China Motor with HP Carb Barely Running Only With Choke Off

Philip A.

New Member
Feb 14, 2021
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I have a basic Chinese motor. recently, I upgraded from my leaking NT carb to an HP carb. I put the new carb on and go for a ride, it runs great. the next time I ride I heard A lot of clicking, coming from around the piston cap. I bought a new piston cap gasket and double stacked them, I was already having some compression issues with it because I'm unable to get one of the bolts down so tight. going on my test runs I started with choke on and I could hear my piston physically turning, but it had zero power. the throttle cable in both the handle and the carb ends are functioning fine, I believe I have the idle screw tuned right but I'm not familiar with what this carb likes. while I was riding with choke on/halfway, I noticed that really compressed air was shooting out of my carb, shooting in bursts consistent to the throttle I was giving it. I immediately bring my bike home and take off the cover of my carb to see black oil and gas splattered around. I take off the carbs bowl, the gas inside is still pure, and rinsed my jet (I'm unsure of my jet size, I have heard people take the jet from the NT carb and put in in the HP, But I haven't tried.) the jet and needle going up looked clean but I gave it a few blows (pause) anyway. I check carb to manifold, manifold to motor, motor to exhaust, all gaskets and ports are fine (one thing I didn't check was the end of my exhaust.) At this point I still cant get power from my bike. I then took off the motor cap, spark plug is a good chocolaty/mocha color, so it the top of the piston but lighter in color. I then took off the cylinder housing (I don't know proper terminology, But the piece of metal that has exhaust/manifold ports) everything looked clean, debree free. I then rolled my bike with the cylinder housing on, cap off, and saw that my piston seemed to be working fine pumping up and down.( Something I did notice but I don't think matters was that the piston head would get low enough to reveal the exhaust port when looking from the inside, but never gets low enough to show the carb/manifold port.) I put the cylinder housing and cap back on and go for another test ride. keep in mind, I didn't actually do anything when I had the cylinder housing off, I just took it off, inspected, and placed it back on. all the sudden, my bike actually gives me a little power, just enough to get 10-15 mph's, which is way better because before I was getting nothing, and the pressurized air stops shooting out of the carb. I am only able to get it to give me power with choke fully off, if chokes on it bogs down/has zero power.

I have no idea what's wrong. I thought about the spark plug being bad but it worked before I had the HP carb on and for a single ride with it on, can they go bad that fast? I read a little into someone's similar issue and they discussed how piston heads can blow air back through the carb which is what I was experiencing, I just couldn't really understand what they were talking about in what types of carbs work for whatever types of piston heads. I have experienced gas in the magneto case, but not very recently and with the NT carb. I have no Idea what's keeping my engine from giving me power. the few things I can think of is that there's still scuffs/debree in the cylinder, I dont have a piston head that matches with the functionalities of my HP carb, the spark plug or cdi isn't functioning properly. I haven't tried putting the NT carb back on but I really don't want to use it. Does anyone think they got any advice to help me out? i really want my bike to work.
 

wrench

Active Member
Aug 20, 2019
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So Cal
I was already having some compression issues with it because I'm unable to get one of the bolts down so tight.
Gotta fix that first. A compression leak will cause all the problems your talking about.
If ya got the acorn nuts change em to regular nuts
The acorn nuts might be running out of threads (bottoming out) and preventing you from tightening it up all the way

Hope ya haven't warped the cyl head, ya better check that.

If it is warped, Put a piece of 220 sandpaper on plate glass and sand it flat again

If ya don't have any plate glass you can use any known 100% flat surface
 

wrench

Active Member
Aug 20, 2019
212
99
28
So Cal
Even if your not sure if the head is warped and want to make sure the head is flat.
First mark up the sealing surface of the head with a black sharpie pen.
While sanding check your work and you'll be able to see any low spots.
Sand until all the black sharpie marks are gone. Now it's perfectly flat again.

You can start with 120 to speed up the process but switch to 220 to finish
Sandpaper has to be fresh and new with NO wrinkles in the paper
 

Philip A.

New Member
Feb 14, 2021
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Thank you for the fast reply wrench, I apologize for not getting back to you sooner.

I have not been a fan of these acorn nuts, having an excuse to buy some different ones sounds great. im only having an issue with one of the bolts, and its because I stripped the threads it goes it to completely. I don't have a tapping kit, I am guessing my best bet at getting that fixed is using some JB weld? what do you think?

sanding down my motor cap sounds like a great idea, and ill start that project soon.

In the mean time, I've gotten a consistent 10-12 MPHs with choke off. I disassembled my motor again, now whenever I try giving it some upper mid - high throttle ill get a nice pop. I cant associate this noise to anything good or bad, I can just tell that its coming from around the motor cap, I believe the issue with pressurized air shooting out my carb has stopped, but not 100% sure. let me know of any other fixes you might recommend

Thanks.
 

Philip A.

New Member
Feb 14, 2021
8
0
1
22
Gotta fix that first. A compression leak will cause all the problems your talking about.
If ya got the acorn nuts change em to regular nuts
The acorn nuts might be running out of threads (bottoming out) and preventing you from tightening it up all the way

Hope ya haven't warped the cyl head, ya better check that.

If it is warped, Put a piece of 220 sandpaper on plate glass and sand it flat again

If ya don't have any plate glass you can use any known 100% flat surface
 

wrench

Active Member
Aug 20, 2019
212
99
28
So Cal
I don't understand what ya stripped is it the stud .
Thank you for the fast reply wrench, I apologize for not getting back to you sooner.

I have not been a fan of these acorn nuts, having an excuse to buy some different ones sounds great. im only having an issue with one of the bolts, and its because I stripped the threads it goes it to completely. I don't have a tapping kit, I am guessing my best bet at getting that fixed is using some JB weld? what do you think?

sanding down my motor cap sounds like a great idea, and ill start that project soon.

In the mean time, I've gotten a consistent 10-12 MPHs with choke off. I disassembled my motor again, now whenever I try giving it some upper mid - high throttle ill get a nice pop. I cant associate this noise to anything good or bad, I can just tell that its coming from around the motor cap, I believe the issue with pressurized air shooting out my carb has stopped, but not 100% sure. let me know of any other fixes you might recommend

Thanks.
Replace the stud if the studs striped
You must fix the head bolt before ya can tell if something else is wrong
 

Philip A.

New Member
Feb 14, 2021
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I don't understand what ya stripped is it the stud .

Replace the stud if the studs striped
You must fix the head bolt before ya can tell if something else is wrong
what's stripped is the hole under the cylinder that the bolt would go into, not the top of the bolt. I have seen the inside smoothed over and had to clean the broken threads out of the hole.
 

wrench

Active Member
Aug 20, 2019
212
99
28
So Cal
Sorry to hear this,
The threaded hole in the case for the stud is striped. JB weld will not fix that.
I forget do you have 6mm or 8mm threads? New threads can be installed
Your gonna have to put a HeliCoil in that.
This requires buying a Helicoil kit $30 It comes with about 9 Helicoils and all tools needed.
A Helicoil is new threads, The stripped hole is drilled and tapped for the Helicoil Ya use their tool to screw in a coil to make new threads.

Good thing these motors are cheep Most people just buy a new motor.
 

curtisfox

Well-Known Member
Dec 29, 2008
5,371
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minesota
If you do use Heli Coil make sure you drill straight, and the motor top you call a cap is called a head, best.........Curt
 

Philip A.

New Member
Feb 14, 2021
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0
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Sorry to hear this,
The threaded hole in the case for the stud is striped. JB weld will not fix that.
I forget do you have 6mm or 8mm threads? New threads can be installed
Your gonna have to put a HeliCoil in that.
This requires buying a Helicoil kit $30 It comes with about 9 Helicoils and all tools needed.
A Helicoil is new threads, The stripped hole is drilled and tapped for the Helicoil Ya use their tool to screw in a coil to make new threads.

Good thing these motors are cheep Most people just buy a new motor.
My bike uses 8mm bolts

2 of the bolts/acorn nuts are detachable and in good shape, the other two bolts/nuts are bound together, one of the case holes the bolts go into is stripped.

I'm gonna try to free the bolts with a vice and some grips. if they are too damaged, ill order new ones. I'm also gonna trash all the acorn nuts and replace with whatever heavy duty hex nuts I can find. (do 8mm studs take 14mm nuts?)

The retaining Allen head screw on the carburetor --> manifold and a single manifold --> cylinder screw have a rounded out head. I'm also going to try backing them out with grips but they have a circular head. if I'm not able to take my carburetor off I wont be able to lift the cylinder entirely off the case. it makes enough clearance to apply JB weld, but not enough to fit in a drill or tap to insert a Heli Coil.

The best path I can take is using a Heli coil if I'm able to get the clearance I need for a drill and tap, and once I get that set up with hex nuts and a sanded down motor head I should be in a better situation.

The nuclear path is not getting carburetor or manifold off and having to apply JB weld ( I've seen videos of its application working, and I don't see why it wouldn't work in my case.) Id still use Hex nuts instead of Acorns but instead of sanding down the head id just buy a new head. I don't want it to go that way but if I have to I will.
 

wrench

Active Member
Aug 20, 2019
212
99
28
So Cal
Double nut the stud so ya don't have to clamp vice grips on it to get the acorn nut off.

I suggest removing the motor from the bike and take the cyl off to repair case threads
You are not gonna repair the case threads with the cyl on.


TIPS: Rounded out allen head bolts
Socket head allen bolts ya can grab with vice grips
Button head allen bolt if ya cant grab it with vice grips. Easy out works Or take a Dremell tool with cut off wheel and put a slot in the head so ya can use a flat head screwdriver to remove the bolt. If that fails shave the sides of the button head so ya can clamp on some vice grips.

I don't understand why you would be afraid of rubbing a cyl head on sand paper to check and see if it's flat. While your waiting for your new head to come in the mail resurface the old head and learn how to do that.

BTW the same method is used to resurface many other Items
Welcome to working on small engines
 

Philip A.

New Member
Feb 14, 2021
8
0
1
22
Double nut the stud so ya don't have to clamp vice grips on it to get the acorn nut off.
Double nut the stud so ya don't have to clamp vice grips on it to get the acorn nut off.

I suggest removing the motor from the bike and take the cyl off to repair case threads
You are not gonna repair the case threads with the cyl on.


TIPS: Rounded out allen head bolts
Socket head allen bolts ya can grab with vice grips
Button head allen bolt if ya cant grab it with vice grips. Easy out works Or take a Dremell tool with cut off wheel and put a slot in the head so ya can use a flat head screwdriver to remove the bolt. If that fails shave the sides of the button head so ya can clamp on some vice grips.

I don't understand why you would be afraid of rubbing a cyl head on sand paper to check and see if it's flat. While your waiting for your new head to come in the mail resurface the old head and learn how to do that.

BTW the same method is used to resurface many other Items
Welcome to working on small engines
Thank you Wrench I appreciate you're input, It will help me out a lot.

what do you mean when you say double nut the stud? where would I place them and how would they help me get a solid grip?
 

wrench

Active Member
Aug 20, 2019
212
99
28
So Cal
Double Nut - Tightening one nut against another nut is a common way to install and remove studs.
This avoids damaging the threads of the stud with pliers
 

wrench

Active Member
Aug 20, 2019
212
99
28
So Cal
youve either got vacuum leak probably in crankcase or piss poor compression.
Philip already figured this out, It's a compression and vacuum leak
The Head/Cyl stud striped out the threads in the case
He's gonna put in a HeliCoil
 

Philip A.

New Member
Feb 14, 2021
8
0
1
22
Sorry to hear this,
The threaded hole in the case for the stud is striped. JB weld will not fix that.
I forget do you have 6mm or 8mm threads? New threads can be installed
Your gonna have to put a HeliCoil in that.
This requires buying a Helicoil kit $30 It comes with about 9 Helicoils and all tools needed.
A Helicoil is new threads, The stripped hole is drilled and tapped for the Helicoil Ya use their tool to screw in a coil to make new threads.

Good thing these motors are cheep Most people just buy a new motor.
Hey Wrench,

Ready to go out and buy my Heli coil set, I'm having trouble knowing what size/type to buy. What I think I need is 1.0 pitch, M8 fine thread for the case. does this sound right? I don't know the visual difference between fine thread/normal thread so I don't know what what my bolts would call for. I also don't know for sure what my proper pitch would be between 1 and 1.25.