Hyosung SF50 2 Stroke Running and Starting Issues

[bvulic1]

Hello all, I am in need of some help on this scooter I am working on. It is a 2009 Hyosung SF50. It has about 150 miles on it. I purchased it not running and without a CDI. Since purchasing, I have purchased a new OEM replacement CDI which works well, went through and cleaned the carb. There was a gas leak in which gas was flowing out of the air box. I fixed that by putting in a new float and plunger which I had laying around. Fuel is properly flowing to the bowl. 

Issue I am having is regarding the starting/running. It won't fire up consistently at all and when it by chance does, the idle is very low, any twist of the throttle kills it and then wont restart. If you leave it running, it'll run that way for about 10 seconds and die. I have the carb and idle adjustment exactly 1-1/2 rotations out. No matter what I do, I cannot get it to start or run properly. Very strange. I am guessing my carb adjustments might be off? Thats just about my only guess.

If anyone has any insight on this, I would greatly appreciate it. 


 

[classacted]

I'm sure you made certain the gas is flowing good and it's not contaminated.

everything here points to the carb as not working 100 percent. I don't like to tell people to clean the carb and check all of the passages because I have done that myself hundreds of times and it doesn't always work. a new carb doesn't always work either. there are other possibilities, but I would start there if that was my bike.

[aeroxbud]

I would probably try and use the original parts you had in the carb to begin with. Even something like a different weight of float could throw the running off.
Of the carb was originally overflowing. It's usually a stuck valve. Try cleaning out the seat. And make sure the rubber tip on the needle is in good condition.

[classacted]

I'm glad aeroxbud mentioned the float because floats sometimes become gas-logged or crack. it's rare, but happens.

[bvulic1]

Aug 2, 2023 14:15:04 GMT -5 classacted said:

I'm sure you made certain the gas is flowing good and it's not contaminated.

everything here points to the carb as not working 100 percent. I don't like to tell people to clean the carb and check all of the passages because I have done that myself hundreds of times and it doesn't always work. a new carb doesn't always work either. there are other possibilities, but I would start there if that was my bike.

Hey there, you are correct, I drained the old gas, have a full tank of new fuel and its definitely not contaminated. I usually like to eliminate as many variables as possible. It's interesting to see because I initially thought it could be the carb settings as it was very inconsistent, etc. I am unfamiliar with the Hyosung carb settings so I stuck to a 1-1/2 rotation out as a starting point. 

Something is telling me the carb isn't working properly. While the carb was leaking into the airbox, if I took the filter and airbox cover off, it would start and run decently. 

[classacted]

back when I had ambition, I used to test the float mechanism by blowing in the hose at the fuel inlet while raising/lowering the float (of course the carb is mostly disassembled) to verify the float level, the valve and float is moving freely and the flow and shutoff of fuel is taking place.

[bvulic1]

Yes, I agree, that is exactly what I did as well, everything seems okay when testing the float. When lifted, the passage is entirely blocked off, when blowing into the gas inlet with the float down, air flows however not entirely freely, it seems to be restricted a bit, I'm not sure if this is how it should be or if it should completely blow through freely. Also, when the carb is installed, the bowl is full of fuel, when I unscrew the bowl screw, gas flows out freely.

[pinkscoot]

How does it run now with the air box off? If it ran well that way with the leaky valve then you are way too rich.

[bvulic1]

Aug 4, 2023 13:52:00 GMT -5 pinkscoot said:

How does it run now with the air box off? If it ran well that way with the leaky valve then you are way too rich.

I have it at a point where I removed the air filter element from the air box, the air box is secured and closed just without the element. The scoot will start on the second kick. It will stay running for about a minute. If I give it throttle, it’ll rev out, and return to a really high idle. It’ll stay like that until the revs drop on their own and the scoot dies. Very erratic behavior, it’s driving me a bit nuts.

[pinkscoot]

You definitely have an air leak then. It is either where the carb bolts to the manifold or where the manifold bolts to the case. When its running try to spray carb cleaner or starting fluid around those connections to find the leak.

[classacted]

I suggest setting up an independent fuel supply for test purposes using any bottle and clear tubing. it sounds like a fuel problem to me.

[bvulic1]

Thank you for the ideas. I will try testing for a leak if not, I will try going with a straight fuel source.

I was messing with it today, here is the exact running situation:

-Starts up no problem, first kick
-It will idle normally for about 2 minutes, tire just barely spinning while on the center stand.
-After the two or so mins, The RPM will gradually increase up until the point where it wants to fly off the center stand. rear tire rotating at a strong speed.
-At this point, I give it some more throttle to see if it will ultimately settle down on its own, almost immediately after that, the RPM's drop rather quickly until it dies out fully.

Hope this helps and provides enough details.

[classacted]

well, you have things to do. when you find the solution come back and post it, please.

fyi, a two-cycle engine will rev when it runs out of fuel. I can't speak for 4-cycle.

[bvulic1]

Aug 6, 2023 10:21:08 GMT -5 pinkscoot said:

You definitely have an air leak then. It is either where the carb bolts to the manifold or where the manifold bolts to the case. When its running try to spray carb cleaner or starting fluid around those connections to find the leak.

I tested this theory first, started the scooter, let it idle, sprayed some carb spray directly on the connection of the carb and the intake manifold. RPM's dropped instantly which leads me to believe there is a massive air leak at that connection there. Later tonight, I will be able to place some copper gasket maker on it that I have laying around. Hopefully this helps seal that location properly as I do not have any actual gasket element. 

[pinkscoot]

I would try out a paper gasket, they always worked well for me. You run the risk of pulling the copper gasket maker into the intake. Be sure to clean Bothe surfaces well and be sure that there are no rough spots.