Rochester carb jetting?

Hey all, looking for a little guidance. The original carb for my '68 Riv is currently being rebuilt. At the same time I'm building a new engine. Engine is a .030 over 455 with TA forged production pistons, notched version advertised at 10:1. Block has been decked .010 and the same taken off the factory iron heads, Felpro head gaskets. Cam is a TA 290-94H , 287/303 and 514/530 at 1.60 according to the cam card but I'll be running 1.55 rockers for now. Edelbrock Performer and shorty headers.

Do you think I'll be okay with the stock jetting? Is this even something that can be figured out based on the info above?

 

Edelbrocks will generally run on about anything, but they are often on the rich side. With headers and cam, you'll probably be close enough to enjoy it but tuning it would make it better.

 

You'll need to re-jet the carb. The 430 carb will be too lean. I believe the 430 will have 71 jets and the 455s used 75s. I cant remember the rods though, which might make them net closer. You'll need to do some work.

 

Rochester carb, original to the 430. Edelbrock intake.

 

Thanks for that, do you by chance have that same chart for a 70 455?

 

Thanks, I do have the chassis manuals, didn't even think to look for carb specs.

I am confused though, based on those charts it looks like the '68 430 had bigger jets than even the '70 Stage 1.

 

If the carb is unmolested, just up one size on the primary jet with the stock primary rods. On the secondary side, make sure choke pull off is not releasing too quickly, have some tension on the air door. I used to run richer rods on the secondary because more fuel must be better? Not always so. A good place to start is a set of CV’s. They have 0.053 tips. Good luck. When Qjet is set up well, they sure perform great. Get your distributor set up to complement your combo as well. You’ll be glad you did.

 

The biggest mistake folks make in this hobby is to INCREASE jet size or "fatten" up a carburetor for any particular application, larger engine, more powerful engine, etc.

We do "recalibrate" if/as needed and often add some fuel clear across the load/speed range simply because it takes about 5-7 percent more of this new piss-water gas to do the same thing as older fuel did.

As for richening up a carb for a new engine build, most if not all of those mods are because we used a larger camshaft and now making LESS vacuum at idle speed and the throttle is cocked open further due to all the overlap the cam has introduced to the scenario.

In reality, IF we made "improvements" to our engine during the "build", it may actually need leaned up some, not richened up. These "improvements" include higher compression, tighter squish, improved combustion chamber shapes, improved head flow, cam events (improved cylinder filling), induction and exhaust mods, etc. When the engine becomes a better air pump, and VE is improved BSFC goes down allowing it to effectively burn LESS fuel or a leaner A/F ratio for best efficiency and most power.

So what to do. For sure the carburetor will need idle system improvements for that much camshaft. Stock engine produced a LOT of vacuum at idle and stock carburetors were set up accordingly. Making more idle fuel available also adds fuel at light part throttle as the transfer slots are players in that deal.

Since the carb is being "built" your "builder" should know these things and be able to install a calibration to get the carb where it needs to be right off the bench, or at least pretty close. That particular cam isn't "huge" in a 455 build with 10 to 1 compression and it's ground on a 112LSA so should idle pretty decent. I'd at most open up the IFR's slightly, stock main jets, and employ the APT system in the baseplate so I could tune part throttle A/F without taking the car apart and changing primary metering rods......