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ST 2500 issues

Old 05-27-2018, 03:16 PM
  #1  
planenutzz
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Default ST 2500 issues

G'day Bill,
not sure if this formum is still being monitored by superTigre support but here goes.
I have a ST-2500 which is fitted to a JMI P51 of 82” span, an old but lovely model.
The engine has run beautifully for many years but last 2 or 3 I have had nothing but trouble with it leading to many deadsticks and several on climb out leading to model / retract damage.
  • Fuel mix is 12% Klotz/Coolpower oil, 5% nitro or sometimes no nitro at all
  • It also has a Cline regulator fitted
  • OS 7D carb fitted
  • Engine is inverted
  • Onboard glow system fitted
  • Plugs tried:
    • McCoy MC-8 & MC-9
    • OS F
    • SuperTigre Medium
Orignally swung a Bolly 17x10 but broke that some time back and now is fitted with a 18x10 Bolly prop which may be a bit much for it. It had the 17x10 for around 20 years!

Symptoms/things tried:
  • Can’t get a proper tune, engine will bog right down and die as soon as throttle goes past end point of the onboard glow driver. It’s like a switch is flicked. this presents for both engine upright and inverted
  • Sometimes I can tune it out but it is not reliable
  • Engine will start easily with a backward flip on the spinner
  • Needle valve is not responsive
  • I’ve rebuilt the Cline regulator with new walbro diaphragm and needle. Same results
  • Tank pressurises fine with no leaks
  • I’ve put all new fuel lines throughout
  • Friend gave me another OS 7D and same issues present
  • Rebuilt the carb as far as I was able to, no change - same issues with both carbs
  • New carb O ring on the base
  • Removed Cline and set up as a normal fuel system tube direct to carb, no tank pressure. Same issues present so it's not the cline regulator
  • Bench ran upright and initally had it running on the bench ok or so I thought but did not seem to peak out. I used same tank etc from the model. Engine appeared to be very hot
  • Used same set up in the model. Same tank. Sometimes it will run for a while at full RPM but will die. Other times it quits just past glow driver cutting out with.
  • Starts easily
  • Very little response on the needle valve to peak out RPM
  • Have tried a range of plugs, OS F, Supertigre medium, McCoy MC-8, MC-9, Thunder Tiger hot
  • Have tried my normal fuel mix for other glow models, 20% oil, 7% nitro. Engine sounds very bogged down at low to medium RPM but will run at full throttle some of the time but not peak out and needle valve is not responsive. Very unreliable
At my wits end with the damn thing and seriously considering retiring the model due to total frustration with it. It’s a simple engine and can’t figure out the problem.
I am thinking it has overheated a few times and the piston / liner has become varnished leading to further overheating.


Hoping you can shed some clues?
Are parts still available for these? Piston and liner? The bearings are fairly new.

thanks in advance
P

Last edited by planenutzz; 05-27-2018 at 03:21 PM.
Old 03-10-2019, 01:19 PM
  #2  
planenutzz
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Default

I have an old JMI 82 " P51D. this is the lineage that teh CJM P51 was developed from and dates back to late 80s or early 90s.It's a good flyer and I have it well decked out with scale goodies.

Last few years I have had nothing but engine woes, rebuilt the ST2500 once and have had multiple rough arrivals as a result or dead sticks on climb out.

Of note is that the model has a Cline regulator fitted which takes crankcase pressure via a one way valve to pressurise the tank to about 10psi and the regulator has the diaphram section from a walbro carb that allow suction from the carby to open the needle via the diaphram.

I've been experencing a huge dead band that I could not tune out, starts and idles up to fast idle fine but will not transition to full rpm. I had read these engine can glaze up and overheat causing the same symptoms so managed to track down a new in box engine as well as all the parts to rebuild, cylinder and liner.

So I fitted the new donk and fired it up on the bench. Ran like a dog with the ST carb and put the OS 7D on with just normal tank suction. Seems to be ok upright. Inverted though it played up. Tried a range of plugs and was out of the Mccoy MC-8 which this engine likes for inverted running for some reason.

So packed it away and forgot about it for nearly year, finally got some more MC-8 plugs and pulled it out again on Saturday.

Engine ran but very rich transition and kept quiting. Suspect the regulator supplying too much fuel as I could not tune it out. Also was not sure about my old OS 7D carb so fitted antother I have that has remote mixture control via a servo control. By this stage it was late afternoon and had had enough of it as I'm sure the neighbours were sick of hearing the engine and me swearing at it [img][/img] [img][/img]

Sunday after copius quantity of coffee I tried again but this time I got out my old cline regulator and inspected it. Had antoher diaphram and fitted that. Tried it and seemed to run quite well though could still be rich. I

Interestingly by pulling on the edge of the regulator it would lean out and go on song. Hmmmm suspect needle sticking open or possibly diaphram a bit stiff.

I'll get a new diaphram an needle from mower shop during the week but as it is now I could fly it. When it comes on song it is pulling like a horse.

You tube clip of it running.

fun and games!

cheers

P

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