Dual B & B Carb tuning

[Cold Blue]

OK guys - my 48 Plymouth has dual B & B carbs bolted to an Offenhauser aluminum intake. The carbs leak after I turn the engine off thru the bottom shafts. Not much, but enough to worry me about a fire.  Will a carb rebuild solve the leaking problem? And another thing - it seems that it is very easy to flood the motor when starting it if you ain't real careful. For sure don't pump the accelerator pedal!  Are dual carbs hard to get right??? In my thinking, 2 carbs are overkill when I think that one carb came on the motor to start with...is my thinking flawed??? She idles great, runs down the road just fine, and accelerates as she should. Thanks in advance....

[HotRodTractor]

I have limited experience with dual carbs - but so far it has been different than yours. I have to do about 2 pumps of the accelerator pedal and she lights right off.

 

You also shouldn't have fuel leaking out the throttle shafts on shut down.

 

In 1954 Dodge released the hemi for its large truck line.... at the same time they offered a factory dual carb and dual exhaust option for the 25" long flathead I6.... the six cylinder had more ponies and it was in very fine print compared to the new marvel V8 engine. lol

 

You might just have a pair of carbs that aren't well matched to this application - or they have enough issues to cause you a fit. Leaking gas on shutdown seems to me that you are getting too much fuel somewhere... perhaps on startup as well. Are your floats set right?

[Andydodge]

Theres nothing wrong with dual carbs if setup correctly........have you had them synchronized?.........which is done by attaching a metering or synchronizing thingy(?) above each carby and adjusting each till they flow the same, in both idle and power circuits......as for the fuel weeping or leaking out of the lower or throttle shafts that is usually a sign of worn throttle shafts and the only way to repair this issue is to have the throttle shafts re bushed to stock specs or bored and oversize shafts installed......either way its not for the amateur to do, carby rebuild shops maybe able to help as this is a common problem with Holley 4 barrels, I don't know if single barrel Carters are rebuildable tho'...check with a carby specialist.........I was going to install an Offy intake & a pair of Oz Stromberg single barrel carbies and while one is good, two is better..............but the best thing I ever did was to install a Langdon Stovebolt Engineering HEI distributor..........instant starts to the extent that I could lean in thru the closed door, open window and turn the key (it had been changed to a key start when I got the car) and it would start immediately, no winding over, no touching the accelerator or choke, just varroom!........best thing I did to the car............andyd 

[torqczar]

"seems that it is very easy to flood the motor ",  the motor is flooded before you start it..." The carbs leak after I turn the engine off thru the bottom shafts",  high float level caused by  float level too high, , trash in the needle and seat , sunk float.   because both carbs. leak  ...I would head to the  fuel filter, trash made it to the needle and seats.  you need to get new ones,  they come in the carb. kits   Or the float level is just set to high   , from past repairs.   pull carb. top off   like 5 or 6 screws...you can push float tang against the needle  and check height ,  level or less . . as for  synchronizing carbs ,  you need  2 vacuum gauges..  synchronizing  carbs.  is not that important,,  because they share a common  plenum.... make sure they open at the same time. and go to full throttle at the same time ,set mixture too  highest vacuum..Oh yea,, does oil smell like gas???.

[Cold Blue]

Thanks so much Andy and torqczar! I will get rebuild kits and check all that is mentioned in your posts. Thanks!!!  I am not getting gas in my oil, thank goodness.

[Adam H P15 D30]

You may or may not have worn throttle shafts but you definitely have one or more of the following:

1. the float setting too high

2. sunken float

3. cr@p in the needle and seat

4. fuel pressure too high

What fuel pump are you running?  Most electrics have too much fuel pressure (5-7psi) and I have seen some of the repop stock mechanical fuel pumps as high as 9psi, so may want to check.  B&B's want about 3-4psi.

As Torqczar stated, you do not have to meticulously synchronize multiple carbs on a manifold where they share a common plenum, but set the throttle openings and mixture screw base settings as close to the same as possible.  For example if you lean one down, lean the other the same amount.  Same goes for the throttle linkage and idle speed screw settings.  If you have separate plenums, you need to synchronize them with the tool, it's just not as critical when running them on a common plenum.

 

Adam

Edited October 30, 2018 by Adam H P15 D30

[Cold Blue]

I am running what appears to be a stock looking mechanical fuel pump. How do you check the pressure on a fuel pump? This is my first "old" car, so I have never had the need to check fuel pressures. I will check the float settings. I was a teenager in the '60's - first car was a 64 Impala SS 409 4 speed (wish I had that car now!) and built quite a few V8's back then, but this is my first date with a flathead six. I have experience with carbs, but only on motorcycles. Two-stroke motorcycles - Mikuni carbs - Yamaha RD350's and Suzuki TS185's.   I am sure the Carters are quite different, but I feel good about working on them. Thanks!

[Adam H P15 D30]

You will have to rig up a 't' between the pump and carb, attach a pressure gauge and run it.  I would check the float level first and set it a little (tiny bit) lower than spec because of the fuel we have now that tends to percolate after shut down.  If you know Mikuni's the B&B is a walk in the park, just follow instructions and keep an eye on the check balls.  If you end up replacing them, look at the Carter/Webers Tom Langdon sells on the Stovebolt 6 website.  I am very happy with the driveability they provided me.

 

Adam

[1951plymouth]

Hi, I have two stock carbs on an Edmunds intake manifold on 218 flathead.  They work great.   A rebuild will not fix the leaks from the throttle shafts.  They will require fixing by having the shafts bushed.   However, before you do that double check the float level.  Also set the accelerator pump on the lowest setting or you can try the medium setting, I have used both depending on the weather.   Most important to succeeding with dual carbs, 1) your linkage must be sturdy, consistent, adjustable and well adjusted to open and close the carb butterflys equally and 2) you must adjust the idle airflow and mixtures using a synchronizing tool like a Unisyn.   My carbs work great ,slightly rich but not unacceptably so.   My engine idles nicely and drives smooth.   I am building a 230 so I will wait to change jetting til then if necessary.