Jacobsen Engine ‘The Twin’ Found at Last

Author Photo
By Andrew Mackey | May 11, 2020

Jacobsen “4-Acre” reel type mower with 24-inch cut and 1-1/2hp engine. Aptly named, the “4-Acre” meant the user was able to mow four acres in an 8-hour day.

This story actually starts 56 years ago, when I was 8 years old. My parents took us kids to many antique shops all over the East Coast. At a shop in Lafayette, New Jersey, I happened across a 1952 Jacobsen Lawn Queen reel-type mower half-buried in weeds behind the shop. It seemed to be all there, albeit sun-faded and a bit rusty. I went inside and asked the owner if it was for sale. He seemed to not know it was out back, so he asked me to show him. He thought about it for a minute and said, “five bucks and it’s yours.” “Sold,” was my reply, and I nearly ripped my pants trying to get my hard-earned mowing money out of my pocket.

My dad was not too happy with my purchase — it nearly didn’t fit in the trunk of his Peugeot 403! When I got home, I cleaned the mower off, mixed some gas and oil for the fuel, filled it up and gave the recoil starter a pull. To my surprise, the Jacobsen 2-cycle engine started right up and purred like a kitten. For a few years I had been using a REO Royale reel-type mower to cut grass for spending money. The reel and bed knife were well worn, and the mower was not doing a great job. I decided to give the Jacobsen Lawn Queen a try on my next job, and I was hooked. It was a lot lighter, seemed to be more maneuverable and was just as quiet as the REO model 211 4- cycle cast iron engine.

I actually used the Jacobsen for nearly 10 years and finally sold it after I bought a Jacobsen Turbo Vac rotary mower. The Turbo Vac had an 18-inch steel disc with four small triangular blades mounted on the rim, giving the mower a 21-inch cut. It had an aluminum deck which made it light and maneuverable in tight corners. It also had two blower fins that added air movement to the discharge, making the mower a vacuum at the same time – hence the name. It did a better cutting job and a much better job picking up clippings than a friend’s new Snapper mower.

I ended up selling the Lawn Queen to a neighbor who still uses it to this day. As far as I know, it has never been or needed to be rebuilt, and has been decoked only twice. Decoking is the removal of carbon from the exhaust ports, a common problem with older 2-cycle engines that used a relatively heavy oil-to-fuel ratio. In this case, 16:1 or one cup of oil to one gallon of gasoline. I have been hooked on Jacobsen engines and mowers ever since.

Turbo Vac modifications

My next mower was a 30-inch cut Jacobsen rotary mower. A huge beast with a steel deck, it was no lightweight, but it did cut a wide swath which made for fast cutting of large lawns. A friend and I used it many times in the course of our landscaping jobs. It did not do a very good job blowing out the clippings and only made some mulch out of leaves encountered on the grass. I decided to try mounting four of the blower fins that were mounted to the Turbo Vac mower to “The Beast” to see if it then would do a better job with the cleanup. My big mistake here was not doing the job of installing them myself. I had bought them and asked my friend John to install them and I arrived at his home just in time to see if indeed the mower did a better job at cleaning up.

The first problem we encountered after John installed the fins was the fact that the mower would not speed up to its governed speed. I had to make the fuel mixture rich for the engine to get the necessary power to bring the mowing disc up to speed. “The Beast” had a disc about 24 inches in diameter and was about 1/8 inch thick. It had four triangular cutting blades mounted on the rim of the disc. Even before we put the blower fins on it usually took a minute to get up to speed with the throttle open. After I opened the fuel needle about a quarter of a turn, the engine accelerated. I had turned my back on the machine, when I heard John yelling, “Turn it off!” in a screeching voice. I turned around to see the mower rising off the ground, about a foot in the air and still going up, and the engine was still accelerating!

John had both hands on the handles and could not reach the throttle to shut the engine down. By the time I managed to get my hands on the throttle and cut it off, the mower was nearly 5 feet in the air and at about a 30-degree angle. It took about a minute to settle back down to the ground, and another minute for the engine to stop entirely. We were both scared nearly out of our pants! John, actually, had to go change after that. When I turned the mower on its side, I found out why it had lifted off. John had installed the fins upside-down and mounted them to the bottom of the blade disc as well. “You dummy, you put them upside-down!” I said. “They wouldn’t fit,” John said. We took them off, installed them in the proper direction and on top of the disc, and tried to run the mower again. This time, there was no lift-off but there was a loud howl as the engine ran. It was not just the engine noise. The mower then generated a tremendous amount of wind blast out the side chute. It was enough, that in the fall we could blow a 3-foot-high pile of leaves with it.

We had only one problem using it. If we left the engine running at high speed and left the mower in one spot more than a minute or two, it would suck the grass and dirt right off the ground, leaving a hole on the lawn under the mower. After the first few times, we learned to shut it off right after we were done using it.

“The Twin” exists

Made of steel and cast iron, the Jacobsen “4-Acre” was “Recognized as the most practical all-purpose Heavy-Duty power mower” according to this vintage advertisement.

I have used many Jacobsen mowers and equipment ever since. I have had Jacobsen lawn edgers, snow blowers and others. I still have an engine off my Sno-Jet. The machine had been totally wiped out, but the engine is still good. I also have several 521 and 321 type engines as well. When I was a little kid, I even mounted a 321 engine to a two-wheel scooter. Quite a few years ago, I picked up a Jacobsen “4-Acre” reel-type mower. It had a 24-inch cut, a vertical 1-1/2hp engine and was steel and cast iron construction. When they said you could do four acres in an 8-hour day, they meant it! Even at nearly half throttle, you had to practically run to keep up with the machine. I used to run and demonstrate it at many shows I attended.

While I was at the Jacktown show in Bangor, Pennsylvania, about 20 years ago, I had a gentleman come up to me and say, “I have one like that but it has a 2-cylinder engine on it.” I told him Jacobsen didn’t make a 2-cylinder engine, and he walked away.

Several months later I had another man tell me he had a seen a twin-cylinder Jacobsen at a show in Sistersville, West Virginia. Later, I came across an advertisement in National Geographic from October 1931, showing “The New Heavy Duty Mower, by Jacobsen.” It showed a twin-cylinder engine in the photo.

1931 advertisement for “The Twin” boasts “Another Jacobsen Achievement.”

Search and acquisition

I spent the next 20 years looking for a Jacobsen twin. In June 2018, my friend Austin Wilcox on Harry Matthews’ SmokStak forum, sent me a message. He had seen one for sale at a show in Portland, Indiana. I was very interested so he gave me the seller’s information. That evening I left him a message to contact me about the engine.  He gave me a good price and I said, “SOLD!” He said he would deliver it to the Coolspring Power Museum in Brookville, Pennsylvania, as they were having a show soon. We set a date to meet a day before the show. George, the seller, called me the day before we were to meet and asked if I’d be interested in a second Jacobsen twin. The second engine was stuck but mostly there. Twenty years of looking and now I had two of them in under a week!

The original exhaust on the twin was missing but someone had modified two brass 40mm Bofors gun cartridges, and mounted them on 1-inch diameter iron street 90-degree ells to make up vertical exhaust stacks.

Coolspring Power Museum is a great engine show and the museum itself is a wonder. When I met George I had my first look at the engines. The first was mounted on a small wooden cart along with a magneto and a recoil starter. The engine was an upright twin-cylinder and actually looked like two “4-Acre” mower cylinders mounted on a single common crankcase.

The engine had been repainted silver from its original battleship gray, and George told me it had been sitting in a barn for nearly 20 years. It had been described as “nearly running” and it looked it with the exception of both spark plugs broken off at the porcelain insulators. It was very hard to turn over due to what I thought was only bent and misaligned tinwork. Otherwise, it was as described over the phone and I completed the purchase.

The donor engine was in several pieces. It was stuck fast, but was mostly there. There were several broken pieces too, including the carburetor intake and one of the flywheels. The magneto was with it, but it was busted. Nothing that couldn’t be fixed. I loaded both engines into the back of my SUV and set out to see the rest of the Coolspring show. I was disappointed as a lot of exhibitors had packed up and by noon Saturday and nearly 3/4 of them were gone. I had missed a lot of interesting exhibits. The next day, I began work on “The Twin.”


The first thing was remove the recoil starter. Next to come off was the magneto. The wooden mounts for both the magneto and the starter were broken and the mounting lag bolts had been ripped out of the wooden cart rails. The bolts were too short. The magneto was not grounded to the engine. Even if the engine had turned, there would have been no fire to the plugs. The next items to be removed were the tin shrouds from the upper tinwork on the cylinders. I found screws were missing or stripped out and the tinwork was bent.

The magneto and starter were first to be removed.

As I had the tinwork from the donor engine and figured I would just clean up the tinwork and swap it for the busted one. There is a third piece of tinwork between the cylinders that is held in place by two screws. It, too, was bent. After the upper tinwork was removed, I went about trying to get clearance back between the flywheel and the lower tinwork, which was badly bent. This was achieved by placing a flat blade screwdriver on the flywheels and going around prying the tin away from the flywheels. This took some time and effort to accomplish. After about four hours of work, I had enough clearance to get the engine to turn over cleanly. There was only about a 1/16-inch clearance between the lower shroud and the flywheel rims.

As I turned the engine over, there was a dreaded sound. Clanking and clunking as the engine rotated. This meant there was excessive clearance somewhere inside the engine. Pistons, rods or wrist pins were worn, and now I had to go inside and see what was loose. The engine did not turn over easily. I was disappointed, but in the long run, it was a blessing in disguise.

The magneto side upper tinwork had been bashed in and hammered back into a crude shape. It was also fractured.

To get inside, the first things to be removed were the flywheels. They are made of cast iron and the hubs are split by about a 3/16-inch gap along the sides. The hubs are drilled to allow a bolt to crimp the sides of the hub to the crankshaft through the center. This was pretty straightforward, just remove the hub mounting bolts and drive a wedge between the sides to slip the flywheels off the crankshaft. The crankshaft is straight, not tapered. A large Woodruff key keeps the flywheels off the crankshaft. The flywheels felt quite heavy for their size.

They had eight cooling fins that were about 3/4 inch thick and seemed to be facing backwards. At first I thought they had been reversed. I was wrong. The shaft diameters are different for both sides, and the ends are different too. The magneto side is longer and thinner, about 11/16-inch, with a Lovejoy coupling mounted on the end. That side of the flywheel has a short hub. The starter side of the crank is short as is, 3/4-inch in diameter.

It is also the side that the power is derived from the mower, using a 3/4-inch-diameter steel shaft. Power to the mower drives are taken off the shaft by means of two 530 chain drive pulleys. The outer end of the shaft is supported by the starter mount assembly. There is a bronze bushing with a grease fitting at that end. The power side flywheel has a longer hub and is drilled out in two places. One bolt anchors the flywheel to the crankshaft. The starter/power shaft is anchored to the power side flywheel with another 3/8-inch bolt that goes through the long hub on the starter side flywheel.

Outside view of donor flywheel.

I decided to look at the donor engine flywheels to see if they were the same. The cooling fins seemed to be reversed meaning the cooling air was drawn through the cylinder fins and shrouding and was discharged through the hollow flywheel centers. This is the reverse of modern day air-cooled engines where air is drawn in through the flywheel and discharged over the cylinder fins. The exception was the Vacuflo system used by Onan for a few years in their generators. Another difference between the flywheels was that they had different weights. The near-running engine had heavier flywheels. The donor engine had flywheels that seemed to be at least 1-1/2 pounds lighter each.

Inside view of donor flywheel. Note the coolings fins seem reversed.

As I pulled the starter side flywheel off I found a nice, shiny cadmium-plated 5/16-inch nut lying between the flywheel cooling fins and the lower shroud. Now where did that come from? After the flywheels were removed, I found another problem. Several mounting screws for the lower shrouds were missing and there were sheet metal screws (the wrong size) screwed into the block. This would have to be addressed during reassembly. In the meantime, disassembly would continue. The lower shrouds were then removed. See Part 2 in the next issue of GEM.

Jacobsen History

Knud Jacobsen, Henry Ford, Eli Keikoffer (of Disston and Evinrude 2-stroke engine fame) and Jerome Increase Case were close friends and close in business, as well. Jacobsen made patterns for Case, and they worked hand-in-hand for many years. In 1921, Jacobsen came out with the “4-Acre” mower. It was an immediate success. After a few years, Jacobsen started making other purpose-built mowers. At first, all equipment was painted battleship gray. This remained so until 1936 when Case decided to change from gray to Flambeau Red, an orange color. Although Case didn’t release the color change until 1939, Jacobsen started about 1936.

Lawn Queen wheel painted vibrant Flambeau Red.

They bought their paint from Case in 55-gallon barrels, adding a quart of IHC red to brighten it up a bit. That color is very close to today’s Case Power Red paint. The twin-cylinder mower was introduced as “The Heavy Duty,” in 1931. In or about 1936, the name was changed to “The Twin,” at about the same time as the color change. The 2-cylinder mowers were built until 1941, a run of about 10 years. Jacobsen built only 807 of these machines, starting with serial no. 101. The last one built in this series had serial no. 908. Originally, the 2-cylinder engine was rated at 3.5hp at 1,190rpm. Later in production, it was re-rated at 4hp at 1,260rpm. Definitely a low speed, but very torquey engine. Even the upright single-cylinder engines had different ratings, although nearly identical internals. They were rated at 1.5hp but speeds varied from 980 to 1,260rpm depending on the equipment on which the engines were placed. Over the years, twins had at least three different magnetos, and two different carburetors. “The Twin” mower weighed in at 396 pounds dry, and cost $470 in 1936.

Contact Andrew Mackey at P.O. Box 347, Rockaway, NJ 07866 • (862) 432-1552 • mackmotr@aol.com

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