Sluggish B Carburetor

[JRN1967]

I just installed a B cam and B intake in a 1500 mile model a engine.

I have an A carb that runs excellent, GAV is left at 1/4 turn and forget about it. Never stalls and starts right up cold without any choke or gas needed.

I also have a B carb that gives a noticeable power boost. I harvested this carb off a double B. Last year I threw a carb kit at it, and it ran good enough on my yard jalopy.

Now that I have the bigger cam and intake, I want this B carb on my driver. It starts a little hard, I have to give it a little gas to get it going. I can close off the GAV and it will stay running, but it's sluggish. If I open up the GAV two turns it becomes much more responsive, but the idle comes up with it as well.

Yesterday I disassembled it completely and cleaned all the jets and air passages. Even the passage in the throat I reinstalled the plug with the vertical slit facing in. The cleaning improved the way it starts but nothing else.

Why is it sluggish with the GAV closed? Why does it improve when I open it up two turns?

Thanks, John

[Jim/GA]

Could be a slight vacuum leak where carb mounts to intake or where intake mounts to engine.

I have to run my B carb open 1/4 turn at the GAV when warmed up. Needs to be open 1/2 turn when cold.

Double-check the float level.

Look for wear on the throttle shaft and where it passes through the carb throat.

You could try flow-testing the jets, one of them may be to small or plugged with a small bit of something that you cannot see but it's affecting the flow.

Good luck!

[Marshall V. Daut]

Check your "Private Messages" in the upper right corner of this web page.
Marshall

[JRN1967]

Originally Posted by Marshall V. Daut

Check your "Private Messages" in the upper right corner of this web page.
Marshall

Thank you Marshall. So a set of flow tested jets is the starting point. I'll Bite-the-bullet and order a set, then post my results.

John

[DBSHELTON]

Is there any way I can tell if my B carb has original jets?

[ursus]

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Originally Posted by DBSHELTON

Is there any way I can tell if my B carb has original jets?

Original Zenith jets have a number stamped into the base.

[JRN1967]

After a little research on modelabasics.com I have a good idea of why the car responds better with GAV out two turns.

I believe the Cap Jet gives the quick acceleration when I rev the engine off idle. It gets its gas from what’s stored inside the secondary well. When the GAV is closed the secondary well gets its gas from the compensating Jet.

If the compensating jet is too small, it won’t allow enough gas into the secondary well to match the cap jet. But if you open the GAV it will allow extra fuel from the bowl into the secondary well giving the cap jet enough gas for quick acceleration.

This is why the jets need to match each other for low speed driving.

[JRN1967]

My B Carb is no longer sluggish. I received my new Renner jets yesterday and couldn't wait to install them. Visually they looked almost the same as the ones I removed from the kit last year. But I know the orifices aren't sized or flowed the same as Renners.

After I replaced the jets and installed the carb, I set the GAV at 1/4 turn CCW and it started right up. With a little adjusting I set the idle low where I like it and as it warmed up I was able to close off the GAV completely. I like it 1/4 turn out for best idle and throttle response.

Finally was able to take it up to the state line for a little cobweb clearing, and I'm very happy with the increase in power from the B cam, B intake and B carb together. The car has very a noticeable increase in lower end torque (especially on hills) and top speed yesterday was easily obtained at 65. I'm not looking for more top speed, my goal was a little more power for modern traffic.

I want to thank everyone for their tips and advice.

John

[old31]

JR, do you have a standard head?

If you do, then get a Snyders 6.1 Head for more modern traffic get up and go. I have the 6.1 with B carb with renners kit, and bored out intake and it makes a world of difference in performance.

[JRN1967]

Originally Posted by old31

JR, do you have a standard head?

If you do, then get a Snyders 6.1 Head for more modern traffic get up and go. I have the 6.1 with B carb with renners kit, and bored out intake and it makes a world of difference in performance.

Yes old31, I have the 6:1 head with an RTS transmission to boot. My next improvement will be radial tires

[Jack Shaft]

Its a slippery slope,be careful..

[Purdy Swoft]

I run 5.9 heads and dual updraft model B carbs on some of ours . I have NO problem with flooding . I'm happy with the way that they run .

[JRN1967]

Originally Posted by Jack Shaft

Its a slippery slope,be careful..

Nice Jack, I suppose the quest for hp can lead us astray from our humble beginnings. I will heed your warning!

[JRN1967]

Originally Posted by Purdy Swoft

I run 5.9 heads and dual updraft model B carbs on some of ours . I have NO problem with flooding . I'm happy with the way that they run .

Purdy, do you need a fuel pump when you run dual updrafts?

[Purdy Swoft]

Originally Posted by JRN1967

Purdy, do you need a fuel pump when you run dual updrafts?

No fuel pump is needed with updraft carburetors . The carburetors are at about the same level as original and gravity flow works perfectly for me .

[old31]

JR, I also have a 4 speed trans and radials.

The radials give it a softer ride, and the car does not want to follow a track or groove in the road. The car stays straight and true with the radials.

I bought my WW 19" from Coker.

[AL in NY]

Here's an excellent flow chart showing the fuel flow in the Zenith A carburetor. It illustrates what JRN1967 was stating about his problem.