Mods on a 7041262 for a RAIV 455 engine. Are they OK?

[MattK2]

I modified a 7042262 to the "3rd recipe" for my motor, and it worked great when I first got it going last fall, but it dripped.  Ended up having a micro crack that would open when hot.

I sent my carb out to be serviced, he confirmed the crack, and found a donor 7041262 body and air horn.  It was a 11 day turnaround with shipping.  This "newer" carb has slightly different guts than the one I built.

My engine is a 455, 0.030" over, RAIV cam (adv.4*), 9.3 CR, 6X-8 heads.  400 hp/ 500 ft-lbs

The first carb: jets 74, rods 37, tubes 38<-------cracked carb
the second carb: jets 77, rods 43, tubes 36 <---------current setup.

It idles at about 13" vacuum, cruises at about 19"

Total timing is 37*, initial 15*, vac advance 15+*, curve is very generic and being addressed.

I'm still experimenting between ported and manifold vacuum signal to the vac adv. can.  At the moment I'm using manifold. 

Idle is poor when cold, seems like a choke issue, better when hot.

Restart is weak when hot, better when cold or warm.

At a 20 mph roll, if I drop the hammer in 1st, the tires break hard and it goes like hell, so that's good.

[Cliff Ruggles]

77 main jets are HUGE in one of those carburetors.  I'd go back to 73's or at most 74's with 42 or 43 primary rods.

Idle tubes will need to be .038" for the RAIV cam at that compression ratio...

[MattK2]

A 73/43 combo. OK. I need to purchase some jets and tubes.

Any thoughts on why a hot start seems overloaded with a weak vacuum signal?  That’s my assessment of it.  It starts soon enough, but doesn’t come alive like you would expect from a hot engine.  No quick pop off. I could set the idle up to 900-1000 and it’d start better but then it could develop runon at shut down.

And, it would be great to get the idle around 700 and not be so rich. I lean out the mix screws and it’s not helping much. The. I start messing with vac advance signal to help. Going in circles.

[Cliff Ruggles]

Check to see if there is enough vacuum at start-up to fully employ the vacuum advance.

In many cases to effectively use MVA you'll need a VA that fully applies much lower or all of the timing may not be in at start-up or even at idle speed.....

[MattK2]

Soon I’ll be swapping dist. Current one has a long curve. Generic.  Going to try 1000-3000 range.  Letting the builder decide on the rate.

Swapping jets and idle tubes once they arrive. Going to measure everything when it’s apart.

Given that carbs were spec’d to specific engines, does it make sense to modify a carb similar to a 69-70 RAIV spec’d carb. those carbs are around. Rare, but those specs are out there, right?  It’d be a good starting point.  Small tweaks from there.

[Cliff Ruggles]

Distributors are easy to modify, provided it's an OEM unit.  No need to replace it otherwise as you can put any curve you want into any unit.

Low compression with a big cam likes more initial timing and quicker curve starting right off idle and all-in by 2800rpm's or so. 

No need to look for another carb, then one you have will work once it's dialed in for what you are doing.....

[MattK2]

I may know why it starts poorly when hot. The choke isn’t staying wide open, the choke rod is wrong.  Just got it out of storage. It won’t come close to the index notches. I admit I never noticed them before as a way to calibrate the air. It was sold as a Pontiac part. The one with the hairpin bend in it. Old Service Manuals from 1970 show it with single kink. The Rochester parts diagram is the same. So all this time I’ve had a choke rod 1/2” too long.  Cold starts should improve given improved full closure on a cold start. I’ve got the vacuum break at 0.330+/-.  Now I just need to find the correct one and see if things greatly improve.

Still need to change out the rods, jets, and tubes too.

[Cliff Ruggles]

I stock everything you would need including high performance quick release choke pull-offs, links, jets, metering rods and the correct power piston spring for the RAIV cammed 455......

[MattK2]

I have new jets rods tubes and pull off I bought last year from you.  Correction, not the “choke rod” ,but the “choke stove” rod.  Sorry.

Since I’m mixing 68 intake with a 71/72 QJet. The choke stove rod is too long in this setup. Was the cam follower different pre1970?  I have 70-74 Delco Bulletin for Pontiac carbs so I can see the stat/rod setup won’t attach to my intake, it’s rotated 90d.

Still researching old part numbers to see if I can find one that works.

[Cliff Ruggles]

Pontiac used several different arrangements for the divorced choke carbs from 1967-72.  They had three different choke pull-offs and several different fast idle cams.  Mixing and matching parts typically doesn't work well.

I don't stock or sell the choke coil to carburetor links, only the choke pull-off to secondary airflap shaft links.  So not sure where that came from or if it even the right part. 

In any case they are easy to make from an aluminum TIG rod, much easier to bend and modify that steel rods.....

[MattK2]

Thanks Cliff. I’ve done a full history of manifold stat coils from 67-72. The 67-70 and the 71-2 which sat on a diff intake design.  Found a Delco Bulletin from ‘71 with all the part numbers.

I may be dealing with a choke rod that’s over bent and not close to the 2.5” end to end length. Will confirm. If it was over bent the other part of the choke setup would not be able to compensate since the cam follower would clock differently. More CW than it should be which would make the stat rod appear to be much longer than it should be at full choke. Just a theory at this point.

[MattK2]

Rod was overbent. Had to make it straight to be at 2.5”. The outer rod now sets in R notch. So it’s getting better.

[MattK2]

Installed the new jets and tubes I bought from Cliff last fall.

To recap it’s a 7041262

Jets 73, Rods 43, Tubes 38

Idles nice down to 650 RPM

No bog.

But I have a new issue. It bucks at low speed off idle. Not sure what that’s telling me. Too lean?

It wasn’t bucking prior to swapping out the jets and rods per Cliff’s recommendation.

[Kenth]

Is the upper adjustable main airbleed screw under the triangular retainer seated?
Do you have at least .052" down channel restrictions (may need .055")?

[MattK2]

Not sure about the down channel restriction.  I’m assuming they’re fine.  As for the bleed screw under the triangle. My understanding was that it didn’t do much on the 71-2 carbs.

There was zero bucking until I after I swapped jets and tubes yesterday. So frankly I don’t know what improved. Yes the jetting is leaner than before but low speed driving has diminished.