380SL has developed a engine murmer
 

I have a '84 380sl and just recently it has developed a slight murmer at idle. It's a little miss-fire I guess but it's enough to shake the car a bit. I have replaced the plugs, and added some techron to the gas. I have ordered the metal connectors to the plugs. I am hopeful that this will help.
I was also replacing the air filter and noticed a lot of gunk on the butterfly value of the carb. I increased the idle and sprayed in some gumout. This cleaned things up nicely but didn't change the problem.

So any suggestions about next steps before I deplete my checking account?

Thanks in advance for any help you can offer
Captgrail