High RPM Knocking!?

dylandiel

New Member
Aug 19, 2012
8
0
0
Belmont, CA
when i get into higher speeds on my 80cc beach cruiser, I hear a fast knocking sound and would like to know if I should be worried about that? or what is causing it?
 

maurtis

New Member
Dec 14, 2011
707
0
0
Kyle, TX
If it is a rat-tat-tat-tat burst, it might be 4 stroking. I had 4 stroking at high RPM in short bursts, maybe 1s long or so per burst, which was also accompanied by a temporary loss of power. When I went to a slightly leaner jet, the 4 stroking went away.

If that is what you are experiencing, it will not cause any real harm, just means your tuning is just a bit off.
 

dylandiel

New Member
Aug 19, 2012
8
0
0
Belmont, CA
with the bike off. i can jiggle the chain and it sounds like that might be it.. but when i give it full throttle, it will have that knocking sound and jerk with power then it will feel like its not giving its all. I feel like my bike is not giving its full potential...

I just replaced the carburetor. new fuel..

im not sure that the spark plug is the right one. because i bought if for my previous 49cc bike that got stolen so i used it on my new 80cc bike.
 

GearNut

Active Member
Aug 19, 2009
5,104
7
38
San Diego, Kaliforgnia
It is not my video, just a good one I found on youtube.

1 gallon of gas to 4oz of oil is a 32:1 ratio of mixed fuel.
32:1 is an excellent ratio to use for petroleum pre-mix oil and an engine that has been broken in.
If your engine is still new (less than 500 miles) you should use a 25:1 ratio.
5.12oz of oil to 1 gallon of gas.

Rich is relating to how much fuel the carburetor adds to a given volume of intake air.
Rich = too much fuel.
Lean = not enough fuel.

A basic starter for beginning to tune the carburetor:
To lean out the mixture raise the clip on the slide needle. Doing this lowers the needle into the needle jet on the floor of the carburetor's throat and reduces the fuel added into the incoming intake air.
Make this change one needle notch at a time until it runs better.
After the engine runs good, you need to do a plug chop to determine if the mixture is not too lean.
Operating an engine with the carburetor tuned too lean can quickly do serious, permanent damage.
This is why you do the adjustments one needle notch at a time to sneak up on the perfect tune, not jump a couple of notches at one go.

Again, this is just a basic starter to begin tuning an NT carb.
NT carbs are stone simple to work with if you are willing to learn.
http://motorbicycling.com/showthread.php?t=302

http://www.google.com/cse?cx=partne...gsc.tab=0&gsc.q=tune NT carburetor&gsc.page=1

This is also an excellent read:
http://www.motoredbikes.com/showthread.php?10392-The-Crash-Course-for-MotoredBike-N00Bs
 
Last edited:

dylandiel

New Member
Aug 19, 2012
8
0
0
Belmont, CA
It is not my video, just a good one I found on youtube.

1 gallon of gas to 4oz of oil is a 32:1 ratio of mixed fuel.
32:1 is an excellent ratio to use for petroleum pre-mix oil and an engine that has been broken in.
If your engine is still new (less than 500 miles) you should use a 25:1 ratio.
5.12oz of oil to 1 gallon of gas.

Rich is relating to how much fuel the carburetor adds to a given volume of intake air.
Rich = too much fuel.
Lean = not enough fuel.

A basic starter for beginning to tune the carburetor:
To lean out the mixture raise the clip on the slide needle. Doing this lowers the needle into the needle jet on the floor of the carburetor's throat and reduces the fuel added into the incoming intake air.
Make this change one needle notch at a time until it runs better.
After the engine runs good, you need to do a plug chop to determine if the mixture is not too lean.
Operating an engine with the carburetor tuned too lean can quickly do serious, permanent damage.
This is why you do the adjustments one needle notch at a time to sneak up on the perfect tune, not jump a couple of notches at one go.

Again, this is just a basic starter to begin tuning an NT carb.
NT carbs are stone simple to work with if you are willing to learn.
http://motorbicycling.com/showthread.php?t=302

http://www.google.com/cse?cx=partne...gsc.tab=0&gsc.q=tune NT carburetor&gsc.page=1

This is also an excellent read:
http://www.motoredbikes.com/showthread.php?10392-The-Crash-Course-for-MotoredBike-N00Bs

Thank You very much. this information is really helpful. I will give it a try and respond back.
 

dylandiel

New Member
Aug 19, 2012
8
0
0
Belmont, CA
you guys are awesome lol.. my bike runs great now. Thanks. I move the clip up one notch to lower it and there is no more 4 stroking. clean idle and smooth acceleration. Thank You
 

GearNut

Active Member
Aug 19, 2009
5,104
7
38
San Diego, Kaliforgnia
I had to tune to the extreme. Solder the Jet shut and redrill it to a smaller Jet number and keep doing that till i got it right.
This procedure is very commonly needed and I am glad to hear that you were willing to keep at it until the jet was sized correctly.
Some folks end up pulling their hair out and quit after a few tries.
 

MeniacleMechanic

New Member
Sep 4, 2012
16
0
0
Virginia Beach Virginia
This procedure is very commonly needed and I am glad to hear that you were willing to keep at it until the jet was sized correctly.
Some folks end up pulling their hair out and quit after a few tries.
lol, thanks man. it works like a champ. I build cars and do alot of custom fabrication work anyways. So its nothing to keep at it till its right.

I did end up running it a little lean at one point and that gave me issues in the long run. Fixed now tho.