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-   -   used 91 ST heli engine wont run (http://www.rcuniverse.com/forum/ys-engines-support-197/11267899-used-91-st-heli-engine-wont-run.html)

Gregsrc52 10-18-2012 04:43 PM

used 91 ST heli engine wont run
I have bought a used heli and it has the YS 91 ST engine in it. Ihave never used a pumped engine before so this is a learning experience for me.

I have been trying to get the engine started but not having very much luck. Itook it apart cleaned the carb all fuel passageways.
At first it had no compression (the ring was stuck). I replaced the ring and cylinder liner.
I got it to run once but the battery died in the heli and it just idled until it ran out of fuel.
I have not got it started again,

Now Ithink Ihave a fuel issue. The engine turns over fine at first but it soon goes to hydrauic lock and stalls out my starter motor.
I remove the new OS#8 glow plug and it turns over fine, put the glow plug back in and the same thing happenes..
The person who built the heli put two ball valves in the system to aid in filling up the tank and relieving the pressure.
Yes it has pressure. the check valve is 10CM from the engine's rear port. I am using30% heli fuel.
Since it haspressure Iam asuming the fuel tanks are fine (not leaking).

I have pulled everything apartand cleaned it in new fuel except the screw on the side of the regulatorassy. (the one that says set at factory).

I am not sure what to try next, please help Iam running out of hair.

mike31 10-19-2012 02:01 PM

RE: used 91 ST heli engine wont run
The brass screw adjusts the fuel similar to a needle jet. You should be able to download instructions on the net.

Gregsrc52 10-22-2012 01:15 PM

RE: used 91 ST heli engine wont run
Does the regulator diaphragm wear out?
I am thinking I should replace it and the fuel check valve.

Gregsrc52 10-24-2012 05:01 PM

RE: used 91 ST heli engine wont run
Well I ordered some new parts so I hope to have them by next week.
I sure wish I unserstood how the fuel system works on these engines,   (hint, hint)

petec 10-28-2012 05:55 AM

RE: used 91 ST heli engine wont run
Sorry I missed this, my day job has been keeping me busy 8 days a week. The factory set screw is the regulator adjustment screw and I really wouldn't suggest messing with it.

Yes the diaphragm can wear out, but they usually last a long time. If the engine is getting flooded to the point of hydraulic lock you probably got some crap in the regulator that is not letting it close. There are good fuel filters between the tank and the engine? Fuel filters are your best friend with a YS.

Here is a link to your instructions:

Get everything set back to the starting point and make sure you do in fsct have the correct check valve for that engine. It will save you headaches.

The fuel system is basically mechanical fuel injection. The crankcase pressurizes the tank through the one way valve. The regulator comes off it's seat with pressure pulses and allows a metered amount of fuel to pass and get to the needle valves.

Gregsrc52 11-03-2012 05:35 AM

RE: used 91 ST heli engine wont run

Thanks Petec and Ed ( who e-mailed me to explain how the YS works,  which was very kind of you Ed.)
I found a small tear in my diaphram when I went to replace my regulator. 
I had ordered a new unit but found out that RON's Heliproz south are out of stock.   
So I am waiting and waiting...  


Gregsrc52 11-12-2012 06:24 PM

RE: used 91 ST heli engine wont run
The story is almost over.
I got the new diaphram, put it in and took my heli to the field and it started.   yes!!
I ran a half tank through it, it all seamed to go right,     but    
I could not start it again,  the engine was flooded again and it would not turn over until I removed the glow plug.
I did not get it started again that day.
Not knowing what else to do I took the engine out of the heli and took the regulator apart again.   nothing seamed wrong.
I flipped the diaphram over and put it back together.   ( I put the flat side of the diaphram facing down towards the side making contact with the lever.)
I was able to start the engine 4 times after that.    One of the times I let it sit for 30 minutes before I refueled it,   it was flooded on the first try.   I removed the glow plug turned it over a few times and put the plug back in and it ran fine.  
I think if I remove the pressure after each flight it will not happen again ( I think I will need to figure why this has happened).
But for now I will be able to fly my heli for the first time.    I am hoping for one more warm weekend this year.


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