50cc carb problem

[benhonda]

So I've had this problem with multiple bikes, multiple carbs...all 50cc chinese stuff.

The bike, after sitting unused for a couple of weeks, will start easy enough but if you give it throttle it will bog and if you don't let go it will die.  But let it idle, and keep trying to rev it up and eventually (sometimes takes 2-4 minutes) it will run fine.

This has even happened on bikes running non-ethanol fuel!

I am going to look into trying to adapt a throttle slide carb, common to 110cc kids' atv and pit bikes, as they seem to give less issues than the CV carbs on these scooters.

Anyone have an idea what part of the CV carb is either getting clogged or not working right that causes it to bog badly when you give it throttle...but goes away once the bike warms up?

[GrumpyUnk]

The engine bogs because the fuel:air mixture is off from what it likes. It is either too rich or too lean.

The electric 'byvalve', or choke, forms a separate path for fuel:air to get to the intake. The fuel flows through a small jet embedded into the float bowl(for most but the latest), and into a vertical tube built into the side wall of the float bowl. If the jet gets plugged, which is easy to do, the mix will likely be too lean, and cause the engine to stall readily until it is warmed up. I have used straight pins to poke through that jet and insure fuel can fill the tube. Once the engine starts, the flow into the tube is limited by the jet, and is less limited upon startup as the tube is full of fuel(as high as that within the float bowl). The engine can get a good 'snort' of fuel from the tube as firsts start, and then is limited by the jet.(and additionally the normal idle mixture/speed settings).
I think the slide carb may not perform as well as you want with a CVT rather than a gearbox, normal on slide carbureted cycle. It may work, but may have funny response to the more constant rpms of a CVT system. A slide carb will have(most) a mechanical choke. The Keihin used on a 2T Kymco Super 9(8) has a regular enricheniung mechanism the same as a CVT carb... The no-name slide type on a Sym 150 has a regular choke.
If you let them sit, it may pay once you have them fully clean, to cut the fuel supply from the tank, and drain the float bowl completely. That would help prevent buildup of 'evaporated' fuel remains which are likely the cause of the poor running on startup.
I would not want to try to set up a slide on a CVT as I'm not sure how it would respond.
tom

[tortoise2]

Apr 23, 2020 23:54:10 GMT -5 benhonda said:

after sitting unused for a couple of weeks, will start easy enough but if you give it throttle it will bog and if you don't let go it will die

A contributing factor MAY initially be an issue with stratified water at the bottom of the carb bowl . . main jet intake is lower than pilot jet.  During the initial unused duration, that water is sealed by the slowly evaporating gas floating above it.



Probably doesn't help that the initial replenishment gas from gravity flow tanks may also contain stratified water from the bottom of the tank.

Consider adding a little Iso-Heet and Lucas Fuel Treatment when refilling the gas tank.

[mikehailwood]

Like Grumpy says, I'd definitely make sure those jet passages are clean, particularly the idle jet and, in your case, the enrching circuit.
Does the carb have an electric enriching device/bystarter? If it does, shortly after start-up does the engine idle very high and after a couple minutes drop down to a proper idle speed? It should; if not, I would test this device (as well as making sure the passage is open) .

[benhonda]

all my CV carbs have electric choke/bystarter
I had assumed they were good, since I thought if they were not then the bike wouldn't be able to start
But then the symptom going away after the bike warms up makes more sense if it is the choke being plugged up

Though, the idea of water in the bottom of the bowl makes some sense to...as it wouldn't effect startup or idle but would cause it to bog when I open the throttle...and since I keep trying to open up the throttle as the bike warms up, I may be slowly using up the water in the bowl until there is only fresh gas in it and then the bike can respond normally to throttle.

I had not considered that the CVT could impact how the carb works. So is the general idea that you need a carb that can vary the air intake based on load...and not just throttle input...since the CVT varies the load as you drive?

I still think it would be fun to try a slide carb and see what it does, and I think I have a 45 degree metal intake tube off some 110cc atv, if it can be made to fit than the only other change Id have to make is to bypass the vacuum petcock. Im pretty sure I could do it without making it so that I couldn't easily go back to the CV carb.

[mikehailwood]

1-Have you checked jet passages as we have suggested?
2-Shortly after start-up does the engine idle very high and after a couple minutes drop down to a proper idle speed?

[GrumpyUnk]

Most carbs have a drain that can be opened to let the float bowl run dry. You could direct the drain flow into a glass jar or can, and empty the bowl of any content. It won't be absolutely empty, with a small amount of 'whatever' remaining at the very bottom, but should be empty enough that fresh fuel would be supplied to the jets when it was replenished from the tank.
A second alternative would be to disconnect the fuel supply from the carburetor inlet, and direct it to a container. Apply suction to the vacuum operated petcock, or open the petcock, and capture the flow for a minute or so. Close the flow, and inspect what has been caught. If you find 'pearls' wandering around the bottom of the catch can, they are likely water. Water will fall to the bottom, and aggregate into globs. The globs will move around as you direct the catch can, with the globs following the lowest point.
If you have globs/pearls, you have water in the fuel supply. You can repeat the drain process, inspecting as you go, until you get fuel without contamination showing up.
Given that it runs ok, and you can apply full throttle after warming up completely, the likelihood of fuel contamination without any sypmtoms when running faster is, in my mind, rather low. If there was water in the fuel, I think some would be fed to the carburetor when running warmed up and at higher speeds, not only when the engine is cold...
tom

[donaldsonta]

Some folks over at the "Chinese Project Scooters" Facebook run slide carbs. I don't run one because I've read they're a lot harder to dial in, and require a lot more tinkering to keep dialed. The guys that run them claim a nice performance gain at the expense of fuel economy, and more intentional throttle use.

It's a Facebook group though, so there's a lot of bad advice. An interested person has to sift through a lot of people claiming to own 1000 50cc scooters that are all perfectly jetted and run 60mph underwater.

Editing to add:

My advice is If you're just wanting play around and experiment. If you're riding 50s for your day to day transportation Id def stick with a CVK.