Posted Sunday, August 30, 2009 @ 04:24 PM  

I'm beginning to wonder if it's mogas. In particular I'm beginning to wonder if the specific gravity of modern mogas is significantly different from old mogas or avgas. Could it be lighter causing my float to ride lower thus raising the float chamber level? Coincidentally, one of my friends who is in a Jodel 1050 group has been complaining of rich running just recently and talks about how leaning the mixture has helped the situation. They run on mogas all the time. I don't have the luxury of a mixture control, though.

Posted Thursday, September 3, 2009 @ 07:53 AM  

How do I know I'm running rich? If I pull the plugs they are very sooty but as well as that when flying at full throttle I can see the exhaust. By that I mean I get what I can only liken to a sort of flickering shadow at the edge of my vision. I have seen that before when very high where the air is thinner but now I get it at moderate levels. At the same time the engine feels and sounds rough as if it's slightly misfiring. If I back off the throttle it all evens out.

Posted Monday, September 28, 2009 @ 06:43 AM  

You won't want to hear this, but I don't think you've yet found the problem.

Are you running with 83mm cylinders, ie, do you have a 1400cc motor? If so, we have found that you will need a bigger main jet.

Like you, we measured a standard '1.25' main jet and found it to be oversize at 1.26mm or so.

We are currently running both WGN and HDO with main jet orifices measured at 1.28mm (the diameter of a 1.29mm bit) and they're both running at a perfect mixture -- and the fuel level in both float bowls is deliberately kept high, because I use that fuel for inverted flying and need as much as possible. Our float levels are around 5mm below the joint face.
I repeat, they're both showing a nice plug colour, and we burn 10 litres per hour cruising at 3100 rpm, 7 litres per hour at 2800 rpm, although that's with our coarse Australian propellers.

I have spoken at length to several VW experts (your nearest is John Maher at John Maher Racing in the Outer Hebrides). He says that the graph of mixture versus performance is steep on the lean side of the curve but very shallow on the rich side, so you can run quite rich without it affecting performance very much.

Plug colour does not denote mixture. It denotes combustion temperature. Usually 'cool' combustion (dark plugs) is a sign of richness, but it can also be caused by excess oil and several other things.

The ONLY way to discover whether your engine is running rich or weak is by checking your fuel consumption.

Fill your tank to the brim. Fly at a precise constant rpm for exactly two hours. Refuel. See how much fuel you uplift. Then compare your consumption to the book figures if you are running a 1200cc motor with a standard propeller, (or multiply the book figures by 14 and divide by 12 if you're running a 1400cc motor). Of course you will have calibrated your tacho first, because rev-counters are the second most unreliable gauge on the aeroplane.

Alternatively, you can run at full throttle for two hours. If you read the Rectimo manual graph, that gives full-throttle consumption (I can't remember it off the top of my head, but I think it was around 14lph for a 1200cc motor). That way you don't need to rely on your tacho.

For your engine to be so rich the plugs are black, it would need a main jet of around 1.3mm or 1.4mm diameter, and you would hear the motor going rumpty-rumpty-rumpty (like it does with the choke pulled out).

Try opening the throttle wide on the ground (with the brake on, of course) and turning off the fuel. Does the rpm increase just before your engine stops? If it does, the mixture's too weak. If it doesn't, then the mixture's OK.

If any of the above suggest your mixture's OK, you are probably burning oil (piston rings, valve guides etc).
That might cost you a whole £500 to fix -- not a problem.

Good luck.

Posted Monday, October 5, 2009 @ 11:32 AM  

I guess what REALLY intrigues me is that you say this richness gradually developed.

It would take millions of hours for the main jet to open out through the flow of fuel, so I can only suspect that something became loose or wore.

If as you say the float level is correct and hasn't changed, I'm really stumped as to what might cause a gradual increase in fuel flow and richness.

My mind keeps floating back to your problem in odd moments, but I regret I really cannot come up with an answer. I would really be interested to know what you learn alnong your diagnostic way.

I brought HDO's carb over from Aussie with me this summer and have been running it on WGN. Swapped them back last week with no apparent cahnge whatever (two test flights, less than two hours apart), although WGN's carb has a 1.28 main jet orifice against's HDO's 1.29 measured main jet orifice.

All other components are exactly as found. I'm even re-using for the umpteenth time the original gaskets, although now I have a new set, I'll replace them at the next dismantling.

Do let us know how you get on, because presumably we'll all eventually have the same trouble.