mike109

G'day

A friend has two ASP four strokes. One is a 52 and the other a 91. He has had both since new and has been having a weird problem. He has been arriving at the field, filling the tank and having a perfect first flight. He then fills the tank but the second flight is less successful and the third is even worse with the engine beginning to run roughly and eventually deadstick.

So today we did an experiment. I asked him to drain the tank after the first flight. The fuel that came out was noticeably dirty. He had previously noted similar contamination in the 91. I did a similar check on my Saito 62 but its remaining fuel was fine after a similar flight.

So we started to theorise. It would seem that the engine is putting rubbish back into the tank via the pressure line and that this is making the fuel unusable. We wondered what this rubbish is. Water? Carbon - it is black? Or possibly wear products from a loose muffler or internal engine wear? The engine runs fine when the fuel is clean.

Laser (English manufacturer of high quality four strokes) strongly suggest that muffler pressure not be used with their engines and so we decided to see if the little 52 would run (inverted) in its Tiger Moth with no muffler pressure. The fuel tank/carby relationship is ideal so we figured it was worth a try. It worked perfectly. The engine needed to have the low speed needle opened about 1 full turn and the high speed needle also opened. The engine now runs for the entire tank, it sounds happier in the mid range AND the fuel is perfectly clean at the end of the flight (as would be expected). The model flew all the normal maneuvers that a Tigger would be expected to fly - loops, and just flying around.

Any ideas/comments etc.

Mike in Oz

TomCrump

Just curious, but what fuel was he using ?

mike109

G'day

It was "home brew". 5% nitro, 5% castor, 15% Klotz 200, 75% methanol. It is almost the same as the brew that I use. Mine is 10% nitro and the rest the same.

Mike in Oz

TomCrump

Sounds good, and it (at least in my mind) takes away my thoughts that it was fuel related.

1QwkSport2.5r

Originally running rich because of muffler pressure; extra carbon formation, carbon mixes with oil and into the fuel tank it goes. That's my theory. My engines when running at ideal mixtures tend to spew some darker oil through the pressure line. If I flush the line out before the next fill, it doesn't make it to the tank.

pe reivers

Scrutinize the position of the muffler pressure connection.

Sport_Pilot

Good point, a muffler connection at the low position of the muffler will pick up more carbon and moisture. Lots of water in the exhaust which can cause problems if you make a lot of short flights and refill the tank each time, especially if you don't check the needle setting each time. I would not pump the fuel back to the fuel jug till the engine is broken in. Also try a fuel filter on the pressure line to catch worn metal and carbon from the muffler.

SrTelemaster150

I always try to place my pressure tap on top or @ least the top 1/2 of the muffler diameter & close, but not too close to the inlet end.

The higher the better

Charley

My Feedback: (2)

I put a fuel filter in the pressure line, close to the muffler, on my larger engines or when I see blackish discoloration in the pressure line. Just blow the filter out now and then.

CR

Clay Walters

I try to keep a filter between the clunk and carb since any glow engine with exhaust pressure to tank will do this. Keeping the tap "on top" helps if you fly upright but not so much if you don't.

petergomes786

check  the Lead gasket ( bewteen engine  exhaust & muffler) which I think is made of soft lead which has come along with my ASP .61 must be slightly loose causing slight rubbing thereby letting black oil into muffler.  

mike109

G'day Thanks everyone. I have not been able to look at the engine since I made the posts. I will say that lately I have been arranging the muffler on my Saitos so that the muffler points down. This started because one came loose and fell this way during flight and I noticed that with the muffler pointing down, the amount of oil on the model had been greatly reduced so now all my Saitos are set up like this where possible.

The motor in question is a four stroke so it does not have a gasket between the muffler and the engine. If it were a two stroke, I would be thinking that this could be the problem.

I will have a good look tomorrow at where the pressure tap is but I suspect the owner will have disconnected it and set his ASPs up like my Lasers which do not use pressure and which run perfectly.

Thanks to all who replied.

Mike in Oz

tipstall 131

This an interresting thread. I have a Magnum .61 4-stroke (essentially ASP) which does the same thing. I noticed black rings in the tank after I emptied it although the fuel was not noticeably discoloured. I have not paid much atention to this as the engine runs perfectly well inverted on a 1/5 scale J3 Cub. Just curious what might cause this. I am using 20% castor fuel.

tipstall 131

This weekend I took the head off my Magnum 61 to see if it could be excessive carbon build-up that might be the cause of the contaminated fuel. I was pleasantly surprised. The piston crown was a nice light honey colour and there was no visible carbon in the combustion chamber or the valves. There was a slight film of blackish soot in the combustion chamber though, but that was easily wiped off with a soft rag. This was after approx. 10 hours of running. Interestingly, I had to disassemble and ASP 52 2-stroke after a crash and the piston crown was completely covered in hard carbon and the combustion chamber was also completely black. Both these engines use the same fuel (15% Nitro, 20% Castor).

mike109

G'day

Further to our original experiments - both engines, ASP 52 and 91 four strokes, have now been converted to "naturally aspirated" operation. That is, the muffler pressure lines have been removed from the muffler and repositioned so that they vent forward just behind the prop. The engines needed the low speed needles to be opened about 3/4 of a turn and the high speed needles also opened about a turn. both engines are now very happy runners. We have had no more deadstick landings, no deterioration in run quality with each flight and the engines actually seem to be running better generally though they are no more powerful than before with a fresh tank of fuel. They are now very reliable. And, of course, the fuel in the tank is now remaining clean.

So what does this all mean? We really don't know the cause of the problem. My Saitos do not dirty up the fuel when a pressure line is used (same fuel) but the ASPs do and it makes a big difference to the way they run. The mufflers are both drilled for the muffler pressure fitting close to where the exit pipe from the head enters the muffler. Saitos are about half way along the muffler. I remember my OS four stroke mufflers (Surpass) being similar to the ASP design. I did not have this problem with them.

And as previously mentioned, my Laser engines (70, 100 and 150 singles) also do not use muffler pressure as Laser recommends against it and does not fit muffler pressure fittings. They run perfectly.

So for the moment my friend is going to run his ASP four strokes "open". One is in a 60 size trainer (mounted upright) and the other is inverted in a Tiger Moth bi-plane.

If any thing new comes up, I will post it here.

If you have ASP four strokes you might like to try what we have done and also check your fuel after your first flight of the day to see how dirty the remaining fuel is.

Cheers

Mike in Oz