engine starts easily now. it takes 30-50 quick flips to prime it while fully blocking the muffle and carb holes. usually doesn't need to be primed when engine isn't stop for too long. there are bubbles, don't seem to bother much of the engine running , but maybe causing the tomorrow to fluctuate.

Turk,
when u said close the HSN, the LSN still opens, right? how would this stop the fuel from feeding the carb while LSN is still open?

Originally Posted by kentli22

Turk,
when u said close the HSN, the LSN still opens, right? how would this stop the fuel from feeding the carb while LSN is still open?

That glow type carb. controls fuel mainly only by HSN.LSN is only for metering of that main allowance.
You dont need to close muffler because there is no pressure relation with tank unlike glow.It seemed too much flip to me to prime.Make sure on WOT while choking the carb mouth during priming.(be very careful for no ignition on priming)

I see. Thx!

Originally Posted by kentli22

engine starts easily now. it takes 30-50 quick flips to prime it while fully blocking the muffle and carb holes. usually doesn't need to be primed when engine isn't stop for too long. there are bubbles, don't seem to bother much of the engine running , but maybe causing the tomorrow to fluctuate.

That seems to be a bit much.... but that may be caused because you block the muffler too: this causes the engines pumping function to cease.

Try only closing the carb inlet, leave the muffler open, and depending on fuel tubing length, it should prime within say 5 flips, allowing for some air leakage Theoretically, a 9 cc engine should prime way over one metre of fuel tubing per flip, in reality maybe 10-20 cm is achieveable, so it should only be a few flips to fill the fuel line and get some fuel in the engine.

Brgds, Bert

hsn 6 turns out, lsn 2 turns out, idle is stable at lower 2000rpm range. but at any throttle position, rpm is relatively not constant, it always quickly jump up and down within somewhere within a 500prm range. wonder if this is due to the bubbles feeding to the carb or because the engine is being still in the run in period? But otherwise, the engine is running fine.

Yeah the bubbles can cause the engine to run more lean momentarily.
Have you positioned the pump so it is isolated from the engine vibrations?
That usually stops the bubbles from forming.

the pump is now hanging by the 3 fuel line behind the firewall, suspended freely in the air. it seems to vibrate less than previously when I had it mounted on the back of the firewall with a thin layer of foam in between. but due to the fact that it asks to keep the fuel line short, i have not yet figure out how to have it properly isolated from the engine yet. now I have about 6 inches of fuel line from pump to carb. if it is ok to have this fuel line longer, then to isolate it form vibration will be much easier.

Brutus,
when I flip 60 times, engine will start easily each time. when I flip 50 times, it starts but may quit immediately, when I flip less, it wont fire. As I look at the fuel traveling in the line, I see that the fuel advance to the carb as I flip, but as each flip motion ends, the fuel will draw back out of the carb, so as I am flipping, this in and out motion contiunes, so it seems the net amount of fuel enters the carb per flip is not that much, thats why I think I need to flip so many time to accumulate enough to get it going.

when I flip, I fully block the carb valve opening. I no longer block the muffle.

Originally Posted by kentli22

when I flip, I fully block the carb valve opening. I no longer block the muffler.

Ken,


Any internal combustion engine is essentially an air-pump.

If it were a glow engine with a muffler pressurized fuel-tank, then when you block the muffler exit, or the carburettor intake (not both at once); you are doing virtually the same thing...

Plugging the carburettor intake and turning the prop over, will obviously cause fuel to be sucked into the engine through the carburettor.
Plugging the exhaust and turning the prop over, will cause air to be pushed into the tank; in turn causing fuel to be injected into the carburettor.


With a spark-ignition gas engine, which has a pump-equipped carburettor; including the NGH 9cc; one would normally not use the exhaust to pressurize the fuel-tank.

If this is the case, I see no point in plugging the exhaust; if the goal is priming the engine to ease its start...

DarZeelon,
Ok, I got it, thank you for the explaination.

Originally Posted by kentli22

Brutus,
when I flip 60 times, engine will start easily each time. when I flip 50 times, it starts but may quit immediately, when I flip less, it wont fire. As I look at the fuel traveling in the line, I see that the fuel advance to the carb as I flip, but as each flip motion ends, the fuel will draw back out of the carb, so as I am flipping, this in and out motion contiunes, so it seems the net amount of fuel enters the carb per flip is not that much, thats why I think I need to flip so many time to accumulate enough to get it going.

Possible.... but it seems strange. If I prime a normal glow engine of around 8.5~10 cc, I typically draw 5 to 10 cm of fuel with each flip, and fuel will not really flow back (maybe 1 cm) as long as I keep my finger on the carb.

Your engine has a pump that contains at least two non-return valves.
So basically, you have a slight air leak somewhere (which might also cause the bubbles in the line. Ideally there should be none) and the non-return valves seem not to do their job properly.

I would check these points and most likely, you will be rewarded with less flips for priming and a vastly improved RPM stability.

Brgds, Bert

ok, i will check for air leak. but I remember the fuel line behaved the same way since before, now is with brand new fuel lines.
you mentioned to allow some air leakage when priming? does it mean I should not fully plug the carb intake when flipping the prop, but instead leave a tiny opening to allow for some air to go inside?

Originally Posted by kentli22

ok, i will check for air leak. but I remember the fuel line behaved the same way since before, now is with brand new fuel lines.
you mentioned to allow some air leakage when priming? does it mean I should not fully plug the carb intake when flipping the prop, but instead leave a tiny opening to allow for some air to go inside?

No... that was referring to the choke valve of most engines: they do not close fully, because they are usually intended to be used in combination with some form of "powerstart" and then the smell opening serves to prevent hydrolock.

For priming you need to close the carb inlet fully with your finger.
The fact that your hoses are renewed does not mean that you solved every leak, and the fact that it behaved like that before, is also NOT an indication that all is OK.

The pump/regulator can leak as well, and there can even be a small airleak in the area of the tank or the clunk

Please check if you can see (with running engine) where these airbubbles originate. For example, do you see bubbles after, but not before the pump, or do you see bubbles entering the pump coming from the tank, etc etc.

On ANY engine, airbubbles in the fuel line is not a good thing, but some engines can handle it better than others.

Brgds, Bert