Dual B&B carb jet sizes

[Normspeed]

I've heard that you can solder shut the main jets then drill them out to a smaller opening than original. I have a few extra jets lying about and might do some experimentation on this. Has anyone here got any info on alternative jet sizes, or drill sizes for doing this?

On my recent trip, my average MPG was 15.0 pulling the trailer, and that corrects to 14.1 taking odometer error into account. I looked at a couple of the plugs when I returned and they were not fouled or sooty at the electrodes but a little sooty at the base of the threads, and my tailpipe is black inside, so I think I might benefit from leaner main jets. Any info appreciated.

[Don Coatney]

Norm;

My plugs were totally black running the dual carter B&B's (rebuilt by George Asche) until I doubled up my air cleaners. Having done that my plugs are now clean everytime I have pulled them. Not sure of the jet size in my carburetors.

Using a drill of known sizing you should be able to see what size jets you currently have. It will be interesting to see the outcome of your soldering and re-drilling experments. I have a pair of Carter Webber progressive two barrels ready to install on my engine. Problem is my engine runs so well I hate to make changes.

[Normspeed]

Good suggestion Don. Mine is only a 218 so probably sucks less air than that monster long block. I do plan to switch to washable foam elements soon, they should flow more air than the paper ones. Car runs great, I'm just a little disappointed in the highway MPG.

I'll let ya know how the jet work comes out. Actually, being a JB Weld fan, I'm thinking of closing the jets with JB instead of solder, then drilling them out. I think JB is gasoline proof once its cured.

[James_Douglas]

Be careful,

If is a common misconception that you need to run smaller main jets on a duel carb set up. Often you need to run larger jets in a duel carb set up.

Why you ask? Because by adding two venturies you have doubled the venturi area and you will have a vacuum signal loss do to the larger hole the engine is sucking through.

If the vacuum signal is weaker, you would need larger jets to compensate for that loss. The only way to really set the jets in a duel carb set up is to use an exhaust analyzer and have a bunch of jets handy.

James

[martybose]

(snip)

.... and my tailpipe is black inside, so I think I might benefit from leaner main jets.

Be careful about making assumptions about tailpipe color when running unleaded gas; it will be black under almost any condition. From what you described, I think you're actually pretty close.

Marty

[greg g]

Before fiddlin with carb innards, make sure its gettin proper amount of air. Any chance your choke butterflies aren't opening all the way?? Maybe run it 25 miles or so with no air cleaners and see if the plugs straighten out.

[Normspeed]

Great tips, thanks. Here's a plug that I just pulled. Just under 8,000 miles on the motor and the plugs, and this is after a 1400 mile highway trip. Plugs look much the same as they did at 2,000 miles of local driving.

I don't think the chokes are a factor, I'm running dual wonder chokes.

edit..the goopy stuff near the threads is nevr sieze that squeezed up off the threads when I installed the plugs.

I tried to go to photobucket for the pic at 2,000 miles but the bucket server seems to be down.

Also, I wonder if the very cool running thermostat might be part of it. I run a 150 stat and I don't think it ever really closes, because in cold conditions the temp barely gets up to the running temp on the guage.

[greg g]

Conventional wisdom states that a warmer engine runs cleaner than a cool one. Mine runs in the 160 to 180 range. Should have pulled a plug on mine take a look after out trip to NC. But I didn't.

[Don Coatney]

Norm;

Your plugs look about the same as my plugs. With my dual air cleaners my plugs look like this every time I pull them.

[PatS....]

This might help a bit:

[bob westphal]

Hey guys, That's a new one on me! I never heard of a Dodge truck engine with duals from the factory! I'd like to see what it looks like!!

[Plymouthy Adams]

wait till you see the 413 cu in six...

[Don Coatney]

See the red truck in the background.

Here is the engine.

Here is another factory dual setup.

This engine even has dual points.

[Normspeed]

Pat, that data page has a nice little footnote about balancing the dual carbs. pretty much the way I did mine. No carb balancer used.

Well, I got into my photobucket album today, and these pics show the plug condition at 3,000 miles of mostly local use, and then at 8,000 with highway driving. I guess it's not so bad. What I might try first is a warmer stat. and when I run outta paper elements, a pair of those nice washable foam filters.

3,000 miles:

Same plug at 8,000 miles:

I notice from an earlier post that at 3,000 miles my gap had increased from .028 to .031, and I reset the gaps to .028. Now, at 8,000 it's back up to .030. So frequent regapping seems like a good idea. I run Autolite #306 plugs.

[randroid]

Gents,

Last year I was fine-tuning my '67 VW Bug and visited the Solex web site where I found some fascinating info on swapping main jets. It tells you how and why particular carbs on particular engines get particular jets and what to expect when you change them. There's some math involved and a few of the equations have some goofy symbols that don't look anything like a number; don't let that scare you because the formulas can be simplified and still be accurate for our purposes.

What you'll learn will allow you to be able to match carb barrel to the venturi to the jet to your engine, all dependent upon how you want to drive your car, and if anyone here is contemplating such an adventure it's a viable read regardless of the brand of carb. Spend twenty minutes reading and you'll likely come away knowing more about the subject than most people who do it for a living, in my observation.

Drilling-out a jet is a labor-intensive way to spend your time until you fork-out a few bucks for new jets. Read the info from Solex and you won't need to depend upon the memory of somebody who did read the article.

Carbs depend upon a convergent\divergent nozzle (the venturi) that must operate within laminar flows <5000. If you drill the jet you'll need to then ream it smooth or it will be outside that limit of flow, so figure the correct jet on paper and do it properly the first (or second) time.

Sorry if I'm sounding as though I'm preaching but I got trained in fluid flow in the Navy and it's stuck with me. Thanks for taking the time to read this.

-Randy

[Dodgeb4ya]

Here are a couple pictures of the big 413 six cylinder truck engines.

Bob