Wayne Miller

Hi XJet,

Thanks

Wayne

Squid

I too have this problem. Running 5% nitro, 16% pure castor, rest Methanol. Dont want to up the Nitro, too expensive here. Started with the not being able to shut off problem and cutting at full throttle. Sealed the carb, fixed the cut off. Put the engine cuts down to not fully run in/overheating. Indeed after a couple more flights got it to run well. Then suddenly (after removing a shim to get more power), it ran like a pig, after peaking, it would cut if held vertical of splutter if ran a little rich. Also the main needle could be turned +- 1/4 turn without a noticeable difference. Fuel system checks out ok, no bubbles in the feed and no leakes under pressure. Just tried using some silicone tubing to further seal the needle (the o rings were lookin a bit worn, esp the outer one which doesnt get much lube). HAvent run it yet to see if it helped. I was also thinking maybe a cooler plug to compensate for the increased compression until I read this thread.

I shall investigate the holes lining up, and fix it if so (without drilling any nipples) and report back in a couple of days.

Wayne Miller

Hi Squid,

Thanks, I look forward to your report. Good luck!

Fly4Fun,

Wayne

cheechukranch

Wayne,

I've got a new GMS 32 for a combat plane. I have not run the engine yet. Should I go ahead and assume the thing will leak around the carb and muffler and seal these areas with red RTV?

Thanks!

Squid

Well, guess what guys, the hole in the brass insert was exactly half blocked by the carb body! Seems like they didnt pay enough attention when assembling the carb and they probably passed it on doing an air flow check (blowing through the hole) which would not pick up the restriction. This 'anomaly' has GOT to restrict the high speed running fuel flow!

I drilled out the hole only with a 1.5mm drill using the inlet nipple as a drill guide. If I had not promised to leave the nipple alone (!!) to determine if the this one mod made a difference, I would have drilled the whole thing through with a 1.8mm drill. Watch out for breaking the drill bit, I had mine snap off inside as the hole completed and the drill fell to the back end of the brass insert. MAnaged to pick/flush it out phew! Cleaning is very easy blowing through with some wd40 using the plastic tube extension into the holes. Remove the barrel before drilling and remember to backflush from that side afterwards, then use a strong light one side to check all shavings removed. Use a smidge of locktite on the nipple threads, needle holder threads and barrel retaining screw on reassembly.

For y'all out there with suspicious running motors, this is such an easy check to do. Remove the fuel inlet nipple with a 5mm spanner, shine a light in the hole and it will become immediately obvious if the hole in the brass sleeve is in alighnment with the carb fuel inlet or not, a 2 minute check.

Will run and set up the motor this evening, may not pick up the motor cutting, but should be able to see the motor leaning reaction to nose up test and needle sensitivity. Test flight report will only be available monday unfort.

Squid

Cheech, if your motor doesnt cut smartly then the throttle barrel is fully closed, then you need to do this. If the motor is out of the plane, it may be easier to just do it anyway and save yourself future hassles.

On the muffler side, I did not use the paper gasket supplied, they always compress after a while and blow through or cause the muffler to fall off! Just a thin smear of RTV sealant is all thats required.

Spicoli

Wayne,
I think you are right.
I checked the inlet on 3 different carbs.Every one looked restricted to varying degrees.It looks like They pounded in the brass insert with the hole already in it and didn't check to see if the hole lined up.
One hole looked so bad I dont know how the fuel got through.It looked like the carb air intake hole when the throttle is set to idle.
I drilled all 3 of them out and will test them on combat spads this weekend [weather permitting]
All of the carbs came from engines with the silver muffler and with no fins.

You the man!

Kelly

Wayne Miller

Hi Squid and Spicoli,

Thanks for the update, I'm glad to hear that you were able to confirm the blockage. Did you drill out the muffler nipple, or are you going to try the carburettor fix first, then the exhaust nipple if required?

Also, you may want to review the "fix" post (number 38 in this thread) for the carburettor adj. - if you find a better method, let me know. By the way, if the engine hesitates when transitioning from low to high, I normally have to lean out the low speed adjustment for most engines. The GMS seems to like it better if I richen it a little instead - why? I don't know. You may want to also verify this with yours.

I'm anxious to find out how you make out.

Thanks so much for all your help.

Fly4Fun,

Wayne Miller

Edited for typo

Spicoli

Wayne,
I drilled the carbs.I am going to leave the mufflers alone.They work great on my OS46FX's

Squid

Hi Chaps,
Had a good go at it yesterday evening.

I was able to simulate the leaning overheating problem quite easily in the garden so was able to do all the tests necessary yesterday. It appears my motor just cuts when it gets too hot (measured over 210deg C on the head!), so now to find the reason for the overheating.

It appears to be excessive leaning when the model is in a climing orientation. In its current configuration, this motor appears to be much more sensitive to this than others (eg Irvine) which I have. So lets see if these mods help at all:

1) Ran the motor with only the new hole in the brass insert, 1.5mm, needle more sensitive (could have been the sealing of the needle end with fuel tubing), no change to climb/leaning sensitivity.
2) Drilled out the exhaust pressure nipple to 2mm. No change in anything.
3) Drilled out fuel inlet nipple only to 1.8mm (brass insert hole still 1.5mm), needle the same, slight reduction in climb/leaning sensitivity.
4) Drilled out the brass insert hole to 1.8mm to match the fuel inlet nipple, needle the same, slight reduction in climb/leaning sensitivity.
5) Replaced head shim (total 3). Starting easier (less compression), no apparent loss of power, no change in needle or leaning sensitivity. (running 5% nitro at sea level, 24 deg C)

After all this, the excessive leaning was still a problem. but could be tuned sufficiently for flight, about 30% power loss in horizontal orientation when peaking in the verticle. Still not acceptable.

The one thing I didnt try was a baffled muffer. Im running the standard noisy instrument. This in conjuction with the previous mods would have helped the most I suspect.

My conclusion is that although these mods helped, they have not fully solved the problem in my case. I took another look at the fuel tank location and decided to relocate it closer. Previously, the tank centre was 8 inches (about 210mm) from the carb. I have relocated it to 6 inches (170mm). I ran out of light for testing, but am hoping those extra 2 inches will make a difference (!!).

Wayne Miller

Hi Squid,

Thanks for the input. With the tank that far back, I'm sure increasing the tank pressure from the exhaust will help.

By the way, I found that when I was relying on tank pressure alone, I only needed the back baffle (near the exhaust exit) show in the earlier post.

Again, thanks for your help, I'm sure one of us will find the problem and solution!

Fly4Fun,

Wayne Miller

Squid

Ran the motor again last night with the tank moved forward. That seemed to do the trick, only about 5% drop in power between horizontal and vertical orientations now which is acceptable. I guess the low muffler pressure due to lack of baffles combined with the rearward tank location was causing my leaning out blues. GMS really should fit a baffle, they are noisy without, and the running improvement would certainly be worth a few hundred rpm drop imho.

Flyer Freq

I don't think we totally have the problem solved yet, but we are close. I understand why many would believe there is a need for baffles in the tuned muffler, but it really shouldn't be necessary. The exhaust outlet is smaller than the typical tuned muffler. This is what offers the back pressure. My friends GMS has the new style muffler, and it has the same problem that you are attributing to the tuned muffler. I believe it is stricktly a carburation problem. I should have my friends engine to play with in a couple of weeks. I am not commenting, as I am, without first hand experience. I have one of the older GMS .47's, and it is an awesome engine. It has been run so much, you can't tell that the head was ever gold. Instead of the performance decreasing, it keeps getting better, and I have not removed any of the gaskets in the head. As for the noise, I agree that it appears the engine is noisey, but if you run a hose from the exhaust port to another muffled canister, so you get the engine cumbustion noise away from engine, you will find that most of the noise is coming from the propellor, turning incredible RPM. When my prop is turning 16,200 RPM, the prop noise is incredible. In the air, when the engine can really unwind, there are some undiscribable sounds that come from the plane, that I believe are the prop tips going supersonic. If I could run a bigger prop on this plane, I would, as this is a lot of punishment for the engine. But in that regard, I am in my 5th year with this engine, and it is still going strong. Each .47 should be capable of this performance.

Thanks Wayne, for putting this organized effort together to find the problem. It will pay dividends for all of us!

Andy

Wayne Miller

Hi Squid,

From reading your post, I wasn't quite sure whether you enlarged the hole in the exhaust pressure nipple and/or where you installed a baffle(s) as well as moving the tank forward. I'm interested to know what finally worked for you, can you let me know?

The newer engines seem to work well - GMS must have identified these problems. The newer engines have the carburettor holes lined up, and they have also modified the mufflers (silver center is older, dull finned is newer).

Just a hint, one time when I was trying baffles, I used a strong spring as the rear spacer (near the exhaust exit) on the bolt. By doing this, when I tightened the long muffler bolt with the nut, it compressed the spring and pushed on the baffle. This took up any slack between the baffles and spacers. It worked well and I thought I should pass this trick along.

Thanks for keeping at it.

Fly4Fun,

Wayne Miller

Wayne Miller

Hi Andy,

Thanks for your input.

I think with your thoughts it will be interesting to see what your analysis will bring to the table. We need all the input and help we can get!

I think we all agree the GMS engine is a great engine when it works well.

I look forward to the time when you get the engine to "play" with, please keep us informed of what you find and hopefully we can help other modelers.

Thanks for your kind words.

Fly4Fun,

Wayne

Flyboy Dave

My Feedback: (21)

I need to have a look at my .47....for the fuel inlet solution. I don't
think the issue has anything to do with the "tuned muffler".

FBD.

Wayne Miller

Hi Flyboy Dave,

Thanks for jumping in, let us know what you find.

Look forward to your input,

Wayne

Spicoli

Wayne,
"I think we all agree the GMS engine is a great engine when it works well. "
All engines are great engines when they work well.
Problem is a lot of GMS's don't run well.

XJet

Yeah, one of our club members has just fired up his new GMS 47 and it seems to suffer from air-sickness. He hasn't got a single flight of more than 60 seconds before the engine dies.

He had *no* problems at all with exactly the same plane, tank, fuel-lines, etc when it had a TT46 Pro in it -- so go figure.

I should be firing up my GMS32 next weekend so I'll report what I find but, given that I can feel the bearings grumbling straight out of the box, I'm not holding my breath that this will be one of those rare (around here anyway) GMSes that "work well". I'm ordering a TT42GP to take its place just in case. The TT is cheaper, lighter and more powerful anyway.

Wayne Miller

Hi Spicoli and XJet,

I think we all agree. I'll be glad when we get to the bottom of the problem.

I think its great that modelers are joining together to try and solve the problem.

Again, thanks for your help.

Fly4Fun,

Wayne

captinjohn

Just a thought....try a perry pump... it sure keeps the lean-out problem away....and adds a lot more RPM. Give one a try. Capt,n

Squid

I agree that the GMS is a great engine and is well priced. It starts better than any engine I gave ever owned and certainly delivers power, I dont think thats an issue. What we are trying to do is address the running consistancy which seems overly sensitive to the aircraft orientation (compared to other motors) and results in lean running and unexpected engine cuts.

Some manufacturing quality issues have been found, such as leaking carb interface, needle seals and now the fuel inlet hole lineup. From f/b, it appears GMS are addressing at least the hole lineup. Hopefully the other issues will be addressed too as I believe they do contribute to the overal running consistancy to a greater or lesser extent based on other factors such as tank placement , % nitro etc.

With the mods I have done to date, the biggest improvement was made by moving the tank closer to the carb. That is not to say the other mods did not make an improvement and im glad I did them. The one thing I have not tried, and I belive will make a significant improvement is increasing the muffler backpressure. It appears that before moving the tank previously, the pressure was insufficient to raise the fuel the 8 inches or so required without excessive decrease in flow. Ive eliminated fuel restictions with the mods, so it follows that the insufficient pressure must be the culprit. I am talking about leaning out at full throttle between the horizontal and vertical atiitudes here, so I dont *think* part throttle carburation is the issue here.

Others may have other issues, I can only comment on my findings here, hope it helps!

I am now happy with the running with the mods so am ready to flight test. My next step would be to insert a baffle or different silencer to increase the muffle pressure. (im using the dull finned one). But maybe not required now. It would have allowed the tank to be placed more rearward, at the loss of a few hundred rpm.

Wayne, I drilled the exhast to 2mm right at the beggining (see my detailed post) and it made no difference. I dont think air has a problem flowing through a 1.5mm hole, fuel however does seem to!

Captn, I use a perry pump in my 1.60Fx equipped 1/4 scale lazer, I cant seem to justify (in my head at least!) spending on one for a GMS in a spad type plane!!

Will hopefull get some test flying in this w/e, good luck to you all!
R.
Mark

Wayne Miller

Hi Mark,

Thanks for the update. Let me know how you make out on the weekend. I'm guessing you may need more tank pressure as well.

Fly4Fun,

Wayne

Squid

Hi Wayne,

Had two good flights on sat. More power than I every had before due to being able to lean out better in the horizontal plane, and no cut outs due to leaning in the verticle plane or when the fuel level drops, and that was a HOT day, 28 deg and 99.99% humidity (there was condesation on the carb on landing!)

I think if one can reduce the fuel flow restriction by doing the mods we did, and keep the tank close in to the carb, then theres no need to restrict the exhaust outlet further to obtain more pressure. If its not possible to get the tank close enough, a baffle like yours should certainly help matters.

I would now say that the GMS is performing comparably to my Irvine 40, in fact it starts better and is half the price, so Im very happy!!

Thanks for making the info available on this thread. I would still be struggling and cursing GMS otherwise!!
Regards,
Mark

Wayne Miller

Hi Mark,

Thanks for the update, I'm glad your engine is working now. What did you find was the best method for tuning your carburettor?

As a side note, I measured the 2 mufflers again. The "silver" muffler body is bigger than the "finned". The exhaust outlet on the "silver" muffler is smaller than the "finned" muffler. You would think the smaller outlet on the "silver" muffler would create more pressure and baffles would not be needed.

I was thinking, if the problem is the muffler to tank pressure, one could probably add an exhaust deflector which would cause a small restriction, or add an exhaust restrictor such as a small pipe extension on the outlet slightly pinched. NOTE: The muffler outlet can not be pinched - it would break - therefore the reason for a small extension pipe. If I run into the problem again, I'll have to give these a try.

Thanks for keeping at it, it's great to see modelers helping modelers, especially at such a distance.

Fly4Fun,

Wayne