Bike revs high at slow speeds

C

Carson2000

New Member
So I completed building my first motorized bicycle. The problem I noticed is that at slow speeds like 10 to at most 15mph the bike revs at high rpms. I'm am wondering if it is the carb I need to adjust and if it is the carb what adjusting should I be doing. The carb is a NT speed carburetor

Thanks, Carson
 
Last edited:
Sounds like your clutch is slipping to me. Kind of like you are accelerating then all of a sudden it revs really high?
 
YesImLDS said:
Sounds like your clutch is slipping to me. Kind of like you are accelerating then all of a sudden it revs really high?
Click to expand...

Well not exactly. I just have a very short acceleration period. It just will gradually rev high at about 12 mph and I don't want to keep going cuz I'm afraid I will burn the engine. This a brand new engine so I'm in the break in period and I did notice I have a rich mixture.
 
If the throttle cable is adjusted correctly and not binding I have to say your symptom sounds like an air (vacuum) leak somewhere downstream of the carburetor. The carb must be tight where it attaches to the intake manifold and the manifold must be sealed well where it attaches to the cylinder. These are the two most common areas with a new engine/install. A fast idle, or faulure to return to a slow idle is a symptom of excess air entering the engine causing a lean fuel/air mix.

As for being a "rich mixture". This is also very common with a new engine that isn't broken in and a stock carburetor that hasn't been tuned or jetted. I wouldn't be concerned until the engine has about 200 miles on it. Then you can start tuning for better performance.

Tom
 
2door said:
If the throttle cable is adjusted correctly and not binding I have to say your symptom sounds like an air (vacuum) leak somewhere downstream of the carburetor. The carb must be tight where it attaches to the intake manifold and the manifold must be sealed well where it attaches to the cylinder. These are the two most common areas with a new engine/install. A fast idle, or faulure to return to a slow idle is a symptom of excess air entering the engine causing a lean fuel/air mix.

As for being a "rich mixture". This is also very common with a new engine that isn't broken in and a stock carburetor that hasn't been tuned or jetted. I wouldn't be concerned until the engine has about 200 miles on it. Then you can start tuning for better performance.

Tom
Click to expand...

Thank for the info. I'm guessing you have experience with these engines. My question is it normal to have an engine running the way I said with it being a new engine. Also I think I may have a rich mixture because the exhaust was very smokey and thick. The engine itself also was starting to smoke while the bike was idling (wasn't moving) after 30 seconds.
 
Make sure you are running a mixture of 24:1 maximum for breakin, though i always ran 32:1. 40:1 after break in.

Do not use the 16:1 mixture the 'instructions' suggest.

If the engine speed is very high with the clutch pulled in, check the things that 2Door mentioned, and keep us posted with your results.

Did you try turning down the idle screw, and physically looking in the carb throat to confirm that your slide is coming back down all the way when you release the throttle?
If not, besides the idle screw, check the cable adjustment where it goes in the top of the carb, it should be just loose, like 1/32" of free play.
 
Last edited:
Bikeguy Joe said:
Make sure you are running a mixture of 24:1 maximum for breakin, though i always ran 32:1. 40:1 after break in.

Do not use the 16:1 mixture the 'instructions' suggest.

If the engine speed is very high with the clutch pulled in, check the things that 2Door mentioned, and keep us posted with your results.

Did you try turning down the idle screw, and physically looking in the carb throat to confirm that your slide is coming back down all the way when you release the throttle?
If not, besides the idle screw, check the cable adjustment where it goes in the top of the carb, it should be just loose, like 1/32" of free play.
Click to expand...


Thanks I will. What you said about the throttle may be true. When I finished the throttle assembly I felt to make sure the throttle moves right and I noticed I don't have a lot of throttle movement. Maybe at most 1 1/2" of movement. I just feel there should be more. The spring in the carb is already compress a bit from what I think lack of cable.
 
I'm lost here - no idea what OP means by 'revs high' - lots of smoke when rings haven't seated yet is normal - if clutch is pulled and engine takes off at high revs, that's an air leak, if engine revs higher while bike doesn't speed up, that's clutch slipping - what this guy has could be anything without more info.
 
crassius said:
I'm lost here - no idea what OP means by 'revs high' - lots of smoke when rings haven't seated yet is normal - if clutch is pulled and engine takes off at high revs, that's an air leak, if engine revs higher while bike doesn't speed up, that's clutch slipping - what this guy has could be anything without more info.
Click to expand...

Agreed, but it sounds like you got the bases covered for both right here in this post.
 
It's a very simple subject of what I'm trying to say here - Sorry I'm not providing enough information. What happens is I start riding the bike on flat ground and it only reaches about 12mph with high rpms. At that point I can't go any further without intense rpms
 
Last edited:
My guess? Clutch cable is too tight. The clutch isn't fully engaged because of it and slipping.
Typically you don't see a clutch this far out of adjustment from the factory so that takes us to cable adjustment.
Whatever you do, don't start adjusting the clutch 'flower nut' until you have a complete understanding of what that adjustment does, and why.

Here's some help to see if this is your problem > http://motorbicycling.com/showthread.php?t=22726

Tom
 
Last edited:
my guess is that he doesn't mean rpms, but rather intense vibration which would probably be a loose motor mount - unless of course, he has a 20inch wheel on the back and really isn't going that fast while engine revs
 
Carson, have you had any luck with adjusting your clutch cable etc.? If you don't have motorcycle experience it can be hard to describe what the problem is but there's nothing that can't be fixed or replaced if needed. If you got your bike together then you have the skills but maybe not the knowledge and that's what this site is about. Lots of folks here willing to help but they need a bit more to go on. If you can make a video of the bike in action it would go a long way to help diagnose the problem. If you know someone who tinkers with motorcycles get them to check it out.
 
.trlrl. The
Chaz said:
Carson, have you had any luck with adjusting your clutch cable etc.? If you don't have motorcycle experience it can be hard to describe what the problem is but there's nothing that can't be fixed or replaced if needed. If you got your bike together then you have the skills but maybe not the knowledge and that's what this site is about. Lots of folks here willing to help but they need a bit more to go on. If you can make a video of the bike in action it would go a long way to help diagnose the problem. If you know someone who tinkers with motorcycles get them to check it out.
Click to expand...


Thanks for the info. I'll try to make a video when possible. I read that these bikes get to about 30mph. My engine is new and i got 16 mph and im not a heavy person. When it reaches 16mph the rpm seem to high. Is the slower speeds just a result of a new engine breaking in?
 
Hey Carson, these motors will gain more power as they break in but the speed you're getting is way too low so there's a problem elsewhere. It really sounds like the clutch is slipping and needs adjustment. Best to try the clutch set up thread that Twodoor posted the the link for rather than messing with the flower nut.

If you send me a personal message with your phone number I can call you free with my phone plan and maybe we can get to the bottom of this. I'm no expert but I might be able to help you get the issue more clear and then the real experts can give you the info you need to fix it.
 
another thing that can make it seem too high is if it is 4 stroking and suddenly hits a point at which 2 stroking starts
 
Bike won't go full speed

I have a new engine that has entered the break in period. I read these are suspose to get to 30mph and I got to 16mph. The engine is at high rpms as if it has already reached its top speed.

Thanks, Carson
 
Chaz said:
Hey Carson, these motors will gain more power as they break in but the speed you're getting is way too low so there's a problem elsewhere. It really sounds like the clutch is slipping and needs adjustment. Best to try the clutch set up thread that Twodoor posted the the link for rather than messing with the flower nut.

If you send me a personal message with your phone number I can call you free with my phone plan and maybe we can get to the bottom of this. I'm no expert but I might be able to help you get the issue more clear and then the real experts can give you the info you need to fix it.
Click to expand...


Thanks. I tried the link that 2door posted and what it did was not let the engine go into complete gear. I'll be posting a video
 
You must log in or register to reply here.
Back
Top