Ethanol is made from corn.
Ethanol promotes gum. And you will find gum forming in the jet assembly in your carbs, especially if you use your Spridget infrequently. Note the photo below. Pull the choke and the jets come down to richen the mixture. Let go of the choke, and if you are unlucky, gum prevents the jets from returning. Thus your mixture is permanently rich, even though the choke looks like it is in the off position on your dashboard.
These pictures are from Dave in Washington, who reports fouled plugs. No wonder. You can see the gold jet tubes exposed in the photo. This indicates his jets are stuck in the hyper rich position, likely due to ethanol in the fuel and the resultant moisture the corn absorbs and gummy residue that results.
Ethanol free fuel is best if you can find it. We can’t get it here in the Northeast, but look online to see if it is available in your state. Otherwise, check your choke operation. Spray them with carb cleaner as you cycle the choke. Make sure the carb jets slide easily and take them apart to clean the jet tubes. Worst comes to worst, push the jets upward so that their shoulders rest in the proper position, which will effectively turn the choke off.
In the top photo, you can see the left carb jet is lower than the right one. So we also have asymmetrical mixture as a result of this issue. Remember, when you loosen the nut shown right above the jet, you richen the mixture. It should be about 12 flats down from screwed all the way up tight. These carbs would have 50 or more flats screwed down were you to adjust the mixture intentionally to this setting. No wonder the plugs fouled!
We have a few videos about this on youtube that would be worth watching is this is still muddy for you. We have had many questions about this issue, so I wanted to highlight this situation to help people better understand the importance of a healthy choke! We sell new jets and rebuild kits in our catalog if you need them…