plymouthcranbrook

Low speed miss or hesitation

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Although I haven't had any difficulty with my 52  Plymouth Cranbrook I have noticed as the weather has gotten cooler in the mornings there is a tendency for the car to stumble or have a low speed miss while driving in gear at below 40 mph or so.  Car is stock with the flathead 218 cu in 6 cylinder. Car runs great when cold and also after being used for 15-20 minutes it evens out and runs well again. It is in the area between stone cold and fully warm the stumble/hesitation occurs. I decided that since it hadn't had a tuneup for about 5 years(maybe 1000 miles or so) I would just up date everything to start. So, new plugs, wires, cap, rotor, points and condenser.  One thing I did notice that I did not remember seeing before was a slight variation of the timing mark at idle i.e. a bounce, maybe 1`-2 degrees) . I don't have a distributor machine but did use the  timing light to make sure that the vacuum advance was working. Mark moved about 12-15 degrees or so at mayby 1500 rpm.  I don't remember if this is unusual or ? so maybe someone can let me know.  That said even though the issue seems like a fuel problem I checked vacuum(18-20 steady with a good drop when the throttle is opened and a leap upward when closed). Compression on all cylinders is between 100-110 psi.  I replaced the intake/exhaust manifold gasket about 3 years ago and did check for leaks by spraying carb cleaner on the carb, carb base and the intake manifold without a change in speed.  When you push the gas down at almost any speed (over half way) the car picks up speed right away with out hesitation.  The carb was rebuilt by a local old car repair shop in 2014 and has seemed to work well since until the recent issue. I have adjusted the choke according to the manual 3 or 4 times and while watching it from a cold start until operating temp it seems to smoothly  open until vertical.  I have not opened the carb up to check any internals and before doing so I though that maybe someone has dealt with this type of problem and could give me some ideas where to look. Thanks in advance.

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Variation in timing can be caused by slop or play in the bush or bearing in the distributor in which the distributor shaft runs. The shaft moves around, causing timing movement.

 

Carb. icing will affect you after a couple of minutes running then will cease after 10 minutes or so of running. My Dodge 8 does it in cooler weather. I can't get out of 1st gear while there is ice below the venturi.

Edited by Spinneyhill (see edit history)

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5 hours ago, Ben Bruce aka First Born said:

 

  Carb icing up?

 

  Ben

 

Exactly what I was thinking , classic symptom.

 

First few minutes of running causes a large chill factor across the throttle plate,  due to  low pressure and high velocities through the venturi, mix this with cold fuel, cool air and moisture you've

created a perfect refrigeration effect.

 

As the manifold and carb base warm up the symptoms disappear,  modern fuels are part of the problem as they are more prone to this condition. 

 

If you are able; first start of the morning feel the carb base on and off for a minute or two, you will feel the chill as it happens, in severe cases you can actually see an ice ring form around the carb base on the outside.

 

Without wanting to bang on too much, throttle ice is most likely to occur 10 to 15 degrees ambient  above freezing point and is influenced by the moisture content in the air, any of you aviators reading this will know what I'm talking about. 

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The carb icing had not crossed what mind I have left even though I have dealt with it in the distant past. I am using the original oil bath air cleaner. Do you think that this could contribute to the problem by allowing a longer and perhaps faster Venturi effect? I will look and see if I can see any icing this weekend. I didn’t think it would be cold enough for that to occur yet although with Lake Michigan about two miles east humidity is often an issue here. Thanks for the idea.  What have you done if this is the problem to help minimize it?

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1 hour ago, plymouthcranbrook said:

The carb icing had not crossed what mind I have left even though I have dealt with it in the distant past. I am using the original oil bath air cleaner. Do you think that this could contribute to the problem by allowing a longer and perhaps faster Venturi effect? I will look and see if I can see any icing this weekend. I didn’t think it would be cold enough for that to occur yet although with Lake Michigan about two miles east humidity is often an issue here. Thanks for the idea.  What have you done if this is the problem to help minimize it?

 

The oil bath cleaner is not significant, the issue really centres around the throttle plate area, the laws of physics dictate that a venturi affect, small throttle opening, moist air and the chill effect of vaporising fuel create the ideal conditions for ice to form and build up around the throttle plate, this will only last as long as the combination of the above effects prevail.

 

What changes these conditions is warmer air around the throttle plate as the engine itself warms up, a combination of manifold heat plus warmer under hood air entering the carb intake.

 

You mention not being cold enough, the fact is that the temperature of the air will drop rapidly as it passes through the venturi (physics again) with a moisture (humidity) content carrying the necessary icing component, I reiterate that the ideal conditions are about 10 - 20 degrees ambient above freezing to 5 - 10 degrees below freezing, the humidity content is a major influence in this range  In fact in the Arctic the problem isn't as bad simply because the air is dryer, certainly colder but dryer thus no moisture to freeze.

 

So what  to do; the easiest thing is introduce some warm air into the air cleaner intake. I have (a not very pretty) flexible duct with one end just above the exhaust manifold and the other pushed into the air cleaner nozzle. With an oil bath cleaner this isn't easy to do, but I have seen where someone drilled some holes in the upper out side of the casing and fashioned a fixture to accept a piece of flex duct to the side of the air cleaner, again not pretty. With this approach of course, you don't want it there when the weather warms up as the other laws of physics could lead to detonation with the intake of too much hot air.

 

Finally, forgetting about the physics, just live with the problem and adapt your driving to accommodate it. In all of the above of course, your problem might not be icing at all !!

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28 minutes ago, hchris said:

 

The oil bath cleaner is not significant, the issue really centres around the throttle plate area, the laws of physics dictate that a venturi affect, small throttle opening, moist air and the chill effect of vaporising fuel create the ideal conditions for ice to form and build up around the throttle plate, this will only last as long as the combination of the above effects prevail.

 

What changes these conditions is warmer air around the throttle plate as the engine itself warms up, a combination of manifold heat plus warmer under hood air entering the carb intake.

 

You mention not being cold enough, the fact is that the temperature of the air will drop rapidly as it passes through the venturi (physics again) with a moisture (humidity) content carrying the necessary icing component, I reiterate that the ideal conditions are about 10 - 20 degrees ambient above freezing to 5 - 10 degrees below freezing, the humidity content is a major influence in this range  In fact in the Arctic the problem isn't as bad simply because the air is dryer, certainly colder but dryer thus no moisture to freeze.

 

So what  to do; the easiest thing is introduce some warm air into the air cleaner intake. I have (a not very pretty) flexible duct with one end just above the exhaust manifold and the other pushed into the air cleaner nozzle. With an oil bath cleaner this isn't easy to do, but I have seen where someone drilled some holes in the upper out side of the casing and fashioned a fixture to accept a piece of flex duct to the side of the air cleaner, again not pretty. With this approach of course, you don't want it there when the weather warms up as the other laws of physics could lead to detonation with the intake of too much hot air.

 

Finally, forgetting about the physics, just live with the problem and adapt your driving to accommodate it. In all of the above of course, your problem might not be icing at all !!

Thank you.  Still will check for it just in case.  Otherwise maybe something in the carb itself out of kilter.

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Do you know how AC works?   A liquid under pressure injected into a lower pressure area . Liquid turns to a vapor and becomes cooler.   Well, intake manifold is a low pressure area, maybe 16 inches VACUUM.  Gas being sucked into this are as a liquid, is at at least atmospheric pressure. Big difference.   Solution? heat up the intake area. I am not familiar with Mopar stuff. General Motors, specifically Buick,  heats the area beneath the carb with exhaust gas.  Bet Mopar does s well. Check this out.

 

  Ben

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30 minutes ago, Ben Bruce aka First Born said:

Solution? heat up the intake area.

 

And this is what the Heat Riser does on older cars. You know, the rusty part everyone wants to remove now to prevent valve burning......😉

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My Dodge 8 is a devil for carb icing. So I start it, idle for 5 minutes or more, shut down, go and get my jacket, lunch, camera, comfort stop etc. When ready, start up and drive away. The heat from the exhaust manifold and engine has warmed the carb enough that the temperature drop below the venturi is not enough to reach freezing. I just live with it.

 

As the air plus fuel expands below the venturi, it has to absorb warmth to expand (make the molecules move further and faster and thus spread apart). Energy is taken from where it is available, including the carb body. Expanding gases must absorb warmth; when you compress a gas, it must give off energy to slow the gas molecule movement (to pack them together more tightly), so the compressing system heats up. I seem to remember something called Brownian Motion in relation to this.

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The heat riser is free and working on my car.  I checked it again yesterday to be sure. I plan to find time this weekend to see if I can confirm any icing. After the idea was raised I remembered a 1963 Corvair that some dummy who will not be named to protect the guilty decided that what a stock 95 hp or so engine needed was a Holley 500 cfm carb on a log style manifold. Not too bad until winter started. Could have supplied enough ice to keep a small bar in business by just scraping off the manifold.  

Edited by plymouthcranbrook (see edit history)

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36 minutes ago, plymouthcranbrook said:

I remembered a 1963 Corvair that some dummy who will not be named to protect the guilty decided that what a stock 90 hp or so engine needed was a Holley 500 com carb on a log style manifold. Not too bad until winter started. Could have supplied enough ice to keep a small bar in business by just scraping off the manifold.  

 

That is so true!

 

People are always asking how to get those center mount carburetors to run right. I tell them remove it and install the original carburetors, problem solved!😁

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Looks like you guys might be onto something. Today after working on my wife’s Kia I started my Plymouth and once it idled smoothly I took the air cleaner off and felt around the base of the carb.  It was noticeably cooler to the touch and continued to be so as the engine warmed up.  I could not see any ice buildup and shutting the engine off left a cloud of fog in the Venturi area.  Thanks again to all who helped

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Final update on this. I drove it to work today with temps in the low 80’s F and had no problems.  Nice to know what the problem is

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