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Ethanol free Mogas went bad?

dothetime

Well Known Member
I have an 0-200 that I have been using ethanol free Mogas 90 octane for 7 years, 400 hours.
After finishing an annual from **** that took me 10 weeks, my carb was flooding after shutdown and during running causing erratic operation and black smoke.
Engine was new in 2006, so the carb(ma-3spa)is 15 years old, I took it apart and found no issues, so after putting it back together, the problem was still there. So I sent the carb out for repair.
Reinstalled repaired carb and the issue was still there. Checked the primer, no issue. So I decided what the heck, and I drained the Mogas and put in 100LL.
Flooding stopped. I couldn’t believe it, but there were other signs, my fuel pressure sensor got erratic and my flowscan sensor also quit.
I’m just putting this out there for discussion and to see if anyone else experienced something similar.
Fuel was at least 2 months old, maybe more counting storage and delivery to gas station. I usually fly a couple of times a week, plane is hangared 100% and use marvel mystery oil in the fuel.
Dave
N601XP
 
Last edited:
Do you have the blue epoxy unsinkable float before amd after?

Was the fuel level set correctly in the bowl or changed?

Float could be higher on 100LL and maybe the density of the mogas in your carb was just different enough to partially sink your float and run rich?
 
Mogas issue

Yes I have the blue epoxy float
I never changed the float level as carb was original untouched for its life.
The carb came back from overhaul shop that set the float level to spec.
I had the same thoughts about the density of the fuel, or it’s specific gravity.
But I have been using this fuel for 7 years and 400 hours, I’m thinking the fuel went bad, and it’s density changed?
 
Bad Mogas

I also considered some one may have tampered with my fuel but I see nothing or smell anything unusual in the fuel samples.
 
10 weeks in a warm hangar in Florida *might* cause a bunch of the high vapor pressure fractions of the autogas to evaporate, leaving you with something...different...

I'm curious about what happens when you use up the 100LL and refuel with mogas.
 
MOGAS starts to deteriorate 30 days after its refined. You might have passed the threshold of what your aircraft could tolerate. There are so many variables you can’t really make comparisons from past use. Your other symptoms sure seem like bad fuel.
 
Thanks for replying and I'm glad you had all the info and ideas covered.

Winter's 90 octane no ethanol fuel this year near me seemed poorer than usual sooner than usual, too for me.

I found some vapor issues at only 7500' on a cool day that was not carb ice and was repeatable before Sun n Fun week.

I run 50/50 with 100LL to put the lead where I want it. Never had a problem in 20 plus years of no ethanol mogas use til this early spring.

Current stuff acts like normal summer cut, again. Thankfully.
 
Bad mogas

Update:
Have alternately used 100ll and fresh Mogas in separate tanks with no issues
So the mogas definitely was the problem with the carb float leaking
 
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