B Carb Runs Lean @45 MPH

[Stage1gs]

Hello.. have a model a with a 6-1 head, big valves, touring cam, lightened flywheel, T-5 and a model B carb. Jets were flow tested but that’s just a starting point. I ran at 45 mph steady for 5 miles and turned the key off and coasted to the side of the road, and then pulled the plugs. White as can be. So I opened the main jet up by about 15 ml’s and did it again. Plugs are still really white. Looking for some tan color.

Is the main jet the correct jet for me to be adjusting in this scenario on a B carb?

Stage1gs

[Richard Knight]

What plug are you running. Going to colder plug might get the color you want.

[Synchro909]

I only have one B carburettor (they are not common here) and I've found it likes a bit more GAV than the old Zenith. Maybe yours does too. I realise the GAV affects the mixture t all revs. Have you done a similar test at any other speeds?

[Stage1gs]

The plugs are champion W18s.. I have some spare main jets and will open one up by about 20 more mls and see what happens..

[mcgarrett]

I would respectfully ask; how do you know that it's running lean after only 5 miles of running? If the plugs were new when you started out, they might need a little more run-time to display the tan color you're looking for. Did you notice any other symptoms that would lead you to believe it was running lean? If you are using a paper air filter I would suggest removing it.

As Synchro says, you might want to start by opening the GAV more while driving at a constant speed to see if the condition improves - the engine will tell you what it wants. If you don't see any change in the plug color, a slightly colder plug might be the next move.

[Richard Knight]

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If you don't get the tan color you can try W16s

[Y-Blockhead]

A while back we had one running lean, carb to manifold leak. New gasket cured it. Another one had excessive throttle shaft clearance.

[Stage1gs]

I hadn’t really considered the GAV.. I will try that. I also suspect a vacuum leak as I have used this same distance and speed for years to check my carb.. I can get the plugs to go tan in this distance. But I ended up replacing my carb because it rusted through in one of the ports. I bought a set of flowed jets but it seems to be running real lean in general. I could also have a issue with the throttle shafts.. how would you check to see if they were good?

[Jim/GA]

I have pretty much the same engine setup in my Town Sedan. I have to run the B carb with the GAV open 1/4 turn after warmed up (open 1/2 turn for cold start). Use the GAV; that's why it is there. I think you are better off doing that than modifying jets.

Also make sure the passage and jet that feeds the throttle "power jet" circuit is clean and open. It richens the mixture when the throttle is open more. (IDK what your throttle position is at 45 MPH, maybe you are not into that jet yet.)

If you really want to know what the mixture is while driving, I suggest you weld a bung onto your exhaust pipe and put a Wideband Air/Fuel Ratio gauge on the engine. Look at it while driving and adjust the GAV. I have been thinking about doing that for a while. If I were more motivated, it would be installed now.

Vacuum leak at the throttle shaft is possible. Try to jiggle the throttle shaft in the carb upper body. If it feels sloppy, it is probably leaking a little. Vacuum leak at the throttle shaft usually shows up worst at idle. It makes it hard to keep a consistent idle mixture. The little bit of leakage you get there is washed out at 45 MPH, though.

HTH.

[Oldgearz]

Try a set of Autolite 3076 plugs....they don't cost that much. Mine are a perfect tan color with my B carb.

[Phil Brown]

Jim/GA has the best suggestion with the wideband air/fuel gauge, modern unleaded gasoline won't really color the plugs like the old days of leaded gas.
Plus gives you real-time readings so no long drives, cost to the side of the road to pull plugs.

[SAJ]

Stage, my motor is the same as yours. Flow tuned jets, B carb, stipe cam, large inlets, 6 to 1 compression. Running 95 or 98 octane non-ethanol gas. With GAV open 1/4 turn W-6Y plugs were barely off white. I started to burn inlet valves at this colour. I now run another tiny bit more open on the GAV ( previously at half past on a clock face. Now right at 22 minutes past the hour). Plugs look grey in daylight. Camera makes them look mid to light tan for some reason. In photos Plug chop done after running about 1 mile at 55 MPH. Petrol consumption at this GAV setting 22.5 imperial mpg. at touring speed of 55mph. Goes up to about 23 mpg at 50 mph with GAV setting at leaner half past GAV setting
Car is 1930 roadster with full luggage and tools and 2 passengers, 3.78 diff, 26% Mitchell o/d and Excelsior Stahl radials at 38 psi front and rear.
With modern unleaded fuel, my exhaust pipe is black not grey or tan, and the old tan colour of plugs is now more grey than brown. See photos attached.
SAJ in NZ

[katy]

Originally Posted by Jim/GA

I suggest you weld a bung onto your exhaust pipe and put a Wideband Air/Fuel Ratio gauge on the engine.

Can the sensor be mounted at the tail end of the tail pipe? I was thinking of a temporary set-up w/a clamp on bung.

[Jim/GA]

Originally Posted by katy

Can the sensor be mounted at the tail end of the tail pipe? I was thinking of a temporary set-up w/a clamp on bung.

No, it must be mounted as close to the exhaust manifold as possible.

You could pull the O2 sensor from the welded bung and plug the hole off when not in use. But it must be up close to the engine.

[katy]

Thanks, that makes sense.