Leaky DCNFs

Yay! Some progress today based on the excellent input from this forum. The carbs no longer leak when the car reaches operating temp.

I sealed off the enrichment circuits, and that did it. I made no other changes. So now it is drivable around town -smooth idle, no gas smell, no stalling at idle.

Still no effect whatsoever on the off idle tip in stumble or the backfiring when you drift down the rpm range after letting off the throttle.

It's not clogged jets/idle circuits - The stumble hasn't changed a lick with five sets of carbs and dozens of jetting combos. Its not a carb cough when it stumbles, it just stumbles momentarily. If I give it any throttle at all, it smooths right out. It always smooths out right around 2000-2200 rpms.

The AFR ia about 14 at idle, about 14.5 at a steady throttle, and about 13.8 at WOT.

The backfires also have not been affected at all by any of the many measures I've taken.

Looking forward to any tips at all. You guys have already solved one of the three issues I have had with this motor.
 
Glad that you solved one of the problems. However, that implies that all of those carbs may have had leaky cold starts. Sounds like a quality issue. What did you use to seal off the cold starts?
 
Regarding the "off idle tip in stumble", have you tried a different acc pump cam?
Or probably better, a fatter pump jet?

Is the "backfiring when you drift down the rpm range after letting off the throttle" really a backfire through a carb, or a lound pop from the exhaust?
 
Let me try to answer the questions that came up yesterday.

I used blank enrichment tubes to seal off that circuit. I had a set of four in my "bits" carb box form about 40 Years ago!

The AFRs are what the gauge is telling me, but the jetting is a little fat if anything. Currently running an F36 with 135 mains and 175 air correctors and 52 idles. Pumps are 45s. I just use the AFR to see what changes when I swap out jets to try to lick the issues.
.ever "coughs" through the carbs. And the backfiring isn't just a rumble and pops, it really very louds bangs.

I have new fuel pressure regulator and gauge on the way. I want to make sure the pressure is what the gauge says. I run it at 2.5-3.0 psi.
 
Whoops. I meant to sat it "never cough" through the carbs.

We took her out last night and drove her quite a bit. Now that he carbs don't pour gas through the venturis, it's really very drivable. I can control the stumble (no carb coughs when it does that) with the gas pedal/clutch, and if I pop her into neutral when lifting ff, the backfires stop.

But I don't like unresolved issues on my cars, so really hope you guys can see something I'm missing - again.
 
Banging on deccel can be an air leak in the exhaust. My X with headers and home made exhaust pops and bangs on deccel and I'm sure it's a leaky joint in the exhaust. In my case it's kind of a fun sound so I don't mess with it.

Does the A/F gauge show a leanness right at transition where your stumble is?
 
No. It gets momentarily rich and then pops right back. I've had dozens of idle/main/E tubes jetting combos and pump jets in her from 35-50. Zero affect.

And tip in when accelerating from normal speeds (just normal acceleration in normal driving circumstances is always smooth and the response is immediate. It's just off idle and only when the engine is warmed up. When I cold start it and drive off in the morning, she's really smooth and she doesn't backfire. Drive her mile or so and both the backfires and the stumble show up.
 
sptcoupe said:
Yay! Some progress today based on the excellent input from this forum. The carbs no longer leak when the car reaches operating temp.

I sealed off the enrichment circuits, and that did it. I made no other changes. So now it is drivable around town -smooth idle, no gas smell, no stalling at idle.

Still no effect whatsoever on the off idle tip in stumble or the backfiring when you drift down the rpm range after letting off the throttle.

It's not clogged jets/idle circuits - The stumble hasn't changed a lick with five sets of carbs and dozens of jetting combos. Its not a carb cough when it stumbles, it just stumbles momentarily. If I give it any throttle at all, it smooths right out. It always smooths out right around 2000-2200 rpms.

The AFR ia about 14 at idle, about 14.5 at a steady throttle, and about 13.8 at WOT.

The backfires also have not been affected at all by any of the many measures I've taken.

Looking forward to any tips at all. You guys have already solved one of the three issues I have had with this motor.
Click to expand...
AFR of 13.8 at WOT seems a bit lean, best power on these engines is generally about 12.6 from everything I have read.
 
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