Carb issues on 1950 Desoto Custom

[jumpinjimmy]

Hi, I'm having some running issues on a 1950 Desoto Custom. The car runs great while the choke plate is closed or partially closed, but once warmed up, it hesitates and stumbles when accelerating and sometimes pops in the carb. For me, this sounds like it is starving for gas.....would you guys think that it's an issue with the accelerator pump or float level setting?? I've also noticed that the spring for the dash pot is all floppy and loose (located on the shaft where the summer/winter settings are on the accelerator pump cam) and the dash pot doesn't seem to work or move at all.

When the car is at cruising speed, is hesitates a little, but when going up a hill or really giving it the gas, it runs just great.

Here's a photo of the carb. The bit of linkage hanging down is part of the fast idle linkage that I can't seem to figure out where it connects. I don't think there is anything missing. I have a manual for the car but the photos of the carb and description are really vague.

Also, I have already set the timing, dwell, etc. and I'm fairly certain that they are not part of the problem.

CLICK ON PHOTO TO ENLARGE

[jumpinjimmy]

I had a thought.....does the idle mixture screw have an effect on the lower rpms of the motor and then transfer over to the main jet when more accelerator is applied?? I have adjusted the idle mixture screw and it doesn't really seem to have any effect. When fully closed, I expected it to shut down the engine but it didn't, nor did it cause the engine to idle poorly. It seems to idle perfectly no matter where the screw is set, unless I remove it completely and then the engine shuts down.

For whatever the reason, I have a sort of dead spot in acceleration when first giving it gas and am getting the occasional pop from the carb.

[mribbich@wi.rr.com]

For starters are you sure it is the right carb for your car????? My only other guess woulds be to check your vacuum advance, plus, cap, rotor, points, wires and condensor. Hope this helps.

Marv.

[jumpinjimmy]

All set in the ignition dept. and this is the right carb for my car.

[c49er]

Some pictures of your carb but a E7A1. Sounds like it definately is running lean. Fuel pump pressure and volume needs to be ok. Float level has to be correct. Accelerator piston leather cannot be shrunk up and loose in the bore. Step up piston has a small thin gasket underneath it that seals vacuum at idle-is the gasket there? Is this piston free? Check main jet and emulsion tube for blockage.
Idle issues can be affected by the top cover sealing over the idle air vent passage-warped top. Idle problems are also caused by a idle oriface tube being plugged up.
You need to probably carefully and properly go through the carb if it ends up being a carb issue. The factory shop manual is a excellent guide. Also there is a carb guy online who has the factory detailed info on your carb and parts. Cannot remember where or who he is right now.
All this advice is worth as much as you paid for it!
Bob

[jumpinjimmy]

Bob, that's fantastic info. Thank you very much. I'll have to order a gasket set before I take it apart, but I've rebuilt a few carbs before and I feel confident about taking this one on. Mike's Carburetor Parts seems like a good supplier. Thanks again.

[JACK M]

I have used Mikes Carb parts, usually has the kits for older carbs and ships right away.

[carbking]

The linkage hanging down in the picture connects to the fast idle cam.

The fast idle cam is a metal gismo that slips over a pivot rod sticking out horizontally from the throttle body right behind the throttle arm. There is a hole in the cam; and that is where the rod goes. The cam is held in place by the throttle arm. The cam controls the positioning of the curb idle adjustment screw. The fact this is not hooked up is probably why the idle adjustment does nothing.

When ordering kits, you will need the letter/number combination STAMPED on the airhorn. Orient the carb such that the fuel inlet is on your right. You will then see the letter/number combination near the top of the vertical strengthening vane on the airhorn.

Grinding my own ax - we have kits.

Jon.

[jumpinjimmy]

Thanks Jon, I checked out your website and looked at the troubleshooting page. When I look down the throat of the carb after it is warmed up, the is a very strong spray of fuel from the pump jet when I work the throttle. So, in reading further your description of bog, should I possibly be raising the idle rpms to open the butterfly more? or less? My understanding is that the fluid drive desotos should have a very low idle speed. I don't know if you read my first posting, but the car does not have this hesitation when first started (choke flap partially closed position), but once warmed up, it starts hesitating.

[NTX5467]

When the fast idle and slightly-closed choke butterfly are happening, the additional richness can help cover for the leaner condition you seem to be having once the choke "comes off". Then, when you throttle into the carb, heavily, you're getting the "acceleration power enrichment" function happening.

I had something similar on a much newer Chrysler 2bbl, just more intense . . . it wouldn't curb idle once it came down to "base idle" when hot.

What I might recommend is that, perhaps, the main system fuel has some sort of slight restriction somewhere in it . . . OR a drilled orifice in the fuel feeding the venturi is a little restricted. With the carb disassembled AND a Chrysler shop manual nearby, look at all of the diagrams of the various carburetor functions and fuel circuits. Then, you might inspect them for any accumulation or deposits at or near the orifice. I'm not that familiar with that particular carb, but here's what I did with the Carter BBD on my car.

As noted, once it warmed up, it would not idle, just die . . . even at 60mph when slowing for an off-ramp. I cleaned the carb with spray Berryman's B-12 cleaner and it all looked good, even the flow through the smaller tubes in the bottom of the venturi cluster. When I put it all back together, it did good, until the B-12 was used from the idle tubes, then it dies. When the fast idle cam is operational, it would do well as the main fuel was coming from the main system rather than the idle system. Get the rpm low enough that the idle system should work, it died.

On my particular carb, there were to brass tubes extending downward from the venturi cluster. One for each side, idle and main fuel. I'd seen a spec for "Low Speed Jet" in the service manuals, but finally found ONE manual with an arrow showing where it was . . . a drilled orifice up inside of the idle feed tube. I probed it with a bent-wire spark plug gap gauge to see what size it was, but with each larger wire, the hole got bigger, several times. I then got a selection of hand twist drills and used them to clean the hole out, until I "got brass", then I used the carb cleaner to clean the tube from both directions. I put it all back together and that was the last of that problem.

I suspect something of this nature might be happening in your situation, but not quite to the same degree.

One OTHER side issue . . . I'm not sure where you're located. Reason for this curiousity is whether or not you have "good" gasoline or "blended" gasoline in your area. "Blended" meaning 10% Ethanol gasoline.

The normal fuel mixture for the gasoline your vehicle was designed for . . . NOT octane rating . . . was for "stuff" that was pretty good for its time. This would mean a cruise fuel misture of 14.8 to 1, which is pretty much the chemical ideal mixture for all gasoline-fueled engines. The E10 fuels, with their lesser heat content rating, need more physical volume of fuel to run, which means E10s optimum fuel mixture is 14.2 to 1. Therefore, it it's metered for 14.8 (as I suspect it is), then with E10 its running leaner than it would on E0 fuel. Typically, this extra leanness is pretty well tolerated on modern engines (and better carburetors and intake manifolding), but it might not be on the older engines and carburetors.

A few years ago, when we just had gasoline with MTBE and E5.7 blends, a friend had his 1930s Oldsmobile in another friend's mechanic shop. Tony drilled the main jets a few thousands of an inch due to the fuel issue. The owners drove the car on a weekend cruise, about 100 miles or so. They raved about how much power it now had, compared to what it used to be.

So, if all of the drilled orifices are mechanically clean and their circuits flowing as they should, then perhaps a cautious and conservative "more jet" situation might be the ultimate cure. Just about .002" at the most, as in going from a '63" jet to a "65" jet. OR . . . to me a better way to do it would be to seek out some oversize jets which can be screwed in rather than drilled. I'd feel safer about using that approach as the original jets would not be "forever changed" that way.

Just some thoughts . . .
NTX5467

[jumpinjimmy]

I think the fact that my idle mixture screw isn't having any effect would indicate a blockage somewhere. I've tried blowing through a bit of compressed air but with no effect, but I was going easy as not to damage anything. Other than the hesitation and popping when accelerating, the car is running great, idling great, and no annoying clouds of black smoke.

I do believe I'm missing a part off of the linkage for connecting the fast idle arm and I'm going to post a photo to get advice on this sometime today.

[jumpinjimmy]

oh, I'm only using non adulterated gasoline. Ethanol gas is available, too but is clearly marked and I don't touch it.

[jumpinjimmy]

I'm pretty sure my carburetor is missing something for attatching the fast idle arm. I would really like to be told I'm wrong, but I can't figure out how this should attach. Any ideas. If I'm missing a part, would anyone like to sell me one??

Please have a look at the photos and opinions are welcome. Click on image to enlarge. The dangling rod is the fast idle arm.

[c49er]

Your fast idle cam is missing-it slides over the shaft. Look at my picture above.
Bob

[jumpinjimmy]

If anyone can supply me with this fast idle cam, I am a happy buyer.

[carbking]

Chrysler Corp used several different fast idle cams. When ordering the cam, it will be necessary to know the carburetor identification number. See post number 8 in this thread.

Jon.

[jumpinjimmy]

I'm assuming these are the numbers to identify my carburetor but it seems rather vague.

[c49er]

Nope! Not enough letters and numbers. Should be stamped something like E7L1 or E7L2 ect. Look on the other side of the sail.

[jumpinjimmy]

Yes, I figured out with the help of Jon that it's on the other side. Thank you.

[Dave Fields]

My 50 Desoto runs rich, had the carb completely and professionaly rebuilt. I blew out the new muffler, and 40 mph is topson a good day. The tail pipe is black, and now there is a grey smoke ( totally rebuilt engine, professionally done with corect parts, no corners cut. The is a miss, and sounds like a lifter hanging or valve may be hanging up. There was a3 or 4 year period of time before the engine was built, and it was started. I have had 3 good mechanics look at it, and they are stymed.

[jumpinjimmy]

Dave, sounds like stuck valve or pushrod has come unseated. As for my issues, I rebuilt my carb and found that whoever had rebuilt it before used one too many check balls and had inserted one in the cavity of the main jet! Also, float incorrectly set. My car is running like a top now.