Ice Cream Truck stalling when hot

I am the proud owner of a beautiful 1974 Ford P350 Van Ice Cream Truck / Step Van. It has a straight 6 300 4.9L and a single barrel carburetor with an automatic choke. We have replaced a lot of parts (new alternator, rebuilt carburetor, ignition coil, distributor cap, rotor, wires, spark plugs, fuel pump, fuel filter, transmission gaskets/seals, motor mounts, transmission mounts, thermostat, break booster check valve, gas cap, radiator cap and exhaust manifold). The carburetor is hooked up a little weird. The intake manifold carb mounting area has the one large hole and then a small hole next to it. There is a gasket and then a piece of thick sheet metal on top of it which covers the small hole and then another gasket and then the carburetor. When I search carburetor gaskets for the truck, they all have just the one big hole? So anyway, it runs great until you get off of the highway and then it wants to stall at the first light that you stop at. When I arrive at places after a longer drive, it’s hard to keep it running. It will also often stall when warm and you shift into reverse or drive. Short drives are no problem.

When it stalls, you sometimes have to hold the gas pedal down to restart it. It was running hot and once after a highway trip on a warm day, coolant was “bubbling” into the overflow. I have since installed a fan shroud (since it didn’t have one) and now it seems to be running cooler. My mechanic used his IR thermometer and didn’t think that it was running hot, but it sure seems to heat up fast and get pretty hot. When it’s hot it also seems to be “idle hunting” and then sometimes stalls while its in park or neutral.

I have tried adjusting the idle screw and the fuel mixture on the carburetor. When it’s hot, I have to turn up the idle enough so that I am worried about damaging the transmission, so I have kept it lower. No matter what settings I try, I can’t find an acceptable setting.

I had my mechanic check for vacuum leaks and he said that he didn’t find any, however I tried spraying some starter fluid and found some problems. When I spray it at the base of the carburetor or at one spot on the intake manifold seal it immediately stalls. I thought that when you find a vacuum leak this way, the engine revs higher when you introduce fuel / richer mixture into the engine. It’s also really hard to tighten the carb mounting nuts.

I have thought about trying to get it running cooler with a new fan clutch or a new radiator. I have also thought about a thicker carb mount gasket or maybe 2 gaskets. I don’t know what the extra hole is in the intake manifold and if I even have the right carb mounted.

Any ideas or help would be greatly appreciated. My knowledge is limited, but I’m always learning. Anyone who helps and is near Albany, NY gets a free ice cream!

I should add…I guess I’m wondering if a small vacuum leak at the base of the carb can cause the stalling issue. The carb gasket(s) are new and a mechanic installed the carb most recently when the new manifold was installed. I’ve tried to tighten the mounting nuts but they are so hard to get a wrench on. There’s not enough room for my socket wrenches. Is there a special kind of wrench for these situations? Thanks.

This old, a few thoughts. One is the gas line is too close to the engine or a manifold leak is blowing hot air on to the exhaust line causing the fuel to kind of boil in place creating a similar issue. Some other ideas are make sure choke is totally open when it is in limp mode, do you have a temp guage and if so is it hot at failure time, Have you had a pressure test on cooling system? Replaced thermostat and coolant hoses? Often times a bad lower radiator hose will collapse when pump demands more than the radiator can provide causing overheating and run and cooling problems. Also mechanical advance in dist may be sticking. My top dogs, got any cookie dough ice cream? Not sure about gasket leak but fix it anyway.

I was worried about vapor locking too, so I insulated the gas line so I don’t think it’s that. The choke is open all of the way when it acts up. My mechanic re-hooked up the temp gauge with t new sensor and it does read pretty high (7/8 up) but I don’t necessarily trust it…maybe I should. Does anyone know the best place to take a temp reading with a IR gun and what it should read when its “hot”? Thanks.

BTW, I don’t “drive around” and sell ice cream. We go to events, park it and sell from there. Otherwise I’d be in real trouble lol.

This may be what you need to tighten those nuts.

That leak would easily cause the idle to be off.


Awesome…thanks. I will by myself a set. They look very handy!

Sounds like the automatic choke is not opening. Yes it looks like you have found the leak. If you fix the leak the engine may not start after it stalls, unless the choke problem gets fixed. A new choke thermostat may fix the problem unless the hot air is failing to get to the choke. You may have to replace the quarter inch tube. You may be smog exempt. If so, you might be able to get a hand choke conversion kit for it.

How was the carburetor rebuilt? Done right, this means it should be completely disassembled and allowed to soak in carburetor solvent for several hours. The parts are then removed, washed out thoroughly, and all passages blown out with compressed air.

Those idle passages are very tiny and it doesn’t take much to obstruct one.

If not an idle circuit problem or vacuum leak I might lean towards vapor lock.
How thick are these gaskets between the intake and carburetor? Sometimes an insulator block is needed to combat vapor lock problems.

I think the Thick metal the Op mentioned is the Insolator block that OK4450 mentions, and a gasket should be on both sides of this.

I would also suspect at the ago of the vehicle, thast most of the vacuum lines are brittle enough that there could be a few leaks from those. I’d replace all the vacuum lines.

Then you may want to find an older mechanic that has been around and worked on more carbs.
No insult to the younger guys, but many don’t have much experience in carbs these days.


The OP referred to a thick piece of sheet metal and I’m not sure that’s the insulator block. The one I’m referring to is made of fiber, is about 1/4" thick, and the purpose is to prevent heat soak from the manifold into the carburetor which then causes vapor lock; or boiling gasoline.

Several Fords I owned back in the day had those fiber blocks and one Ford that did not got one soon after I got the car as it had vapor lock problems in 100 degree OK heat.

You probably need your carburetor rebuilt and don’t forget to buy a new float.

The choke is working / opening fine. I sent my carb to a carburetor exchange place online. They sent me a rebuilt one. It seems like it was rebuilt well. Where would I get an insulator block / heat shield for the carb? I looked online and it was hard to find one for my vehicle. Tomorrow I will try to seal up,the carb gasket leak with my new crowfoot wrenches :). Btw, it ran great today in the 40F weather. I’m still wondering if I need a new radiator or fan clutch to get it to run cooler. I’ll get some temps with my IR gun and post them soon.

I used my new crowfoot wrenches to tighten everything up and it seems to be running much smoother. I need a warmer day with a decent ride to see for sure. Those wrenches are great! Not sure how I lived without them!

I also ordered some Mopar Combustion Chamber Cleaner to try to get rid of any carbon deposits. Has anybody tried this? I read a lot about the water method but couldn’t bring myself to try it lol.

Glad the wrenches worked good for you @mobilescooper, I’ll take a dish of mint chocolate chip!!!


Still stalling when fully warmed up. Measured carb bowl temp with IR gun at 180F. I imagine that’s too hot. Ordered a phenolic carb spacer / riser. Still waiting for new gaskets to come so that I can install it. I can hear the gas boiling in the carb when it’s hot. Top of radiator was 220F. Wonder if I need a new radiator. Maybe I should have it flow tested. Well, I’ll try the carb riser first and keep my fingers crossed.

Some of the Asian cars of that era have sight glasses on the sides of the carburetor float bowls. It’s common after shutting one of them off in heat to see the gasoline boiling away in there like a coffee pot.

Some Toyotas of that era (I forget which) actually had a tiny fan a few inches in diameter mounted by the carburetor. When the temp was a certain point the little fan would blow air on the carburetor float bowl in an attempt to keep the gas from boiling.

I have an early 70’s Ford truck w/a 302 V8, not sure how similar that is to your engine. But I have had similar symptoms over the years. For me the solution that usually worked was to fix a leaky vacuum hose or vacuum device. There’s a lot of them, so it is a bit of a tedius job, and you have to remove stuff so you can see the connections clearly.

Now’s a good time to draw out a diagram of the entire vacuum system btw.

If it isn’t a leaky vacuum hose or connection, there’s one vacuum device that tends to fail on my truck and create a vacuum leak, it’s the gadget that directs warm air into the intake when the engine coolant is cold, and cold outside air when the coolant is warm. Located on the air cleaner ass’y on my truck. In 1974 on the 302 equipped trucks anyway I think Ford used some kind of thermally operated switching gadget in the exhaust manifold too, for this intake air-heating function. If you have that, make sure it is working correctly.

So check all your vacuum hoses and all the vacuum operated devices using a hand-held vacuum pump.

I’ve also had problems with air leaks where the carb bolts to whatever it bolts to. It sounds like you are already on to that one.

And I’ve had problems with passages getting clogged up with soot in the area immediately under the carb. I had to remove the carb and the spacer to inspect and clean those passages as I recall.

I’ve also had problems with the EGR valve sticking, so that’s something to check too. While the carb and spacer is off that’s a good time to remove the EGR valve and make sure it is seated properly, the associated passages are open, and it moves freely and isn’t stuck.

With all this stuff removed, it’s a good time to check the PCV also.

The carb spacer definitely help keep the carb bowl a lot cooler, but what stopped the stalling after the highway was new points and condenser. As with many old vehicles, as you fix one problem a new one starts lol. So now, the truck idles at a very low rpm. The idle screw seems to do very little. If it’s in park, and warmed up it is idling at a very low rpm. If I tap the gas the idle will sometimes increase and then stay up. So now for the big problem. When you put it into drive, you have to hit the gas very very lightly or it will stall. It will shudder for a while, and then once you get going it’s fine (same with reverse). It will also stall when you brake too hard at a red light. I feel like there may be a vacuum leak somewhere, but I can’t find it. I just changed the fuel filter and have a new air filter coming. The transmission fluid level is good. I’m worried that it’s the tranny or the torque converter. Also, when on the highway going over 45 mph, it lurches a bit as if there is an intermittent loss of power or its slipping. At least it’s been getting me to all of my gigs, so I have some money to get it fixed :slight_smile:

Congrats on your progress :slight_smile: … hmmm … well, on the other problems, if your idle speed is too low, that can cause hesitation on acceleration from a stop. Try to find out what’s causing the low idle speed first I think. On my truck there’s a solenoid that pokes out when the engine is running which keeps the idle speed up, then when I turn the key off it goes limp and the throttle plate closes completely. That’s to prevent the engine from running-on after it is turned off, dieseling in other words.

The butterfly valve on your truck must be doing something if you can go 45 mph, so you need to figure out why turning the idle speed screw isn’t causing the butterfly valve to move. If you have a similar “run” solenoid, that’s something to check. One possibility, you may be turning the wrong screw, for example you may be turning the high-idle screw. Or a mixture screw.