yard-dart

My Feedback: (35)

I just purchased an enigne (brand not important) NIB. I'm finished with the break in on it, and it runs great. I've just one problem with it, it won't die when the carb is fully closed. In fact, I can put my finger over the carb opening and there is very little change. I have to disconnect the fuel line in order to shut it down. It's quite obvious that there's an air leak. I've removed the carb and replaced it, pushing down on the O ring as hard as possible before tightening it down. I've considered taking off the backplate and replacing it using RTV sealant.

Is there anything I'm missing? If I seal the carb and backplate with RTV, is there any other place that can be letting air in?

John

Hobbsy

My Feedback: (102)

If it is held on by a pinch bolt, it is most likely leaking there, it's not uncommon.

stuk_at_work

My Feedback: (111)

Is there a rubber gasket between the carb. and the engine? Does the engine have a remote needle valve? Is it an "ait-bleed" carb.? Manuf. & model actually are important pieces of information to have if you want your problem diagnosed properly.

yard-dart

My Feedback: (35)

Hobbsy,

The carb is held secure by a pinch bolt.

Stuk,

The engine is a Mutunuc Millenium .61, an OS SF clone. It was distributed by Mecoa at one time. Yes, there is a rubber gasket aroung the carb kneck. It doesn't have a remote needle valve, the needle is on the side of the carb.

Flyboy Dave

My Feedback: (21)

It may have a loose fitting barrel (carb) in which case you will have to
live with it. I use a clamp on those pinch bolt carbs....when I see the o-ring
is pressed down, and distorting, I tighten down the pinch bolt.

FBD.

N1EDM

My Feedback: (2)

If none of the suggestions above seem to help (they're the first place to start), there's a chance that you could be leaking air via the front bearing (assuming that the rear crankcase cover is airtight). The cure would be to replace the front bearing. The folks at Boca Bearing will be able to help

This (bearing) situation is a little more obscure - try this only after you've eliminated the other suggestions first.

Hope this helps,

Bob

yard-dart

My Feedback: (35)

I'll start by RTV'ing the backplate. If that doesn't work, I'll RTV both sides of the pinch bolt. If that doesn't work, I'll RTV around the carb, which I really don't think is the problem area. It's pretty darn tight right now. If that doesn't work, well, I'll just have to pull the fuel tubing off the nipple, cause this engine runs too good to get rid of.

Thanks,

John

JoeAirPort

My Feedback: (41)

One time I loosened the spray bar arm to change the position for the throttle servo and caused an air leak. All I did was put soapy water around the carb and blew into the air intake to find the leak. I could hear it as well as see it. I just tightened the spray bar nut and all was well.

piper_chuck

My Feedback: (12)

I'd concentrate on the carb area before things like the backplate. A leak in that area would be more likely to result in an air/fuel mixture capable of supporting idle.

N1EDM

My Feedback: (2)

I like that idea about the air bubble mixture... I suppose you could use dishwashing liquid and water... I've got an irvine that has been giving me problems. That might help find it.....

Bob

yard-dart

My Feedback: (35)

Update!!!! I mixed up some dish washing liquid and water for trying to find the air leak. Well, it worked, but the leak wasn't where a few of you had thought. N1EDM came the closest to calling the problem, although it was his last idea. There is a pretty major leak at the front bearing. I opened up the carb, doused the area with soapy water, and blew through the carb intake, while holding my finger over the fuel inlet nipple. And there they were, bubbles for days, appearing around the area where the shaft exits the front of the engine.

This has to be the problem area. When I ran the engine for the first few times, I noticed that it was a lot harder to shut down the engine after it had ran a while, while it was hot. When I would only run it for a few seconds and shut it down, it came a lot closer to shutting down, but not completely. I guess when the engine and bearings heat up and expand, the gap around the bearing and maybe the gap between the shaft and bearing, gets larger, causing the leak to be worse. This is the only thing I can think of that could cause the leak to be worse at that time.

Anyway, that's what I found. Now, it's quite obvious that I need a new bearing. I can probably get one from the local bearing shop. What do I need to put into consideration when buying another? Does it need to be slightly larger in diameter on the outside/inside? Does it need to be a sealed bearing? What do you think?

John

C_Watkins

My Feedback: (10)

You'll likely have to get the same sized bearing or it won't fit properly.
A sealed bearing might be helpful, if you're lucky. If you do purchase a sealed bearing, you
probably want to at least pull the seal off the "inside facing" side of the bearing, for lubrication.

Hobbsy

My Feedback: (102)

It will leak less if you remove only the outside seal, I have many Fox engines and they have no outside seal, they do not rust and they do not leak.

yard-dart

My Feedback: (35)

Hobbsy,

How does the bearing get lubricated if the inner seal is left in place? I would think that the inner seal should be removed, for lubrication like Watkins said. Another question. If I were to get a replacement bearing with no seals at all, will it leak? Are bearings without seals supposed to leak air, or are they built to where air doesn't pass through them? Where do you think the current bearing is leaking? Where could it leak?

To make this a lot more simple, what type of bearing do I need to buy? If a sealed one, which seal, if either, needs to be removed?

This is new to me, so stick with me on this one,

John

Hobbsy

My Feedback: (102)

My oldest Fox .74 has many many hours on it both as a glow and as a Diesel and the front bearing looks like new. In your engine there is most likely a channel cut in the crankcase under the carb that goes to an area near the front bearing. This channel stops just short of the front bearing on most engines to get oil near the bearing but not directly expose the bearing to crankcase pressure. On some engines this channel is straight, the Fox's have a spiral groove so that the crank rotation works to screw the oil back toward the carb and away from the front bearing. It sounds all wrong but sure does work well.

Hobbsy

My Feedback: (102)

John, here is a picture of a couple of engines with no front seal, Fox .74 on the left, Fox .46 center and MVVS .91 on the right. I tried to take a picture of the groove in the .74 crankcase but the camera would not focus that close. The oil groove in this .74 crankcase is straight and not spiral like the old one.

yard-dart

My Feedback: (35)

Hobbsy,

Are all of those bearings sealed on the inside?

Hobbsy

My Feedback: (102)

John, yes they are, some Boca Bearings come with a green seal on one side and a Black seal on the other, on a one seal application they tell you to remove the green one. I never asked why though, I should, it would good to know. If you ever examine a seal closely they are beveled outward a little from the shaft to keep things in the bearing, I think they would let oil under a little pressure enter. The MVVS front bearing leaks an occasioal drop of oil which kinda backs up my thinking.

BillS

John,

Since ball bearings never form a very good seal you might want to compare the leakage to another engine in your collection. It is suspected that you will find that they all leak to a certain extent.

Bill

yard-dart

My Feedback: (35)

Bill,

I'll check some others. If this is the case (all do leak to an extent), what do you think is causing my engine to not shut off. It has to be from an air leak, and a pretty big one. Like I stated earlier, when I put my finger over the carb opening there is very little change in the RPM's. The engine is evidently getting air from somewhere. When I did the leak test with the soapy water, the front of the engine where the shaft exits was the big leaker. There weren't really any other spots that bubbled up, which isn't odd since all of the air pressure was leaking through the shaft output area.

Any other advice as where to look?

John

Flyboy Dave

My Feedback: (21)

Try a small rubber o-ring on the shaft before you put the prop stud
washer on. Also you can clean the outside of the bearing up rear good
and use some medium CA around the outside of the bearing to seal it
to the case.

Is your bearing unsealed, or unshielded ? A bare ball bearing ?

FBD.

yard-dart

My Feedback: (35)

I really don't know. I haven't checked it yet. That's what I've been discussing with Hobbsy, wether or not to use a sealed bearing as a replacement. And which side the seal needs to be on. The super glue idea isn't bad either.

jaka

Hi!
Sorry! But its not the front bearing that is the problem...all front bearings leak fuel, that's the way they are sopposed to work...but no air is going in that way.
The problem you have with leaking air got to be either the carb or the rear cover that sucks air...most likely the carb.
Regards!
Jan K
Sweden

yard-dart

My Feedback: (35)

Jan,

Then what do you reccommend to be the fix, if the carb is the problem? I know what to do if it's the backplate that's leaking.

Motorboy

Better to have a little leak in front bearings to lubricate and cooling the plain bearing.
Not possible to get air inside bearing cause (indolence?)/ inertia oil + little tolerance in bearing.

Jens Eirik