Tuning a Gas Engine to Run Slow

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Fast-forward a couple months, and here in England we just had the first shows in May, so my husband, Jim, prepared my newly acquired Baker Monitor for her first outing. As usual, the cart design depended on available materials, which in this case was a set of four large, narrow, thin-spoked wheels that Jim's brother acquired during a family holiday in Belgium. The cart is quite stylish, as befits such a stylish engine! It's also nice and easy for me to move the engine about, too.

Show season anticipation has inspired folks to take their engines out for an airing and to tune them up correctly, which brings us to our topic: show season fine-tuning.

Recently, an engine found its way into my garage and needed a little tune-up. Getting her to run well was no problem, but it ran faster than I like. Getting her to run well at a slow speed was a bit of a head-scratcher.

Even though the throttle was closed all the way, air leaking around the various shafts and butterfly keeps the engine running too quickly. This wouldn't be an issue with the engine running at its rated speed or under load, but this old girl will spend the rest of her days lazily lumbering along at engine shows. The adjustment that had the most dramatic effect was decreasing the intake valve lift. By reducing the lift, the rpm was cut in half.

Isn't that pretty much the same as 'throttling down?' I mean, you're reducing the amount of mixture that's taken in, which seems pretty logical except the intake valve is the final word on how much is admitted.

I agree with you that running an engine at a slow speed at a show is the way to go. In the United Kingdom, people typically run engines at the rated speed even at shows - but I've always figured that was because they didn't know how to slow them down!

Running at a slow speed is, of course, more of a problem with a throttle-governed engine than with a hit-and-miss engine. I once owned a Witte T/G that I couldn't slow down regardless of what I did to it. I examined the carburetor and discovered the butterfly didn't completely close. Whether it was due to bent or rough edges or a worn shaft, who knows?

Looking back on the situation, I think the manufacturer never intended the engine to run at any other than the rated (high) speed, so there was no reason to machine either the butterfly or the inside of the carburetor to any great extent.

Never overlook the linkage: On another engine, the throttle linkage was just too short. I lengthened the throttle rod so the butterfly was just barely closed when the governor weights were all the way out, and the engine ran well.

In a car or tractor carburetor, you usually have an idle passage - a way to get just a tiny bit of fuel to the engine to keep it from missing when the throttle is closed. I set the linkage so the throttle plate is open just a tiny amount, maybe 10 thou (0.010-inch), when the governor weights are all the way out.

Now, let's say you have the throttle rod set real long so the plate is shut tight, and the governor weights still have room to move. That allows the governor to put a lot of stress on the linkage. It can shut the fuel off tight, making the engine miss and lope, too.

If you want the engine to run slower, change the springs on the governor so that it takes less speed to get the weights to fly out all the way - don't try making big speed changes with the length of the throttle rod. You want it so when those weights are all the way out, the butterfly is darned near shut, save a few thou.

If you take your time, play with the linkage a little and see that everything fits nice and doesn't bind or bear down hard, you'll have a better understanding and can tweak it better.

Most engines don't seem to mind running really slow, and I like to see an engine that is running effortlessly. With that said, I think it's possible to run certain engines too slow.

Most farm engines are built really heavy, and I don't see much threat in running them really slow. Keep the mains snug enough so the crank doesn't jump up and down when it fires and listen for nasty thumps. Watch the top surface of the flywheel rims and see if they wobble when it hits. If you see things flexing, then speed it up a bit.

Another thing to remember is to retard the spark when you slow it down. You don't want that mixture burning real good when the piston is still coming forward, trying to compress it.

Sometimes you want to slow an engine down, and you just can't get it to vaporize gasoline properly. That's when you need propane. I love propane! Some engines are really easy to convert to propane. Any engine with a Lunkenheimer mixer is ready to convert. Feed the propane in where the gasoline line connects, gag the airway off, and you're ready to go. Other engines may be more difficult.

I know one guy who uses demand-flow regulators, so the engine has to pull a little vacuum to get the gas flowing. I know another guy who just lets it seep into the mixer. The problem with doing it that way is the gas keeps coming if the engine stalls, and a cloud of flammable gas wafting around your engine isn't cool.

The earlier comment about engines always running at rated speed in the U.K. was certain to attract attention from someone on this side of the pond, and our national engine habits were defended!

We do run a lot of our engines at lower-than-rated speed where appropriate, but a lot of early engines are splash-lubricated and need a bit of a rpm to get the oil distributed inside the crankcase. Plus, we also show engines that power implements a lot of the time, so to be fair I think we're probably a 30- to 70-percent ratio of 'slow' to 'normal' speed at shows.

The other factor is the variability of running times. A lot of people show an engine most weekends through the season, and it may be the format of the show precludes one or the other type of running speed, depending on local conditions.

The Ruston diesel I have runs at 375 rpm, which is relatively fast by your standards, but it will run down to almost stalling speed if I wanted it to. The dyno display units, which will go with it, will not be too happy at tick-over. So, when they're ready for the circuit we'll probably keep the engine at the nameplate speed.

There you go! A few tips on how to tweak an engine and fine-tune its running speed to get the optimum performance at a show are always nice to know. Have fun!

Engine enthusiast Helen French lives in Leicester, England. Contact her via e-mail at: helen@insulate.co.uk You can join the Stationary Engine List on the Internet at: www.atis.net