Engine won't start, runs only on choke and almost fully lean

Otto Neumann

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My engine barely runs with the carburetor needle set at the second notch from the top side, and runs exclusively with choke on; if the choke is lowered, the engine dies out immediately, the engine dies out as well if I roll to a stop.

Do you have any idea what could be the cause?

I suspect that it's either the coil or the stator to have something wrong:
It might be a weak spark caused by the high compression environment, thus its incapacity of handling richer mixtures, I'm definitely going
to swap the stock plug with an NGK B6HS, which is already on its way to me.

Shall I swap the stock CDI with a better one, such like one from a GY6 scooter, which I got readily available at my trusted store?
Shall I get a new stator coil?
Sadly it will take a while for a new stator to arrive down here, motorized bicycles are not that popular in EU, and many spare parts are hard to get in short times.
 
Crassius is on point as usual. The fact that it only runs with choke on usually means it is extremely lean likely due to a clogged jet or massive air leak. Cutting off air by choking it increases the fuel ratio since there is less air and the same amount of fuel.

I have my own clogged jet story as well...I made the mistake of trying the Kehein clone carb+reed box and the bike would barely accelerate with the choke off and had to almost go WOT to move. It topped out at ~15mph and I turned around before even making it 1 block during testing. I tried changing the main jet and it made no difference, and when I removed the slow jet to size it, it was basically closed from the factory...barely a pin prick orifice since it was blocked off with a burr or something. Replacing it with the same sized new slow jet from the kit I got instantly cured the low to mid range issue. Note that even after fixng that the top end sucked, and after testing 5-6 different combinations of jets without breaking 25mph I ended up throwing a bone stock NT carb and it instantly did 28mph again with much better mid range. I even tried removing the reed petals and it increased by 1mph but was still slower than stock.

Two morals to this story - check your main jet for obstructions, and the stock carb is easy to tune and plenty good for 90% of builds :).

*edit* Your post beat mine while typing, but if you are sure your jets are clear and properly sized than the next step would be checking for vacuum leaks. Don't forget abnormal spots like throttle cable cap housing and even crank seal.
 
instead of b;owing, I turn it upside down and look at the shy thru it - should see a nice, round hole (if not looking round, there may be something in it still)

NOTE: if motor isn't firing and you have fuel, the plug will be REALLY wet even with choke off
 
Crassius is on point as usual. The fact that it only runs with choke on usually means it is extremely lean likely due to a clogged jet or massive air leak. Cutting off air by choking it increases the fuel ratio since there is less air and the same amount of fuel.

I have my own clogged jet story as well...I made the mistake of trying the Kehein clone carb+reed box and the bike would barely accelerate with the choke off and had to almost go WOT to move. It topped out at ~15mph and I turned around before even making it 1 block during testing. I tried changing the main jet and it made no difference, and when I removed the slow jet to size it, it was basically closed from the factory...barely a pin prick orifice since it was blocked off with a burr or something. Replacing it with the same sized new slow jet from the kit I got instantly cured the low to mid range issue. Note that even after fixng that the top end sucked, and after testing 5-6 different combinations of jets without breaking 25mph I ended up throwing a bone stock NT carb and it instantly did 28mph again with much better mid range. I even tried removing the reed petals and it increased by 1mph but was still slower than stock.

Two morals to this story - check your main jet for obstructions, and the stock carb is easy to tune and plenty good for 90% of builds :).

*edit* Your post beat mine while typing, but if you are sure your jets are clear and properly sized than the next step would be checking for vacuum leaks. Don't forget abnormal spots like throttle cable cap housing and even crank seal.

The jet is the stock one, shall I drill it wider? I'll make sure to check it out again
 
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instead of b;owing, I turn it upside down and look at the shy thru it - should see a nice, round hole (if not looking round, there may be something in it still)

NOTE: if motor isn't firing and you have fuel, the plug will be REALLY wet even with choke off

I'm giving it a try, luckily my float bowl is still empty, I need to get some gasket maker or an o-ring to shove in the carburetor
 
Oh nope, before cranking up the engine for the first time, I dropped through the spark plug's opening a dab of 2 stroke motor oil just for prelubrication purposes, and I carefully turned it by hand with the spark plug tool, I cannot risk to damage the new piston
 
Crassius is on point as usual. The fact that it only runs with choke on usually means it is extremely lean likely due to a clogged jet or massive air leak. Cutting off air by choking it increases the fuel ratio since there is less air and the same amount of fuel.

I have my own clogged jet story as well...I made the mistake of trying the Kehein clone carb+reed box and the bike would barely accelerate with the choke off and had to almost go WOT to move. It topped out at ~15mph and I turned around before even making it 1 block during testing. I tried changing the main jet and it made no difference, and when I removed the slow jet to size it, it was basically closed from the factory...barely a pin prick orifice since it was blocked off with a burr or something. Replacing it with the same sized new slow jet from the kit I got instantly cured the low to mid range issue. Note that even after fixng that the top end sucked, and after testing 5-6 different combinations of jets without breaking 25mph I ended up throwing a bone stock NT carb and it instantly did 28mph again with much better mid range. I even tried removing the reed petals and it increased by 1mph but was still slower than stock.

Two morals to this story - check your main jet for obstructions, and the stock carb is easy to tune and plenty good for 90% of builds :).

*edit* Your post beat mine while typing, but if you are sure your jets are clear and properly sized than the next step would be checking for vacuum leaks. Don't forget abnormal spots like throttle cable cap housing and even crank seal.

I got an update!
I checked the jets as suggested, they are perfectly clean; I improved the seal around the carburetor by wrapping around the outlet
some aluminium tape, and then I placed the brass piece back above it, now the engine fires even more readily, at almost fully rich mixture too this time.

While I were fitting the carburetor back into the collector, my eye fell on this

photo_2018-08-20_16-47-37.jpg
photo_2018-08-20_16-47-33.jpg

This seems to be the only anomaly around the whole crankcase's gasket, how shall I proceed?

I know that I'll have to open the engine, and I'm kinda reluctant, I don't know how to proceed.

I might bring the motor to my trusted mechanic, which fixes any gender of bikes, and I'm sure that if I'll show him the problem,
he will get rid of it
 
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