Mogas - Technical Question

[PB]

After a moderate period without use (six weeks), my VW engine was hard to start and ran roughly. Inspection of the carburettor revealed small, soft yellow/brown particles roughly the size of small sugar grains inside. There was also a yellow/brown residue on the ends of the needles.

The engine has been run on Mogas (with occasional Avgas).

In so far as I am aware I have not used fuel with alcohol in it, but is it possible that undetectably low levels of alcohol have caused the problem? The fuel system is all constructed from supposedly alcohol resistant materials but the tank is GRP.

Can any of the fuel-experts on here provide a proper technical explanation of what is going on?

[cardiffrob]

IIRC, Mogas has a 'shelf life' measured in weeks. My Morris minor Sidevalve (1949, 6.6:1 comp ratio) needed a squirt of easystart to fire up but ran alright following 7 months of dry storage. My VW (7.8:1, 1835, Leburg) wouldn't start at all after 3 hours of swinging until someone suggested a single thimble-full of fresh Mogas through one plughole. Only stood for 8 weeks.

What carb do you have? I have an elderly spare CD150 Stromberg that had water inside after a pressure washing and it turned the alloy a dusty white with a blue/brown crud of corroded brass on the needle.

[G.Dawes]

I think this is residue from the old fuel and various cruds that have accumulated, if it was warm the volatiles would evaporate quickly, best maybe to run the engine dry of fuel at end of game and the float chamber would be dry. The life in a tank would be much longer as it has a bigger bulk.
My lawnmower doesn't suffer this and it has fuel from last year which still ran OK.

[Roy Nerou]

On the subject of Mogas, I need to check the float level in our Stromberg carb, float level is dependent on specific gravity of the fuel used, any ideas on a ball park figure for Mogas & 100LL?

Cheers

[Steve Brown]

I understand petrol /mogas is around 0.73-0.74 SG whereas Avgas is nearer 0.70-72 SG.

These values vary with batch composition & temperature.

In reality I'd suggest you'd have a tough time measuring the difference in float level - it may even be that the slight difference in float level may compensate for the slightly different calorific value (energy per volume) of the 2 fuels. Ie denser fuel (with likely more energy/vol) would cause the float to ride higher hence the fuel level would be lower so you'd get a weaker mixture.

Either way I'd set it to the manufacturer's setting.

[gasax]

I really would not worry about these tiny differences. If I were faced with trying to measure them then working out a method would be very difficult.

Float levels - completely irrelevant!

Today I was searching the net for some info. Unconnected up turned a CAA AN for Mogas. Dated 1986 it was for a G registered aircraft based in Abu Dhabi. The AN allowed operation on Mogas 97 at temperatures up to 43 deg C ancd 9000 ft altitude! Somewhat different to the conditions which were then current for Mogas of 20 deg C and 6000 ft.

The conservatism of many of these assessments needs to be understood.

[Nick Allen]

The AN allowed operation on Mogas 97 at temperatures up to 43 deg C ancd 9000 ft altitude!

I've been a car where the outside (shade) temperature was 45 deg C, and then proceeded over an 11,500ft pass (where admittedly it was somewhat cooler): the engine worked fine throughout!

[WBerry]

Speaking of Mogas, which aero engines can you run on it.

I know Mogas is actually recommended for Rotax engines, but specifically I wondered about the bottom end Continentels/Lycomings. Are they easy going engines, or require mods (trivial or otherwise) or is it Avgas only?

Cheers.

[Nigel Ramsay]

My A65 is approved for Mogas, it was fitted with hardened valve seats from new apparently. I don't know if this applied to all or just certain years; anyone know more about this?

[gasax]

The vast majoirity of low compression Lycon are approved for Mogas. CAA Note 98 gives the full listing. In the vast majority of types the only 'modification' is a logbook entry and some stickers.....