G'day engine lovers. I friend was given a GMS 47 which the original owner could not get to run reliably. He put it into small Sukhoi aerobatic model with an Bolly 11.5 x 6 and tried to get it to work. His fuel had some nitro but not much, and the oil had some castor but the engine just did not sound happy and worse, the muffler kept coming loose and eventually fell apart in the air. So after several deadsticks he too was ready to put the engine in the too hard basket.

The new owner is no newbie and was having no problems with several other similar engines.

Another of our club members bought a couple of Macs Mufflers and offered one to try on the GMS. The muffler was fitted, but this time a Bolly 12.5 x 5 was fitted (because it was all that was available) and the motor was given one last chance. Same fuel, same model just different muffler and slightly different prop.

Chalk and Cheese was the result. The motor is now totally happy, runs the tank dry if given the chance and hauls the little Sukhoi round with ease.

Any theories as to why a change of muffler and a relatively small prop change would make such a huge change?

PS. He is very impressed with the low cost and high quality of the Macs product.

The Macs muffler is not a tuned muffler like the original GMS muffler.

The Macs muffler has much more internal pressure and puts much more fuel up to the engine

The engine will have a lot less power with the Macs One Piece muffler than with the original GMS muffler

Yet the Macs muffler will never give any trouble

Maybe this is one of the GMS that had the fuel inlet missaligned.

Take out the nipple just at the carb and look if the brass insert is missaligned, if its, then redrill it (with the nipple as a guide).

There was an old thread about the GMS 47 problems...most of them were cured this way.

I absolutely agree with the mis alignment theory. i had 2 of these engines and a quick check and re drilling made all the difference in performance, corrected all issues. i suppose the added pressure from the macs pipe is overcoming the mis alignment problem. you would be suprised how small the opening is when this brass insert is in wrong.

And the .32's as well, must be the same carb or the same spraybar assembly. One sloppy little mistake in carb manufacture and the engines suffer a bad reputation as a result. Not like it was a difficult engineering problem to overcome either, just sloppy manufacturing and QC.

Did the rpm drop with the change from the 11.5x6 to the 12.5x5? If it did, remember the fuel flow rate would then drop so the problem is reduced owing to that factor in addition to any other changes.

MJD

Not only that, but that prop is way too much of a load on that engine with its high revving porting/timing, especially when running the stock muffler, which was designed to operate at much higher rpm levels than any 12" prop will ever let it rev.

The GMS .47 engines were noted as being screamers when they first hit the market some years ago. A 10x6 to an 11x5 is a much better choice. Try it some time and you will see what I mean.


Ed Cregger

Once I got the .32 running it ran like a bear on a pipe, almost 18k static on an 8-7 and without fine tuning the pipe yet. It acted like it was not a lugger for sure, the pipe boost was significant. Shame about the ****ty carbs. If I found one cheap I'd use it for sure, but put a Perry or other spare carb that works on it.

MJD

G'day

Thanks everyone for the comments. I hope to see the engine and its owner tomorrow and I will pass on your comments.

My feeling is that the owner is now so happy with the way the engine is going that he will probably leave well enough alone though he may investigate the carby input problem.

The ultimate test for an engine is whether it flies the model it is in. In its original form it did not. It was un-reliable and difficult to handle and did not fly the model at all well.

With the larger prop and the Macs muffler it is a different engine and suits the model very well.

I will suggest we do some testing and see what happens when the carby is inspected and then suggest he try some smaller props.

Thanks again for the suggestions.

Mike from Oz

I'm not saying not to use the 12" +- prop if it is working well along with the Mac's Muffler. I'm just saying that its specialty is running at higher rpm, which probably isn't what the owner wants to do with it.

I've heard lots of stories about the carbs. Haven't heard of many fixes other than replacement. Sometimes one can find a Perry carb on eBay that is cheap enough not to tip the total price/value scale too hardily. I used to ask fellow club members if they had surplus carbs that they wanted to sell. Many times they would just give them to me. They don't have to be pretty to work right.

I have a NIB GMS .47 that I have been holding on to for a few years. I should pull it out and run it at least enough to break it in and go over it for air leaks.


Ed Cregger

I have a tower trainer arf and I used to have an os 40la in it. My instructor said it flew like it was underpowered. Picked up a GMS 47. Now it flies like a champ at partial throttle. Great to have spare power for goofing around!