Did first full flights with a saito .72 in a strega arf today. Got quite a suprise. I'm using the ultraproducts control line manifold with super tigre needle valve assembly, muffler pressure, sullivan 6 oz clunk tank rigged normally, zinger pro 13 X 6 prop, max rpm was a bit low at 8400 and I launched at 8200.  Lap time was slow as expected at 6.3 seconds on 67 ft lines. Engine run full tank was 2 minutes. That got my attention, especially since I was inverted the first time it quite. Pulled the motor and confirmed the plumbing. Everything there ok. Entire engine compartment was dry. Next flight I confirmed 6 onces in. Engine died at same time and sounded like a normal engine cut off. Confirmed zero fuel remaining in the tank and the engine compartment still dry. So we uncowled it and decided to start it inverted and watch to see if we could tell what was happening. What we saw was a shock. Each time by the way engine started right up normally on the electric starter. (sorry, I don't hand start 4 strokes even with a chicken stick, but I earn my living with my hands.) When the motor started so did the spew of raw fuel... coming out of the breather tube! The guy holding the plane has 20% nitro deoderant to prove it. Hope no one gave him a cigarette to hold today. We were bot befuddled and could not think of anything else to try but to disconnect the pressure line from the muffler and let it run suction. Fired up and no fuel came out of the breather. Run went normal duration. Max rpm went up to 9300 rpm, a more normal range for the motor. 

Sure would be glad if someone could explain this. Only conclusion I could come up with was that the muffler was providing more volume to the tank than the motor could use. In RC we would normally have a 10 to 16 oz tank for this motor. I do not think the original runs with the OS FP needle valve did this. At least they did not on the test stand where I had muffler pressure on but it was a 14 oz tank.

All I can say is that is what happened. I thought it might help someone not loose a plane to it. 

Bob Branch

Somethin ain't right, have no ideas but that has to be the strangest thing I've ever heard.

Don't have a 72 but if I would let it, my 56 will turn that prop at over 9 grand. Heck my 40 will turn an 11-7 at 10 grand. Can't imagine how it would cause it but did you check the valve clearance.. Muffler not plugged up, groping at straws but it should run fine on muffler pressure, my guess would be something not right with that particulat 72.

Bob, i dont't know what is going on with your .72, but if you can get in touch with Brad Walker or Steve Moon i bet they can help. Ron.

I would suggest that you check Valve timing and lifter clearence, I have used  4 strokes like  Y. S 91 and O S 120, OS 48& 20& 26  and all of them requier valve check at least once a year, depending on use.
Good Luck. 

In any of the possible scenarios above which might allow fuel to enter through the muffler pressure-tap, that situation can be absolutely prevented by adding a small check valve in the line between the fuel tank and muffler. I think this addition is crucial also to prevent a reverse engine run, in which any positive outcome is impossible.

Bob,

Out of curiosity, if you close the needle down, can you blow air through it (from the fuel tubing intake), or is it really closed off? The reason I wonder is that it sounds like you aren't able to really close the needle down far enough (with the ST spraybar) to handle the higher fuel pressure that you get with muffler pressure. If I understood you correctly, it would work with an OS spraybar. It might be that the seat is damaged in the ST spraybar, and it allows fuel to still get by even when you are needled down.

Still not sure how it is blowing by the ring or the valve seat. But that is another story.

My GUESS: Cam is out of position:

* 9300 RPM on a 13-6 sounds low.

* If it had anything to do with the needl/spray bar, THAT MUCH fuel would make the engine hydraulic lock LONG before the escess fuel blew through the piston/cylinder.

* Like one of the earlier posters mentioned, the ONLY path for a LOT of fuel to get into the crankcase is ABOVE the valves and through the pushrod tubes, through the cam and through the breather.

* If the cam is out of position about 1 tooth the engine will run but could actually be SUCKING fuel through the muffler if the exhaust valve is still open when the piston descends.

BTW, exhaust pressure (and thus muffler pressure) is the same whther you use a 1 oz tank or a 100 oz tank - but the 1 oz will come to stable pressure FASTER!

Seem to remember Bob Z (or someone) posted instructions on how to check the timing of the Saito to Bobs 4 stroke group some time back, not sure if I still have the post but will look.. Seems as though this was an issue with the 72 a while back.. Comming from the factory with the timing not set properly..

When the motor started so did the spew of raw fuel... coming out of the breather tube! The guy holding the plane has 20% nitro deoderant to prove it. Hope no one gave him a cigarette to hold today. We were bot befuddled and could not think of anything else to try but to disconnect the pressure line from the muffler and let it run suction. Fired up and no fuel came out of the breather. Run went normal duration. Max rpm went up to 9300 rpm, a more normal range for the motor.

I have been running the TT 75 on the bench.

It appears to have more STUFF coming out of the spooge tube than I have ever seen (oil and fuel).   There is a HUGE 4 foot spot on my driveway from the spooge tube exit (that is not right).  As a side note , the engine is not smoking like I would like to see.  I think all the oil is ending up in the case somehow and not going out the exhaust valve.  It also does not run smooth.

BTW, the TT 54 runs great and smooth with a beautiful plume of smoke and almost nothing coming through the spooge tube.  Like it should be.

I have run a few 4 cycles on the bench...  I have never seen an engine pump out fuel and oil through the spooge tube like this.