As a beginner with three months soloing under my belt I appreciate any assistance and suggestions. I think I'm having a fuel foaming/air in fuel problem. Background - I'm flying a WM Mach I high wing trainer powered with an Evolution Training Engine. Since getting my wings I've gone to a two bladed prop vise the three bladed that comes with the Evolution Training engine. My problem occured with both props and I seem to have the engine tuned properly and verified by club members. The engine runs fine at low and mid speeds, but when I increase it to 3/4 and full throttle, both grounded and airborn, it revs up and down, isn't consistent and air is visible in the fuel line. Additionally, the foward left and right fuse around the engine vibrates significantly. Originally, I didn't have any padding around the tank, so as advised, I pulled the tank, replaced the lines, checked the tank, reinstalled it with padding. Still zero luck. Again engine runs great at half speed! Is this a fuel foaming problem and should I replace the tank with a better quality one or is this the price of buying a $70. trainer airframe? Thanks for any help.

Nitro Dew

Did you replace the lines inside the tank and check the metal tubes for cracks?

I did not ... inexperienced! If I go that far, would you suggest just replacing the entire package - tank, internal lines and metal tubes?

I have a tank which was having the same problems. I replaced the lines then the brass tubes and still had the problem. It pressure tested fine but still had bubbles. Replaced the whole system and the problem went away. Didn't make sense, but problem is solved.

“air is visible in the fuel line� is always a bad sign and usually means that the engine can never be tuned for consistent performance.

When the tank is repacked look for the smallest sign of chafing on the tank. It is also best to use vibration-absorbing foam such as latex foam rubber and use care not to pack too tight.

Bill

If it's a standard style tank, replacing it is an easy and fairly inexpensive way to solve the problem. If it's not something you can just pick up at the LHS, try replacing the 1/8 brass tube and clunk line. It actually wouldn't be that difficult to just try doing that first and will help increase your abilities to troubleshoot your airplane.

When bending the brass tubes be SURE not to kink the lines. Put a braided wire inside the tube or a spring type tubing bender on the outside. That is likely the source of your problem right now.

lines or brass tubes can have holes in them inside the tank and still pressure test fine. I had a split line inside the tank that let pressurized air inside the tank go into the fuel line but the tank still tested good.

Also, i have found an inline fuel filter helps a little for bubbles.

Many thanks to all, sounds like an easy fix, at least I hope so.

the inline fuel filter can help some by sometimes at least making the bubbles smaller and it will definately keep your needle valve cleaner.

You mention the fuselage sides vibrate. Are you sure the prop is balanced? It sounds like an engine vibration problem rather than a fuel tank problem. I would also check the engine mount, firewall, etc. to make sure everything is secure.