usually be heard right around the exhaust when the engine is under a large strain. Max torque on the Hus 142 is at 4500 rpm. If you're at full throttle and the engine can't do at least 4500 rpm then engine strain will develop. You are correct in when it gets to this point your engine is about to blow.

I don't need to edit. If he plans to keep his current engine using a single drive ratio then he's going to have to settle for lower speeds in order to have hill climbing abilities. Otherwise for more speed, he can build a shifter bike using the current engine or get a more powerful engine if he plans to stay single drive ratio.
 
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