Engineboy

This isn't a third gen thing but it applies to all rochesters.....

How can I make it run REALLY rich?? (besides setting the choke so it doesn't come all the way off )

I have a 1970 500 caddy that has a lean miss at idle. To make a long story short its not an intake leak or a bad carb. All are new and sealed correctly, trust me.

I put my 750 Holley of my truck on it and the problem went away, but if you are familliar with cads, you can't run a conventional carb on a stock intake because of linkage differences. (the throttle hits the intake runners)

This thing will miss at idle and if you close the choke just a little it runs great.

I can't get any more adjustment out of the mixture screws or they will fall out of the carb!!

I just need a little more fuel at idle, thats all.

The carb is correct and there is nothing vac. actuated on the intake (I've disconnected everything including the brake booster)

Sorry so long, just looking for ideas that I may have overlooked.

Thanks in advance.

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ASE Mechanic/Machinist/Smog Tech

1999 NBM Trans Am
1986 Chevy 3/4 ton pick up
1981 corvette
1995 Kawi ZX6R

GO #3

[This message has been edited by Engineboy (edited November 09, 2000).]

Damon

Got you covered, man. Here's how you fatten up those lean idle circuits. It's a necessary mod if you got a little 305 carb that was probably marinally lean even on the smaller motor. Put that same carb on a big fat Caddy and you get what you got here.

CHeck one thing first before you go any further- your float setting. the far end of the float farthest from the intlet valve should be only 1/4" lower than the topf of the float bowl with the inlet valve fully seated and the fulcrom rod it rotates on held firmly in place with a 3rd hand. Float setting is accomplished by simply BENDING the float arm- there is a special pre-weakened spot in the arm to bend it.

Here's the procedure to fatten things up. Part one talks about basically just drilling out the idle discharge ports to a larger size which is the FIRST think I would try. They are often made intentionally too small so a bone-headed car owner can't accidentally over-richen his idle. If that doesn't get it doen for you then proceed to the second part about idle channel restriction enrichemtn which will fatten up the curb idle AND off-idle transition circuits. Here goes:

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94 Firebird Formula M6- No options but Z rated tires. No mods over $10. 13.5@105.
79 Malibu "beater" w/junkyard 400 SBC, tweaked QJet 4bbl, finally a decent set of heads, a few other tricks. A maddening 13.000001 @ 108 on the motor with little traction. No nitrous runs with the new heads yet.
Grandma's old 78 Malibu (33K miles!) soon to have the powertrain from the 79 put in it.

"One of the last remaining QJet tuners on Planet Earth!"

[This message has been edited by Damon (edited November 10, 2000).]

Damon

Curb idle enrichment procedure:

If you want to do just the curb idle then take the throttle body base off the bottom of the carb first. Look at the holes where the idle mixture screws stick through into the primary throttle bores. That's what you want to dirll out a little larger. Take the idle mixture screws out, find a drill that is just a little smaller in diameter than the mixture screw itself. Typically a common 1/8" drill will be just about perfect.

Drill out the discharge port a little bigger by going from the same side that the mixture screw would come in from- from outside the carb inward towards the primary bores. Its not tough since the original hole is there to guide the drill bit. Press lightly and steadily. In about 5-10 seconds you'll be through (its just aluminum). Clean out all the shavings thoroughly.

Damon

Full-on idle channel restriction mod. Proceed carefully- you can make things REALLY fat REALLY fast by drilling stuff out too far!

The path that fuel takes in the idle system is like this:

1. Goes through the main jet (which is not a restirction to it) and into the main fuel channel (same one the primarys use) 1. A littel of the fuel is sucked up through the "idle orifice tube" that has a tiny little .035" hole in it (think of it as the equivalent of a tiny little Jet just like the primary system has)
2. Gets to the top of the idle orifice tube and turns 90 degrees (horizontal).
3. Goes past the idle air bleed where it is mixed with a small amount of air tuning it into something with the consistency of Cappuccino foam.
4. Makes another 90 degree turn and heads back down toward the curb idle and off-idle discharge passages via the idle channel.

This is where the idle channel restriction is placed- after the air bleed, before the discharge ports. Opening it up allows more "Cappuccino foam" down to the idle and off idle ports. When you take the airhorn off the main casting of the carb you will see several holes in the main casting near where the main booster venturi stalk attaches to the main casting. Hope that makes sense.

Now look on the gasket that goes between the airhorn and the main casting in what would be that same general area. You will see a little "kidney bean" shaped hole in the gasket that goes between 2 of the holes. There is one "kidney bean" per primary bore.

OK, match that kidney bean up with the main casting and it will outline 2 holes. One of the holes is the idle orifice tube (which you can't modify easily cause the restriction is about 2" down in the orifice) and the other is the idle channel restriction (which you CAN modify cause the restriction is near the top where you can drill it out easily).
The holes that are slightly farther FORWARD and closer to the SIDE of the carb are the idle channel restriction holes. If you were to run a small wire down it you would see the wire pop out the bottom of the main casting. If you run it down the other hole, it won't pop out the bottom- it will bottom out in the hole.

Better to check two or three times BEFORE you drill cause you don't want to mess this one up!
OK, so you're sure you've got the idle channel restriction properly identified. Now you need a set of precision drills. Sears sells a set of drills that go by "drill number." Each size is only a few thousanths of an inch apart. Drill numbers #1 (1/8") to #60 (real freakin' tiny!- .035")

Use the drill bits, sliding them into the idle channel restriction by hand until you get to one that fits. Then go up a size until you get to one that WON'T fit. Using this GO/NO-GO method you can determine the size of the restriction within a few thousanths. Usually the restriction will be in the range of .045" - .055" stock. Try opening them both up by ~.002" and see how you like it. Go slowly in small steps cause there's no way to "un-drill" that hole again. Usually .003"-.004" over stock will richen things up enough that you will have to close the curb idle mixture screws 1/2 to 3/4 of a turn to get the curb idle back to the proper mixture. In short, a little difference in the channel restriction makes a BIG difference in A/F ratio.

As with the other mod, cleaning all the drill chips out is of primary concern in the operation. The idle channel restrictions are typically pressed-in bronze bushings about 1/4" long. Just thought you might like to know what you're drilling through. Try to keep that drill as perpendicular as possible while drilling (a drill press would be best if you have access to one)

Engineboy

Thanks Damon, I'll give it a shot.

I've got total access to all tools needed (I work at a shop) so none of this should be a problem.

I've been all in and around rochesters and have rebiult many for customers but I have never tweaked on one.

I'll give it a shot.

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ASE Mechanic/Machinist/Smog Tech

1999 NBM Trans Am
1986 Chevy 3/4 ton pick up
1981 corvette
1995 Kawi ZX6R

GO #3