Carby catastrophy & confussion

Greetings,

I have had a 49cc 2 stroke motor installed for a while, the motor is ridden in, however I had to get a new carby because the carby seal around the intake manifold had broke after being tightened too much. It worked well and still works well but leaks like a brick toilet through the bowel gasket. This carby was easy to tune. It is sealed well regardless of the fuel leaks. The c clip was set on the second pin setting from the top, rich or lean?

I installed a new carby, the same NT size carby, this one seems to behave quite differently at exactly the same pin setting. When I accelerate it chokes a bit at low speed. When I disengage the clutch and stop accelerating the revs take a long time to reach low idle.

I am assuming this means there is too much air coming in. But after checking the seals and no air coming in I think it might be the pin setting. Maybe the pin setting is too lean?

As far as I remember, the c clip set high on the pin lowers the pin deeper into the jet, making the jet closed up, allowing less fuel into the engine, and making the mix leaner. The higher the C clip setting to the top, the leaner the fuel.

And the lower the C clip on the pin,the richer the fuel, allowing more fuel thru the jet, is this correct?

So if a C clip is on a higher setting, say the second one from the top, does this make more air as well as less fuel, and cause the engine to continue to rev slowly down in idle? I have been wondering if my engine needs less fuel or more fuel. or is it more complicated than that.??




cheers

You've got the clip idea right, the higher the setting the leaner the mix - but nothing will increase/decrease the airflow save the restriction of the airfilter.

As for your revs taking a long time to return to idle, could it be simply a binding throttle cable? The rest sounds like just a matter of dialing in the mix, something you seem quite capable of... but that slow return is an odd one...


With the engine off, if your throttle cable is set up properly (slack taken out w/adjusters) you should be able to hear a "tink" as the slide hits the top at full throttle and another "tink" as it hits bottom when you release the throttle. It's faint and a little hard to hear - but if you don't hear that sound of the slide bottoming out as soon as you let go the grip, there could be drag in the cable housing... lube (inside the grip too) and make sure there's no sharp bends. There's always the big slide return spring inside the carb, there's also the possibility the little throttle cable nubbin isn't seated properly in there and is dragging a lil, but it's unlikely that these are the probs.

I hope it's as easy as that

Thanks barely awake.

It is a bit odd and the throttle and cable works fine, the spring works fine inside the plunger, and the response is good on the throttle, its just the slow return. Perhaps the engine needs more fuel, a richer mix. This slow return thing also happens with another carby I have for 80cc engines, the larger carby has a bigger jet and plunger, allowing more fuel. But this larger carby also has a slow return. I had such a crappy time trying to tune this one I was convinced it was faulty, now I'm having issues with the new one

I guess I'll eventually figure out whats going on, I haven't adjusted the throttle screw, would this make much difference to the pin setting?



I use 50ml oil to the litre, I have been advised these engines like to run on around 30ml to the litre.

not really, that jus changes how low the slide seats, allowing in a little more (or less) fuel and thereby providing your idle speed - but only when there's no tension on the cable and the slide is down completely.

Oh wait... throttle screw? I thought you meant idle screw... forgive me - what do you mean by "throttle screw"?



I'm not really convinced your slow return has anything to do w/mix tbh, it should pretty much just drop to idle speed - even IF yer mix is all wrong, it would just run poorly (misfiring etc).

There are far wiser gearheads than me about though, sooner or later they'll wake up (or get back frm a ride) and I bet they'll be able to help ya


partic when it comes to yer weird Australian metric mixes o_O

Too much oil will make the engine run leaner.
Just because the last carb was set at the second notch means nothing, you have to look at your spark plug to get a "baseline" and then adjust the C clip from there.
I does sound like you are running lean.
The plug color you want is medium brown or darker gray.
A white or very light gray plug means you are running lean.

__________________
If it ain't broke, and you mess with it long enough, it will be.

Yeh, The throttle screw is where the throttle cable enters the carby, you had it right.

I assumed it would lift the pin up when the screw tightens the cable and allows more fuel to enter the plunger area. I think you right, it might have nothing to do with oil mix or pin setting, it could be that I have two faulty carby's or air is getting in from some unknown area not yet found somewhere inside the carby, I checked the intake manifold seals on each end, the carby is sealed with silicone gasket sealant around the manifold and the gasket for the other end where the manifold attaches to the engine. I did read somewhere a week ago that too much air or an air leak somewhere causes this to happen, but when I spend a long time already sealing everything it beats me what can be happening. Thanks for your help,

cheers

Hi Bikeguy Joe, thanks for your advice, So can a lean setting cause a slow release??

I did check the sparkplug, it's a dry dark brown color, it did have a little bit of oil on the tip. I will try changing the C clip down a notch, its not too bad and use a 40 ml/litre mix to see if it gets a quicker release, brings the throttle release down quicker to idle.

Perhaps there is too much oil, maybe 50ml to the liter is too much as I did have green oil coming out of the gasket around the previous carby bowel. Strange, the oil separating from the fuel inside the carby bowel.

skuzzy...this new nt carb has a bigger intake, a bigger slide...do you have a o ring in the carb where the intake slides in...you must really seal this up tight...to my knowledge there isnt a larger intake for these carbs....after you get it running let it idle and spray carb cleaner or even water around the carb, intake and exhaust to find air leak...if none...pull the choke up and see if the engine stops...if it does you are tight...i have one of these new carbs...it worked great but allways had a air leak...took it off and went back to the original but used the square filter cover for air...i might try a straight intake from manic machanics...it has a o ring around the intake tube...good luck and let us know how things go...Dennis

Hi Dennis,

I just purchased an intake manifold from manic mechanic for its o ring seal around the edge of the intake tube for the carby insert.

I have two new carby's, one is the larger NT carby which you are talking about and the other one is the smaller NT one. The larger one has a bigger jet, jet hole and plunger/slide. Neither of the carby's have an o ring inside the opening where the intake slides in, but has a C clamp to tighten it up by factory default. It has three teeth with gaps for tightening and about a 3mm seal inside. I usually silicone seal the gaps between the teeth. The larger carby had a slow release as well as the small one.

So what I will do to dignose this is,

Seal with new intake manifold from manic mechanic,

Check everywhere for air leaks with water or carb cleaner and pull choke up to check for tightness. (does motor stop if using water?)

If no change

Use less oil in the mix, 40ml/litre ( am using 50ml/litre now)

Lower the C clip on the pin to a richer setting.

After I do this I will post the results.

Thanks Again

good for you...i'm going to send a couple of pics...keep them on hand...yes the engine shouldnt die...but if there is air leaks the rpms will slow down when you spray those areas...if its sealed nothing will happen...take your time and do it right...its worth it...Dennis...also ThatsDax has the o ring for the carbs intake...

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