houckj

I have a used GMS 47 that has been sitting in its box for about 2 years. I started it about a week ago in my back yard and it seemed fine but I did not advance the throttle because I did not want to be a noisy neighbor. I put into a plane for the first time and tried to start it at the field.
After I started the engine I advance the throttle and the engine would quit very suddenly. If I slowly push the throttle it sometimes advances to full throttle but just as often will quit suddenly. I have tried switching plugs (Fox to OS #8 and K&B ?) but no change. When I first started it at the field the engine had thick black oil spraying onto the wing but it cleared up after a few minutes.

Any ideas. Not sure where on this forum to look for general engine behavior problems.

Thanks.

Mr Cox

Sounds like the main/high speed needle is too lean. The factory settings means nothing, you will always have to tune both needles your self and they should also be checked each time you're flying (in particular the main needle) to adapt to daily changes in daily weather etc.

MJD

My Feedback: (1)

Yeah, exactly. Step 1.

Failing a needle setting issue, blow out the spraybar and lines, check all the lines for leaks, including in the tank. Congealed oil sometimes can cause partial blockages restricting flow, and lines can cause fits for any reason they feel like.

MJD

houckj

Thanks. I will check it out tomorrow. I had the needle valve opened about 4 turns and thought that should be enough.
Also. I was noticing that the line from the fuel tank would clear after priming (covering the carb and turning over the prop 5-6 times). The line from the needle to the carb would stay ok. I thought it was because the fuel draw is a little long vertically because of the plane design. Would trying to raise the tank have any effect.

Bigshark

My Tower .75 exibits this behavior when flooded. The engine will idle all day BACKWARDS and will usually die with any throttle application at all. Just a thought......

MJD

My Feedback: (1)

Sure, if you cranked the needle open from where it was with no effect then start hunting. If the fuel can run back from the needle to the tank but does not take the fuel in the line to the carb with it, does that not suggest an air leak in the area of the RNV? You're sure there are no hairline splits or whatnot in the lines there? Can you seal the NV with fuel line over top the needle to spraybar area?

Are you the guy that used the engine? Or someone else?

How much fuel lift is there? Normally, you should be able to tune the engine fine in a level condition even if the tank is a bit low, but you would see issues in the air.

MJD

jimmyjames213

My Feedback: (2)

your idle is way to rich, highspeed prob to rich as well.......you are just flooding the engine. lean your low speed about .5-1 turn if it doesnt help turn it more.....if it still doesnt help you were too lean on the low end and reset it back to where it was but richen it half a turn

houckj

I did not make it to the field to test because of bad weather.
The engine was purchased used from a flier that had moved on to bigger airplanes.
I checked my tank and found that the black plug was a little loose so I tightened it. I also decided to move the needle valve from the back to the front and replaced the tubing. That should eliminate the possibility of air leaks. I also verified that the tank was secure. My next step will be to adjust the carb so that it angles slightly back so I won't be sticking my fingers in the prop while trying to adjust the HS needle valve. I plan to start with the needle valve open about 4 turns to start with. It may be a day or two before I get a chance to try it. If I still have the problems I plan to remove the engine and modify the model so that I can raise the tank. I could mount the tank above the battery with a little enlargement of the firewall hole between the tank and engine.

Thanks for the advice.

jimmyjames213

My Feedback: (2)

your going to have to adjust the engine when its running, just be careful and you wont hit the prop, even if you hit the prop u just get a little knick (think paper cut) from the back of the engine. just be safe about it and you will be fine, i know its indimidating...been their...

houckj

Tested my engine out Saturday after converting to a front mounted carb, replacing the line and removing and cleaning the carb. I had one of our locan engine experts help. The think he noticed is once the engine started there was little or now response to changed in the HS needle valve. We kept looking for air leaks and found none.
When putting the carb back on the engine we noticed that when you tightened the bolt to secure the carb the carb rotated clockwise. The needle valve got closer to the prop. Inspection showed a gouge in the carb where the tightening pin was trying to grip the carb. I suspect it had been overtightened once in the past. At this point I plan to disassemble the engine and look for any loose volts etc. I may have to find a new carb.

Mr Cox

If there is no response to changes in the HS needle then there is most likely some obstruction of the fuel delivery that limits the flow rather than the HS needle.

A small mark on the side of carb throat is nothing to worry about. The important thing is that the gasket seals.

MJD

My Feedback: (1)

This is a GMS after all - have you looked into the fuel inlet of the carb to see if the through hole in the fuel inlet nipple lines up with the hole in the spraybar? There have been many reports of mis-drilled holes causing fuel flow restriction - have you seen those? The cure is to match drill the hole through, deburr and clean it.

estradajae

You may have the fuel and pressure lines interchanged....... check that first....if the tnak is ok and lines ok...then you should go to check if the brass insert below the fuel nipple in the carb is missaligned...and redrill it if necessary.

the Wasp

My Feedback: (1)

LOL
one of my flying buddies had a GMS 61 and it too Flamed-Out as the Throttle was advanced, it would run up to about 60% Throttle but over that it would Flame-Out, well I wanted to stay out of it but after 3 guys worked on this Engine for 3 hours it still would Flame-Out, so after 3 hours I could not stay out of it any more, so I walked over and saw it had a Wood Prop so asked the Owner what size prop he has on it and he told me "a 9x4", I told him the Engine is not being Loaded correctly and he needs an 11 or 12 inch prop, so he said he had an APC 11x8 so he put the 11x8 on and his problem was instantly gone !!!,, the 11x8 may not have been the perfect Prop for this 61 but it was much better than the super light/small 9x4,,

now, this is not to say this is an GMS problem only, because I helped another guy that had a 160 4st that would Flame-Out as he went to an Idle (on the ground and in flight), simply put a 13x6 Wood Prop is too Light and too Small for a 160 4st

so the question is what prop do you have on your 47 ??
the OS AX46 will take an APC 11x6, so if you 47 is as powerful you could use the same, if you want more Top Speed you could use an APC 10x8 maybe a 10x9..

hope I have helped !!!!!
Jim

MJD

My Feedback: (1)

I sincerely doubt this has anything to do with propellor choice. The only possible help a prop change could make is to lug the engine down and reduce fuel demand, thereby masking a fuel delivery issue that should otherwise be fixed.

GMS engine have genuine issues with carb quality control however.

houckj

I have been out of pocket with family concerns. I am not quite understanding what to look for with the needle valve. If I stick a pin into the fuel inlet nipple on the needle valve it should come out in the center of the carburetor spray bar hole. Does this illustrate the problem?

(0) is fuel inlet nipple.
=== spray bar hole

Good

carb body

(0)=========

carb body



Bad

carb body
(0)
=========

carb body


The prop is an 11/6 apc. Same as I use on my Magnum 46.

Is there another carb that would be suitable for this engine? Perry? Perhaps an old Magnum 46 would work. I guess I need to measure.

Could raising the tank help? I appears low to me the way the plane was built. The key thing that sticks in my mind is that adjusting the HS NV has almost no affect on the engine.

Mr Cox

If there is no response on the HS needle then there is another obstruction for the fuel.

To me it sounds like just some dirt in the carb. To clean it you will have to take it apart.

The (0)===== etc. says nothing to me...

goirish

Take the HSN out completely, don't hook up the glow igniter and turn it over with an electric starter. It should blow fuel our of the carb. Sometime if there is some crud in the carb this will clean it out. Worth a try.

MJD

My Feedback: (1)

That gives you light pressure. I usually turn the prop so the intake port on the crankshaft is closed, then hook my fuel pump directly to the carb inlet to flush it, both with the NV in to blow the outlet in the spraybar clean, then with the NV out to clear anything that doesn't get blown through the spraybar. If there is any crud that ought to get rid of it. Hoilding the model so the excess fuel dumps somewhere you want it to rather than into the model is a good idea.

After exhausting this and other maintenance/installation possibilities (which I hope your engine expert did), the business about the fuel linlet on GMS carbs could still rear it's head. The issue here is that the hole through the fuel inlet nipple, where you connect the fuel line to the carb, meets up with a hole in the spraybar so fuel can flow enter the spraybar and turn 90 degrees towards the carb intake where it flows from the spraybar into the air stream. However, on a number of GMS carbs, the hole in the spraybar does not line up well with the fuel inlet, so what happens in some cases is that at full throttle, the misaligned holes become the primary restriction in the fuel system, preventing the needle valve from permitting enough fuel no matter how far you open it.. so the engine starves for fuel at full throttle and dies. Some people experienced this in flight, meaning the mixture was almost good enough on the ground but went too lean in the air, or others found they simply could not set it right on the ground at all. The .32 I worked on could not be richened enough on the ground to allow some room for unloading, i.e. could not be backed off from a too lean setting. The problem gets worse with smaller props, higher rpm & hp and thus higher fuel demand, but should not be there to begin with.

Your symptoms, where the NV has no effect in either location at full throttle, are completely consistent with both an air leak which you say is not the case, and with a fuel flow obstruction in the spraybar assembly. If there in no crud in there and no air leaks, I'd be checking the fuel inlet/spraybar situation. When present, it is a case of the round hole at the base of the fuel nipple and the hole in the spraybar not lining up well and restricting fuel flow.

I don't guarantee or even "strongly suggest" this could be your problem, but if all else fails I do suggest you examine the possiblity. It has bit a number of people. Anyone who has a good GMS will say hogwash, they're great. Well, they do run great if they can get enough fuel.

MJD

the Wasp

My Feedback: (1)

MJD, sincerely, why such a reply , surely all engines need to be loaded correctly within reason, and sincerely that's one "one" reason why they make different props in the first place, and surely and sincerely an APC 11x6 does not "lug" a 61 as you suggest , and surely~ if a 61 is spinning a APC 11x6 at 13,200 it would spin a super light Zinger 9x4 way out of it's power efficiency range, and surely that could "could" create fuel demand issues ,,
and to add, yes, in "houckj" case he did not have to small of a prop, but how the hell did I know that ,,, so I asked ,, so first things first !!!, surely I will NOT tell someone to remove his cab off his 46 size Engine and modify it before I know what prop is on it !!..

BTW, that GMS 61 with the 11x6 runs fantasic !! no problem with it's carb, but I do feel it should have a 12x6 on it [X(]

Jim

MJD

My Feedback: (1)

Of course they need a reasonable load on them, I agree, but how often does such an extreme example happen?

I did not suggest that 11-6 prop lugs a .61. What I suggested was that if the OP had marginal fuel draw due to a partial obstruction such as a misaligned fuel inlet, increasing prop load to lug the engine down in rpm and thus down the power curve would reduce fuel requirement and could reduce it below the threshold where it cannot be needled rich enough. The reason I suggested this was because I witnessed this phenomenon first hand just a little while ago while diagnosing a GMS .32 with marginal fuel draw due to the known carburetor QC problem.

In all my years of flying, I have never seen anyone mis-prop an engine as much as your example, that's weird. Which is why I considered the probability of that far lower than either the very common fuel delivery problems due to crud in the spraybar/NV assembly, or the now fairly well-known GMS carb QC problems that have resulted in tinkerers remedying the problem and ending up with a decent engine, or others putting the engine away on a shelf or peddling it in the RC community.

A .61 will run great an an 11-6, sure, and a 12-6 too. They;re both in the bracket.

MJD

buzzard bait

I'm surprised no one has suggested richening the LOW speed needle. Adjusting the HS needle is not having the expected effect...classic case of the LS needle being too lean and affecting the high end.

Jim

DarZeelon

Houck,


I agree with what Jim just wrote...

The fact that in you post #4, you mention 'the needle valve' in singular, hints that you did not adjust the low-speed needle; or maybe, that you don't know its true effect...


Although it is possible for the high-speed needle; being too closed, to prevent the engine from advancing to full-power; I believe the issue you are facing is a badly incorrect setting of the low-speed needle.

There could also be a lot of gunk (dried-up castor oil residue), from the rather long period of disuse, lodged in the low-speed nozzle; but having gone to high-RPM, even momentarily, suggests any such obstruction has already been washed away.
So, you should proceed to adjusting the low-speed needle, for optimum transition, before disassembly for cleaning.

Please note; I did not call it 'the idle needle' because it simply isn't that.
It is a constantly changing mixture-control valve, which determines the fuel mixture curve; from idle to ~80% throttle.
The high-speed needle, if not closed too far; only affects the absolute maximum amount of fuel available to the engine...

At anything but maximum RPM, its setting is thus irrelevant.


The main and primary fuel-control of your engine, is the low-speed needle!


Since you are facing a problem in transition, from idling to a higher RPM; and because your engine can (occasionally, with some manipulation) reach higher RPM; it is quite obvious the high-speed needle setting is not the problem...


I assume the engine has enough time on it to be broken in...

Put it on a test-stand, if necessary. Open the low-speed needle some (1-2 turns) and adjust both needles' settings, for optimum operation and transition; beginning with the high-speed.

MJD

My Feedback: (1)

The advice to bench run the engine is very sound. "When in doubt, whip it out".. to eliminate any variables beyond the basic engine and tank setup. Jett preaches this regards their own well-regarded engines, and any time I remember to follow my that advice myself the bench is invaraibly the place where things make sense.'

MJD

houckj

My next step is to remove the engine and put it on a test stand. It will be easier to look for leaks and adjust the low speed needle valve.

Thanks,