Stromberg and car gas

[a64pilot]

OK long story short I’ve been burning 100LL for a long time due it storing well and availability and car gas just stinks. I have a 280 gl tank in my hangar now for 100LL.

I have run straight no ETH car gas before and it ran beautifully.

Due to the insanity of fuel prices I’ve decided to put up with the hassle to save some bucks, initially everything was fine with a blend of the two, but once I ran the port tank down to 1/4 and filled up with car gas at mid range 1500 to 1800 or so it hesitated and even seemed to have a slight miss. So I thought somethings wrong with the carb, it’s after firing slightly just in that range, runs fine WOT, so I spent yesterday going through the carb to not find anything, it has been almost 15 years since I was there before repairing a leaking float and I was sort of happy but disappointed to find everything clean and no throttle shaft wear etc. However I did blow out the mixture assy with carb cleaner and got some what looked like dirt out, so I thought great that’s it, slightly clogged mixture bleed hole would make it go lean.

Also read the thread where car gas will cause a Stromberg to stumble and thought maybe it’s normal, almost convinced myself of that.
Went out and ran it just now, some mid range stumbling and even some slight missing, but ran fine at idle and WOT, ran it awhile thinking maybe getting hot it would get better and playing with the ultra sensitive mixture to see if it helped, it didn’t and it didn’t improve when fully warmed either. Switching mags didn’t point to either mag being an issue.

So I started looking, with my finger and calibrated spit I determined 1,2 and 4 cylinder head would fry spit, but number 3 wouldn’t, it was too hot to touch, but not fry spit hot. So I did have a weak cylinder, further looking and you could see the inner rubber out near the head on the intake tube had slipped down, getting a screwdriver further confirmed the intake leak hypothesis as the clamp wasn’t real tight. I loosened the outer rubber slid it down, repositioned the inner and reinstalled the outer and tightened the clamps.

Engine runs beautifully now, no stumble. I can even open the throttle very fast from a high idle to wide open with no stumble and no miss.
So I don’t think a stumble on car gas is normal, mine didn’t use to, and apparently once I fixed the vacuum leak it still doesn’t.
So if your engine stumbles on acceleration you may have something wrong, she just might be trying to tell you something, do some looking

[6277]

Nice work!

[8224]

Boy, this is the kind of information that is great to have!!!

[6478]

So who's your spit calibration guy? Mine is past due

[8342]

I find the weak cylinder with a squirt bottle of water.

With the engine uncowled, I have a qualified person run up the engine.
Switch to the rough running magneto and carefully squirt water on the exhaust of each cylinder where it connects to the head.
The one that doesn't sizzle is the "cold", misfiring cylinder.
Once that is determined you can continue trouble shooting from there.

Rick

[4004]

Be more professional, , Harbor Freight has their laser thermometer on sale this weekend for nominal $20. Yeah, I know it's not to NTSB standards, but ok for troubleshooting and comparison among cylinders, etc.
and letting your dog or cat chase the red spot on the floor.

[a64pilot]

I have an IR thermometer but didn’t go get it. At that point I was just looking trying to find something. It being cooler got me looking, up until that point I hadn’t considered a weak cylinder, thought it was the whole engine leaning out. Once I saw the rubber I thought bet that’s it.
But the takeaway is she will take throttle as rapidly as any other airplane with no bog, if there is no vacuum leak

[chewie]

Great info, thanks for the post.