BlueMax3

My Feedback: (1)

A glow engine can be primed by putting a finger over the end of the muffler and turning the prop to draw fuel into the carb. Does this technique work with a gas engine that has a Walbro carb? I am trying to think of ways to more easily prime my Roto25 and I don't recall this approach being covered.

Thanks,

Travis

fisher1648

I prime my glow engines by opening the throttle to full and covering the intake with my thumb and flipping quick a couple times - not the exhaust. Wouldnt covering the exhaust discourage fuel draw ??

I prime my Brison 3.2 basically the same way - just enough to get some fuel in the line and entering the carb - with the ingnition turned off of course

pe reivers

This won't work.
A glow engine can be primed this way, because the exhaust pressure thus generated squirts fuel into the carb.
A gas engine has a regulated fuel pressure, so tank pressure will never get to the fuel nozzles.
You gotta use the choke, or directly inject fuel into the carb or crankcase.

BlueMax3

My Feedback: (1)

Thank you for the replies. I had never seen this mentioned, so assumed it wouldn't work, but thought I would ask. Thanks for confirming.

Travis

BlueMax3

My Feedback: (1)

One more thought I had: how about putting a fitting on the muffler near the exhaust exit so that a little gas can be put into the cylinder?

fisher1648

Doesnt covering the venturi with your thumb do effectively the same thing as the choke ?

3dd

My Feedback: (6)

yes

RCVFR

My Feedback: (7)

bluemax3

You do not want to have raw gasoline anywhere near the exhaust system (muffler). Gas engine exhaust is much hotter than glow engine exhaust. Gas is MUCH more volatile than glow fuel. Keep gas away from the muffler. Prevent forest fires.

cu. in.

Right on! Safety first.

Bruce

BlueMax3

My Feedback: (1)

I was thinking about it only for the first start of the day, when the engine is cold. That's the only time it's an issue for me right now.

Thanks for the warning though. I try to be safe.

sillyness

My Feedback: (25)

With the types of qustions you are asking I am a little concerned about your setup. You don't have the vent line from your fuel tank connected to a fitting on the muffler do you? The fitting on your muffler is NOT for the fuel tank vent line... it is ONLY there if you want to use a smoke system. I don't use smoke, so I unscrew the fitting on the exhaust and replace it with an appropriately sized screw to block the port.

Run your vent line around your fuel tank once (fore-aft - to prevent siphoning) and out the bottom of the fuselage into free air... not connected to anything.

BlueMax3

My Feedback: (1)

Sillyness,

This is my first gasser, but I am pretty sure it is setup corrrectly. I am using a two line system. The line from the clunk in the tank goes to the carb, via a "T" connection with a filler line with a fuel dot. The vent line in the tank wraps once around the tank, goes through the firewall and out the bottom of the cowl. I asked a lot of questions and had an experienced gas modeller check out my plane before the first run, just to make sure the setup was correct.

The engine runs great so far and is getting better and better. It's smooth, has a rock solid low idle, good transition and clean high end. It is also producing plenty of power for my application on a KMP Hurricane. The one issue I have is that it takes a lot of flipping to get it to fire the first time in a day, with choke on, full throttle and ignition on. I have tried various approaches suggested by others and haven't found the right procedure for this engine. It's clearly having difficulty drawing fuel into the carb because if I squirt a little into the carb, it fires on the first try. Once it fires with choke on, I turn the choke off, reduce throttle to just above idle, and it runs after five or six flips. On subsequent flights in the same day, with choke and full throttle it fires within two or three flips, so it's really just the first part of the starting procedure when the engine is completely cold that I am trying to work on. Since a gas prime directly into the carb works so well, I was thinking about other ways to do this with the cowl on. That's why I asked about a fitting on the muffler near the engine exhaust. I was wondering whether anyone else had done this so that a little gas could be squirted in there before the first flight of the day and then closed off. When the engine is running if I richen the H mixture needle, the engine rpms drop off significantly, so according to what I have read, the fuel flow is good. Please let me know if I have missed anything or if you have any suggestions.

Thanks for your concern. I really appreciate all the help and suggestions that everyone provides on this forum.

Travis

tripp3d

I would think a small squirt bottle would work instead of a choke. Usually once you can get one to "hit" on a shot of gas they start picking up fuel. I'm familiar with gas 2 strokes but not in an airplane...yet.

BlueMax3

My Feedback: (1)

That has been my experience. The engine on my plane is inside a cowl and the carb is not easily accessible. That's why I was exploring alternatives.

I ran my engine today and it took about 30 flips to get the engine to fire with choke on and full throttle. Not a problem really. I know what to expect. But it would be nice if it was easier.

Travis

Flypaper 2

An easier way to prime it is to stick your finger in the throat and pump one prop blade up and down like a pump handle. Just the piston going up and down will pump fuel without turning the motor over. Once your finger is wet then crank it over. Primes quicker with the throttle wide open, just don't forget to just crack it open to start.

davewallace

My Feedback: (29)

Hi,
If you do this with you will only get a burnt fingerLOL

Dave

pe reivers

An alternative for choke valve priming in a very cowled in setup is to press on the regulating diaphragm while flipping the prop. This can be done mechanically, or by blowing on a tube you have connected to the hole cover plate (the one with the 4 screws) to this end, solder a small tube on the cover hole.
It works like this:
the regulater keeps the fuel pressure at the needles at exactly atmospheric pressure, and thus does not allow the fuel flow pulses from the pump section to reach the jets.
Depressing the membrane lifts the fuel regulating needle, and thus allows the pump to inject fuel directly through the jets.
In the past, I have tried both systems, and they work if the carb is wet with fuel. If the carb has been run dry, blow on the tank vent to get fuel into the carb first. Thus the carb pumping action is initiated.

TLH101

My Feedback: (90)

What motor do you have that you are you having problems drawing the fuel on?

BlueMax3

My Feedback: (1)

Roto25

TLH101

My Feedback: (90)

OK, not familiar with that motor, but if there is a hole in the choke plate, you can solder it closed and it will help with the choked fuel draw problem. I did this with an FPE 2.4 and it made the initial start-up much easier. Does not effect the running of the engine at all, just the helps cold start.
There were some ZDZ motors that were sold with Bing carbs that had fuel draw problems when choked, and the US distributor swapped the carbs out for Walbros and cured the problems.

pe reivers

Not all carbs have a choke plate. In general, drawing fuel with a dry carb is a Pumper Carb problem (Walbro-Zama-etc.), not a motor problem.
If these carbs are cowled in you cannot "thumb choke" the engine, which serves to get the fuel into the carb, AND to get extra fuel into the engine for starting.
To bypass this problem there are only two ways.
One is to inject fuel into the engine using a syringe or whatever and hope the engine pulses will get the carb pump going,
and two is to get the carb to do the job for you with a bit of tank pressure help. That is what post 17 meant to explain.