Need Carb Guru Helb

[48mirage]

'49 Dodge Coronet 230 ci with fluid drive and 3 speed.  For several years I have been running a holley 94 from a '56 ford.  This carb was originally spec'ed for the 272 V8 with automatic.

 

Because my original carb was starting to show some miles I replaced it with a like carb.  The only change was to lower the jet size from a 52 to a 49.  The car idles well and accelerates well but wants to buck when I try to hold a steady speed.

 

Is this because of the smaller jets?

[martybose]

'49 Dodge Coronet 230 ci with fluid drive and 3 speed.  For several years I have been running a holley 94 from a '56 ford.  This carb was originally spec'ed for the 272 V8 with automatic.

 

Because my original carb was starting to show some miles I replaced it with a like carb.  The only change was to lower the jet size from a 52 to a 49.  The car idles well and accelerates well but wants to buck when I try to hold a steady speed.

 

Is this because of the smaller jets?

Most likely.  The idle has it's own jet system, and acceleration is a combination of jet size and power valve.  At steady speed only the main jets are in play.  I'd put the 52 jets back in as an experiment to prove the hypothesis.

 

Marty

[Plymouthy Adams]

big difference in revving an engine without a load than maintain speed under load..the smaller jet may be your culprit....lets see..230 divided by 6 is 38.3 per cylinder and 272 divided by 8 is 34 per cylinder ... while this is not absolute math or fuel ratio compression formula but given rule of thumb..you need x air and x fuel by volume...so the larger jet is needed

Edited March 20, 2013 by Plymouthy Adams

[48mirage]

Marty and Adams,

 

You guys were right.  I put the larger jets in and the trouble went away.  Adams, I tried to address the difference in displacement differently but I see my mistake now.  I was thinking of the gas and air flow as a continuous process, but your explanation made me realize that the carb was processing fuel in batch mode.  Thanks guys.

 

Jim

[greg g]

Assuming the car is a 2bbl with both butterflies opening equally.  Wondering if you put one 49 and one 52 in??  Since everything goe into a common manifold you might do a way with your stumble but pick up a bit of mpg as the is very close to Tim' s equation What did your plugs look like when you were running the old carb?  I would be a little concerned about running different mains if this were a V8 with dedicated intake runners but since the carb feeds a common plenum in a 6 is should affect a steady mix throughout to each cylinder.

[48mirage]

My engine has 90k miles on it.  Oil fouling because of the rings prevents a good read of the plugs.  I'm thinking my focus should be on the idle jets.  I want to slow down the idle a little but there is no more adjustment left in the idle screw.  I think it is just a little high.  When I have a load on the engine like with the car in gear and holding the brake so the engine is working against the fluiddrive it sounds like an automatic car in gear held with the brake.

[greg g]

What kid of fuel pressure are you running.  If I remember correctly those ford carbs only like about 3.5 psi or so, stock mopar pump is around 7.  at idle maybe you are overflowing the flat chamber cuz it the flat cant stop the flow.  If you can't adjust the pressure, maybe lower the float level a 32nd or so, and see if ti makes any difference.  Is you idle smooth or lumpy?

[48mirage]

Added a pressure regulator today, set it at 2 1/2 lbs.  Didn't help the idle but also didn't have the flooding problem I ran into yesterday.  I'm also sure that the lower pressure will reduce the seeping problem I had in the past.  Idle jets are not available in multiple sizes but since I have some spare parts I might try to size one, solder and redrill with a pin drill.  Not a priority so I think I'll leave it as it is until weather is better and time is available.