Fuel Filters

Greyhead · 16-01-2005

In a reply in another thread, Four Stroke Torque, I mentioned that I use multiple fuel filters and thought that an explanation as to why would not go amiss, so here goes:

I used to only have a fuel filter in the feed pipe to the carburettor but one flying session was cut short because the engine died when pulling up for a loop. Doing a full throttle / nose up test the engine leaned out and no matter how much I opened the needle valve I couldn’t get it to run satisfactorily; obviously a serious “plumbing” fault I thought.

On stripping every thing down at home the problem was solved; the fuel filter was virtually blocked with fine grass cuttings and this was restricting the fuel flow. The flying site hadn’t been mowed recently and the model had done a good impression of a “strimmer” on the take off run, the ends of the prop becoming stained green. As fuel was sucked from the tank, air and in this case grass, was drawn in through the tank vent to take its place. The answer is a fuel filter in the tank vent line, each time the tank is filled the filter is “reverse flushed” so keeping it clear.

This started me thinking; as the piston moves down the contents of the crankcase are expelled via the crankcase breather, as it moves up it sucks air into the crankcase. It is normal practice to extend the breather with a pipe exiting the bottom of the cowl, right in the contaminated air. Another filter required.

If debris can get into the system through a vent pipe or breather just imagine what the carburettor could suck in, the RC car boys always use an air filter, I know their models are (nearly) always on the ground churning up dust, but for the price of an air filter I can’t understand why every body doesn’t fit one.

One thing to be aware of if you use the fuel feed line to fill your tank, make sure you fill straight into the tank and not through the filter. If you do the latter any bits in the fuel will be held on the engine side of the filter to be sucked straight into the carburettor as soon as you start the engine.

**wonwinglo** · 16-01-2005

As an addition to this Grahame I always use a fuel filter in the tank pressure feed on my two strokes,that black gunge getting back into the fuel tank can cause a lot of problems as well.
Fuel filters do need an eye keeping on them especially in situations such as freshly mown grass etc,check those commercial ones as well,sometimes the seals are not very good.

**wonwinglo** · 16-01-2005

Watch the breather pipe length as well,it can be critical with some four-strokes,dont ask me why,I cannot answer that one ?

Greyhead · 18-01-2005

Barry

The filters I use, which go by the poetic name of “Crap Trap”, are made of a clear plastic tube with 2 filter elements and aluminium ends. They have a couple advantages; you can see if they are getting blocked and they don’t have a sealing ring to leak. To clean them out just reverse flush them, which is the correct method for any filter, I’ve seen people attacking them with pins, not a very clever idea!

As for the breather extension pipe, it’s the internal volume that’s important.

As the piston moves down it pushes the crankcase waste products out of the breather, if the volume of the breather extension pipe is greater than the sweep volume of the piston (the engine capacity) as the piston moves up it will simply suck the “gunge” back into the crankcase. Provided the volume of the breather extension pipe is only a small proportion of the engine capacity the amount of waste sucked back into the crankcase doesn't cause any problems.

Grahame

**wonwinglo** · 18-01-2005

Thanks Grahame,something else learned today.