View Single Post
Old 01-09-2007, 09:36 PM
Senior Member
My Feedback: (1)
Campy's Avatar
Join Date: Jan 2002
Location: Baltic, CT
Posts: 3,613
Likes: 0
Received 0 Likes on 0 Posts
Default RE: clunk/ tank question

ORIGINAL: Avalanche2

I have a Mayhem with an OS 70 4-stroke that dies when inverted. I got lots of help and ideas to try and fix that but, after examining the tank set up (I bought this used) Ive noticed that the tank clunk lays about an inch and a quarter from the rear of the tank..?? In level flight this would seem okay but, flying it vertical... or 3-D..ish might be a problem.... I seem to have good movement with the clunk in the tank as far as flopping or sliding goes... I have searched but cant find any good tank "set-up" help... also the manual says this line to carb other is vent/refuel line. Mine is used as a refuel line but has a brass plug for affixing during flight. Does this hender the "vent" part or does it get enough vent from the pressure line? (to muffler) aarrrggggg ... thanks for any advise...
1. Your clunk should be about 1/8" from the back of the tank AND have free movement side to side and up and down. If you are concerned about the clunk coming forward in the event of a hard landing, add a section of brass tube inside the tank between the stopper and the clunk.

2. There are two basic setups for your fuel tank, a 2 line system and a 3 line system.

A 2 line system has 1 line that attaches to a brass tube that is bent 90 degrees and goes up and stops just before the top of the tank (frequently there is a little bubble there and the end of the tube goes there) A tip/hint - file a "V" groove in the top of the tube. It will prevent against the vent tube being pushed against the tank and blocking the vent pressure.

The other line has a clunk on it. That line is attached to the carb. and is used as a combination fuel line and fill line (disconnect it from the engine to fill the tank ).

A 3 line system has 2 lines as described above plus a 3rd line for fueling/defueling. This 3rd line needs to be set up just like the clunk line in a 2 line system. The end of this line is plugged when not fueling/defueling the plane. There are several systems out there (DuBro Fuel It and fuel dots immediately come to mind ). A 3 line system does not have any effect on tank pressure as long as you put the plug back in the line after filling the tank.

I suggest avoiding the one piece metal fuel fillers. I, and almost everyone I know that has used them, have found that they tend to leak after about a season. They also require a special fueling tip that is brand specific (one brand will not work with another brand ). If you lose or misplace the fueling tip........well you know the answer to that one already.

One other thing. Make sure that AT THE FUEL TANK the line to the carb is within 1/4" - 3/8" of the same height as the SPRAYBAR in the carb. Many planes require mods to get the fuel tank location changed for an inverted engine. If you can not get the tank at the correct height a Cline regulator or Perry pump will take care of the height problem.