Mayonnaise gets too thin after adding solids I make a mayonnaise by slowly whisking a cup of extra light olive oil into two eggs yolks and a tablespoon of dijon mustard. The mayonnaise is not as thick as store bought mayo but it is thick enough to spread. 

However, when I stir in solid ingredients to make an aioli the emulsion suddenly becomes very thin. It doesn't break, but it turns into a liquid. This has happened with minced garlic and with chopped clams. Sometimes after stirring in the solid ingredients it looks as if there are bubbles coming out of the emulsion. 

What causes this, and how can I prevent it?
I'm going to step out on a limb based on experiences I've had. I suspect that adding garlic and or chopped clams after you have emulsified your other ingredients is simply adding moisture and making your mayo/aioli thinner. (Especially since you said that it doesn't break.)

I would suggest putting your garlic or clams in at the beginning with your other ingredients before you emulsify with the oil. If you still find it to be too thin you can whisk in another whisked egg yolk or a little more oil to get your desired consistency.

Re the comment about acid, I don't use any. Also there would likely be vinegar in the Dijon mustard, so you are not sans acid.