Carter AVS Lean Spot

[Scooter]

I'd try Air door tightening. Had a similar transition bog from my 625 Holley Demon. Thought I was doing something wrong but kept going 1/4 turn at a wack. Took nearly 2 full turns to get the spring tight enough, bog is officially gone.

My2c.

[bennydodge]

I'd try Air door tightening. Had a similar transition bog from my 625 Holley Demon. Thought I was doing something wrong but kept going 1/4 turn at a wack. Took nearly 2 full turns to get the spring tight enough, bog is officially gone.

My2c.

Damn, 2 full turns... Either the door was super loose out of the box or the secondary jetting is to lean.  :alan2cents:Those Street Demons are nice , I've got the 750 cfm phenolic float bowl model-pretty good carb

[Scooter]

^^^ Been pretty satisfied with mine once adjusted properly, think it was just loose from the factory. Got the phenolic float bowl hoping it would help with my CA blend gas burning off in the bowl after shutdown.. did not make a difference. Car never runs over 180(big alum rad and switched fans), when I shut it down you can pop the hood and hear the fuel boiling in the carb. Might see if I can get a non metallic spacer in there this summer.. not much hood clearance tho.

Sounds like OP has very similar air door bog issues. Trying the adjustment first is 0 cost, that's where I started after making the initial idle adjustments with a vac gauge. Took about 30 minutes to get mine adjusted properly. Just kept running up and down the highway from one exit to the next rolling on the throttle. Get out.. pop hood, turn screw1/4 counterclock. Rinse, wash, repeat.

GL OP
-Scoot

[734406PK]

Yes, metering rods do make a difference as do step up springs. You could run the first step of the rod factory-sized but make the mid and power steps smaller and this would make the transition smoother. Stock step-up springs are rated at 10" which should be fine for stock/stockish engines.

The OP's carb is an ECS carb, I think this stands for evaporative control system??- also referred to as a "California" carb. The non-ECS 340 carbs have richer secondary jetting and slightly different metering rods.

  Evaporative Control System carb for CA requirements in 1970. Jetted lean from the factory can be expected. 3 step AVS metering rods appear to be scarce, might have to try 2 step rods.

[kawahonda]

My jets are:

Front 489

Rear 120

[bennydodge]

My jets are:

Front 489

Rear 120

You should see a "120" and a "396" stamped around the top of the jet-I've seen factory carbs that are missing the size stamp. 120 is stamped on one side of the jet and 396 should be stamped on the opposite side. Your metering rods should also be stamped "16-574".

[kawahonda]

Those are the sizes I recorded when I rebuilt the carb...

I'm driving it now. I would say that I made it worse from where it was.

I don't think we need to worry about the air door yet. Down low it's worse.

I'm going to put the linkage back to the center and reset the plunger height. Reassess then.

I may cheat and just install the square bore AVS. Gain a few ponies too.

Going to do another stop-go test ride. May make another video.

[734406PK]

The FSM claims Carter AVS #4937S is equipped with a hot idle compensator valve. Do you still have this valve in place and check that it is sealing? https://www.allpar.com/photos/mopar/carburetor/carter/hot-idle-compensator.jpg Just a thought.

[kawahonda]

Yea, that valve is in place. Not sure how to test if it's "sealing" though.

I drove it today some more and its works pretty good. I went over to my engine builder's house and had him drive it. We went on a 4 mile run, and that "hesitation" happened twice--much more often that it "didn't happen.

He thinks it has nothing to do with the squirter size. He was able to get it to do that while "barely" pushing on the pedal by accident. It has no carb hesitation at all from cruise. I think I said that it did yesterday because it probably wasn't fully up to normal engine temperature.

We both agreed that it is "minor" and no one should be losing sleep over it. He said he would just drive it and not tinker with it.

He mentioned if I wanted to toy with it, then I should set the floats higher (use a smaller bit). Basically, the idea is to go as high as you can before it leaks. If you can picture the "float", there is a "seam" in the center on the top side. Do you guys incorporate that "seam" when sliding a drill bit behind it, or do you keep the drill bit on the bulk of the top surface (and ignore the protruded "seam")? If my floats are set accordingly, is there a recommended "next size" drill bit to try?

He also said that I should back my timing off. I'm at 15 initial now. I may try 13 or 14 initial. It's basically right on the cusp of being a little too much. If it's just me driving and if I "get on it" I don't get any pinging. But with me and him in the car (more load), you can then get some faint pinging noise if you stress the engine. So I'll lower the timing a tad bit (probably drop it one degree) and reset everything. Then proceed to maybe alter the floats a little bit...

[kawahonda]

Of course I can "drive" it to where it won't do this, but this is another video showing the stumble (me purposely trying to do it). The first stumble was pretty bad that you'll see. You'll see the others are very mild. There was a couple times where it didn't do it.

This video is .046 squirter in the most "rich" part of the lever. I would say that going from .043 to .046 did not create any "for sure" change.

1) Reduce float height

2) continue to "up" the squieter size, try .050.

Maybe this is not squirter related. The video above I was pressing the throttle 10% and not being very aggressive. Definitely nothing to do with secondary door.

[734406PK]

 With the air horn inverted and gasket in place, use a 7/32" drill bit or gauge between the top of the float seam (in the front) and the gasket. Use a smaller drill bit to raise the level. Check the float drop and alignment as well. The specs are in the '70 FSM pages 551-552 or 14-124 and 14-125.

[kawahonda]

With a 7/32 bit rolling underneath the float (including that thicker "seam" part and gasket in place) the float will move slightly. So it's actually set slightly smaller than 7/32...

[734406PK]

With a 7/32 bit rolling underneath the float (including that thicker "seam" part and gasket in place) the float will move slightly. So it's actually set slightly smaller than 7/32...

Slight contact is fine, not an exact science here. Remember this is liquid fuel and its sloshing around like mad when driving. Imagine the action in a full cup of coffee on the hood. You might want to invest in a quality AFR gauge at this point. It's a real time saver in carb tuning. The secondary air door adjustment is in the FSM. 2 turns from wide open, I would go a little tighter until you get the primary side figured out.

[734406PK]

Quote: Yea, that valve is in place. Not sure how to test if it's "sealing" though.

The hot idle compensator is a bi metal leaf that should normally seal a vacuum port drilled through the carb body. Pushing down the leaf should not change the engine rpm if it is closed already. There shouldn't be any movement in the closed valve either. If this thing is sticking open at random or opening at a low temp, tuning will be impossible. Its worth looking at, I would plug the port. 

[kawahonda]

If this is the thing that leads to that "larger" exterior port on the left side, then yep, there is a plug on her already.

I'm wondering if my jetting is just a bit too lean to begin with. I wonder if this has nothing to do with the accelerator pump. I would think that the pump "tuning" would come more into play for "quick' transitions when the pedal really moves, not for slow (but "purposeful") 10% foot movements.

Again, I'm no expert. But a .046 bored nozzle is getting pretty damn up there--it's larger than Edelbrock's largest nozzle that they sell @ .043. My engine is only a 340, it ain't a big block.

Is it common that people go even larger as one way to solve this?

I would love to see what AFR my rig is pulling. If I ever went that route, I'd go for a temp install. It isn't cost-effective for a temp install.