Carb Jet questions

[undysworld]

I've got some lingering issues with my newly-rebuilt carbs not idling smoothly, and wonder if anyone can offer some insight.

In a recent discussion I had with a local guy, he commented that our Zenith's have trouble with new gasoline. He commented that it had to do with a change in the specific gravity of the fuel due to various additives. In particular, this was said to effect the midrange between idle and the high rpms. He indicated that the idling problems were related to a lean condition.

He has recommended the following modifications: He suggested boring out the Idle Jets to 65 (next drill size up), running 135-140 for the Main Jets, and replacing the Air Correction Jets with 170-180 (SAV price $55 each!). He tells me to expect the idle adjusting screws to be at only 1/4 - 1/2 turn out.

I'm a bit reluctant to go modifying the jets and spending $220 on new jets, if they're not necessary. But the guy who made the recommendations is pretty savvy on carbs (Tom Zat of Alfa Romeo fame). So, can anybody here comment on these proposed changes?

Thank You,
Paul

[edzz]

What is your current jetting? What he is recommending is close to the “6x6 standard” and “4x4 and 6x6 tropical type” listed in the operator’s manual on page 203. For what it’s worth my 710k is running the 6x6 standard jetting and runs richer than I would prefer, no black smoke when at operating temperature however the exhaust pipe is very black and sooty and my spark plugs indicate too rich a mixture. How do your spark plugs look?

[undysworld]

I've only got a few miles on since the carb rebuild. So I can't really get a current status from it yet. The plugs have historically run a bit rich looking.

Thanks for the data. I obviously missed that somewhere.