I have an old lawnmower engine (finally!), a 1978 Briggs & Stratton 4-stroke single cylinder with a vertical crankshaft. I've disassembled it fairly well and reassembled it, and spotted problems and their solutions. But one thing that still puzzles me is in the carburetor.
The air filter screws on top of the carburetor, and below it is the choke plate, and below that is the feed from fuel tank. Past that, turning to the right, is the throttle, and then the pipe that leads to the intake valve. Anyway, with the choke, there is no sign of any knob that would control it at all, and it looks as if there never was one on any of the Brigg's & Stratton engines that I have seen pictures of. Also, the screw that holds the air filter in place goes straight through the little choke allowance opening in the center of the choke plate. So how does one control the choke plate, and how does any air at all get through the (now blocked by the air filter screw) choke plate opening? The choke plate is automatically closed, by the way.
Sorry if that was too confusing - I'll draw diagrams if necessary.
If you take a closer look, the choke valve has small bevels on either side, allowing the choke to turn upright along the sides of the air filter bolt. The mechanism that controls the choke is nothing more than the throttle itself. You wont be able to run the engine at full throttle without activating the choke... at least that's the way most of them are. I don't think I've ever seen one with a manual choke.
There is no apparent linkage from the throttle to the choke. So when you start the engine, the choke is automatically closed, and then as the piston moves down on the intake stroke, it sucks in fuel and air , which also sucks the choke open?
Some engines have a lever that sits near the magneto. The wind from the magneto fan moves that lever and controls the choke on those. Some have linkage directly from the throttle. Some have a link inside the carburetor. One way you could tell is by moving the throttle back & forth while looking at the choke. If it opens and closes, then you know it's ok. If it doesn't, then there is a problem. I've never heard a vacuum choke on a lawnmower engine, but that doesn't really mean anything. :lol: You might try to Google the engine model... or the manufacturer... or just small engine carburetors in general.
Well, when I open and close the throttle, nothing happens. Nothing happens when I spin the flywheel, nothing happens when I pull the pull cord, nothing at all. And there seems to be absolutely no visual evidence that there ever was a linkage between the choke plate and anything else.
But do vacuum-operated chokes exist? This is a four-stroke, whereas most lawnmower engines are two-stroke, so maybe the manufacturer decided to do something fancier with the choke like for instance make it vacuum-operated.
There's no way you could spin it fast enough by hand to create the wind that's needed to move that. Is the lever there? It just looks like a flap, connected to a rod, right near the flywheel. It should be made of plastic.
Anyway...
It's definetly possible that it has a vacuum mechanism... and back in the 70's, they used alot of vacuum devices on cars. As a matter of fact, my truck has a vacuum choke.
Can you take some close-up pictures of the carburetor and linkage at different angles and post 'em?
OK, it's most likely a vacuum choke, now.
There is one other thing, though. The carburetor has relatively large tube connecting to it just above the choke plate that comes from just beside the exhaust port. My only reason for this would be that it recycles exhaust. There is always still oxygen in exhaust, and exhaust is hotter than outside air, and the hotter the oxygen the better efficiency the engine will get (unless it's *too* hot, of course). Anyway, if I'm right and that's an exhaust-recycling port, maybe the exhaust pouring in at relatively high velocities creates a strong enough wind to push the choke open? Thing is, that doesn't make too much sense because the warmer the engine gets, the less exhaust produced, so the less the choke is open whereas it should be the other way around: the warmer the engine, the more open the choke plate is.
I'll try to take some pictures, but I warn you, they'll be webcam quality.
Thanks for the help so far. The pictures I'll probably take later today.
[EDIT]
I looked in a manual that covers Briggs & Stratton engines. I looked in the carburetor section of the book, and it covers automatic chokes - I was wrong, there is linkage between the choke and something else. Thanks for the help though.
One more thing - do you think I'm correct in my theory that the carburetor recycles exhaust?
Oh yeah... That would make it turbo.
You mean it's turbocharged? I don't think so - turbochargers use the exhaust stream to spin a turbine that compresses air and hence allows the engine to fit more air into the cylinders and therefore more gasoline. Hence more power for the same sized engine. I think this just uses some nice hot exhaust as air to mix with the gasoline as opposed to using 100% relatively cold outside air.
I must have mis-read your other post. I thought you said it had parts missing from the tube. One more thing though... there will always be some unburned fuel in the exhaust. That device is
probably meant to make it more fuel efficient.