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

Mr Cox 06-01-2011 09:50 PM

RE: GMS Engine Tuning Problem
 
You don't need any check valves, just place the tank at the right height. The little Norvels have great fuel draw too with their stock carbs.

longdan 06-01-2011 10:45 PM

RE: GMS Engine Tuning Problem
 

I am going to try a Vess 12x6 next flight
12 x 6 is a lot of prop for this engine. It will load it up a lot. The .47 likes to scream.

I agree with moving the remote needle valve to the carb. Very easy to do - it just screws right in.

Flyer Freq 06-02-2011 04:41 AM

RE: GMS Engine Tuning Problem
 
I can't believe this thread is still alive either. Drilling the carb was never a consistent fix. The problem always was that the hole in the spray bar was drilled in the wrong location on several engines. The hole is partially covered by the carb body it fit into. The only real fix was to replace the spray bar assembly that was only $8.00-$10.00 at the time. I don't know if you can get this part anymore or not. Many wanted to argue with me that you could fix the engine by drilling the carb. Every engine I put a new spray bar in is still screaming somewhere today. I received many PM's from people verifying they fixed their engines with the new spray bar. Unfortunately many people drilled and still aren't happy with the results. Best advice I can give is to change to a Perry Carb or try to find a new spray bar somewhere.

adji 06-02-2011 10:14 AM

RE: GMS Engine Tuning Problem
 
Ok, some guys talking to their .47 size and others on 1.2 size . but I only have problem with my 1.2 size.I don't have any problem to my .47 size. for my 1.20 size I have drilled the carb inlet, drilling muffler pressure nipple but nothing help. My 1.2 size has run quite well for years after instaling the check valve until I convert it to gas a month ago for economical reason. I think the best advice I'v read is installing the perry carb and perry pump, even it's a big money for a cheap engine.

longdan 06-02-2011 12:00 PM

RE: GMS Engine Tuning Problem
 

ORIGINAL: Flyer Freq



Many wanted to argue with me that you could fix the engine by drilling the carb
For many, myself included, this DID fix the problem. The flaw was obvious and the drill fixed it. The engine has run like a swiss watch ever since.
I would still argue that it works. I've seen it work on several other GMS engines too. Never seen it not work.


Uncas 06-02-2011 07:16 PM

RE: GMS Engine Tuning Problem
 
So far, just drilling the carb has worked for me. (just drilled it a week ago and have about 5 flights on it... so far so good)

The 12x6 Vess prop runs about the same as the 11x6 - maybe a little more pull on the up lines. The engine runs slower - but did not bog down, yet still a disappointment. I got much better performance from an APC 12x4. Definately more pull on the up line and at the top of a loop at a cost of some air speed. But pattern planes are supposed to fly at a pretty constant and not incredibly fast speed - I think.

Next I will try a 12x4 Zoar and the APC 12.25x3.75W 3D prop.

Flyer Freq 06-03-2011 03:51 AM

RE: GMS Engine Tuning Problem
 
You will notice that I said, "Drilling the carb was never a consistent fix". While it worked for some, it didn't work for all. I could go into an explanation why this may be, as I do have an opinion, but it isn't worth the controversy.[:'(] I abandoned this topic long ago. Not sure why I allowed myself to be pulled back into it.[X(] The long and short of it is, drilling the carb may fix the problem but it also may not. You shouldn't have to modify a carb for an engine to run correctly. GMS had a manufacturing goof up and the best fix is to replace the part they goofed up. I have sold all my GMS engines and moved on to gas. They gave me many good memories. My wish for everyone here is that they find what they need in these many pages so they can enjoy their GMS engines. That is what it is all about. Have fun guy's, and good flying!!!

Remember the pilots prayer! Lose not thy air speed, lest the ground rise up and smite thee!

Uncas 06-03-2011 06:24 AM

RE: GMS Engine Tuning Problem
 
Flyer Freq,
Thanks for your comments. All information is useful. I totally agree, a new carb is probably the best answer. Hopefully mine is fixed, but if it gives me trouble I will likely replace the engine with a stronger/bigger one as I need more pull.

dhempy 06-08-2011 09:18 PM

RE: GMS Engine Tuning Problem
 
Since there are some relatively new posts here, I'd like some advice on a GMS .76. This engine came on a used warbird airframe ... and has a Pitts style muffler. The tank is mounted directly behind the engine but the fuel lines run through a filler fitting so there is about 9" of fuel line between the tank and the carb. I fired it for the first time today and experienced similar problems to those posted about the .46 .. stuttering, dying after running at WOT ... sometimes after idling. IT did fire immediately and ran fairly well at idle even after pulling the ignitor. And it may be that the issues popped up after it "warmed up (maybe it was too lean?). Needless to say I didn't fly this bird today (it would've been a disastrous maiden).

After I got home I pulled the carb and looked inside .. there may have been some obstruction in the spraybar ... since cleared. It seems to me that the carb on the 76 is different ... the spraybar goes all the way across the carb throat. The cutout on the spraybar is on the bottom (i.e. you cannot see it unless the carb is out) and the LS adjustment goes inside the spraybar. So many of the mods posted here do not seem to apply.

I did remove the fuel barb and noticed that the bottom inlet hole is not directly aligned ... but as posted WAY back by Wayne, the inlet holes are recessed in "slot" on the spraybar and the base of the barb is about 1/8" away from the hole. I'm not sure I'm going to drill first but I may later do that as an option.

I have new fuel and a new plug

So here is my plan ...

1) Check for leaks (tighten screws etc.)
2) Replace all of the lines (should've done this first anyway) and clean out tank
3) Do a blow test (first from pressure outlet on muffler, through tank to carb inlet, then on carb / spraybar itself).
4) Reset needles ... 2.5 for HS, 1.75 for LS (these seem to be a good starting point)
5) Mount the engine on a test stand and try the tuning procedures mentioned before.
6) I will take an IR temp gauge and see if it is overheating.

Will have to go out and look at prop size and fuel particulars (I know that the fuel is Wildcat but not sure of percentages).

I think that the engine will be fine and have plenty of power for this aircraft once I can get it to be more reliable. If anyone has anything to share that would be of help, I'm all ears.

Thanks to everyone that has contributed over the years.

Dan

RC-Bearings 06-09-2011 05:17 AM

RE: GMS Engine Tuning Problem
 
The biggest hurdle you will have getting this thing to run right will be the Pitts muffler. Most of them have too little internal volume and compensate by having too large of an exhaust opening. If yours has two outlets, try plugging one of them. If course, performance will suffer but you might find that the engine will be more reliable.

opjose 06-09-2011 08:35 AM

RE: GMS Engine Tuning Problem
 
I agree, especially during the engine break in phase.

Once the engine has been run for awhile the Pitts Muffler problem seems to diminish to a degree.


dhempy 06-09-2011 10:03 AM

RE: GMS Engine Tuning Problem
 
Thanks RCB and Opjose.

I will run down my list first and then work on the muffler.

BTW .. thanks again RCB ... I bought a set of bearings from you a few months ago for an OS 50 heli engine .. it is running very sweet now!

Dan

P.S. Wayne, in keeping with the spirit of the thread I will post my results back here.

dhempy 06-09-2011 11:53 AM

RE: GMS Engine Tuning Problem
 
Can more pressure be generated by tapping the back plate? Seems to me that in years past engines were tapped at the back plate for pressurizing tanks ... just a thought. Seems that there is a raised area in the center of the back plate that looks like it was made to be tapped.

I haven't had a chance to get the engine mounted on the stand yet .. I did tighten the head down though ... a bit over 1/2 turn per screw!

Dan

Dad_Roman 06-09-2011 02:14 PM

RE: GMS Engine Tuning Problem
 

ORIGINAL: dhempy

Can more pressure be generated by tapping the back plate?


I haven't had a chance to get the engine mounted on the stand yet .. I did tighten the head down though ... a bit over 1/2 turn per screw!

Dan
No. A 2stroke motor utilizes compression IN THE CRANKCASE on the downstroke to charge the combustion chamber.

Triple ditto on the above....its a gimme for you....the way to increase your tank pressure is to plug an outlet on your Pitts muffler. A common fix.

1/2 a turn? Wow....thats huge!

Good luck...wll be watching;)

dhempy 06-10-2011 11:57 AM

RE: GMS Engine Tuning Problem
 
Reporting back:

First, I probably DID NOT tighten the head 1/2 turn each, turns out that the shaft on my handle based allen was rotating in the base making it look like I was tightening the head. More on that later.

I did a blow test on everything ... seemed like the fuel filler might be a slight restriction. Opt to test on the stand with fresh line only.

The prop on this motor is a MAS 14/6.

On the stand started with a fresh tank, fresh line, a fresh plug, and fresh fuel. Note that the length of the fuel line and pressure line is about 12" or so on my stand ... longer than what is on the airplane. The Wildcat I was using before was 15% and I picked up some 10% Omega (lowest nitro I could find at the LHS). For needle adjustments, I started out as someone else mentioned, two turns in from flush on the LSN and 2 1/2 out on HSN. The engine fired immediately and ran relatively well. I did a pinch test and noticed little change in speed until much later when it leaned right before dying. Fired immediately back up and adjusted HSN until I was getting a consistent 9500 RPMS. I'm going to have to check my tach because it seemed that a good idle speed was around 2000 and that seems fast to me ... 9500 seemed about right with the 14/6.

So next step is to test with the other things that were in line. I put the filler and filter back in line with smaller diameter fuel line. Fired up but running like crap (thought I had my AHA moment). Put the fresh line back on and still running like crap. Drat.

I decide to check the head for tightness. Turn my allen driver a quarter turn each and get suspicious. I go get a different wrench and notice differing tightness on the head bolts so I pull the head clean up the screws and put 'em back in with a little blue locktite, do a star pattern tightening and all seems well. Refired engine and it is just a little better. Go back and check LSN and cannot tell if it has moved. Pull, put a mark on it, and reset at 2 turns in from flush. Seems like it is pretty easy to turn so I'm suspicious that I need to find a new O-ring. Anyway, refire, and seems back to normal. Decide to check temps ... ran it up for about 5 minutes and check head temps ... 205-210 ... not too hot. RPMs back at 9500. Take original line and put filter on. Runs great. Add in filler, still running great. Check LSN, does not appear to have moved. Refire and run up ... place gloved finger over one port of the pitts, noticeable drop in RPMs and it doesn't seem to run any better. Drop to lowest idle and after about a minute or so, finally dies. I wouldn't trust the engine to run consistently at that idle ... the best idle seems to be where you can just see the barrel open between 1/32 and 1/16 inch looking down the throat of the carb. Again, my tach indicates that this is around 2000 and that seems high to me. I'll have to see if I can find a manual AND do a double check with someone else at the field with a tach.

At this point the problems I had when I first fired it seem to be gone. I'm gonna go run it on the stand once more ... if all seems well back into the plane it will go and we'll try the standard run up techniques at the field (including the big test ... holding the plane vertical).

Again, I'm not sure if there was an obstruction in the spray bar that was since cleaned ... I thought that I couldn't see light through it and blew it out quickly before confirming it was blocked. IF it was obstructed, this may well have been the problem all along. If not, then the other things in part or combination were the answer (Fuel, Plug, Needle Settings, Reseating the head). I don't think I had any air leaks in the engine / carb based on my blow testing.

Since my purpose was fix the running characteristics of this motor AND being the 76 with a different carb than the smaller engine that the majority of the thread is dedicated to, I cannot say for sure where the issue was. I wish I hadn't been so quick on blowing out the spray bar. I can confirm that the initial LSN and HSN settings as mentioned are relatively close ... I haven't touched the LSN since the second reseating (2 turns in from flush) and I tightened the HSN about 3/8 of a turn to get max RPMS. I also want to run a tank of the 15% through and check the temps again. I doubt I'll need the extra power though ... the motor will pull the warbird with authority I am sure.

I am going to go in search of a new O-ring for the LSN ... it seems way too loose to me and I don't want to locktite it ... does anyone know what size it should be?

Last, the 14/6 may be a little big ... I think the manual says 11/8 or 12/7 ... but this sits in a Corsair with a big cowl. Ground clearance isn't an issue. I may swap in a 13/6 to see if there is any noticeable difference. Ultimately though I am going to look for a 3 blade to enhance the scale look.

This thread was a great help and thanks to those that commented. I am still all ears and welcome further comment.

Dan

Quick Update: Ran the last of the tank through it .. everything ran quite well. Temps immediately AFTER the run were 270 .. seemingly more in line (205-210 is probably low) .. and I did run it HARD .. mostly high speed, lots of run ups from idle after 10 seconds. A good solid 7 minute run. Gonna throw it back on the plane tonight ... will field test next week.

MJD 06-10-2011 12:54 PM

RE: GMS Engine Tuning Problem
 


ORIGINAL: dhempy

Can more pressure be generated by tapping the back plate? Seems to me that in years past engines were tapped at the back plate for pressurizing tanks ... just a thought. Seems that there is a raised area in the center of the back plate that looks like it was made to be tapped.

I haven't had a chance to get the engine mounted on the stand yet .. I did tighten the head down though ... a bit over 1/2 turn per screw!

Dan
You can't pressurize the tank directly like that with an RC carb. You can use that pressure tap to drive a regulated pump. You can use backplate pressure with a venturi, but with a carb it blows the mixture all to heck anywhere below full tilt.

dhempy 06-14-2011 08:26 PM

RE: GMS Engine Tuning Problem
 
Looks like I won't get a final report back until late June ... traveling. Still pending is the vertical test ... first on the ground, then in the air!

Dan

dhempy 07-10-2011 11:02 AM

RE: GMS Engine Tuning Problem
 
Well, still no joy .. back to the stand. Took the Corsair out for a maiden the other day .. all was well in the pits but on the runway just after the tail lifted and I was still increasing power, it sputtered and just didn't sound right. I immediately shut down opting to fare with a poorly running plane on the ground versus one about to deadstick me 5 feet in the air. Poor steering on my part ended the day as the landing gear was slightly bent on the nose-over. I did go back to the pits and try the engine some more .. seemed like it was back to it's old ways ... idled OK but half the time would die on the throttle-up ... sputtering transitions happened frequently as well. It would start right up into a relatively nice idle each time though .. and I couldn't get the radio to stop it .. it would just idle way way down and keep spinning (I have an idle down switch set on the radio). The prop was so slow though that a little pressure on the spinner would stop it.

I still have my suspicions about the filler so I'm going to bypass that in the airframe and test again without removing the engine ... if it still has issues, then I'll be finding that LSNV o-ring and looking at some of the other mods (maybe a temporary muffler exit choke ...)

Will also run though the steps I outlined before.

The engine seems so close .. I'd really like to get it running well enough to trust it with my plane ...

Dan

Wayne Miller 11-09-2011 07:44 PM

RE: GMS Engine Tuning Problem
 
Hi,

I haven't been getting notices of new posts here, I just checked out of the blue and was surprised to find so many new posts.

Just a suggestion, if the engine seems to work fine on the ground, and with the nose up, but quits when in the air, try selecting a one size smaller prop. Fly on that prop for a while until the engine has more time on it, and then go back to the original size prop.

Hope this helps.

Fly4Fun,

Wayne

Dad_Roman 11-10-2011 10:55 AM

RE: GMS Engine Tuning Problem
 


ORIGINAL: Wayne Miller

Hi,

I haven't been getting notices of new posts here, I just checked out of the blue and was surprised to find so many new posts.


Wayne
Ditto

Dad_Roman 11-10-2011 11:02 AM

RE: GMS Engine Tuning Problem
 


ORIGINAL: dhempy

It would start right up into a relatively nice idle each time though .. and I couldn't get the radio to stop it .. it would just idle way way down and keep spinning (I have an idle down switch set on the radio). The prop was so slow though that a little pressure on the spinner would stop it.


Dan
Sure sign of an air leak Dan. Could be anything....like you said, LSN oring, backplate, carb to block seal, etc.

You know how to do a leakdown test? (Extreme cases) Take motor off, put couple feet of clean fuel tube on, finger over carb and exhaust, submerge in sink, blow, observe:D

Minimal leaking at the front bearing is ok, major leaking is not.

Good Luck. I repeat, for the new guys. I fixed mine and its a screaming monster.

larrysogla 04-07-2012 09:49 PM

RE: GMS Engine Tuning Problem
 
Wow!!!!!!!!!!!! This thread is timeless..........just this morning I re-visited this thread and refreshed myself of the timeless engine tuning, fuel/compression/prop load/matching issues, etc. etc. All the good, where the rubber-meets-the road remedies that somehow are able to fix these sometimes cantankerous misfiring glow engines and turn them into consistent reliable performers. This thread is a time-saver for many faced with these engine troubles. Gentlemen....I am sure there are still other engine issues out there in the RC world...............so I am sure this thread will keep on breathing and helping RC'ers for years to come. Good luck to y'all and God Bless y'all. Larry.

Dad_Roman 04-08-2012 06:41 AM

RE: GMS Engine Tuning Problem
 
Thanks Larry....I for one appreciate that. Im sure my cohorts here also appreciate it!

Best of luck to you also!

Wayne Miller 04-08-2012 07:40 AM

RE: GMS Engine Tuning Problem
 
Hi,

I was surprised when I got the post notification and was pleased at what I had read. This thread started back in 2004 and I still get thanks from different people who have benifited from the thread.

When I went back and started to read from the beginning, it is amazing how much people know, and how freely they share their experiences and information to help others. We truely do have a tremendous resource here, and I for one really do appreciate it.

Thanks to everyone for making this thread so helpful to others.

Fly4Fun,

Wayne

Tom Nied 07-24-2012 02:13 PM

RE: GMS Engine Tuning Problem
 
OMG, I finally got through all 25 pages of this thread.


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