Well with the cold weather in the minus and unless i can run the engine up on a test stand in a BIG well vented garage I will not be able to test the device until spring... Kinda hope that maybe some one that lives in WAY better climes can test it for us.

I do not think the tank will over pressure as the max pressure at WOT provided by the muffler will be the limit ( or should be) and as the the fuel burn increase so will the volume of pressure be available ..which will be cool when you throttle back down for a long glide and be able to maintain a positive pressure... side benefit will be less lose of fuel from back drain to the muffler as well.

I would think that when the engine is shut off that you may want to release the pressure that may be left in the tank at the connection before the muffler that way the fuel left in the tank would not be flushed out ..( mind you that may be a good thing at the end of the day when you want to make sure your tank is empty.

FlyBoy do you think you would be able to test for us? I think you have me beat for better weather where you live [8D]

XJet, which mod are you talking about? My sons trainer has one of these gms 40's on it and it runs fine in the air for a while and then just dead sticks. This is no fun. He quickly hands control back over to me to land the darn thing. This happens everytime. It is very frustrating. Engine is not hot so it is not running lean which also I hear is a problem with these engines. I think next spring we are just going to change it over to a OS 46 fxi. Mine runs great and never needed any adjustments. Let me know we may try a mod prior to spending 130 bucks on a new engine.

Thanks,
Rick

On the fuel-nipple side of the carby there's a metal insert which also protrudes into the venturi (it's the bit where the low-speed needle slides in as you close the tbrottle.

Unfortunately, this bit seems to be badly designed (or machined) on some carbys and partially blocks th eflow of fuel. If you unscrew the fuel nipple and look down you might see that half the hole is obscured by a bit of this piece. There are pictures earlier in this thread taken by someone else.

Some people have drilled out this piece but I simply pressed it out of the carby (real easy) and ground a divit with a small stone on my Dremel -- then reinserted it into the carb body -- lined up so that the divit was directly below the fuel nipple.

There's now singificantly improved fuel flow (as can be easily proven by a before and after blow-test through the fuel nipple with the carby barrel removed.

It seems that the problem with the GMS engines is that this obstruction makes them very sensitive to changes in fuel-head, or even engine loading. As a result, the simple act of the fuel level dropping or pulling a few Gs is sometimes enough to cause them to flame-out when in the air.

Another solution is to add a restrictor to the muffler exhaust. This works because it increases the amount of pressurization to the tank through the pressure nipple on the muffler -- but it does reduce maximum power somewhat.

Hi,

Post number 38 in this thread give a description of the adjustments made after the insert was drilled out. Reading on through the posts, you see a hand drawn picture of the insert and how it was drilled out.

Hope this helps.

Fly4Fun,

Wayne

Hi,

I recently found a totally unexpected fix to problems I was having with a GMS .47 fitted with a Tower muffler and running a 12.25 x 3.75 APC prop in a 3D model.

The problem was that, even though the engine started easily, idled reliably, transitioned OK and had reasonable power (12,000+ rpm) in horizontal flight, it would sag as if lean when vertical.
The only way to achieve sustained vertical flight was to tune it with the model vetical. This involved opening the main needle an ADDITIONAL 3 turns (i.e. 6+ turns open!!). The engine still started, idled and transitioned OK but was way down on power at full throttle (only 11,000 rpm). It also smoked like crazy when vertical but would sag and die if the needle was closed even a couple of clicks.

This looked like a fuel supply or muffler pressure problem so I did all the appropriate things (many previously mentioned in this thread).
I removed and pressure tested the tank, replaced all the fuel tubing, removed the fuel filter, stripped and cleaned the carb, drilled out the pressure nipple, sealed the carb to the crankcase, replaced the glow plug etc etc.
NONE OF THIS MADE ANY DIFFERENCE WHATSOEVER TO THE PROBLEM!

In desperation and because there was nothing else left to try, I leaned the IDLE needle 1/4 turn (despite the fact that it had already been set apparantly correctly by the pinch test method at break in and the engine idle was already slow and very reliable)

The effect was immediate and dramatic. I was able to LEAN the main needle one full turn. I leaned the idle needle another 1/4 turn and, again, was able to lean the main needle another turn. After a few more cycles of this and with some final fine-tuning I had the engine pulling 12,600 rpm horizontal and vertical, transitioning beautifully with a reliable idle at just under 1,600 rpm.

I refitted the fuel filter and even an 'undrilled' pressure nipple and went flying. In the last couple of months I logged many flights with no problems at all - I haven't had to touch either needle and the engine has never hesitated or even coughed. It is now transformed into a very powerful, reliable, user friendly, 'happy' little 2 stroke.

Remember that all I did to totally transform the FULL THROTTLE performance was to lean the IDLE needle!!!

"That's only one engine - it probably had a bit of dirt in the carb that finally moved --" I hear you say.

NOT SO - Not only is the problem reproducible on my engine if I open the idle needle a turn or so, I have also resurrected another GMS 47 (also fitted with a Tower muffler) that was discarded by anothewr club member as being too unreliable - it kept 'leaning' in flight and stopping. This engine is also now running like a clock. A third near new 47 was also improved considerably by getting the idle needle correct instead of APPARENTLY correct.

So why does the idle needle position dramatically effect the full throttle performance?

A friend (and fellow engine 'nut') and I believe we may have the answer. We think it has to do with venturi effect and fuel atomization.

If you look down the throat of the carb (in the full throttle position) you will see that the idle needle is retracted just clear of the end of the spray bar tube. In the three engines we have 'sorted' recently, the gap between the end of the needle and the end of the tube is about .002" to .003" when correctly tuned. Remember that it is poissible to get a good idle and transition with the idle needle much richer than this - i.e. a much larger gap, but this seems to result in poor and unreliable full throttle performance.

We think that this is probably because the venturi effect at the end of the spray bar tube is compromised if the gap is greater than a few thou. This, in turn, results in less fuel 'draw' which, of course, "looks' like a lack of of tank pressure. We believe it also reduces the atomization of the fuel so that the engine 'acts' lean even while it's smoking.

So, forget about drilling out pressure nipples, changing tank positions, sealing carbs etc at least for a while - It's all too hard and, on my engine at least achieved nothing. Try checking that idle needle to spray bar tube end clearance and working from there to see if that solves the problems.

Good luck,

John.

Hi,

That's great information and I think a good analysis of the proplem. In an earlier post, it described the carb adjustment and interaction between the low speed and high speed needle valve. I think your post went a step further and explains why it works. Thanks.

Hopefully, others will post with information on how leaning the low speed needle helps them.

Some people may still need to drill out the brass insert to improve fuel delivery - there are some of the brass inserts way out of position and off center when they come from the factory, this causes fuel starvation. Drilling the brass insert improves fuel delivery dramatically on these units.

Fly4Fun,

Wayne Miller

Hi guys.

I spent hours reading all the thread and found that it is great for learning about engines, altought I don't have a GMS engine, by reading the thread i wanted to, because it is the cheapest .46 range engine available, but do you have any suggestions?? I have 3 options Tower 46, Magnum XLS 46 (46FX clone) or the GMS 47. Do you have any experience for any advice on them?... I don't want to buy an O.S at this time, very expensive in my country, but I can order from tower... and I guess i will decide buying the GMS becouse of the thread.. Thanks!

I've had all 3 of those motors and all 3 have worked out OK.

The Tower is probably the strongest of the 3 but also the most
tempermental and least user friendly. Expect it to take a long
while to break in and require some expertise on the needles.

The Magnum runs pretty well and is nearly a complete clone
of an OS 46. The one I had killed a rear bearing after only
2 gallons but has run well since replacing that.

My GMS 47 has run almost perfect since day one. I only have
about 2 gallons through it but it seems to run better all the time.
Its presently powering an old beat up Sig Kougar and its hauling
that old thing around with authority. Much better than the old
OS 46 SF it replaced. Swinging an APC 10X7 at around 13500.

Of the 3 I would recommend the GMS. But you may want to
consider the Thunder Tiger 46 Pro. I have 3 of them and
they all run flawlessly. I have one on a Bridi Kaos 40 and
another on a Sig Somethin Extra.

Mike Hammer

I found an easier solution to these GMS engine problems. Dump them in the closest dumpster and get an SK .50 from Kangke. They are so much more powerful and don't have the stalling problems. Atleast I have not had any problems with both of mine. Plus they are a little cheaper. I got mine on sale for 50 bucks each.

Thanks Guys, I appreciate that...
Maybe I'll buy GMS...and first thing I'll check is that piece of brass in the carb. and search for the other things...thanks to this thread!



Jorge

Well it may not be a Extreme speed plane like the SU27 but it does have a U in the letter F7U3 and the colors look familiar. This is my almost finished Cutlass45 that I have been building and have installed my reworked new GMS .47 ...here is the tread on it.

http://www.rcuniverse.com/forum/m_31...tm.htm#3192753

cheers

I was thinking about trying a GMS .32 or there's several OS .25s for about $50 but the GMS seems to have a lot more power. Is this everyone's thinking about these two engines?

I have seen a lot about the .47 but not the .32. Does anyone have any experience on these?

Once again, thanks. Tim.

I have one and I'm just breakin' it in.....The stock muffler would'nt fit into my cowl, so I took one off the Evolution trainer system engine (from a crashed trainer lol) It seems to run well. started with a ms 10/7 We'll see how it does. So far bad weather has
kept me grounded from my maiden.....

I've got one and it runs fine in the stock configuration but when I put a tuned pipe on it I found that the standard carby could not provide enough fuel -- with the needle virtually falling out it was still too lean.

The problem is the bad carby design which also plagues the GMS47 and causes flame-outs in the air.

Easy to fix but a shame that it has to be done in the first place.

Has anyone else set the gap to .002 or .003 and tested to see if it works? I have set mine to .003 but with -20c weather it's a bit hard to get out and test it.

Hi guys. I have a GMS 76 with a serious case of the "slows", plus an alarming knocking sound from it at idle (similar to the condition previously posted by XJet). It is typically operated with 10% nitro Omega (castor/syn blend) & an APC 12-8 prop. So far it has only had ~ 4 gal of fuel through it.

It starts very easily & idles dead reliably, but previously, the HS adjustment was pretty numb -- little response when near the "sweet spot" -- in fact it really didn't have a sweet spot. When it started to peak, I could turn the NV a full turn with no significant change in RPM then it would suddenly go lean & starve when I finally overdid the leaning out. While the HSN was a ***** to set, transitions were good & the LSN was very easy to set. When it seemed to be running OK, if the throttle was backed off from wide open & the revs dropped, the engine would often not pick up again upon opening the throttle. If I then leaned it out a bit it would pick-up OK, but would go lean a bit later.

In flight, with it tuned somewhere safely rich (?) in the "flat spot" it would often sag in flight shortly after take off & act like it had gone lean, but would then pick up mysteriously at times & go as well as it could (not that the @#$%^& thing is a powerhouse).

After reading this thread I checked the position of the spray-bar hole & it was off-center but not really blocked by the carb body. One thing that was immediately apparent was the very small diameter of the hole. I wondered if it was too small for adequate fuel delivery. --- Sooo, using the nipple as a drill guide, I drilled it out to the ID of the stock fuel nipple.

So far I have only done some bench tests, but tuning seems to have been improved noticeably. It now has a much narrower sweet spot in place of the old wide "dead spot" & it doesn't seem to slow down anymore. It is currently taching @ ~ 11,000 RPM on 10% fuel with an APC 12-8. I don't consider this to be particular exciting, but at least it seems to be holding there.

The knocking at idle seems to be louder if I use all-synthetic lube fuel (Cool Power).

Questions:

Is the knocking sound a common feature of this engine? My previous GMS 76 did not make this sound, although it was a performace pig like this one.

Is it a sign of impending doom?

I have looked at the bearings & they appear to be good -- no play in axial or radial directions. I haven't fully dissassembled the engine, (I prefer not to disturb its innards) but I wonder if the con-rod bushings are wearing.

Does the addition of extra castor oil make the sound go away? I am thinking of adding castor to the Omega to bring the total lube percentage up to as much as 25%.

I have worked with 4 GMS .76s and a couple of their .61s. They have generally been fairly quiet at idle; I haven't heard one "knocking" at idle like yours. I have heard one (actually a .61 GMS) with a strange "grumble" at high throttle while flying. When you checked your bearings did you also check the rod big end play?

Does your engine have the finned muffler or the Tower-type muffler? They come in both forms here depending on the shipment. The Tower-type OEM muffler provides quite a performance boost; the GMS .76 I saw with this muffler did 10,400 rpm with an APC 13x8 - not too shabby. Your 11k with a 12x8 seems consistent with what I've seen the GMS .76s with the finned muffler do. If your muffler is finned, have you removed the baffle cone from within it?

If you haven't already done so, read post # 303 in this thread and try your HS needling experiments with different LS needle settings to see if fuel draw and atomisation are affected by the LS setting. John got a big improvment in draw, fuel consumption and power once he'd sorted out the optimum position for his LS needle, due we believe to improved atomisation.

Good luck. Let us know how you go.

I'm familiar with the GMS 47 fix(es) & set-up, having had one of those nasty brutes, & I still own a Tower 46. The behaviour of the GMS 76 is not quite the same -- it doesn't flame out -- it just seem to come & go of its own volition & is quite unresponsive. I have indeed been through the works -- back & forth with high & low needle settings to no avail -- until I drilled the spray bar -- which seem to work (on the ground). BTW, the low speed setting on the 76 is dead easy -- not at all fussy like the 47.

I haven't checked the big-end for play, but I guess that is next. The con-rod shouldn't be failing after 4 gal of fuel.

My 76 has the finned (low performance) muffler & it still has the baffle in it. I have been reluctant to unbaffle it because of the erratic operation -- I had memories of GMS 47's dancing in my head.

I also have a Tower 75, which has none of these problems & has a bucket-full of power to boot. It has a "tuned" muffler -- I'll give that a try on the GMS 76.

Thanks

Hi,

I have two GMS .76's and they both worked great right out of the box.

Is yours OK now, if so, what did you do to make it work in the end?

Fly4Fun,

Wayne Miller

It seems to run better on the ground, but there is too much snow to get it airborn. [:'(] I'll post back when I get a chance to try it out.

Has anyone else encountered the knocking sound?

Hi,

I have also heard a metallic knocking sound at a certain RPM. Note: I have two identical engines and I could only get it to happen on one engine.

I found that if I was very careful, I could duplicate it by slowly increasing the throttle. I talked to our local "engine expert" and he told me that this sometimes happens on an engine, and is caused by the resonance in the muffler, and will also happen on OS engines occaisionally. It is usually more noticable when making slow speed passes.

It was difficult for me to believe that a metalic knocking sound could be made by a muffler. So I tried different things and found I could make it go away with a different size prop, shimming the muffler out a bit, or changing mufflers. You may want to give one of these a try to prove it to yourself.

Hope this helps.

Fly4Fun,

Wayne MIller

Quite a few big(ger) glow engines make this knocking sound -- SuperTigers are one of the worst. In fact the ST FAQ page even mentions this noise and says there's nothing to worry about -- it's normal, albeit they have no explanation for it.

Thanks guys -- that's good news.. I'll report back with tuning results as soon as I can get it into the air

Hey Guy's! I have not had a chance to read back on all the details. Out of the blue, RCU started to contact me again on the posts to this forum, so I have been out of touch.

If it has not been mentioned already, the knocking sound can be caused by high nitro content. So many people believe that if an engine runs well on 5% it will run better on 10% and even better on 15%. This is not the case. Nitro acts like moving the distributor on your 1969 muscle car. The more nitro, the more you advance the ignition timing. If a prop is too small, or you have too high a nitro content, you will get pre-ignition. The .76, which is an awesome engine, by the way, was designed around 5-10% nitro. If you are at the upper end of the propellor range, 10% will work fine. When these engines are manufactured, they are done so within certain tolerances. This is why you can have two engines that apear identicle, but have different characteristics. Wayne's engine that knocks, likely has tolerances built in that mechanically have the timing advanced further than the other engine. If this is true, lowering the nitro content will likely reduce or eliminate the knock, lengthen the life of the engine and improve overall performance. These differences can often be adjusted by machining the cylinder and head combination. There is facinating information on this, on the MECOA website. How the cylinder and head come together is refered to as "squish band". I have a K&B 1.00 that ran awful. You could fry an egg on the muffler, no fooling! When I measured the squshband, I found that the band was so large, I had ignition delayed to the point that it took place in the muffler. It was so far out of tolerance that I could not adjust this with nitro content. I had a few thousandths machined off of the top of the cylinder, and now it runs beautifully on 10% nitro. In your case, I suspect the squish band may be the opposite of what my K&B was. It is likely very tight, so ignition is happening way too early. I doubt it is so far off that nitro content won't solve the issue.

Andy

Hi Andy. Thanks for the comments. I'm quite familiar with pre-ignition & two-stroke combustion timing. Detonation problems typically occur under load. The knock in my engine is occurring at idle, but it is (apparently) not knocking at higher power settings.

I'm not over-propping it, I'm using 10% fuel & a cool plug. While I have certainly encountered pre-ignition, until now I have been fortunate enough to miss out on the entertainment provided by this particular phenomenon.

It seems from the experiences of others that it is not uncommon with some larger glow engines. My engine stable stops at 90-size, simply because I can't transport larger aircraft (I don't want to bother with a trailer, plus the Dragon really objects to a single model filling up an entire room), so maybe that is why I haven't encountered it before.

What is it & why is it? I'm wondering if the flywheel effect of the prop is not quite sufficient at idle to keep the engine running smoothly. When the knock starts the idle becomes erratic.