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-   -   GMS Engine Tuning Problem (https://www.rcuniverse.com/forum/glow-engines-114/2290854-gms-engine-tuning-problem.html)

[email protected] 07-20-2020 08:02 AM

Best solution, throw away that carburetor and replace it with either an OS or Perry of the same size. My .47 runs great with an OS 40 carb, ran great but would quit without warning with the GMS carb. Same for the other GMS engines, the ratio of good to bad carbs is around 1 in 10. Poorly machined needle valves and seats is the problem, lousy quality control. Same solution for Magnum, Asp, Royal etc, all made by the same company.
Larry

the Wasp 07-20-2020 06:12 PM


Originally Posted by [email protected] (Post 12619153)
Best solution, throw away that carburetor and replace it with either an OS

I suggested that to him back a while.

Jim

Alexie 07-21-2020 02:01 AM


Originally Posted by [email protected] (Post 12619153)
Best solution, throw away that carburetor and replace it with either an OS or Perry of the same size. My .47 runs great with an OS 40 carb, ran great but would quit without warning with the GMS carb. Same for the other GMS engines, the ratio of good to bad carbs is around 1 in 10. Poorly machined needle valves and seats is the problem, lousy quality control. Same solution for Magnum, Asp, Royal etc, all made by the same company.
Larry

I should point out that this was probably a Covid lockdown resuscitate an old model from the attic exercise with the main aim being to work with what I already had with minimal spend. So buying new carbs isn't really part of that plan. Having said that I do have an MDS 40 engine that I pulled the carb off. This has run before many years ago and I seem to remember it ran OK then. The venturi barrel hole is the same size as the GSM but I haven't check it'll actually fit in the GSM crank hole. A cheap Russian engine with dodgy bearings perhaps but the carb is OK. Also the carb in my GSM has the full width spray bar unlike most of the engines in this thread so maybe it is OK and just needs tuning ... least that's what I thought before the latest attempt to get it running.

I bought some fresh fuel (12%) and managed to get it running seemingly OK on a test stand with a 10 x 7 prop rather than in the Heli. Idles OK, transitions sort of OK to WOT but the HSN is still 2.5 turns out which is more than the manual.states and possibly it's too lean even then at WOT, just seems to get a bit hot. If you don't lean the HSN back though it 4 strokes at mid throttle and bogs so too rich which seems to the problem here. Idles OK mixture good but when the high end is set right just backed off from the slightly vague peak then the mid throttle is too rich to the point it barely runs. If you fix the mid throttle by leaning the HSN it's probably too lean at WOT but it does run like this just seems like there's minimal margin on the mixture through the rev range. You can't lean the LSN any more to fix the mid throttle because it won't idle, too lean.

Anyway since it was sort of running OK on the test stand I stuck it back in the Heli and had to tweak it some more because the Heli setup is different, engine vertical mounted, tank location lower. It was almost working right except for the transition from idle to mid throttle, just too rich at 1/3 throttle, it 4 stokes which you can clear by pinching the fuel tube briefly but it returns when you release it and possibly cuts, so very rich in fact. LSN is right on the limit of lean at idle, WOT produces plenty of smoke but it "seems" to get hot pretty quickly which may actually be normal, maybe I'm just being over wary of burning more plugs ?. IR thermometer shows 140C but I'm not sure I trust it on a shiny gold anodised head (Reads 60C on a shiny metallic boiling kettle, you'd expect closed to 100C). It will clear the 4 stroking at half throttle and run OK with head temperature of 110C (IR reading). All pretty inconsistent and seems like this full width spraybar carb really has the same problems as the other engines in this thread, the LSN needle and carb in general just can't meter the mixture reliably throughout the rev range, I have a feeling the taper on the needle is almost fully in the spraybar tube hole at idle with the full diameter of the needle essentially doing the metering which means you get a sudden relatively large increase in flow area when the throttle is opened slightly and the taper starts to meter the flow.

Having said that watching videos of 3D Helis flown by experienced pilots the transition from idle to hoover always seems pretty stuttery and smokey so maybe this isn't unusual ... so long as it keeps going to mid throttle you're OK.

So try the MDS carb perhaps, mess with increasing the idle speed to see it it can help improve the low throttle problems, try it with the exhaust tank pressurisation disconnected to see how that affects things .... or give up and stick it back in the attic.

[email protected] 07-21-2020 06:02 AM

Mds carbs had the same problem, lack of quality control and irregular machining . I have had MDS engines that run great with a replacement carb and were impossible to get a consistent mid range with the original carb.
Larry

Alexie 07-21-2020 12:11 PM


Originally Posted by [email protected] (Post 12619383)
Mds carbs had the same problem, lack of quality control and irregular machining . I have had MDS engines that run great with a replacement carb and were impossible to get a consistent mid range with the original carb.
Larry

You could be right there. I did have it running pretty reliably but also have a faint memory that it would do odd things like cut for no obvious reason sometimes. And the MDS carb makes the GMS one look high quality in comparison. There isn't even any O ring on the HSN, no seal at all.

And the connection spigot on the GMS is 13mm whereas the MDS is 12mm so not a straight swap but I think I'll shoehorn it in there all the same and see what happens.

the Wasp 07-22-2020 01:48 PM

I am going back to what I said about loading the engine properly, if your Heli's hub and fan are really light your engine may not be loaded correctly, this will allow the engine to rev quickly in which can make your engine starve for fuel. to test this put a 12x5 prop on it and run it.

I had this same problem on my Century 50 Helis with my OS Hyper 50 engine with the OS 8 plug. and now that I have that same engine in a plane with an APC 11x8 and an OS 8 plug it's the best running glow engine I have ever seen !! it will set and idle for minutes and not load up.

Jim

Alexie 07-26-2020 10:43 AM


Originally Posted by the Wasp (Post 12619688)
I am going back to what I said about loading the engine properly, if your Heli's hub and fan are really light your engine may not be loaded correctly, this will allow the engine to rev quickly in which can make your engine starve for fuel. to test this put a 12x5 prop on it and run it.

I had this same problem on my Century 50 Helis with my OS Hyper 50 engine with the OS 8 plug. and now that I have that same engine in a plane with an APC 11x8 and an OS 8 plug it's the best running glow engine I have ever seen !! it will set and idle for minutes and not load up.

Jim

So I mounted this MDS 40 carb on the engine, not the best fit given the spigot is 1 mm smaller but it seemed snug enough and I don't think the O ring was leaking because you could shut if off when the throttle was closed. It possibly runs slightly more predictably than the GMS carb but really shows the same problems with stumbling rich mid range. Your point about loading the engine is right and it probably isn't helping running it in the heli with just simulated head loading. However, I think the problem is it's over loaded not under when the flybar paddles are turned flat on to the flow and the engine struggles to turn this load at a speed that allows the right mixture at that throttle opening, too much fuel opening not enough air. It runs better with the paddles as they should be when it's underloaded but still stumbles in the mid range regardless.

Which is why I now think the main problem is the exhaust which is a custom fit for the Heli. It's a tuned pipe and seems to strangle the engine causing a rich mixture until it comes on song at which point it clears the rich mixture and screams. Pinch the fuel tubing to lean it up as it's stumbling and stuttering on the point of stopping and it suddenly picks up and screams quite happily even though you've changed no settings other than clear the fuel loaded in the crank. I think this explains why both carbs show similar problems even though they are completely different designs. The GMS is a full width spray bar design whereas the MDS is half width like the engines in this thread. Funnily enough the LSN needle setting ended up with the end of the needle just flush with the end of the spray bar when the barrel was wide open. Funnier still is the fact the barrel rotates beyond the full open position a few degrees which moves the needle fully out of the spraybar providing improved fuel flow. Don't think this is any kind of design feature, more to do with MDS quality I suspect.

I think there's still some scope to tweak the needles on this MDS carb, didn't really get the HSN sorted and think it's still too rich so there's hope yet. And it would be interesting to remove the tuned pipe and fit something else ... not easy given the way the silencer needs to fit in the Heli frame.

Alexie 07-27-2020 04:24 AM

Thought this was interesting regarding LS needle re profiling:

Post 42 of thread ref 10230872 (Midrange issue - glow plug?)

The last bit about sanding away the needle seat to fix a rich mid range was pretty much what I was thinking to lean out the mid whilst still allowing enough fuel at idle.

Tom Nied 07-27-2020 07:29 AM

It just should not be this difficult, even with a "bargain" engine.


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