zongshen 124 runs....... then died

[kenfyoozed]

OK guys i need your help one more time. I got the engine in, and all the kill switch, and key switch worked out. I have teest fired this engine at least 3 times today, and it would idle fine. Now that i have everything done, and the bike back together, it wont run. I took at least 15 kicks to get started and it ran fine for about 2 min to warm up, but then would not come off choke. It finally died, and then would onlyu fire up and run a few seconds before it died. It has spark, and its getting gas.

What should i check? I wanted to get the first run in tonight. Even if i have to make a run late tonight.

The air intake scew should be backed out 2 turns, right?
Ill try again after supper. Drop any helpful ideas you may have.
Neil

 

[TrailPunisher]

I have had this problem before with mine, it is from fuel starvation.
The clone tanks do not get 100% fuel pick-up. Gravity conspires against us and fuel weight pushes gas up-ward against gravity to the carb.
Check your gas cap, make sure the vent is open if it is a vented cap.
Check your fuelline for a kink or bend in the hose restricting the flow.
Check your fuel leve lin your tank, it may be too low. It will look like 1/3 tank but it`s really below the fuel pick-up. Fill your tank with gas.
Double check the engine by spraying a shot of carb cleaner in it and see if it fires up after a couple of kicks.

 

[kenfyoozed]

Thats the conclusion i came to also, late last night. I still have the small fuel lines that came with the bike. I will be switching out to 1/4" id fuel lines today.

Is there anyway to get around this problem of the carb pick up being higher than the bottom of the tank? I dont think i can lower my carb anymore than it is. To bad there isnt some type of small electric pump we could use!

 

[69ST]

I keep hearing about the lack of head pressure resulting in fuel starvation, but I've not experienced it. Here's a way to test for it...remove the fuel line from the carb inlet, then raise end slightly above the level where it normally attaches to the carb inlet then allow the tank to empty. Unless the PRC bikes are different than the original Hondas, the tank will drain completely. The bottom of the tank sits higher in the frame than a lot of people think. If the top of the carb is below the bottom of the tank area (lower seam) of the frame, you're probably alright. As long as the fuel will flow from the tank to the carb, that's all that's needed.

If the carb is too high relative to the tank, there are other intakes or you could have one modified/fabbed to suit. Seems unlikely, lots of these engines have been successfully run in CT70s. It's easy enough to top off the tank and see if that cures the problem...I'm betting that it won't. It sounds more like severely restricted fuel flow, with the lines being the most likely cause.

What I do see frequently are clogged filter screens, crimped/kinked fuel lines, blocked tank vents and carburetor problems including dirt, incorrect adjustments (usually the idle air bleed) and (infrequently)incorrect float level.

 

[TrailPunisher]

With the bigger 125cc Chinese engines, the intake manifold has a riser/spacer and bigger intake manifold itself the carb sits higher than an original one does and the fuel line is actually running back up to the carb.
The weight of gas is the only thing to make gas flow back up-hill.
When the tank gets low there is not enough push to keep the fuel flowing and it starves for fuel even though there is 1/4-1/3 tank of gas left.
I thought about drilling a hole in my clone frame and running the line through it level to the carb.

 

[69ST]

I don't doubt what you're saying, but I have to think that the carb height is "right on the edge". I run the fuel lines below the air filter on my bikes and that makes the fuel flow uphill, too. Since the lines are clear, it's easy to see what's going on. The tank empties completely and only the last little bit in the lines remains. Liquids seek their own level...it's the principal that makes spirit levels work reliably. If there's enough material, cutting & sectioning the intake tube would have the greatest effect. Drilling a hole through the frame probably wouldn't help much since the bottom of the tank would still be at the same height relative to the carb.

I've seen small electric fuel pumps on a handful of custom bikes and the new engine should have enough electrical output to drive one. Unfortunately, I've no idea where to source one.

 

[kenfyoozed]

OK so it was diffentaly a fuel starvation problem. After adding larger hoses, and an in line filter, it all flows nicely. It started up one second kick, first kick with the choke, ran on choke for 30 seconds or so, came off choke, and around the block i went. You can really fill the power difference from the 87cc engine i had. Thanks guys . Off to other problems!