OS 91fx Overheating
 

I know that this is a well discused subject, I read alot about this but Im still wondering how to fix the problem. I installed this engine on a Edge 540 under its cowlling.
My OS 91fx is fitted side ways, with a Bisson pitts muffler. I use an APC 14X7 10% fuel and the remote needle valve is made of plastic.
It had no more than a gallon through it. When run just reach it gets over heated and quits. So I run it very very very reach and runs in normal temps. This ofcourse gives me problems with idle running and no power at all plus it takes it 3-4 seconds to pick to full throttle.
Any one who actually fixed this problem? :eek:

Thanks for reading this........;)

RE: OS 91fx Overheating
 

I would try two things first:

1. Make sure the air outlet hole in the engine cowl is about three times the area of the inlet hole. I tend to cut the bottom out of the cowls to make sure there is enough airflow over the engine.

2. Check for air bubbles in the fuel line at full throttle. This can be due to the fuel tank touching the plane and the transmitted vibrations from the engine frothing the fuel. The air in the fuel causes a weak mixture and can be difficult to detect. Can indirectly cause overheating.

Both these things will be important issues with a new and still fairly "tight" engine.

RE: OS 91fx Overheating
 

You need to make sure that your muffler is supplying enough pressure to the tank to give you good fuel flow. Some non-stock mufflers do not do a good job of supplying pressure to the tank, so the engine gets poor fuel flow and tends to run lean. Many modelers have to restrict the exhaust outlet of their after-market mufflers to insure good fuel flow to the engine.

RE: OS 91fx Overheating
 

Also consider your fuel line. If it makes any type of 90 degree turn coming through the firewall, it could be pinched slightly and will get lean overheat. I have had this happen on other engines.

RE: OS 91fx Overheating
 

One other thing is to check your low end, it might be too lean, I noticed that a lean low end does affect the temp.

RE: OS 91fx Overheating
 

I think now I know what I should do. I already started fixing the fuel tank issue making it not to vibrate and cause the "weak fuel supply".
The question now is should I increase the pressure from the mufller to the tank? The Bisson pitts has two tubes for outlets, is it wise to close the one? Will it affect in any way the engine performance?
I also intend to fly it the next time without the cowlling so that the will be more airflow to cool the engine. If everything ok I will be testing it this weekend.

Thanks for the help.

RE: OS 91fx Overheating
 

I would have to add to what Mick wrote.

Install baffles inside the cowl, to direct the air-flow over the engine and its head, to provide ample cooling.

Just having entry and exit holes in the cowl, will not make that air cool the engine, unless it is forced to flow over it.
This is what these baffles must do.

RE: OS 91fx Overheating
 

So, I changed the tank installed a smaller one (this one came with the kit 320cc) and put pieces of sponge around it. It hardly touches the fuselage. Changed all the silicone tubes and removed the cowlling so I can see if things are going ok. At 8:00 this morning was down the field and tested it. Had the engine re-adjusted to normal good rich. And the miracle was done.
I made three 7 min. flights, no dead stick no everheating just normal engine running. Just on the last flight there was rain..... so packed and left. More testing tomorrow if weather ok.
I will still fly without the cowlling till Im sure that no more overheating is happening.
Now about the cowling, Im familiar with the concept os 1 air inlet - 2 outlet and this is the case with the particular cowling but still I think it will be wise to use a baffle and direct the airfolw exactly on the engine head.
As you can tell Im listening to advices and what I did was to listen to the recomendations from the replies I had .............:D

Thanks guys :)

RE: OS 91fx Overheating
 

Nice one - Good result.

RE: OS 91fx Overheating
 

Ok guys, just for the record did some more testing today. Four 8 min flights (still without cowlling), no dead stick no overheating..................the engine idles fine and sounds happy.

So I would assume that the main problem here was the bubbles in the fuel.

RE: OS 91fx Overheating
 

It's a suprisingly common fault that, I am sure, has resutled in engines being scrapped. It is an infuriating problem: the engine starts perfectly; idles very well, so you taxi out to the strip, open the throttle and it dies. Or you get in the air, open up from low to mid throttle and - flameout. That happens a few times and the model is at risk of getting the big boot.

Often just changing the prop - and hence the vibration period - does the job. I had an engine that would only run on a 12 x 6 wooden prop - I wanterd to use a low pitch wideblade but the engine kept cutting on that - it was great on the wood prop.

Of course, the answer is to isolate the tank completely from the fuselage - difficult to do with the trend of modern ARTF manufacturers who cut a hole in the front bulkhead that exactly matches the tank neck.

RE: OS 91fx Overheating
 

This does'nt make any sense. You installed a smaller tank and the problems went away. First off, those tanks that come with any arf should be thrown away. I never have and never will use a stock tank. You obviously found the problem is the fuel tank which is expected. So why don't you just go buy another decent tankof a better size and more flight time.

RE: OS 91fx Overheating
 

Seems to me that going to the smaller tank makes immense sense. As panicosc said, going to the smaller tank allowed him to put (additional?) foam padding around the tank to better isolate it from the fuselage and therfore from vibration and foaming.

RE: OS 91fx Overheating
 

It's not the ARTF tanks that's the problem. It's how they are assembled and installed in the plane.

Assuming the ARTF tanks are strong enough, what's the problem with using them? Fuel tanks are not rocket science - as long as a few simple precautions are followed they will all work...

RE: OS 91fx Overheating
 

Installing a smaller tank (as "JLNewc" said) gave me space for more foam and therfore reduce the vibration to the minimum.
I know 320cc is a very small tank for the 91 engine, (it just gives me 7-8 minutes max of flight). The idea was to find the solution and fix it. Now all I have to do is cut off some ply wood exactly behind the firewall to make the extra space I need to install a bigger tank with LOTS of foam around it.
I think it does make sense..............