Hot Rods My turn to start a Rochester 2G thread I guess...

Tighten the center header bolts first, then work to front and rear?

 

I always tighten from the center out. Unfortunately it's still backfiring if I let the throttle close all the way. If I very lightly feather it I get no backfire. Engine is definitely idling rich so maybe that's my issue. I also highly doubt it but I have 1 oddball plug on that side of the motor as the store only had 7 Delco's when I swapped plugs last week. The other is a comparable Autolite though so I doubt it. May swap it to the other side of the engine tomorrow just for ****s and giggles.

 

If it back fires when you let off if going quick lean. I’m guessing you still have vacuum leaks.
On the front carb,quite a few answers ago, you didn’t say if you had solved the ****erfly problem. With the idle screw all the way out (if it has one-not mixture) and carb upside down loosen the ****erfly screws and move the throttle rod a few times to seat the ****erfly’s closed and then tighten the screws holding the throttle plate fully closed.The throttle shaft should also not wiggle as the shaft needs to be pretty snug. You should do this on all three to know they seal. When done hold each one up to a light and look with the throttle fully closed. (No idle rpm screw touching the linkage)
A 9-1 compression engine on a Rochester 2GC has 52 jets. Cubic inches and a cam have very little to do with it. It’s fuel/air ratio through the Venturi’s. Occasionally I’ve had to adjust by one size not 8.
Without idling on all 3 you have built in vacuum leaks thru the end carbs throttle shafts and a PCV valve if you have a system installed. With one carb for idle only the idle mixture screws need to be adjusted accordingly as each carb is added and they WILL be out farther than a single carb.
The Carb King here on this site has told us the circuits in the outer 3-2 carbs from the factory are not the same as center carbs and the addition of non-mixture base plates is not always perfect. Same in reverse of adding adjustable base plates to factory outers aren’t correct either.
Not the norm here but I’ve always idled on all 3 and used progressive linkage and all 3 carbs are stand alone from the factory. Dual quads also. I’ve used Charlie’s for matching carbs, all centers, on the 3-2’s I’ve done and they worked just fine.
This my experience and preference, others differ. Currently running a 3-2 and 2-4 engines..

 

The end carbs were both gone through last week as well and are sealing. I can feel a slight pull when I put my hand over the front one still but not much. I'll try to double check my linkages tomorrow between the end carbs to make sure one isn't hanging slightly open.

As for the jets I may in fact need to go down a few sizes.

 

If the carb had a lot of miles it may be a throttle shaft leak. I had that on set I’m currently using. To check I use starter fluid with a brake clean stinger sprayer and just use a very slight spray right at the shaft to the base on both sides. Good ones hardly raise the idle, a bad one raises it a lot.

 

I'll try going around the shafts and bases with MAPP gas to see if I get any idle changes. The carbs and intake are powdercoated so brakleen or starting fluid will ruin the finish.

 

First off, great looking ride, vortec head sbc will be a runner.

I have a machined steel bar that I've used for years, wrap in sand paper and go over decks, heads, header flanges. Can either ****m it up see if there's highs or lows or with very fine paper, or clean up peen type impacts. It's solved some header issue for me in the past.
It will cut thru the ceramic on the inside of the flange, just favor the bottom side when taking the wave out and coat with high temp paint.

No jets in the new outboard carbs - that's ridiculous. Glad to hear you found a great local guy to work with.

 

Just spoke with my new carb builder and he thinks it may be more timing related than jetting. He suggested bumping my base timing up just a bit from the 12 to see if the engine will build a little more vacuum and then play with the advance curve a little.

 

I kinda have 2 different threads going on this but wanted to update this one since it had more followers.

I installed Remflex gaskets and don't hear or feel any exhaust leaks so I'm pretty sure it's sealed.
Also had the carb shop step the center carb jets down from 62s to 60s today as my plugs have been consistently rich... dry black soot, not wet.
Truck is still backfiring out of just the driver side header. I get the typical warble out of both sides I'd expect from a short exhaust with high flow but the backfire is like a shotgun going off whenever I let the throttle snap shut to shift gears.

Engine is the GM 350/357 crate motor with less than 200 miles on it. Compression test showed all cylinders in about the 183-190 range. Base timing they recommend is 12*btdc....I've had it at 12* and even bumped it up to 15* to see if it helped. TDC was verified with a piston stop and I have 2 different tabs with the same markings so timing should be accurate.

I'm at a loss here trying to work this backfire out. Other than that it runs and idles great. I set my mixture screws for max vacuum on the center carb...I may try turning them in 1/8-1/4 turn to lean it out slightly more. Beyond that the only other ideas I have are a bad plug on that side somewhere but don't see how that'd show up on decel, bad valve that didn't show up on a compression test, or my main jets are still too large and loading up the cylinders/exhaust.

Anyone have any other suggestions? Leakdown test possibly?

 

Read that again. Not clear to me whether no vacuum advance only applies to when you are setting the timing, or all the time. I'm running the vacuum advance and car runs real good.

 

GM specifies all the time