Low power - Slows after a while

[bitsnpieces]

Hi, name is David, from Australia.

I've just finished building my first motorised bike and tested it and have some questions.

I'm using a 50cc 200W engine kit from Rock Solid (200W because that's the legal limit before you are required to have a license here in Australia), which took me a while to put together but it's all done and works.

While I was riding, at first it would rev up, but after that, whenever I tried to speed up, it felt like nothing was changing, possibly slowing down even. It sounded like it was choking. When I disengage the clutch, I can rev up the engine easily and hear it do so, engage the clutch, the engine chokes, then runs, sometimes it'll speed up, sometimes nothing.
At one point when I engaged the clutch, the engine just died on me.

What could be the cause of a weak run?

 

[biknut]

These are some things to be suspicious of

Intake leaks, especially where the manifold connects to the head.
Chinese spark plug. It's a good idea to buy a real plug. I use NGK BP6HS
Spark plug cap and wire. Replace the cap and wire with a solid core wire from auto store.
Jetting. A good tip seems to be, these motors are usually jetted rich. Check your plug.
Look inside the carb. When I first looked in mine the jet tube was unscrewed.

 

[MarkSumpter]

Hey Bitsnpieces welcome to the forum.

Your problem sounds to me that you have an air leak somewhere. All 2 cycle engines share an inherent characteristic in that they require a leak free combustion and compression chamber all the way from the cylinder side of the carburetor to the flange of the muffler to run properly. If they are sucking air anywhere they will run lean and produce the exact problems you are describing and if allowed to leak can seriously damage an engine.

First thing to check is that your carburetor has no leaks where it mounts to the intake tube. Norman has a thread that details all aspects of the standard float type carb used on most bikes and this particular post deals with using RTV Silicone to make sure you have sealed integrity when mounting the carb to the intake tube:

http://motorbicycling.com/f4/motorized-bicycle-carburetor-pictures-how-install-195-2.html#post1199

Side note: The entire thread is great and a good read for new people concerning carburetion.

I just installed a brand new engine on my bike and found I had a couple of more entailed leaks and this is the thread where I describe my process to find them:

http://motorbicycling.com/f52/basics-port-polish-port-matching-16646-6.html#post237700

This is also another great thread to read...

Hope this helps you out.

 

[bitsnpieces]

Thanks MarkSumpter.

I've had a quick look at the carburetor one to get the basic idea and will go over it again after work.

I also just remembered that where you put the throttle cable in, there's a small bolt on the left which raises the sitting position of the cylinder thing on the cable. What is that used for? Is it to allow more air to go in when idling?

 

[GearNut]

Yes, that screw is the idle speed screw, and you have figured out exactly what it does.

 

[MarkSumpter]

Bitsnpieces I am not sure exactly what you are speaking of If you can post a picture it would be helpful to clarify what you are referring to.

Some carburetors will have an idle mixture screw to adjust the fuel to air mixture others will use a circlip that is moved up or down in notches on the metering valve to set the mix. And on some carbs an adjustment screw is there to adjust the low idle RPM setting.

I dont know all of the ins and outs on all of the different carbs in use yet. Someone else might have more info.

 

[bitsnpieces]

In the first picture where Norman is holding the carburetor, just to the left of his thumb is the screw I'm talking about.

I think it's what you and GearNut is referring to, the idling air mixture, so my question is if I should have it all the way in to let as much as I can through? Half way? etc.

Other than that, I'll have to check my fuel leakage, tense up the chain a little more, and tighten the handle bar, from what I noticed from my initial test run.

Oh, and anyone know what the mix ration with Valvoline 2 stroke oil with premium (98) unleaded is? Not sure what you guys may call that fuel for those outside Australia.

The handle bar become loose again with all the vibration. LOL

 

[GearNut]

That screw does not adjust the mixture, it only adjusts where the throttle slide stops when the throttle grip is released; to allow enough air flow into the carburetor to allow engine idling.

 

[GearNut]

It appears that you are referring to an NT carburetor. They do not have a specific idle mixture adjustment. They have a jet needle adjustment and a replaceable fuel jet to allow you to install different sized fuel jets.

The needle controls the fuel delivery from 1/4 throttle to 3/4 throttle. The jet controls fuel delivery from 3/4 throttle to full throttle. The idle fuel delivery is static depending on where the needle is set to deliver good mid-throttle performance.
They are an imperfect carburetor, but they get the job done well.

 

[bitsnpieces]

You mentioned 3/4 throttle to full, so I'm assuming when I'm not throttling much, by using the screw to push up the cylinder on the throttle needle, it'll allow more fuel in to allow higher/faster revs/speed? So this would make reaching full speed on the throttle more easier?

At the moment, I'm also using a 35:1 mixture. Is there a way to figure out what the mixture required is?

 

[MarkSumpter]

Gear nut is correct that if you are using the NT carb than the screw on the side is low idle speed adjustment and air to fuel ratio is adjusted by raising or lowering the circlip on the metering valve.

Question though Gearnut. Does the CNS-V1 allow for hi/lo air fuel mix or is it also adjusted by the metering valve to main jet height? I know it has a remote choke but the documentation on it is poor (like many other things for these engines)...

Never mind the question I found the answer:

CNS Racing carburetor is a must for the motorized bicycle enthusiast. With an air fuel mixture screw and idle screw makes this Carb fully tunable. Hand control choke adjust lever that mounts on the handle bars and cone air filter makes this the carb of choice.

Too bad it is only mid range tunable and has only 1 mix screw and not one for low speed mix and another for high. I really am tempted to adapt a Tillotson or Zama Pressure actuated carb.

 

[GearNut]

While I do not have any first hand experience with the CNS-V1 carburetor, I do have alot of experience with close cousins of it as used on mopeds and small motorcycles.
It has an idle speed screw, a low speed fuel screw, an adjustable jet needle, low speed fuel jet and high speed fuel jet. All of these make it tunable across the whole RPM range.
If there are different needles for it as well as different air bleed jets or main jet emulsion tubes (needle jets) then it could be tuned to absolute maximum performance and fuel economy.

 

[MarkSumpter]

While I do not have any first hand experience with the CNS-V1 carburetor, I do have alot of experience with close cousins of it as used on mopeds and small motorcycles.
It has an idle speed screw, a low speed fuel screw, an adjustable jet needle, low speed fuel jet and high speed fuel jet. All of these make it tunable across the whole RPM range.
If there are different needles for it as well as different air bleed jets or main jet emulsion tubes (needle jets) then it could be tuned to absolute maximum performance and fuel economy.

Have also heard alot of people having problems with the CNS-V1. You think it is a design defect or just a lack of experience on the mechanics part?

 

[GearNut]

You mentioned 3/4 throttle to full, so I'm assuming when I'm not throttling much, by using the screw to push up the cylinder on the throttle needle, it'll allow more fuel in to allow higher/faster revs/speed? So this would make reaching full speed on the throttle more easier?

At the moment, I'm also using a 35:1 mixture. Is there a way to figure out what the mixture required is?

1. No. Adjusting the idle speed screw only adjusts the RPM speed at which the engine idles. It will not have any affect on "reaching full speed on the throttle more easier."

2. 35:1 is a little light on the oil for my tastes. I prefer to break in an engine on 20:1 to as thin as 25:1. After break in I run all my 2-strokes on 32:1.

I do not care if the engine manufacturer recommends 40:1 or even 50:1. I run 32:1 and it has never failed me.

There is no real way to figure out the exact ratio required other than listening to the experiences of other folks who run different ratios and report the results they have found.

 

[bitsnpieces]

Thanks GearNut, think I'll stick to the 32-35:1 ratio for now, seems to run alright for now.

On the other news, think I've blown my spark plug from throttling at maximum so I can't start the bike now... Long way from home and I'm not fit either...

 

[GearNut]

Spark plugs don't blow. They can carbon up and short out, or if they are cheap (read factory Chinese spark plug) they can fail internally. Also check your ignition wiring between the engine and CDI as well as the both ends of the spark plug wire.
If you are using the kit supplied kill button, they are notorious for shorting out too.

 

[GearNut]

Have also heard alot of people having problems with the CNS-V1. You think it is a design defect or just a lack of experience on the mechanics part?

I think that it is a jetting defect as initially sold with the kit, the actual sizing of the parts (jets, needles, air bleeds, ect.) as compared to their markings (Chinese quality control), and lack of experience with a new gizmo on the block. I am sure that there are other folks here who have as much experience as I do with small carburetors, it's just that not all of us have that particular carburetor in hand to figure out.

 

[bitsnpieces]

I have another thread detected to this problem now:
http://motorbicycling.com/f34/heard-pop-now-wont-start-25875.html

But basically the magneto and front sprocket were very difficult to turn. I've managed to loosen them so they are easier to turn, but even so, I don't hear any sort of air pumping kind of noise, like when you do when you turn the front sprocket (eg. feeding the chain) and you hear that pumping noise (air compression or something).

The company I got it from said the magneto has probably blown (and I need a new spark plug from the looks of things) but he said that he's soldered the wires down and all, they have probably melted and I can solder it back and get it working, but everything looks in check... At least to me...

 

[GearNut]

Even if the wires shorted together from lack of proper insulating, the solder would not melt. There is just not enough electrical power here to do that kind of damage.
Now, quite a few of the magnetos have poor solder connections from the factory. They are called "cold joints" where they do not get both parts to be joined hot enough to bond with the solder. One part (usually the magneto wire) gets hot enough to melt the solder and the other part (harness wire) is inserted into the molten solder. When the solder cools it will grip the wire but it will not be bonded to it. The wire will vibrate loose over time. Even if the wire does not completely fall out, this can also invite corrosion which will act like a resistor and impede the flow of electricity.
As for not hearing any kind of intake or exhaust sounds, remove the spark plug, look down the cylinder and try to turn over the engine. Can you watch the piston moving up and down?

 

[bitsnpieces]

Bit difficult for me to look down to see the piston given the fact that I can't even get the motor off my bike now (not glued, just the bracket bent a bit much while I was securing it to make sure the engine was snug tight) so will have to find a mirror.