P15 carb adjustment

[bobhavey]

I own a 1949 Special Deluxe, said to have been bought by a nun in California! Being a P15 I guess she got a good deal on a 1948 model left on the lot. 

Imported to the UK in 2015 in fantastic ‘survivor’ condition, and carefully brought up to running order by the guy I bought it off. He was meticulous, but I think he had the mechanical work done by others, and I’ve found one or two minor things make me think they weren’t maybe always the best. 
When I got it, the ignition was way retarded and it smelt really rich. Timing soon sorted but I’m puzzled by the carb. The idle mixture screw was way too far out so I connected a vacuum gauge and tightened it in, but to my surprise the engine still ran when hard tight. I took it out suspecting something lodged in the needle or seat, but seems clean. However it still runs fine, and smells less rich of course, when fully tightened home. 
Am I missing something? How’s the engine getting any gas if the idle jet is fully closed?? Book says half to one and a half turns out. 

 I’m not familiar with this carb, but it seems pretty straightforward, and this one is new or rebuilt, fitted by previous owners. 
Grateful if anyone can tell me what I’m missing here!

[Mark D]

Hey Bob, congrats on the purchase. Glad to see another p15 in The UK, you’re in good company and should get in touch with Robin Weathersbee if you haven’t. He’s got a great event coming up which gathers the p15’s over there for a fun day of pictures and touring. 
 

I am curious about the serial number on your car. I too have a ‘49 sedan and have been searching others to see if anyone has a serial number newer than mine. My car was 4,030 units from the end of production according to production records. 
 

As to your question about the carb, can you confirm that it is a Carter D6G2? 
 

cheers

mark

 

[bobhavey]

Serial is 25064741. 
 

carb is the single barrel Carter. The one in the spares box and the one in the car are both stamped D6G1

[bobhavey]

By the way my name is Bob Harvey but having typed it wrong at registration I can’t correct it!

(any forum administrators reading??)

 

 I got the notification for the meeting but since I’m in Cornwall it’s a bit  if a run. I’m slowly extending the distance I drive, although the cars been 100% reliable so far (barring when I ran out of gas!). I’ve not gone further than 30 miles so far, though I plan to be in Dorset for my son’s wedding next year

[Mark D]

10 minutes ago, bobhavey said:

Serial is 25064741. 
 

carb is the single barrel Carter. The one in the spares box and the one in the car are both stamped D6G1

according to production numbers your car was 6,689 units from the end of production. 

[Plymouthy Adams]

first off...the fuel for all three circuits come from the same well so to speak....the manner it is syphoned in the carb throat is the key to what circuit is being used.  As it is running rich....I would look to see that the main throttle plate is not to far open and bypassing the idle port and defeating the venturi effect you may have other issues that is the cause for the opened throttle like vacuum leaks etc that warrant investigation.    

Edited September 4, 2021 by Plymouthy Adams

[Sniper]

PA is on the right track I think.  A carb has a number of fuel delivery circuits.  The idle circuit you mentioned, the cruise circuit, the accelerator pump circuit are all well known.  However, there is one circuit most don't remember, the transfer slot.  I covers the transition from the idle to the cruise circuit.  If the throttle blades are open too far the carb will run off the transfer slots and the idle mixture screws will have no or minimal effect.

 

An example of where they are and what they look like, the round holes next tot eh slots are the idle mixture holes.

 

[bobhavey]

Yes, I this this must be the problem. It idles a bit too fast and adjusting the idle speed screw doesn’t slow it unless you manually push the linkage closed. I was suspecting the throttle linkage spring was weak, but it sounds more like the throttle butterfly isn’t closing unless forced so it’s idling on air bypassing that and the idle jet is not working. 
I’ll have the air cleaner off and look at it. 

[greg g]

Could also be the step up or power valve is not fully closed, this will make an over rich mixture at all times.  The step up is a intake vacuum controlled jet inside the float bowl that takes over from the accelerator pumps initial shot when accelerating passing or pulling grades.  It is spring operated so it's default is open unless it gets its vacuum signal which overcomes the spring closing it till needed.  There has been several threads regarding this circuit.  Use key word step up to search.  Also from the main page of this site go to the downloads button and click on it. There are two how to documents on rebuilding the various carter carbs.  The step up will b e mentioned as well as illustrated.  The top of the carb needs to come off to check it is moving freely.  The vacuum signal is provided for by a slot cut in the carb to manifold gasket which aligns with a hole in the carb base casting.  Most gaskets these days have 4 slots so the can't be misaligned but earlier ones had one slot that could be 3 positions wrong.  You might want to swap in the spare carb, but it's probably the spare for a reason.  These carter's are pretty simple to rebuild, you can determine  whether you want to attempt it by looking at the documents on the procedure.

[bobhavey]

Some progress, i solved the fast idle - the rod in the throttle linkage was set too short so wouldnt allow the linkage to drop right on to the idle speed adjustment screw. Having unscrewed the rod a couple of turns I'm now getting a slower and stable idle with about 17" on the vacuum guage, and the car sounds fine and smells less rich, but the idle jet still has no effect even when fullly closed!

on the whole, it sounds fine and functions Ok, I'm just a bit puzzled. I havent refuelled it yet to know if the gas consumption has improved!

I think I'll have to decide if I want to tear down the carb to check on the step up - I've got the spare carb [ I've no idea why the previous owner purchased a new carb rather than rebuilding the old one] to practice on before I attack the one on the car.

[bobhavey]

Another question for the experineced owner! The car's running really well now I've fettled all these things, but there's a very very slight irregularity in the motor at steady cruise 55-60mph. The ignition's all new Pertronix, plugs and leads new too, just wondered any thoughts on possible cause??

[Sniper]

irregularity?  Not sure I fully understand what that means.  If you mean it tends to surge that's probably a lean surge.  Might be a bit of crud int eh cruise circuit leaning things out.  Might be jetted different than stock, though with the same number stamped it shouldn't be, but decades have passed and who know for sure without looking.  Heck, it might be the ethanol in the gas.

[bobhavey]

Not a surge, no, more a very slight sound/feeling of irregularity in the engine note. 
it’s a heavy vehicle so no irregularity in speed or progress

[bobhavey]

modify that last reply - I went out yesterday on a 40mile round trip and specifically returned up the highway at a steady 55-60 and there was a slightly perceptible surge/hesitation in the vehicle progress so you mught be right aboutg it being lean, though I still think it smells a bit rich at idle and needs very little choke and for a very short time after startup [though that might be the natural thing with this engine, it's the first one I've owned.

But it's looking increasingly like I might need to re-rebuild the carb; it was bought new or reconditioned by the last owner and fitted 'by a garage in Aberdeen' who I suspect didnt know much about older vehicles.

[keithb7]

When it comes to carbs I like to think about air and fuel leaks. Vacuum is created as the piston drops in its bore. The rushing path of incoming air is to be controlled and directed exactly where it is needed inside the carb. Through several passages. Large and tiny. 
 

Sandwiched pieces of the carb body with tiny orifIces are bolted together. Gasket material between them to seal them all together.  The vacuum is routed specifically through many passages. Some passages accommodate moving air. Some moving fuel.  A concoction of parts all working together, that can work quite well to meter fuel delivery at various engine loads and rpm’s. 
 

However vacuum passages may develop leaks. Gaskets do dry up. Then surrounding ambient air may be drawn into the carb where it should not. Then the very design of the carb is wasted. Precise control of incoming air and fuel is lost.

 

Fuel passages may also develop leaks. Scale or sediment in the fuel system may plug up metered fuel orifices. Obstructing precise fuel flow. 
 

To me, a carb is a fascinating piece of engineering. By comparison fuel injection, although efficient, is boring in my opinion. Engineers spent a ton of time, over many decades, dialing-in carburetors. Many a late night was spent at the lab directing steam and probably smoke thru carbs to understand them. To tweak them.
 

Walter was quite impressed with what was going on over at Ball & Ball. I believe he sent his engineers there to work with B&B (father & son) to further study and develop carbs. Not too long after, I believe Walter acquired B&B? Did it fall under the Chrysler umbrella? Somehow they were pretty close in carb development. 
 

What an exciting time. 

Edited October 8, 2021 by keithb7

[Dodgeb4ya]

Make sure the carb to is perfectly flat.

The idle air bleed hole goes up into the carb top air horn. The top air horn gasket needs to seal tight around the idle air bleed hole and not let outside or venturi air  be drawn into that hole.

Also the carb top has the tiny metered idle air bleed hole in it and has to be clear of blockage.

This might take care of you non adjustable idle mixture.

Edited October 8, 2021 by Dodgeb4ya
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