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Flat-head six cylinder engine in an airport tug: troubleshooting

December 2010

Manolet Ramos wrote from an airport in Manila, Philippines: "We use a Chrysler Flathead Six engine in one of our tugs - the small trucks that pull planes. The engine works fine for a few minutes after start up but misfires when it warms up. What could be the problem?"


The Chrysler Flathead Six was made from 1929 to 1960 for cars, until 1968 in trucks, and possibly until around 1972 in industrial applications (it is possible that production ceased earlier and the company continued to sell from inventory). It was the last surviving engine to have the imprint of Carl Breer, Fred Zeder, and Owen Skelton, the "three musketeers."

The engine tag showed it was model 30-1449-1, serial number E (for "Export"?) 0803 44. The engine had Champion H10C spark plugs. Forum members made many suggestions.

68RT wrote: "Check with your local forklift dealer; the industrial 30 engine was common in them. The sales literature only says 14mm, resistor type spark plugs."

Manolet replaced the spark plugs, but the engine still ran rich, with black soot on the plugs after just one minute of idle. He replied that the mechanic wanted to adjusted the jet, which required opening the top of the carburetor. "The carb seems to have two adjustments. An screw near the base that, I believe, adjusts the amount of air flowing into the carb, and an idle adjustment screw on the other side."


68RT replied: "The only adjustment inside most Carter carburetors is the float level adjustment; one screw adjusts the idle speed air/fuel mixture and the other does the idle speed. You may have other issues inside, like the main jet having come loose.

"A cold engine will run rich because the choke will be on for a while, unless it is a manual choke or you have overridden the spring-applied choke. Unleaded fuel will give a more more sooty look than the older leaded fuel. I agree that you probably are too rich. ... Most of the flathead 6 engines were timed at a low idle with a pointer located in a hole at the rear of the engine that reveals a steel ball embedded into the flywheel. You matched them up and that is it."



On a warm engine, try adjusting the carb mixture screw in the throttle base plate in (clockwise) until the engine idle speed just begins to slow down and falter slightly. This would be a lean idle. If the plugs are carbon fouled, sandblast them clean first, then blast with air to remove any sand particles.

The choke plays a big part in the first few moments of running, enriching the mixture. You don't want it too rich.

Make sure that the spring tension closing the cold choke isn't too strong and that the vacuum pull off opens the plate sufficiently and the vacuum pull-off diaphragm or chamber isn't leaking or binding in any way. All choke linkages must move freely. On a cold start, if you manually push the choke plate open further and the engine speeds up/smoothes out, the choke is too rich.

Always check the distributor point dwell angle before checking timing. For every one degree change in dwell, the timing will change one degree.

dana44 added:

It really sounds like the choke may be closed or partially closed to get the plugs that black that fast, or the timing needs to be advanced. Since the compression is probably in the 6.5:1 to 7.5:1 on the high end, octane shouldn't be a problem at all.

There should be a screw on the baseplate of the carb, located in the front area under the bowl. The second screw should be for idle as noted from previous posts. The one on the baseplate, if it is there, gently turn it closed until it stops, very gently. Then turn it out 1.5 turns. If this screw is more than two turns out, that's where the rich running is coming from.

The timing should be about 6-8 degrees advanced.

How does she drive, as in what happens when you drive forward, increase speed, back off the throttle and coast, things like thet? Black or blue smoke out the tailpipe?
Manolet responded:

The old mechanic didn't replace our carb despite us buying a new one. We replaced it this morning and our tug finally works. Yahoo! We still turned the mixture screw all the way clockwise then turned them back 1.5 turns as you suggested to make sure that we're on track.

Our problem now is fine-tuning it. After letting it idle for 30 mins, we pulled the plugs and they're still covered in soot, and the exhaust is still black. I believe this indicates that we're still running rich. We tried adjusting this by turning the mixture screw clockwise some more to lean it out but it doesn't seem to be affected. The idling stays the same regardless of how many turns we make, clockwise or counter-clockwise. You'll only notice a difference when the screw is totally removed from the base of the carb (turned fully counter-clockwise).

We seem to have a manual choke. It simply pulls the accelerator cable to bring the revs up. Since we didn't have any problem starting the tug, we kept the choke at a minimum for now.

ImperialCrown responded:

With the idle mixture screw turned in, the engine should run leaner and rougher until it reaches the point of stalling. It shouldn't idle at all with the screw turned fully in. If you have no change in idle, look down the throat of the carburetor for dripping gas from the venturi. If it is dripping, then this is the source of gas that is allowing the engine to keep running and not the idle circuit. This could also make it rich.

If you introduce a vacuum leak and the engine speeds up, it is rich. If anything, a vacuum leak should cause engine stumble and loss of RPM.

Gasoline saturated engine oil from prolonged rich operation might also keep the engine running rich. Change the oil if it smells of fuel.

The choke cable that actuates the accelerator linkage should also close the choke plate. If it is only raising the engine idle speed, then it is not a choke. Some of these sixes had an automatic (thermal bimetal spring) choke.

Start each test trial with cleaned spark plugs. They are 'self-cleaning' only if they can come up to temperature, run under a load (more heat) for awhile and have a proper air/fuel mixture. Carbon-fouled spark plugs may not give an accurate response to proper adjustments and will stay fouled at an idle.

After a good drive, recheck the spark plugs for carbon soot. Frequent starting and idle without a warm-up will tend to keep plugs fouled.
Manolet noted that the carburetor is a Carter YF 4371S, which was also used on 1967 Chevy six-cylinder trucks. dana44 added:

The fuel/air adjustment should be under the float bowl; in the picture with the fuel line showing, it would be to the right and underneath near where the carb attaches to the intake manifold. In the picture that has the flathead and spark plugs showing the most, you can see the idle screw. Hemmings Motor News would be a good source of suppliers for parts.

Pull the fuel/air screw out and check the end of it. It may be either bent or not pointed enough. If you look inside the throat of the carb you will see a little hole or slot that is just below the butterfly. This needle fuel air screw should be able to stick through this little hole to adjust the fuel taken into the carb at idle. There may be a telltale visual around this hole that is clean and dirty burned fuel along here. There might be a spring on the fuel/air screw, might be too big, but be careful turning this screw in, they can do harm if turned in too tight. You can use a file to sharpen the end of the screw if needed, to make it shut the fuel down more.

Another thing you can check is the firing order, 1-5-3-6-2-4, possibly a couple are backwards, which will allow it to run, just not very well.

The only other thing I can think of would be, as your mechanic said, lower the float level itself if it is too high. If the float is attached to the top cover of the carb, when the top is taken off, tip the top upside down so the floats are on the top, then make sure the seam of the float is the same distance to the gasket surface of the carb top cover. From there, make sure the tab that allows the fuel needle to open and close is not sticking or stopping where it isn't supposed to, sometimes they can catch on the carb cover and give a false reading. Some carbs have a second tab that once you turn the carb cover over, the float should have another stop to prevent it from going too low, and if it does, bend that tab so it only travels up and down about one inch top to bottom.

The Chevy truck was a 235 or a 250 cubic inch engine, so the main jets may be too large.
(The firing order was correct.)

Don Crider added: "The exhaust headers take the heat from directly below the carb, which is part of the warming up that was part of the original design. Without that, you will need a richer mixture when cold to make the engine run smoothly causing some of your over-richness."

68RT added: "The simple thing is to look down the throat at idle and see if any gas is coming out the main jet which empties into the center of the venturi. If the float is high, you will have dribble coming out the hole and that will overwhelm the idle screw. The rare (but once in a while) thing is the low speed jet air bleed hole is plugged and instead of just air being pulled through at idle, it will siphon fuel though that jet. The nice thing about a Carter is that you can actually follow the circuits start to finish easily."

dana44 added that there was another way to see if the engine is running too large a jet, if no air-fuel sniffer is available: "[With the idle screw,] at 1.5 turns you are very close or right at optimum, plus or minus one quarter turn; as long as the needle itself is in good condition and is able to regulate the fuel bleed below the butterfly correctly, then all the way in without a change indicates the jet(s) is (are) too large, always checked at idle, so a smaller jet is in order if the air bleeds are not plugged and there are no vacuum leaks. "

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3 Posts
i just bought one of these and it is not running i was wondering if it is a 12 volt battery or a 6 volt battery. Also i am wondering where i can buy parts for it i need an radiator cap and radiator hose.

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21,702 Posts
The starter may be stamped 12V. You could begin with a battery eliminator (high amp charger) set to 6V and go to 12V if the lights are dim.
Aircraft was always 12V afaik, automobiles went to it around 1955(?).
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