I currently have my half vw (zenith carb) tuned for 91 octane pump gas, but now that I've gotten a hanger on a field that has 100LL I'm planning on switching over (I have to drive 10 miles extra to fill up a gas can of 91). I expect that the difference in specific gravity is going to mess with the mixture, but I'm not sure how much. The zenith has a ground adjustable main jet needle and I plan on starting with a tank of 4/1 91/100 mix, check the plugs, adjust, go 3/2, check plugs, adjust etc. to 100%. I wish I had egt gauge and inflight adjustable (might do that anyway🤔), but any red flags I'm missing on my planned approach?? The ability to refuel at one of the many small rural airports in my area that have fuel would really open up the areas I can fly.
Speaking for a two stroke Rotax 503, I never found any need to change carb setting when using either 91 or 100LL gas. Good luck on tuning your engine without gauges.
Just discovered that I have to write something in the message area, vs, just posting the pdf. Got an error message about file too big (with out writing anything). Could this be part of the problem some people have had when trying to post pictures?
I don't mess with my carbs! If it's not broke don't fix it. Never have problems that way. I have heard about people that are always "tinkering" with them and wind up "finding" a place to land because of a problem.
Thank you fly dog, and very true words lowflyer. "If it isn't broke don't fix it!" Is actually something I've litterly preached for years at my day job, and a reason I'm going to do the gradual transition while monitoring it rather than a cold quit switch over.
I do not know about jetting for 100LL but some of the guys that use it in small engines (mowers, snowblower, ect) are reporting sticking valves cause of the high lead content of 100LL.I assume that it builds up in the guides. We have 91 octane with no ethanol in it and I use in the mower and motorcycle and it works ok. It is a little more expensive than regular, but about $2.00 per gallon cheaper than 100LL.Why would you change from what you know works to some thing that is questionable?
I got a Rotax 277 (half of a 503) from Steve Beatty at Air Screw Performance, and he recommended running 100LL for the first 10 hours. Then I switched over to alcohol free premium. I've gotten 100LL at airports where I needed fuel to get home, and found no difference in performance.
In general 2 stroke engines won't notice any difference in performance between 91 unleaded and 100LL, and without valves to stick there's only the spark plug electrodes to worry about getting whiskered with 100LL.
In 2 strokes the 'actual' compression ratio is around 6.5:1, although the 'theoretical' compression may be listed as around 11:1. This is because of the ports in the cylinder walls. The actual compression only starts from the top of the exhaust port, so only around 75% - 80% of the sweep is used, and even then some of the incoming fuel will have gone straight out of the exhaust port before it was covered, and some of the last charge may still be in the cylinder when the piston starts rising, thus effectively reducing the effectiveness of the fresh fuel/air charge. In contrast, in a 4 stroke engine the compression starts from when the inlet valve closes at the bottom of the compression stroke, so the 4 stroke gets pretty close to all 100% of its sweep to compress the fuel/air mixture in.
As a result the 2 stroke isn't very sensitive to any performance differences in the fuel, whereas a 4 stroke might well see quite a difference in power. That's why jetting for different fuels isn't so important for 2 strokes in comparison with 4 stroke engines. Because of this I can fully understand why someone with a 4 stroke powered 'max would want to make sure that the jetting is correct for the type of fuel being used.
In response to my rational for the switchover, where I'm based at it a vast expanse of flat corn and bean fields for 40 miles in every direction with basically no senic area within a round trip distance on a single tank worth of gas without cutting the reserve fuel irresponsibly short, but there is a very large number of rural airstrips with 100LL pumps that take cards. Being able to stop for gas for the return flight opens up scenic wooded river valleys, rolling moraines, restored prairies, in short things I want to see from the air.
If you use 100LL you will get a yellow lead monoxide buildup on the spark plug, piston, and cylinder head. If you can find ethanol free unleaded gas then I would recommend that over 100LL. Here is a link to some pictures of an engine run on 100LL showing the deposits....
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