I thought I posted about this before but I couldn't find the post; would have been a couple years ago. When I run mogas I get some preignition when I turn the engine off; the engine runs backwards for a few seconds. Couple years ago I ran mogas for a while and the same thing happened. A tank of 100ll and it goes away. Takes several tanks of mogas for it to start doing it again. Anyone else experience this? Any ideas for a solution?

You might check the timing....probably 25 BTC..might want to back the timing off a little when running Mogas. This was recommended to me by the factory about my Superior Engine.

I thought about that but my mechanic is a stickler. Spec is 28/30 so that is all he will allow.

Just an idea what would happen if you ran say quarter take of 100ll with your mogas since you can't turn your timeing back. Still makes fuel pretty cheap.

What was the octane level of the mogas?

Fuel was 91. These little continentals originally ran on 80 it seems like I shouldn't have to run any 100ll.

I ran mo-gas in my T-Craft, and other then a minor (once you got used to it) bit of vapor locking (just a hiccup) after shut down and restart on a hot day, no problemo. I like the timing thing, it's good that your mechanic is a stickler but maybe doing some further research you could change his mind?

Could check your plugs. Maybe run a cooler plug. idle Mixture to lean. If the Case has been shaved and linebored to the limits that would raise the compression ratio requiring a higher octane fuel.

I'll have to check the plugs. I was thinking about replacing them; they have 500 hours on them. I don't think the case is at the limits. It hadn't been overhauled till I did it at 1300 hrs. Only thing I had done to the case was machine it for the O-200 crank. Still a possibility I suppose. I'll check the idle mixture.

I've been flying on mogas or mostly mogas for about 2400 hours & many years, and never had pre-ignition problems. But every airplane is a law unto itself. My buddy's C180 has a terrible stutter at around 1700 rpm (during mag runup) on mogas of any octane-- it runs OK in flight, just the problem at runup, but no sign of any problem ever using 100LL. So he runs 100LL, His other airplane (with 320 Lyc) runs great on mogas so he uses it in that one. I think in your situation I'd try a 50/50 mix and if that works I'd stick with it.
Speaking of every airplane being a law etc, my airplane has given me several heart attacks when it has stuttered severely right after takeoff. No carb ice, fuel's good, timing & carb good, etc. I've discovered that if I always let it warm up for 3 minutes extra before takeoff, it doesn't stutter. So I let it warm up. Doesn't seem to make sense but it works.

whee it sounds like you may have a hotspot in a cyl.Things like hot carbon deposits will ignite fuel.Auto gas lights easier than avgas.You have what we call run-on after the ignition is shut off.Try lowering the idle speed if you can.It also may be running hot with that timing and mogas.

Bill

I ran my C170 on regular grade 87 octane mogas with the standard 26/28 degree timing, no problem. Currently run my O-320 Lyc on 91 octane mogas with the standard 25/25 timing, again no problem.
I'd suggest maybe double checking your timing to be sure it is indeed set at 28/30. It shouldn't pre-ignite like that (esp with 91 octane) if everything's right.

Whee

I agree with the possibility of the idle seed being too high.
Have solved a run on problem that way before.

One time - on a C-85 - it turned out to be too much thread lube on one of the spark plugs acting like a glow plug.

Chris C

What kind of carb? I have a 0-200 in a cub with a marvel carb, I run auto fuel exclusively. If I don't shut down with the mixture I need to let it idle for a couple minutes to cool down or it will run on sometime backwards. You might try this and also open the throttle when you shut off the mags. You can also turn the fuel off to prevent run on. There is a very good chance you have a Stromberg carburator you might enjoy reading this http://www.bowersflybaby.com/tech/strom ... ecrets.pdf

Monte

whee wrote: I'll have to check the plugs. I was thinking about replacing them; they have 500 hours on them. ....

I wouldn't think it was your plugs. In my experience, bad plugs will cause hard starting and/or a miss/roughness, sometimes at runup (1700ish rpm) and sometimes only at full power. Usually plugs go bad by having too much resistance through them, should be 800-1200 ohms new, discard if/when more than 5000 ohms. Check out the aircraftmagnetoservice.net site for ignition troubleshooting info.

I'll check the idle during the annual this week. Also going to check the resistance of the plugs and reset the timing. Carb is a stromberg. Thanks for the ideas.

hotrod150 wrote:...my airplane has given me several heart attacks when it has stuttered severely right after takeoff. No carb ice, fuel's good, timing & carb good, etc. I've discovered that if I always let it warm up for 3 minutes extra before takeoff, it doesn't stutter. So I let it warm up. Doesn't seem to make sense but it works.

An O-200 that I fly was suffering from an over-rich condition on takeoff if it was not warmed up really well. I actually made a precautionary landing because of it once, but the roughness/stutter went away after it warmed up. It wouldn't happen if I warmed it up gratuitously before takeoff. This was in fairly cold Oregon weather (27 F) and the oil temp was well off the peg, but I guess the intake wasn't warm enough.

Same engine would continue idling roughly well after the mixture was pulled out, but only if I accidentally forgot to turn off carb heat after landing. Another over-rich condition?

How are you shutting it off? If there is no fuel it can't fire or run on, as was said in earlier posts, check idle speed let it cool down pull fuel knob to shut it down, if pulling your fuel knob does not shut it down by turning off ALL of your fuel ask your stickler of an A&P or I&A why it doesn’t. It’s not a bad thing to check your plugs and timing, but they should have no effect on the problem that your experiencing, yours is strictly the right combination of heat, compression, air and fuel to combust on its own, auto manufactures have used fuel shut off valves on their carburetors idle circuits for many years to prevent such run on problems.

Zane wrote:

hotrod150 wrote:...my airplane has given me several heart attacks when it has stuttered severely right after takeoff. No carb ice, fuel's good, timing & carb good, etc. I've discovered that if I always let it warm up for 3 minutes extra before takeoff, it doesn't stutter. So I let it warm up. Doesn't seem to make sense but it works.

An O-200 that I fly was suffering from an over-rich condition on takeoff if it was not warmed up really well. I actually made a precautionary landing because of it once, but the roughness/stutter went away after it warmed up. It wouldn't happen if I warmed it up gratuitously before takeoff. This was in fairly cold Oregon weather (27 F) and the oil temp was well off the peg, but I guess the intake wasn't warm enough.

Same engine would continue idling roughly well after the mixture was pulled out, but only if I accidentally forgot to turn off carb heat after landing. Another over-rich condition?

There is another condition that you may be experiencing, I have seen it many times on VW engines that have had their manifold heat tube disconnected or plugged, although the fuel is being atomized at the carburetor the manifolds are colder than the intake air due to evaporating fuel, the same condition that causes manifold, ice but in this instance the fuel condenses in the manifolds and enters the cylinders in a liquid form, causing the engine to run rough, mimicking a bad accelerator pump, popping and hesitation. Have you tried doing you’re run up with the carb heat on?

Stromberg carburetors do not have mixture shut off so I've always just turned off the mags. I could just use the fuel valve, I know it works.