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I have been running my aircraft on a mix of mogas and avgas, with no problems, but suspect today was an issue due to vapour lock.
Aircraft is powered by a Continental A65, and last flown two weeks ago. I always tend to fill up, just in case, and the last fill up was with mogas, so decided to top up with avgas and put in 18 litres. Start, taxi and run up checks where all good, but got held for about ten minutes before departing. I lined up and went through run up checks again, before rolling. No issues on climbing out, rpm good, temps & pressures all good.
About 15 minutes into the flight, the engine rpm started to fluctuate and the engine was 'hunting' by about 700rpm, as if the fuel was being turned off and on. :shock:
Initially I thought it was a bad case of induction icing, but when the carb heat had nil effect, it dawned on me that this was potentially far more serious. I tentatively checked the mags, no difference. Gently tried changing the power settings and although the rpm's did increase and decrease accordingly, the fluctuations where still there. I fully expected the engine to stop at any time, but was over open, flat farmland, so had plenty of options. I elected to leave carb heat on.
I was at 2200ft and 60kts and not losing altitude or airspeed, so made a pan call and turned to return to the airport. I never envisaged getting back, but thankfully did and landed ok.
I landed from a high right base leg, and expected the engine to stop when I reduced the throttle, but it didn't and the rpm settled. On the ground and taxi back, all appeared fine. I did run up checks before shutting down and all was good. I drained some fuel to check for water, but nothing showing and there was nothing obvious under the bonnet.
I suspect this was vapour lock, but will do a thourough check of the fuel system and engine. Am going to drain and discard the mogas/avgas mixture also and replace with a fresh tank of pure avgas.
Had a few beers tonight :!:
Last edited by seanjd on Sun Aug 07, 2016 10:49 am, edited 1 time in total.
Gravity feed , or lift-pump?..In the case of automotive lift-pumps, I have come across weakened and broken diaphragm -springs, caused by internal engine condensation. can be tricky to spot. With gravity-feed, don't discount a sticking float-needle/seat rare but not unheard-of.....A blocked float-chamber breather took a fortnight and a lot of substitutions (unnecessarily) to track down.
Hi Sean

Like Mick my first reaction was to query the tacho (I recently replaced TIMP's for that reason), but given that's not the case .... I would be looking first at the carb internals. IIRC 'WA was run on Mogas, and had flown very few if any hours in the last year. I believe Mogas does tend to deposit a lacquer which can gum up the works, if left for long periods.

Secondly I would look at the mags. I once had problems with the engine surging ie rev'ing up and down (and misfiring, which you didn't mention if yours did); turned out to be both mags at fault, one had stripped several teeth, the other had shredded the terminals.

As you know, I am definitely no engineer, but that's my 2p worth ....

I do hope you get this sorted soon
The problem with Mogas is not so much lacquer deposition (try a glass jar-full, left to evaporate to atmosphere.)
But the corrosion of brass and copper components commonly found in fuel-systems. Also, "aluminium" carburettors are extremely vulnerable, being die-cast from Mazak alloy which has a large zinc-content. The problem is far worse with fuel containing Ethanol. (AIUI, Esso are the only UK refiner offering a pump-petrol without alcohol added.)
Infrequently used engines will be more prone to internal carburettor corrosion,- here's why.

A warm engine is "put to bed"...fuel is shut -off, so residual heat evaporates part of the float-bowl the air cools, during the night, it contracts, so moist ait is drawn back into the carb.....warer condenses out and reacts with the metal.....repeat the heat/cool cycle of day/night, add in the chilling-effect of petrol evaporating...eventually, all the fuel has gone but you still get the day/night cycle pumping air in/out of the carb, leaving water condensate to attack brass jets and mazak bodies...the corrosion can rapidly restrict or block a small pilot - jet. In my youth, I'd remove the air-inlet , run the engine and crack the throttle wide....allow the revs to build then slap my free hand over the carb-mouth....this would strangle the engine, but give a strong suck on the jets...a quick and very dirty tune-up!.....with practice you could temove the "strangler" hand before the engine died...let the revs build and repeat.
The alternative is to strip (the carb!) and boil for 5 minutes in white vinegar, wash in boiling water,allow to dry and reassemble.
Safely cleans without appreciably altering tolerances and jet-sizes.
Thanks for replies :)
There was no misfiring Rich, just the constant surging. I still think it was vapour lock, but started to check things over tonight.
Drained the fuel, checked the gascolater and plugs. All looked good.
Found a loose jubilee clip on intake side of one cylinder, but that was all so far. Going to check HT leads and mags, and remove and strip the carb next.
May I suggest you carry out an extended air-test after each "rectification" you carry -out.

If you do several things and the result is a cure, all you've done is narrow the balance of probabilities ,to one of those items...whereas you really need to find the specific fault.
foxmoth wrote:What was the OAT? Vapour lock is more likely at high temps - so leaving carb heat on was probably not the best choice if it was!

19 degrees. When it was surging I turned the carb heat off and it nearly cut on me, so I elected to leave it on.
cockney steve wrote:May I suggest you carry out an extended air-test after each "rectification" you carry -out.

If you do several things and the result is a cure, all you've done is narrow the balance of probabilities ,to one of those items...whereas you really need to find the specific fault.

You may, and I hear what you are suggesting. I have yet to find a specific fault, and still believe it was due to vapour lock.

Getting to know the aircraft better now though :)