Running Bad-Need Help!

[alan32433]

My car is running, but not so good. It won't run unless I keep the choke nearly closed at Idle and then it runs PERFECT, no black smoke or anything. It runs fine off idle, again as long as I keep some level of choke set. I've been working on this problem for a solid week. Here's the story.

Engine is back in the car after a rebuild. The valve timing is correct, ignition timing set to 0 deg with timing light. Ignition points were set with a dwell meter. Compression is an even 115 across all the cylinders. It has new guides and valves. Valve tappet clearance was set hot one cylinder at a time (TDC compression stroke) at .0010 IN and .0012 EX.

Since I have to keep the choke on to get it running right, I suspected the carb was running lean. The carburetor is freshly rebuilt using all new jets of the correct size for the year including a new main metering vent tube. The float level is correct (checked at least 5 times). None the less, I swapped in another rebuilt spare carburetor with the exact same results.

I also installed a brand new fuel pump, same result.

At this point I was very confused. I thought maybe I had a vacuum leak somewhere. The intake and exhaust manifolds have excellent surfaces, new gaskets and are torqued correctly. I used a propane bottle to look for vacuum leaks all around the manifolds, head, carburetor and anywhere else I could think of and found nothing. I also plugged the distributor vacuum advance port to eliminate that. Another thing I tried was varying the ignition timing around the specified 0 degrees, didn't help one bit.

I keep coming back to a suspected carburetor problem but I can find nothing wrong with either of the two carbs I tried. I even tried installing a larger than spec main jet hoping a richer fuel ratio would fix the problem, but that didnt help at all. While the carburetor was out, I went through it again, completely disassembled it, checked all the vent passages and ports and blew them out with compressed air. Everything was open like it should be and clean as a whistle. Once again, I verified the float levels, plus checked the level while in the car with the air horn off. Fuel level is fine.

One last thing, during all my tests I have a vacuum gauge installed. As long as I keep the choke set I have around 18 psi at 475 RPM idle and steady as a rock with no drifting or fluctuation. Of course a soon as I release the choke, the vacuum drops and the car stalls.

I have no idea what to do at this point. I'm so desperate I even checked the air in the tires.

Anyone have any ideas, I'm willing to try anything at this point.

[Plymouthy Adams]

your idle circuits may be the problem the main jet is not the cause as you will run good at off idle...suspect that both your carbs may have some residue from sitting...check the idle fuel orifice tube and the idle discharge port..seems like one or both could have some blockage

[JIPJOBXX]

Have you tried a vacuum test on the manifold? By that I mean using a vacuum gage to see if you have any leaks on the intake side of your manifold. Sure sounds like you you have one. Its just like your getting to much air if you haft to choke it that much somewhere?

[knighthawk]

sounds like to me , the problem is in your carb idle circut, just cause it's rebiult doesn't mean its clean. I took apart a Honda generator carb 10 times before I found out I was missing a piece

[alan32433]

Thanks for the quick responses. I'm going to stop pouting and go back out and take the carburetor off....again. A little more history to be more clear. Both the carbs were soaked in carb cleaner, passages blown out, rivets replaced, ALL the brass parts were replaced to include the idle jet tube and step up piston assembly. I have the original carter spec sheet and all the proper parts are in the right places no doubt. I have a 5/8" piece of plate aluminum that I use to sand the air horn mating surface flat, and then checked the fit to the carb body, all good. Also, as I mentioned in my first post, I have a vacuum gauge installed.

The engine doesn't run right at idle or off idle, I always have to apply some level of choke to make it run right regardless of RPM. Seems to me like whatever the problem is, it is affecting the idle and main circuits equally.

I think you guys may be right, something internal to the carb may be plugged. I will let you know what I find.

Thanks for the help!!

[greg g]

is your vacuum port for the windshield wipers cannected and or sealed of? Is the carb tight to the manifold? Running with the choke out indicated it runnig lean, so there must be a pretty substantial vacuum leak somewhere between the carb and the engine.

[Jim Saraceno]

I agree that it sounds like an intake leak however I would guess that lack of fuel could cause the same symptoms.

It might be worth doing a fuel pressure and volume test on the fuel pump to rule out a clogged line or filter.

The other thing to rule out... is there spark at every cylinder in it's correct firing order (we know you have some as the engine would never start without it).

[alan32433]

I agree that it sounds like an intake leak however I would guess that lack of fuel could cause the same symptoms.

It might be worth doing a fuel pressure and volume test on the fuel pump to rule out a clogged line or filter.

The other thing to rule out... is there spark at every cylinder in it's correct firing order (we know you have some as the engine would never start without it).

I took the carburetor off again and completely disassembled it. I blew carb cleaner through all the internal passages and everything was open. I blew compressed air through everything also, nothing is clogged. I have some original documentation showing all the various circuits and passages as reference. I made sure all the vacuum passages through the throttle body and insulator were open and not blocked by gaskets etc. When I initially rebuilt the carb, I resurfaced all the mating surfaces to insure good gasket seals. I reinstalled the carb and got the same results.

To answer a few later questions, my car is a 48 DeSoto and didnt have vacuum operated wipers (its electric). The vacuum advance was plugged off to eliminate that as a possible leak.

The timing is perfect and all the wires are in the correct location. Keep in mind, as long as I have the choke partially closed, the engine runs perfect at all RPM's, and I mean perfect. I can easily get the idle down to 450. As soon as I release the choke, the vacuum drops and it dies (vacuum installed on intake just below carb). At higher RPM's, when I release the choke, it immediately starts running really bad.

As far as a vacuum leak, as I mentioned in my initial post, my manifold mating surfaces were nearly perfect with new gaskets and torqued properly. I also used propane looking for vacuum leaks by placing the propane nozzle all around the intake, head, and carburetor. Didn't find anything. Just to be sure, I loosened the 4 intake to exhaust manifold bolts (ones surrounding the heat riser), then retorqued the intake manifold to block studs.

So, I'm still stuck with no idea what to try next. Maybe I should spray carb cleaner around all the possible sources of vacuum leaks. Propane has always worked good for finding leaks, but maybe this is a case were carb cleaner is necessary.

Any one else have any ideas?

[T120]

Pardon me if I'm asking a dumb question,Does your flange gasket have the slots as pictured?Reason I asked is when I bought my Plymouth it had the incorrect gasket (no slots)..

[alan32433]

Pardon me if I'm asking a dumb question,Does your flange gasket have the slots as pictured?Reason I asked is when I bought my Plymouth it had the incorrect gasket (no slots)..

[ATTACH]15198[/ATTACH]

[ATTACH]15199[/ATTACH]

Ralph,

Yes, mine has the 4 slots exactly like the one you have pictured. I checked to make sure the port was open all the way through the throttle body, insulator and into the carb. You had an excellent suggestion, and thanks for posting the pics!

[greg g]

Do you have a vacuum gauge??? If so attach it to the intake manifold, the wiper port makes a good place, then when you get it runining post the readings.

[1947PLEVY]

Alan:

Are you doing this with the air cleaners installed? Also have you tried advancing the timing a hair? Just guessing????

John

[alan32433]

:mad:As I mentioned in my first post, I have a vacuum gauge installed at the intake port just below the carburetor. As long as I have the choke partially closed, it idles perfectly with around 18" vacuum (sea level here). The needle is perfectly stable and not fluctuating (new valves and guides). As soon as I release the choke, the vacuum drops and the engine stalls. I know low vacuum can be caused by bad rings etc but I have 115 psi compression across all cylinders.

I set the initial timing statically at 0 degrees when the engine was on the bench, then read it at 0 degrees at 450 RPM with a timing light after installing in the car. 0 degrees is spec for my car. My faithful helper (wife) was holding the choke partially closed while I read the timing. Afterwords, I varied the timing around 0 degrees within the physical limits of the distributor and it didnt help at all. Release the choke and it dies.

I am going to keep looking for vacuum leaks. In the morning, I'm headed to ace to buy a length of rubber hose to connect to my propane torch. I think I can do a better job getting closer to the underside of the intake manifold.

What the heck, it shouldn't be this hard should it? I will be awake all night thinking about this!

[alan32433]

Alan:

Are you doing this with the air cleaners installed? Also have you tried advancing the timing a hair? Just guessing????

John

Forgot to answer the air cleaner question. I don't have the cleaner installed but ran the car many times (before this problem) without any ill effect. I did wonder if the oil bath could create enough resistance to richen the fuel mixture? I'm willing to try anything.

[martybose]

I keep thinking that the clue is in the vacuum drop with the choke off, but I'm not sure what it means. I know on my Carter-Webers there is a detail about the throttle plate that says that there is only one way that it goes in due to the beveling on edges. If you install it the wrong way the sealing edge of the plate will be on the wrong side of a fuel passage and the fuel mixture will be off.

I have no clue whether something like this issue exists on the OEM carb, just that the symptom sounds familiar.

Marty

[alan32433]

I think I found my problem. I used carb cleaner to look for vacuum leaks. I sprayed cleaner around the throttle body of the carburetor and immediately the engine started running bad. I could also hear the vacuum around the throttle body shaft sucking the carb cleaner into the intake manifold. I don't have another throttle body unfortunately, so I'll have to find the correct one and wait for shipping. Not to mention finding a good one to begin with.

Anyone know a good source for carb parts, an NOS one would be great but I won't hold my breath.

Thanks for all the excellent suggestions!

[Joe Flanagan]

I have very little experience with this but I'm interested, so here's what is likely a dumb question. Wouldn't the carb spray test tell you more if you were to spray the stuff while your car is running badly? Then, assuming you have a vacuum leak, the sudden intake of the spray mometarily stops the leak and the engine runs normally for a few seconds until the spray has passed through. That is how I found the leak in the intake manifold gasket of my daughter's car. If your car is running well and you shoot carb spray into the engine, isn't it normal for it to start running badly for a short period?

On the other hand, if you have a vacuum leak and your have your choke closed to compensate for it and then carb spray is sucked in through a bad seal somewhere, you get the stumble. Is that what you did?

[alan32433]

Joe, your last sentence is exactly what I did. I had the choke set partially closed so the engine was running perfectly. Then sprayed to look for vacuum leaks. As soon as I hit the throttle body shaft the engine nearly stalled. Just to be sure of the effect, I sprayed carb cleaner in the throttle horn and got the same effect.

I looked at my spare parts and I have a throttle body for a car that doesnt have the same linkage set up as mine does. It's missing the shaft for the fast idle cam. I can't use it permanently but I can at least test to see that it fixes my problem. It has a really tight shaft in it with no noticeable play.

I'll let you know how it turns out.

[greg g]

You might give George Asche a call for parts. He has all mannerof carter coresand parts carbs. When I needed a couple of acc pump pistons, he sent them right out. His number is in the vendors list on the main page of the site.

You should also be able to rebush the throttle shaft or swap your good stuff to the one with the different linkage.

[Young Ed]

Greg and others I found new accel pumps listed separetly at napa when looking around their online catalog.

[knighthawk]

I've used 'starter fluid' to spray around the gaskets and hoses, a little wiff of that, the engine will rev up some. Another guy I knew used soap water in a spray bottle to spray around looking for leaks, the soapy water temporarily stops small leaks, works great on old vacumn lines

[Fernando Mendes]

My car is running, but not so good. It won't run unless I keep the choke nearly closed at Idle and then it runs PERFECT, no black smoke or anything. It runs fine off idle, again as long as I keep some level of choke set. I've been working on this problem for a solid week. Here's the story.

Engine is back in the car after a rebuild. The valve timing is correct, ignition timing set to 0 deg with timing light. Ignition points were set with a dwell meter. Compression is an even 115 across all the cylinders. It has new guides and valves. Valve tappet clearance was set hot one cylinder at a time (TDC compression stroke) at .0010 IN and .0012 EX.

Since I have to keep the choke on to get it running right, I suspected the carb was running lean. The carburetor is freshly rebuilt using all new jets of the correct size for the year including a new main metering vent tube. The float level is correct (checked at least 5 times). None the less, I swapped in another rebuilt spare carburetor with the exact same results.

I also installed a brand new fuel pump, same result.

At this point I was very confused. I thought maybe I had a vacuum leak somewhere. The intake and exhaust manifolds have excellent surfaces, new gaskets and are torqued correctly. I used a propane bottle to look for vacuum leaks all around the manifolds, head, carburetor and anywhere else I could think of and found nothing. I also plugged the distributor vacuum advance port to eliminate that. Another thing I tried was varying the ignition timing around the specified 0 degrees, didn't help one bit.

I keep coming back to a suspected carburetor problem but I can find nothing wrong with either of the two carbs I tried. I even tried installing a larger than spec main jet hoping a richer fuel ratio would fix the problem, but that didnt help at all. While the carburetor was out, I went through it again, completely disassembled it, checked all the vent passages and ports and blew them out with compressed air. Everything was open like it should be and clean as a whistle. Once again, I verified the float levels, plus checked the level while in the car with the air horn off. Fuel level is fine.

One last thing, during all my tests I have a vacuum gauge installed. As long as I keep the choke set I have around 18 psi at 475 RPM idle and steady as a rock with no drifting or fluctuation. Of course a soon as I release the choke, the vacuum drops and the car stalls.

I have no idea what to do at this point. I'm so desperate I even checked the air in the tires.

Anyone have any ideas, I'm willing to try anything at this point.

Ok you did a nice job.I think you are in the correct way.Do not give up.Carburetor adjustments is a very difficult job and requires more time and pleasure to do.I bought a WO carburetor for my jeep and had two months studying and working on it.Good luck. 

[desoto1939]

did you soak the accelerator pump leather gasket in light oil to soften it up prior to installing it back in the carb?  Do you have the correct small ball bearings in the correct holes also check the dog end on the throttle lickage that come across the head and then hocks on the  carb. Sometimes they get a wear spot on them and a lip and this causes issues.  These are just some thoughts not sure what you have done but might check these areas.

 

Also check the breaker plate assembly in the distributor it might just need to be replaced with a NOS one.

 

rich Hartung

Desoto1939

[_shel_ny]

OP has not been on in 5 years.  Not sure if any current suggestions will be much help