Hybrid 7027240/7043240 build

[Rhett]

Trying to make one carb out of spare parts, strictly for drags on my 65 Gran Sport (401, stock cam, auto 3.23s) - Just to see if the additional CFM's of the 73 center section have any effect..

Figured I would use the 67 baseplate for the swich pitch throttle linkage and the 67 top section without idle bypass passages, and run it with not choke assm..

I know I have to close up the upper idle bleeds in the airhorn - and will probably stick pretty close to the 1st build specs in the book for the secondary circuit.

I'm looking for suggestions on how to set up/dial in the idle/off idle/cruise circiuts..the current specs are as follows:

Idle Channel Restriction   0.046
Accel Pump Discharge   0.026
Main Air Bleed (upper)   0.047
Main Air Bleed (lower)   0.042
Idle Air Bleed (air horn)   0.046
Idle Air Bleed (upper)   0.052
Idle Air Bleed (lower)   0.081
Mixture Holes                   0.086

The 7027240 had 70 jets and 44 rods - should I stick with that as a baseline?

Thanks again - Rhett

[Cliff Ruggles]

I stick with the .070" jets to start with with the small MAB's. 

Block off one pair of the upper idle airbleeds, the ones in the airhorn are the easiest.

I'd have to know how big the cam is to provide any other information, idle tubes, down channel, etc?  For most "stock" cams, start out with the stock idle specs, and only add more idle fuel if/as needed......Cliff

[Rhett]

Thanks Cliff -
the car has a stock 090 cam, specs:

EXHAUST DURATION .295°
EXHAUST LIFT  .431
INTAKE DURATION . 295°
INTAKE LIFT .431
SPACING  114°
EXHAUST OPEN BBC  76°
EXHAUST CLOSE ATC 39°
INTAKE OPEN BTC  28°
INTAKE CLOSE ABC  87°
OVERLAP 77°


Thanks, Rhett

[Cliff Ruggles]

Lots of off seat timing but on a wide LSA.  That cam should make power over a very broad range, but still good idle qualities.  I love the old technology, it works like it's supposed to!.....Cliff

[Rhett]

It's a torque monster for sure, and still idles like glass. 2.17 60 ft's on 3.23s and 225 redlines with a 2 speed. In the over 14s class, I 60 foot everyone but then give it up on top end. And that was on the AFB, for other reasons I still havent had a good clean run on the Q-Jet..I have a track rental day to dial it in 9/7.

I got all the parts from you yesterday, I'll get started this weekend. I'll start with 70/44 rods, but is there any benefit to sticking closer to your book and opening up the MABs, and larger jetting (73/43s)? I was going to open up the pump discharge, mains and upper idle bleed... Should I leave them be? Just intending this carb for WOT, not day to day driving....

[Cliff Ruggles]

No real advantage to opening up the MAB's for larger jetting, as that's just the driving side of the carburetor anyhow.  I'd leave it stock for racing....Cliff

[Rhett]

I'll give it a shot - I think I've got 71/44s around here to start with, a K hanger and good selection of secondary rods from CC's to AY's.

With that - most of my rods are long tipped. Will a K hanger bring too much fuel in too early?

[Cliff Ruggles]

Might be, you'll know if it lays down slightly or has noticable transition onto the secondaries.....Cliff

[Rhett]

This junk carb I built turned out better than I could have imagined....Has some off-idle drivability issues, but WOT is crisp and instant.. wound up using a M hanger and AY rods.

Now I'm thinking about how to keep this thing on the car and diaingl in the primaries. But I'd have to configure a hot air choke set up. Has anyone ever been able to make hot ait choke work on a diverced choke body? Is it as simple as finding vacuum passages and using a 75-76 choke body?

[Cliff Ruggles]

It's possible but not practicle.  You need all the parts from a donor 70-74 Old's carb or 73-74 Pontiac carb.  With enough effort they will work but still really not worth the effort, IMHO.....Cliff