Over Jetted?

[Bob Riding]

I've been driving my '40 Plymouth wagon for a few years now (P20 218 with the dual carb setup) and although I have plenty of power, there's always been a hint of unburned fuel that wafts into the cabin. I'm running 2-Carter B&Bs (D6A2 1939-'41) carbs that were rebuilt by a reputable shop when I did the resto in 2015. They both are balanced using an airflow gauge. I'm using George Asche's linkage setup, dual intake and exhaust and a couple of modified Crosley oil-bath air cleaners. Stock fuel pump. Checking the specs for the carbs, I calculated each carb would need approx 250 as the recommended cfm, Carburetor CFM Calculator.

It seems that each carb is providing that amount- so if true, wouldn't reducing the size of the main jets help the problem? 

[Bryan]

The CFM would be for the entire engine. Would the 220 CFM be shared between the 2 carbs?   I would check the spark plugs for mixture, and make sure there are no leaks and the float levels are correct.

[Bryan]

Carter_Carb_Specs.pdf

[allbizz49]

Spark plugs never lie. .. .

[Sniper]

Nice thing about spark plugs is you get to see the per cylinder variations, whereas with something like one O2 sensor in the single exhaust wouldn't do that

[Bryan]

With ethanol in the gas you can't really read the porcelain for mixture. It's the top of the ring above the threads.   

[kencombs]

I plan to install a place to insert a wide band O2 sensor in both exits of my split manifold.  Don't plan on two gauges, just move the sensor and plug the other hole.  That seems to me to be be best way to determine jetting needs.   IMO, jets alone are just part of the puzzle.  Metering rods are a big piece.  Maybe the biggest.  That and the spring under the piston.   All carbs have some such mechanism, some better than others.   Without a way to measure the results one would run the risk of  burning pistons or other damage while not really fixing whatever the problem may be.

 

 

[Bob Riding]

15 hours ago, kencombs said:

I plan to install a place to insert a wide band O2 sensor in both exits of my split manifold.  Don't plan on two gauges, just move the sensor and plug the other hole.  That seems to me to be be best way to determine jetting needs.   IMO, jets alone are just part of the puzzle.  Metering rods are a big piece.  Maybe the biggest.  That and the spring under the piston.   All carbs have some such mechanism, some better than others.   Without a way to measure the results one would run the risk of  burning pistons or other damage while not really fixing whatever the problem may be.

 

 

Because they aren't progressive, they both contribute equally to the fuel supply, correct? If I re-jetted them both to a smaller diameter, wouldn't that potentially solve the problem? How do others with dual B&Bs do it?

[kencombs]

39 minutes ago, Bob Riding said:

Because they aren't progressive, they both contribute equally to the fuel supply, correct? If I re-jetted them both to a smaller diameter, wouldn't that potentially solve the problem? How do others with dual B&Bs do it?

before rejetting, or changing metering rods, I'd want to know if it is rich or not, and if rich where in the speed load range. You can read the plugs to get some idea, but a real exhaust gas analyzer or the setup I mentioned earlier would be better.   Also, I really suspect the smell is not exhaust, but crankcase vapors if you do not have a PCV system, just the original road draft tube.

[James_Douglas]

Check very carefully the step up pistons. Read my old posts on the subject.

 

Also, ethanol requires about an 8% increase in jet size more or less.

 

James

[Bob Riding]

23 hours ago, James_Douglas said:

Check very carefully the step up pistons. Read my old posts on the subject.

 

Also, ethanol requires about an 8% increase in jet size more or less.

 

James

Searched but couldn't find your exact post. What was the title?

[Sniper]

 

[Bob Riding]

Pulled the plugs and they all looked the same- like this

The gaps were around .028" so I re-gapped them at .035" according to the manual. What can you plug-experts tell from the look of the plug? They are Autolite 306s.

 

[Plymouthy Adams]

that they have been removed from the engine........joking aside, there are many plug reading charts in color that will help you interpret this plug by comparing the deposits, glazing and color....just search for plug reading chart.  

[Bryan]

If you are running ethanol, you can tell by the metal ring above the threads.  The porcelain will always be somewhat white.  Heat range from the tip.  On gas without ethanol you tell mixture by the porcelain.  Hard to find good charts.

 

Both are for racing

Tips - Reading Spark Plugs - Ken's Auto Machine Shop (google.com)

Reading spark plugs | Tuning Spark Plugs (enginebasics.com)

 

Most charts are just for showing major problems,  oil fouling, overheating, detonation, etc.   I looked for a while and hard to find a good Youtube video about jetting, mixture and reading plugs. If someone has a good source...

[kencombs]

I see a slightly worn electrode. Normal porcelain color.  No unusual deposits.  All this based on not knowing the age of of the plugs or usage of the engine.

 

As  far as reading for mixture, that really requires new plugs and a carefully controlled process to get good info, and even at that it is not very accurate in most cases on a street driven vehicle.  JMHO

[Bob Riding]

Engine has 5,000 miles since install. Probably 10-12K miles before that. How does changing the gap change performance?

 

[greg g]

Bob double check the step up valves.  If they aren't getting proper vacuum signal they are likely hanging open.  Remember the engine is pumping the same amount od air as it did breathing through one carb, with duals each carb is not flowing about half the flow.  This also supplies to the vacuum signal developed by that flow.  Years back member Bob vanbuskirk was having issues with black smoke, and poor mileage.  He found his step up valve was stuck wide open.  Freed up and provided with the correct vacuum signal the problem resolved.  I think normal vacuum with one carb is 20 to 21 inches, dual carbs will drop that to 17 to 19, so your step ups have less pull on their springs 

 

[Sniper]

One thing I didn't see mentioned, did you balance your carbs?

 

https://www.aircooled.net/synchronize-dual-vw-carburetors-103/

[Bob Riding]

14 minutes ago, Sniper said:

One thing I didn't see mentioned, did you balance your carbs?

 

https://www.aircooled.net/synchronize-dual-vw-carburetors-103/

Yes, they are synced with the same air flow using a gauge.

 

 

[maok]

On 4/25/2022 at 7:03 AM, Bob Riding said:

Pulled the plugs and they all looked the same- like this

The gaps were around .028" so I re-gapped them at .035" according to the manual. What can you plug-experts tell from the look of the plug? They are Autolite 306s.

 

 

That spark plug electrode looks badly affected or are very old, what is your static advance timing? And is your vacuum advance functioning properly?

[Loren]

As much as I like your Crosley air cleaners I wonder if they might be causing an air flow issue.

Oil Bath air cleaners were dropped when Karl Kiekhoffer found they restricted the air flow in his NASCAR Chryslers.

A check could be made by plumbing a vacuum gauge to one of the carburetors under the air cleaner.

If you see any vacuum then carburetor will run rich, like the choke is on.

I have a pair of 32 Ford Detroit Lubricator Air Cleaners that I am considering opening up to install K & N oiled gauze filter inside for my 97s.

It would be a no-brainer except for the fact they are so rare.

The K & Ns have virtually no restriction to them, while even a paper filter has a lot.

Once I had the great idea of using the engine air cleaner as a vent for the transmission.

Well, the cloud of smoke that came from the exhaust told me two things. The transmission 90 wt oil was being burned in the engine and the air filter was too way restrictive.

Just a thought

[Sniper]

Before anyone jumps on the K&N bandwagon, a test to review

 

https://www.project200.com.au/dm-iso5011/

[kencombs]

10 minutes ago, Sniper said:

Before anyone jumps on the K&N bandwagon, a test to review

 

https://www.project200.com.au/dm-iso5011/

 

10 minutes ago, Sniper said:

Before anyone jumps on the K&N bandwagon, a test to review

 

https://www.project200.com.au/dm-iso5011/

I posted a similar comment some time back.     Not a K&N fan as dirt stopping is far more important in normal use than free flow.  If more flow is needed, more filter area, like stacked elements in a modded housing would be my choice.

[Bob Riding]

5 hours ago, Loren said:

As much as I like your Crosley air cleaners I wonder if they might be causing an air flow issue.

Oil Bath air cleaners were dropped when Karl Kiekhoffer found they restricted the air flow in his NASCAR Chryslers.

A check could be made by plumbing a vacuum gauge to one of the carburetors under the air cleaner.

If you see any vacuum then carburetor will run rich, like the choke is on.

I have a pair of 32 Ford Detroit Lubricator Air Cleaners that I am considering opening up to install K & N oiled gauze filter inside for my 97s.

It would be a no-brainer except for the fact they are so rare.

The K & Ns have virtually no restriction to them, while even a paper filter has a lot.

Once I had the great idea of using the engine air cleaner as a vent for the transmission.

Well, the cloud of smoke that came from the exhaust told me two things. The transmission 90 wt oil was being burned in the engine and the air filter was too way restrictive.

Just a thought

Wow, I never thought that the Crosleys might be the culprit! I will check out the vacuum issue, thanks!