Float Carbs

[Dan Forsh]

Apologies in advance, I know this is off the topic somewhat, but I know you guys will have the answers for me.

My 13 Yr old turned up with one of these mini motos he'd bought off his friend. Doesn't work and they'd had a go at fixing it, so arrived with me in a mess, screws missing, seals missing and limited info as to its problems.

I've checked over the bits I'm familiar with; compression, spark, etc.. but this float carb is new to me. I'm used to diaphragm carbs but could do with a few pointers on how these things work. I've taken it apart and think I know how it works, seems pretty simple, but am I on the right track?

The way I see it (and please correct me as we go)the fuel is gravity fed from the tank and the float controls the fuel influx of fuel into the bowl, when the throttle is turned the cable lifts a needle out of the 'jet' and allows fuel into the port?? From there I assume that crankcase pressures do the rest?? There is a small screw with a fibre washer on the bottom of the bowl, that doesn't provide any kind of fixing, so is this some kind of air inlet? Also there is a small tube coming off a nipple that I don't know about, but seems to just vent to atmosphere. What's this for?

Only one adjustment screw which seems to ride against the side of the throttle needle, so I assume this is the idle adjustment. Are these things this simple?

Thanks in advance,

Dan

 

[no longer collecting]

float carb.

too many missing info.....on motor bike they could be 2 cycle or 4 cycle...then the carb would be different....no reed valves on 4 strokes and usually either side draft or down draft carbs.Also if it is a motor bike the main carb jet is activated with a movable cylinder that is held at the top linked to the throttle cable.....totally different from a chainsaw.you would need to post a picture of the carb and model#.On chainsaw gaz is sucked in the engine base first for lubrication then pushed up on the piston top and reed valves prevent back pressure loss in the carb(2 cycle). The other way with 4 cycle engines is a continuous vacuum suction from the manifold that draws air mixture through the idle screws and main jet underneath the bowl. Some chainsaws have an integrated gaz pump diaphragm unit(hl type) and others are gravity fed.
We can talk about this for a long time...have you search for a motorcycle forum on the web?????? they may have your answer........

 

[no longer collecting]

float carb.

Everything you said is right if it is a "mikuni carb". one or 2 vents on each site with pieces of clear tube for overflow....a cylinder hooked to throttle cable with adjusting screw ...that would replace butterfly valve and only one screw on the side to adjust cylinder height for idle speed.Beware of the gaz mixture....it is usually acheived by moving the main jet needle to different setup inside the cylinder and it is held by a spring and "c" clip. There is a number stamped on the main seat and the main jet...you need the manual to get the proper height setting........cant help you anymore...........

 

[TimberPig]

Apologies in advance, I know this is off the topic somewhat, but I know you guys will have the answers for me.

My 13 Yr old turned up with one of these mini motos he'd bought off his friend. Doesn't work and they'd had a go at fixing it, so arrived with me in a mess, screws missing, seals missing and limited info as to its problems.

I've checked over the bits I'm familiar with; compression, spark, etc.. but this float carb is new to me. I'm used to diaphragm carbs but could do with a few pointers on how these things work. I've taken it apart and think I know how it works, seems pretty simple, but am I on the right track?

The way I see it (and please correct me as we go)the fuel is gravity fed from the tank and the float controls the fuel influx of fuel into the bowl, when the throttle is turned the cable lifts a needle out of the 'jet' and allows fuel into the port?? From there I assume that crankcase pressures do the rest?? There is a small screw with a fibre washer on the bottom of the bowl, that doesn't provide any kind of fixing, so is this some kind of air inlet? Also there is a small tube coming off a nipple that I don't know about, but seems to just vent to atmosphere. What's this for?

Only one adjustment screw which seems to ride against the side of the throttle needle, so I assume this is the idle adjustment. Are these things this simple?

Thanks in advance,

Dan

I started typing then decided to let you do some reading instead. You should be able to find all you need to know at this link.

Carb Tuning

 

[hillbillycycles]

Dan,
I'm a new guy here and diaphram carbs are new to me. However, I have worked and tuned tons of two and four stroke motorcycle and atv carbs. The information TimberPig has pointed you to is a great place to start.

After reading up on it post back. I'm sure myself or someone here can help you out on your new adventure.

I managed to get a Stihl chainsaw to work on because I can fix motorcycles/atv's and it looks like you got a bike to work on because you can fix saws. Pretty funny.

 

[JohnL]

For most float carbs, the float opens and closes to keep the fuel level in the bowl at a constant height, it is fed by gravity from the tank. Once in the float bowl fuel is drawn up through the jet, and out a discharge passage in the throat of the carb. It is pulled out by the low pressure created as intake air flows past the discharge hole in the throat of the carb. It is the action of air moving through the carb that feeds the fuel.

The screw with the fiber washer is probably a drain hole , so you can drain out stale fuel. Atleast I know Tecumseh carbs have this.

 

[TimberPig]

For most float carbs, the float opens and closes to keep the fuel level in the bowl at a constant height, it is fed by gravity from the tank. Once in the float bowl fuel is drawn up through the jet, and out a discharge passage in the throat of the carb. It is pulled out by the low pressure created as intake air flows past the discharge hole in the throat of the carb. It is the action of air moving through the carb that feeds the fuel.

The screw with the fiber washer is probably a drain hole , so you can drain out stale fuel. Atleast I know Tecumseh carbs have this.

Yes it would be a drain screw, but the idea is that if you are going to put it away for a while, you simply drain the float bowl and shut the fuel off to prevent varnishing the carb. If it is large enough, as on most Keihin and Mikuni carbs, you can change main jets simply by pulling it and rotating the carb to get the 6mm socket onto the jet. Others are much smaller, and the bowl has to be removed to access the main. You have to pull the bowl to access the pilot. The top has to come off to adjust the jet needle and slide, and the slide has to come out completely and the main be unscrewed on those where the needle jet is removable and can be changed.

 

[ned coed]

mini moto

dan it,s ned again, what and where are your screws missing from, if you can give more detail of the prob perhaps i could guide you, i used to tune and prep my racing two stroke yams so will be pleased to advise-if my memory lets me ha!. regards, ned.

 

[Dan Forsh]

Hi Guys

Posted this and totally forgot that I had until my boy started to pester my about the bike again.

I must look totally ungrateful given the amount of advice that has been posted :-(

I will be resurrecting the bike soon as I can, so thank those of you who have provided info/advice.

Thanks

Dan

 

[Dan Forsh]

For a brief update:

We are talking about a little 47cc, 2 stroke machine.

Ned, managed to sort out the missing screws and the threads that the boys had stripped thanks to our maintanance fitter at work. Carb leaked when fitted back on the bike as soon as the fuel tap was turned on. After some head scratching I realised that the boys had managed to lose an O ring from between the fuel tap and where it screws to the carb body.

Timber, haven't had a chance to check out your link yet, but thanks, I'm sure I'll learn from it.

John L, Exactly what you said is how I'd sort of figured out it worked, but you expressed it far better.

So two points I want to check; the screw with the fibre washer is probably just a drain point and the clear tube off the nipple is an overflow right?

Does anyone know if these mini bikes can take a 200lbs rider? Either way I'm having a go once I've fixed it!

 

[Mr.]

From firsthand knowledge. 200# is not the most one can carry.

Fred

 

[ned coed]

dan, is the screw on the float bowl right along side the plastic pipe or is it higher up or even the other side the bowl ?. ie. if you open the screw are you are letting fuel out the pipe?.

 

[Dan Forsh]

Hi Ned,

I think earlier suggestions are right. The screw is right on the base of the bowl. The pipe, from recollection, is higher up on the carb body. Is this an overflow then?

Dan

 

[TimberPig]

Hi Ned,

I think earlier suggestions are right. The screw is right on the base of the bowl. The pipe, from recollection, is higher up on the carb body. Is this an overflow then?

Dan

It's a vent line to allow air in so you don't develop a vacuum in the float bowl.

 

[JohnL]

It could be an external vent, or, I've seen them use a primer bulb that just blows air directly into the float chamber (Tecumseh).