I was checking out the pnumbers on the carbs that came on my 70 ta auto challenger and they are supposed to be:

4792 center carb (auto)
4789 front carb
4790 rear carb

But turns out that i have
4792 center carb (auto)
And (2) 4790 carbs.

Are the 4789 and 4790 carbs the same carbs just setup different and thats why they carry a different part number or are they different and not interchangeable?

Holley sells 4790 replacement carbs only, no 4789. I bought a set and a 4792 center. The difference is in the metering block between the outboard carbs, besides the part numbers. A '70 FSM should have the metering block orifice sizes listed.

My 4790 carbs are oem.

I guess i could run one those aftermarket metwring plates w screw in jets and id be set???

Domingo, your pair of 4790's will run just fine on a stock 340 with the stock 4790 metering plates, no need to change anything. If you want to tune them for performance (I suggest doing this on a dyno) go ahead with the Promax plates and play with the jetting, or the budget route, drill the metering plates to a larger size per the instructions on this link.

Six Pack Tips

Yes. Thats what i was thinking...just putting the aftermarjet metering pkated w screw in jets and tune em right. I just wantwd to know if they were just the same carb set up diffwrently....which u kust confirmed.

They are goin on a fast syyle 416 toa w w2 heads. Promax plates will come inm handy.

And just to clearify, the 4789 and 4790 carbs have the same metering plates (41's). So other than the list numbers and date codes they are identical.

So other than the part numbers what's the difference between front and rear carbs? I know the idle mixture screw setting was set on a flow bench, then plugged. Would the factory idle mixture settings be different front and rear?

SAME

Ok great. Fired up my new Holley's today and didn't have to adjust the idle mixture one bit. 1/4 out diaphragm side and 1/2 turn throttle factory settings on my outboards.

Why the heck did they use different part numbers then????

I also would like to know why they had different part numbers for the front and rear