scubaozy

Hi all,
I recently bought a second hand plane which came with a 51cc gas engine with walbro carburateur. The plane was never in the air and engine suppose to have 30 mins of run time on the ground. The catch is, last time engine was run, it was 5-10 years ago. I took the plane to the club yesterday and after a lot of flipping and a sour right arm, I managed to get it running. Problem is that the engine is suppose to turn 22x12 prop above 6000 rpm. Best I was able to get was 4800. What weird is that, when I go full throttle, there is a gas leak/spray coming out the engine bay. I believe the leak is around the choke arm but I cant exactly put my finger on it. There is a lot of leak, I suppose this can't be right, perhaps this is also the reason that the engine is not reaching to max power. What do you think is the problem I am having? I am considering: a) order a service kit and rebuild the carb b) buy a new carb. I don't mind paying even a new carb if the engine will run good afterwards but I want to make sure that the problem is in the carb. Btw, I have a brand new ngk plug in it and the color looks ok after running an hour or so.

w8ye

My Feedback: (16)

A 22 X 8 prop would be better for 51cc

scubaozy

Manual suggests 22x12 and or apc 22x14. Surely I will get more rpm with 22x8 but I would like to get close to expected rpms for recommended prop..

captinjohn

Changing the prop may help the RPM, but it will not fix a bad carb leak.

flyinwalenda

My Feedback: (5)

Having not been run for that long equals a carb with hardened pump and metering diaphragms as well as varnish in the metering valve and dirt in the screen. Get a rebuild kit , disassemble the carb, clean it and install the new parts.

All Day Dan

My Feedback: (5)

Rebuilding the carburetor is always a good first step. Be sure to replace the insulator gaskets that are against the cylinder and the carburetor. Dan.

Truckracer

My Feedback: (19)

What engine do you have? Brand? If you don't know, is the engine a rear intake reed valve engine or a side carburetor, piston ported engine? Some piston ported engines are known to spray some fuel out the carb intake especially when transitioning from low to high speed.

At any rate, as W8YE suggested the recommended prop sizes would be an extremely heavy load for an engine of that size. I would also recommend a 22-8 as a starting size.

scubaozy

The engine is BFM 51 with walbro hda 103a. It is an older engine from early last decade. It is known to produce similar output as zenoah 62. I don't intend to reduce the prop size. I would like to figure out where the problem is. I opened the carb and it looked clean, in fact quite new. I also did a blow test to metering valve/membrane and it looks like it is working ok.

Truckracer

My Feedback: (19)

As a basis of comparison, a G62 will not turn the RPMs you are expecting with the prop sizes listed.

scubaozy

According to specs it should.I am adding couple of photos of the carb and the engine.

Bob Pastorello

My Feedback: (198)

Brand of prop makes an unbelievable difference in load, as does the "stated" diameter and pitch. With that in mind, and definitely for what it's worth, there probably has NEVER been a genuine 50cc engine capable of the rpm you're expecting with even the flattest 22 x 12 (Zinger - Probably worst props on the planet, pitch is always shallower than stated, but shape of airfoil is so awful that the rpms indicated never match thrust of other brands similar size/pitch.) One of the best props out there are carbon, Falcon and Xoar are quite good, but they absolutely do not ever flex, which translates to greater load to engine, which means even lower rpm on a given engine.
RPM targets for any given engine have no meaning whatsoever unless there are good comparative readings of same prop, same engine.

flyinwalenda

My Feedback: (5)

Again , as far as the carb, rebuild it with new parts. When looking for a kit get one that has the black or tan fuel diaphragms,,,not the blue one.

Instructor

My Feedback: (16)

30 mins of running on the ground is not enough time to get the RPM's you are looking for. Some of these engine take hours to break in and get even close to those RPM's. A velocity stack on the carb will get you more RPM's and less fuel spraying from the carb. You will have to lean the high speed needle a little more when using a velocity stack. Make sure the 45º angle on the velocity stack is facing forward.......

Larry/Instructor

irocbsa

My Feedback: (12)

The specs are wrong and a number of very qualified people here are gently trying to convince you of that. I'm not nearly as experienced as they are but I can tell you with a certainty that you are overpropping that engine. Listen to the advice you are being given. Rebuild the carb and use a 22 x 8 or a 21 x 10 prop. You'll be much happier.

Truckracer

My Feedback: (19)

From looking at the photos, you may not have a problem. That is a piston ported engine with the carb hanging in a very large opening in the cowl. Airflow through and past that area may be sucking fuel from the carb which is very common for that kind of engine. Certainly make sure the carb is in good condition by doing a rebuild or other means but don't be surprised if the fuel spray continues. As suggested, a velocity stack may help the situation by containing the fuel spray so it can be re-ingested by the engine.

This is a normal condition for piston ported engines when the carb is in the airflow stream. I have seen fuel spray so bad on certain engines that it would coat airplane parts in the airstream behind the carb location. Some people place baffles in front of the carb to reduce this problem and as mentioned, a velocity stack can also reduce the problem.

Edit: Looking closer, I see where that huge opening appears to be a hatch opening that would normally have a cover over it. When you were running the engine, was the hatch on or off? i still suggest most of the fuel spray is coming from the carb intake.

scubaozy

Yes there is a hatch on top. Hatch also contains a small venting whole, i believe the builder was worried of carb starving for air. I tuned the engine first without hatch and the gas was mostly on the firewall and the below the carb on the engine. When running with hatch on, it was leaking from the venting whole on the hatch. The more throttle the more leak.

Navy_Flyer

Did you change out the fuel lines when you got the plane? Fuel lines will get hard over time when sitting that long and you may have a leak under pressure. It will cause spraying and lack of power.

BobH

My Feedback: (2)

Check that lever position in the first picture. It should be NO HIGHER than the top of the carb. Measure with a straight edge to be sure. It looks high to me.. maybe its just the angle of the picture?

scubaozy

Thanks for the comments. I have not changed the fuel lines because they are nice and soft, I also added a clip to the carb nipple to make sure nothing is leaking there. The metering valve lever is flush with the edge, I think the angle of the photo makes it look too high.

shooboy

My Feedback: (52)

You absolutely need to spend $5 and get a rebuild kit. No matter how good the parts look anything that was exposed to gas that long ago will surely have some stiffness that you will not be able see. I replaced just the pump diaphram and gasket on the other side of the carb on an engine recently and it made a considerable difference. The old parts looked fine too. I had to retune the engine afterwards just from those changes to fresh parts. That's the first thing you should do.....Shoo

scubaozy

Yes i already ordered a kit. I am also looking into getting a velocity stack. Truckracer is very right, this engine is a piston ported engine. So, I guess when the hatch is closed, mix becoming too rich and excess fuel is sprayed. It could also be that prop is too big and because the engine cant turn the required rpm, unburned fuel is leaking. But as said, I have read many reports what this engine can do, so for sure 4800 is extremely anemic with the current prop.

spaceworm

Tan is Teflon, blue is acetate. black is rubber. Rubber (black) does not hold up to ethanol laced gas. I suggest either blue or preferably tan, based on my experience as well as others. Maybe you do not have gasoline corrupted with ethanol?

Sincerely, Richard

scubaozy

Yes I use a fuel without ethanol.

flyinwalenda

My Feedback: (5)

Correct.
Black pumps the best but wears out/hardens first
Tan pumps good and wears good......middle of the road choice.
Blue pumps ok but wears the best/lasts longer

Truckracer

My Feedback: (19)

The tan pump diaphragm is pretty much standard equipment in most new carbs these days. The regulator one remains black. Frankly, I have had very few problems with any diaphragm in recent years.