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Mon Nov 09, 2009 8:32 pm

I'm looking some help jetting a zenith for a PLENUM type manifold.
if anyone is running one & knows what jetting he's useing it would be greatly appreciated .

thanks Rick

Mon Nov 09, 2009 10:09 pm

What size engine?

I take it you are near sea level?

Kyle

Mon Nov 09, 2009 10:10 pm

If I remember correctly 135 main jets should get you in the ballpark. I didn't need to change any other jets. I hope this helps.

Mon Nov 09, 2009 10:36 pm

well here's the deal.
24mm vent.
135 mains
60 idles
55 pump
large air 210
& the smaller air jet 140

I've run this same setup with the divided intake on 1776 up to 1914 with great results, but if I put the same carb & same jet setup on a plenum manifold the drivability totaly sucks !
once it gets up to speed it will run smooth but the bog down from takeoff realy sucks.

Tue Nov 10, 2009 9:07 am

The jetting sounds right. I would be looking at the accelerator pump.

Tue Nov 10, 2009 1:54 pm

Rick, that sounds pretty close to what my dad is running. He may have 55 idles though. His ran great for years on both a mild 1776 and a mild 1835. The 1835 had a split runner manifold. I don't remember what the 1776 had. Anyways we tried both style manifolds on his somewhat wild 2180, and it has a soggy bottom end. I think it is the cam and single carb that is the problem with his though. In general, the open plenum style manifolds are good for top end HP and the divided ones are good for low end torque. Something I remember from my V8 days.
--Tim

Tue Nov 10, 2009 6:32 pm

thanks guys ,
but what I'm realy trying to find out is has anyone been able to get rid of the off idle flat spot that seems to go with the plenum intake ?

Tue Nov 10, 2009 7:09 pm

Why not go back to the divided intake if it was working for you before?

Kyle

Tue Nov 10, 2009 7:13 pm

OK this may sound stupid but :oops:

I just want to know how :roll:

Thu Nov 12, 2009 2:52 pm

A zenith will work fine on a plenum manifold. Your jetting sounds correct. Check the accelerator pump, adjust it for more flow and see what that does. If that doesn't work try bigger main jets. I am assuming you have no intake leaks.

Thu Nov 12, 2009 6:28 pm

I have 50 idles, 135 mains, 200 Air Correctors, acc. pump?? in my 2180 with a plenum manifold. My 1835 is jetted about the same with a plenum manifold. Niether one has a flat spot.

Fri Nov 13, 2009 9:23 am

thanks for the info guys . I'm keeping notes.

Fri Nov 13, 2009 10:59 am

There a fair amount of info on Zenith tuning in this book:

http://www.amazon.com/review/RX4BAR03WGE6I

It says the optimum is a "small" plenum under the carb if it doesn't have the slot in the base between the two barrels like this:

Not sure what is meant by "small".

If the slot is there, isolated runners of about 1 1/4" are best, otherwise the mixture needs to be enrichened.

It also recommends using end castings with no slot.

Me, I'm just passing this along from the book. There are obviously guys here who are making it work. Perrib knows his stuff. Keep us posted, my build has a plenum manifold Zenith setup that I'm going to have to make work.

Good luck!

Kyle

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Sun Nov 15, 2009 2:20 pm

I'm running a single 32NDIX set up by Art Thraen on my 2180 and it runs great, but I don't know what jets he put in it. He did ask me a bunch of questions when building the carb like altitude, CC, cam, use, etc, of course. It has the "plenum" slot on the base and it's on a plenum manifold, too. I bought a spare 32NDIX for parts and it does not have the "plenum" cut in the base.....