I have a GMS 120. Its about done with its break in process on the test stand with all most a gallon through it, and runs like a top. But when I hold the test stand up more than about 30 degrees it will stay running at mid to high power then when I throttle down it will starve for fuel and die. When I plug the muffler and carb and blow into the fuel line there is a nice air leak around the carb retainer bolt and nut, its bad enough that when I go to throttle cut off the engine almost doesn’t want to stop because its sucking enough air threw the leaks to keep running. Would this cause it to starve for fuel at high AOA (angle of attack)? I have researched gms and they ar notorious for carb leaks and cutting out at high AOA in flight. I have read of Perry pumps for this problem or sealing the air leaks.
My Question is what’s the best material and method for sealing carbs. Iv heard red rtv. What’s the best thing to clean the aluminum before applying sealant? I would rather try this method first than using a pump. Would simply sealing vacuum leaks help draw more fuel or should I just invest in a pump for reassurance? Thank you…….. Tony

Seal it with silicone from the inside but don;t let any get into the running par of the engine

By inside I mean inside the sock in the crankcase for the carburetor

You need to get rid of all the leaks whether you go to the pump or not. I clean stuff up for sealing with Gumout. Cheap and easily available, it dries with no residue. Ross

Are you sure it's not leaking around the o ring you really have to seat those carbs down tight before you tighten the pinch bolt tower engines have the same problem or the o ring won't contact the carb and engine block
you could take it too the hardware store and find an o ring that fits the carb but has a slightly larger thickness

Now as far as your low speed problem if it doesn't go away after you seal up the carb try richening the low speed while keeping a good transition don't worry to much about how low the idle is till it's on the plane
I think some complaints of idle problems come from people expecting too low of an idle as long as you can get it down low enough for your application that's all that matters I like to keep mine as high as I can get away with as long as the plane doesn't taxi away from me on the ground and will slow down for landing

When I set up my tower engine I open the carb wot then back the idle mixture out till there's a 1/8'' gap between the needle and the spray bar then run it and set the HS needle then throttle down to idle and work the lowspeed
if you set the HS with too small a gap on the low you will chase your tail and get frustrated in a hurry just something learned right before I was going to see what my 30-06 could do to a tower engine
another tip is don't mess with the low speed until the tank pressure has had time to bleed off at idle ohterwise your setting it with higher tank pressure then when it bleeds off your lean again hope that makes sense
GMS and Tower engine don't tune in as easiely as some of the other brands do but if you take the time to learn how to set them up you will enjoy a good running powerful engine.

And don't mess with a pump unless you want to move your tank back on the CG if you can get the tank height correct and the engine has no mechanical issues it should run just fine also run the smallest tank you can for the flight time you need

The best things in life are the simple pleasures.

The pump will create as many problems as it may solve. And in your case you will still have the vacuum leak after you install the pump.

So we need to solve the leak and the idle problem first before you install a pump or you never will get it running correct.

Follow the advise of the previous poster

You need to get the carb seated properly. As the other poster already said.

Take the prop off. Loosen the bolt and nut that holds the carb on. When you loosen the bolt and nut, make sure and wiggle the carb back and forth. It sticks into the case when you initially tighten the bolt. You need to make sure it's nice and loose before you squeeze or smack it down. Take it out if you have too--and put a little 30w motor oil or sewing machine oil on the carb shank first. Any type of light lubricant will work. This will ensure that when you put the carb back in--it'll slip and slide easily without binding up. Now grab the front of the engine--with your thumb on top of the carb. Squeeze the carb down onto the case as hard as you can squeeze it. Hold it down and tighten the bolt and nut. If the O-ring is flat, then you go it sealed up. If it still looks round, then you aren't squeezing hard enough.

FlyboyDave does something similar, but he uses a small hammer and a block of wood. I think he just sets a block of wood on top of the carb and then gives it a few light taps with a hammer to seat the carb down tight. Tighten the bolt and nut.

That should fix your air leak. If it doesn't work, then use the above method of cleaning with Gumout and then use red RTV silicone on the carb shank before setting it into the case.

Tony,


Let's take a look at this one thing at a time:

1. The seal between the carburettor's body and the crankcase... If the carburettor bottom and the crankcase are both quite smooth and the O-ring is flexible and not torn; and the carburettor is pressed down against the O-ring, there is absolutely no reason for anything but an air-tight seal in this location.

If the carburettor cannot be pressed down on the O-ring... See the Tower .75 photo... No Seal!

2. The carburettor pinch bolt/draw-bar... Is the fit of the bolt part in the crankcase so loose that a really bad leak exists in this location?
If so, after the carburettor is held down and the nut is tightened, wipe the area clean with some methanol and after it is dry, put a dab of RTV silicone on both the blind end and the nut. And you've got no leak there after it vulcanizes.

3. Between the carburettor's spigot and the crankcase... If (1) and (2) are sealing OK, this is of absolutely no importance...
No need to 'dunk' your carburettor in red RTV silicone...

4. Between the carburettor's barrel and body... That is the manufacturer's fault, but a dab of silicon vacuum grease on the barrel will solve this problem for ever, even if it does make the throttle servo work a little harder. Use a thinner grade.
Fuel will not wash it off.

5. Between the crankcase and the crankshaft... This is incorrectly thought by newbies (end some 'experts' too), to be a leak through the front bearing.
The front bearing is not supposed to seal the engine! Period.
It is the crankshaft's front rotor that must seal against the crankcase and also to function as the intake valve.
No seal there - there are no excuses.
If there is a leak there, these parts, or the whole engine, must be replaced by the manufacturer, because these parts that must seal are out of tolerance and are not sealing...

And no, a sealed front bearing would not fix that!


You've had more than your fair share of leaky engines, I believe...

Would acetone work as a good cleaner berfore applying red rtv?

No.

Acetone is not a very good solvent of petroleum products. Drop a grease or oil soaked bolt into a bottle of acetone and cover it up to prevent evaporation. When you come back tomorrow--it will look like nothing happened. Already tried it.

You need to use brake cleaner from an aerosol can, or methylketone or 111 trichlorethane. Any of those will dissolve petroleum based products very nicely. I probably didn't spell those last 2 correctly. The brake cleaner will be easier to get.

You did spell them right, Chris...

Tony,


The brake cleaning solvent Chris suggested would work.
However, the goo on your engine is also soluble in methanol, since before it was deposited on your engine's parts, it was a lubricant in glow fuel...

Just using methanol is safer for the carburettor's O-ring, should it come in contact with it.
If this O-ring is silicone, I am not so sure the break cleaner would be that harmless...


...As to acetone, it can be added in low percentages (2-5%) to glow fuel, to reduce the chance of detonation, in very high compression engines...

I would use glow fuel like Dar suggested for general cleaning. If it's heavy crud, a few squirts of Dawn Power Dissolver is safe if left on for less than 45 minutes or so.

The problem with using glow fuel is that it contains oil.

If the carb leak is so bad that he resorts to using silicone to seal the carb--it will be hard to get a good seal because of the residual oil. Silicone doesn't stick to oil. It will form and flow around the oil, but it won't stick to the metal beneath the oil.

I'd use the brake cleaner.

I honestly think he just needs to squeeze the carb down a little tighter. Once that O-ring gets squished down nice and tight--should be sealed.

Chris and Barry,


This is the reason I specified methanol, or another solvent; not glow fuel.

It is also possible to use rubbing alcohol, or those injection swabs saturated with Iso-Propyl alcohol.
It smells awful, but it would get the aluminium crankcase very clean and very dry.

The RTV silicone will stick tenaciously to this surface.

If I'm cleaning a surface, I always use a degreaser afterwards. Should have said that. I didn't know you could still get trichlor.

About the replies to make sure the carb is seated tightly. Sorry I didn’t clarify that in my original post. The carb is seated very tight and like i said when I did the leak check it is leaking around the retainer bolt on the nut side and the blind side, No where around the o-ring could I detect a leak. The tolerances are just very pore with the retainer bolt in the crank case. I will try rtv around this area. Thanks guys very much for your help. Any more advice would be of great appreciation.... Tony