7029207 transition too rich

[bob69]

Cliff, in all of the testing I've been doing with the 7029207, I've noticed that the afr's in the 1000 to 1500 rpm range have been on the rich side. No matter which jet/rod combination I use the afr's in this transition range are in the low 13s. Even when the idle afr's are set to the 14 range the 1000 to 1500 rpm are still in the low 13's.

The 7029207 is known to have generous idle fuel. But what if your combination does not need the extra fuel. Is there a way to reduce the fuel flow in this off idle range. 

[Cliff Ruggles]

Are you noticing negative running symptoms thru transition or just watching a gauge?

There were several different "recipe's" used on the 7029207 carburetors depending on if they were original units or Service Replacement. 

When used outside the original application (350/350hp) you may need to install slightly smaller idle tubes and or DCR's to clean things up at idle/off idle.  I've had to do this a few times when 7029207 Service carburetors were being used on the 350/300hp or engines with smaller cams in them.......

[bob69]

Cliff, drivability is OK but I would like to increase gas mileage. I have a 50 mile pleasure drive that I make often. Speed averages between 35 and 45 mph. The car burns around 3 1/2 to 4 gallons of gas - mpg around 13 - 14 mpg.

Gas milage should be a lot better with a mild sbc 350, 200r4, 3.55 rear. Engine rpms are in that 1100 to 1500 range.

[bob69]

The 7029207 is a service replacement carb. The date code (2602) suggests it was made in 1982.

[77cruiser]

Not likely to do anything with the AFR, but do you have working vacuum advance?

[bob69]

77cruiser, I hooked up a vac pump on the dist vac adv and it holds vac and the points plate moves as expected. All looks ok.

[Kenth]

7029207 idle tubes are in the .038"-.039" range, but i have seen units with .035" as well.
7029202 idle tubes are in the .034"-.035" range.

Only way to reduce idle and off-idle fuel is to replace large idle tubes with smaller.

[quadrajam]

bob69, Hopefully the smaller idle tube will fix you up but consider this.
Your original post says you have a stall converter of 1800 rpm, your cam rpm range is 1600 to 5200 rpm and your getting less than optimum MPG in the 1100 to 1500 rpm range at cruise speed.
Maybe better MPG is just not in the cards for those conditions.  ...
I bet it does better at 60 mph down the open road. Does your converter lock up?

I aint know, I,m just axing...

[bob69]

Kenth, I have four 7029207's. All are SR carbs. Three have .037 idle tubes, one has .038. All have .053 down channel restrictions. Three have .081/.063 upper/lower idles air bleed. One has  .079/.062 iab. The larger iab are on 1982 and 1984 carbs. The smaller iab are on a 1976 carb with the .038 tubes.

The 1976 carb is currently on a 400 sbc and it runs great. The 400 gets better gas milage than the 350 when I keep rpms in the 1200-1500 range. The cams are simular, 214/214 110 lsa on the 400 and 214/224 112 lsa on the 350. The 350 should have better gas milage.

I also have an original 7029202 DH E9 carb. Idle tube -.035, dcr - .053, uiab - .068, liab - .065. It was tested on the 350 and the afrs were leaner than the 207s. But is it due to the smaller tubes, smaller iab, or both.

[bob69]

Quadrajam,  good points. The 200r4 goes into lock up at 30 mph under light throttle. So the stall should not be an issue at the rpms I'm looking at.

The rpms are below the cam's operating range so it's not making best power, but at light load conditions it does not need a lot of power. When needed the 200r4 will down shift nicely. I should still be able to lean out the carb so it gets better gas milage.  I hope.....

[Kenth]

All the 9202 Qjets i have notes on has the same size upper and lower idle airbleeds as the 9207´s, .079"-.081" uppers and .062"-63" lower bleeds.
Down channels in both are in the .051"-.053" range.
Only difference in idle/lowspeed circuit is the idle tube size,

Larger airbleeds equals leaner mixture, just like smaller idle tubes.

[bob69]

So I've made some changes but the carb still runs rich at low rpms. The idle tube was changed from .037 to .035. With no other changes, the afr's changed from around 13.2 to around 13.6. Not as much as I was hoping.

Next, the float level was changed from .250 to .375 in the hope that lowering the float level would lean out the carb. I did not see much of a change in the transition rpms. However the afr's did get leaner some in the 1600+ rpms. The most notable change was in heavy accelerations. Performance felt down.

I checked the secondary hanger to see is the secondaries were too high. They were within specs. I did notice the transfer slots were exposed around 60% at idle. Not sure if this is normal for a 7029207.

Any other suggestions?

[Kenth]

Next step i would open idle bypass air from .073" to .095"-.100".

[bob69]

Kenth, the original IAB was .098. It was opened to .110. The afr's on the initial test were around 14.1 in the 1300-1700 rpm range. Exactly what I was looking for. The idle screw was closed 1/2 to 3/4 turn. The idle needles were adjusted leaner - 1/4 turn.  The IAB holes were .110 with a smaller .098 restriction at the end of the hole. Very easy to drill out.

Now I have a new problem to solve.

After driving around a bit, I did a medium hard acceleration. Afterwards the afrs dropped to the previous 12.9 to 13.2 range for cruze and idle. By chance there was road work up ahead causing a 1 mile backup. Never got on the gas as we crepted along. The afrs kepted getting leaner. After about five minutes the afrs were in the mid 14 range. Idle was deteriorating.

After the road work, the afrs stayed in the low 14 range while driving slow. But as soon as I gave it some pedal the afrs went back to the low 13 range. After that the afrs would move around a bit. In the low 13s at times, then changing to the high 13s at times.

Any ideas as to what is causing the afrs to change like this.

[quadrajam]

Sounds temperature related. Was it hot weather then? Carb
getting hot??