Engine Trouble Mysterious Starting Issue + Bogs at WOT

apsci

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Hello,

I am having some issues with my "70cc" Chinese 2 stroke. So, starting is a pain. I have to pedal with the clutch engaged for at least 200m or 300m total until the engine starts to show signs that combustion is happening, then two attempts later the engine does indeed start. The carburator e-clip i have set to both "low" and "high" positions for rich and lean fueling until 3/4 throttle, but the results are the same. The carburator is the NT Speed, the one with the red back. The spark plug was black and filled with weird fluff when i removed it. It also was black when the c clip was set to lean fueling. I am suspecting i added too much 2 stroke oil to the premix, since i forgot to measure if the gasoline i added really was one liter. The red back I have removed to add airflow, but i get the same issue. Also, the engine begins to die after 20min of operation. At full throttle, it bogs down. At 3/4, it runs best and achieves the highest speed.

To summarize my setup:
- NT Speed Carburator, stock, back removed
- C clip currently one level under leanest setting
- Fuel may have contained too much oil, yet I have also topped it up with fuel without any oil in it, and I have the same issue.
- Running the engine dry, with petcock closed, then turning the petcock on and pedaling leads to the same starting issues, so I don't suspect flooding.

If I kill the engine, it might start again easily without much pedaling, but if I wait 5 minutes, I have to pedal as much as starting with a cold engine.

I would like to thank everyone who took the time to read this, you are all great people.
 
Welcome,

People tend to have nothing but trouble getting the NT carbs to run right. The BoFeng carby is a much easier and user friendly carb to play with.

It sounds to me like you are running lean. Check for vacuum leaks around the intake, the case, and the bottom of your cylinder. The engine dieing after 20mins tells me it's either fouling the plug or is overheating.
 
Thank you for your reply. This is what my spark plug looks like after using gasoline without any oil added. I am suspecting that it is running rich, so you are very likely right about fouling the plug.
Is this normal? I am at ~0m elevation, it is very hot. But is this enough to cause a rich condition even with the standard jet?
What should I need to change in this case, the jet or the needle? I don't have time to order from Alibaba, so I will have to ask motorcycle repair shops what they have in stock.

P.S., I think we have a misunderstanding. Google tells me the BoFeng is the carburator with the red back, or is it the one with the skull shaped black back? Mine is the red one

P.P.S, I forgot to mention, when i choke the engine, it dies instantly, let alone start. So very likely its running rich. But why?
 

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Edit: Sorry for long post

Where I live it is at sea level.. zero elevation… and no hills… flat lands.

I help a few guys build and tune their engines and bikes.

Right now there’s 2 running the NT speed carb. One has a black filter cover and the other has a red filter cover.

Both engines run fine…strong on this type carb and the needle clip is within 1 level of center position on both.

My suggestion to help your situation would be drain all the fuel and mix up a batch with an exact measurement on both oil and fuel.

I would flush the tank with plain gasoline while it’s empty and take the petcock off to be sure the little screen is not restricted with junk.

Clean the air filter sponge.

Get a brand new spark plug… NGK BR6HS. The reference number is 3922.

I would also check the muffler to be sure there’s no build up at the opening. If you’ve been running lots of too much oil it could cause gook to build up in the pipe. I just had one like this. It was so built with carbon junk it restricted air flow.

Then fuel it up and go.

When all the conditions are correct these simple little engines usually fire up and run fine.

I did not recommend an oil ratio. Most run more oil than I do. I run 40:1 from the first drop of fuel but most guys will not agree with this.
Most go with 32:1 or 24:1 mixed correctly.
Some manufacturers recommend more oil so this will depend on preference and manufacturer recommendation.

Never mix the fuel and oil in the bike tank. Always use a clean fuel can with precise measurements and carefully shake it to thoroughly mix the fuel and oil.
 
I run a 24 to1 on brake in then 36 to 1 flatter. Conventional oil or se.i synthetic should be used for brake in, then full synthetic after brake in. Brake in is 2 or 3 tanks of gas
 
Edit: Sorry for long post

Where I live it is at sea level.. zero elevation… and no hills… flat lands.

I help a few guys build and tune their engines and bikes.

Right now there’s 2 running the NT speed carb. One has a black filter cover and the other has a red filter cover.

Both engines run fine…strong on this type carb and the needle clip is within 1 level of center position on both.

My suggestion to help your situation would be drain all the fuel and mix up a batch with an exact measurement on both oil and fuel.

I would flush the tank with plain gasoline while it’s empty and take the petcock off to be sure the little screen is not restricted with junk.

Clean the air filter sponge.

Get a brand new spark plug… NGK BR6HS. The reference number is 3922.

I would also check the muffler to be sure there’s no build up at the opening. If you’ve been running lots of too much oil it could cause gook to build up in the pipe. I just had one like this. It was so built with carbon junk it restricted air flow.

Then fuel it up and go.

When all the conditions are correct these simple little engines usually fire up and run fine.

I did not recommend an oil ratio. Most run more oil than I do. I run 40:1 from the first drop of fuel but most guys will not agree with this.
Most go with 32:1 or 24:1 mixed correctly.
Some manufacturers recommend more oil so this will depend on preference and manufacturer recommendation.

Never mix the fuel and oil in the bike tank. Always use a clean fuel can with precise measurements and carefully shake it to thoroughly mix the fuel and oil.
That's my issue, I mixed it in the tank. I was in a hurry and thought nothing would happen.
Yes, a new spark plug would not hurt either. But here's another issue: That spark plug I posted got so dirty while running on fuel without any oil in it whatsoever. Would you say this issue could go away if I finally start adding properly premixed fuel with the right amount of oil?
Maybe oil got stuck in the petcock ? But I think that's unlikely.
Did these friends you helped rejet their carburators or are they fully stock?
 
You could have had alot of carbon buildup in the cylinder from the oil, that could have fouled your plug. The stock plugs in these kits are horrible, you get a noticeable increase in power with the ngk.
I have attached some photos of the exhaust. If this is not normal, should I clean the cylinder walls, somehow, too?
 

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You definitely have alot of excess oil in there. Use some carb clean to get the majority off. Run some seafoam or som marvels mystery oil and it should clean the cylinder.

You plug definitely looks like it is running rich, do you know how to adjust the carburetor needle for the run mixture?
 
Like others already said you need to check all your nuts and bolts on the engine. Make sure everything is tight and the gaskets are in place. Also the air filter cover can be red or black on the nt and the bofeng so the filter cover color really means nothing
 
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