Converted my pressure washer over. Will take more shots of the finished product, once it's all together. Just have to finish the propane bottle mount.
I couldn't easily use the "Mitsubishi" engine that was on the pressure washer - it isn't a direct knock-off of a Honda GX, so the propane carb wouldn't just bolt on. I chose to swap out the engine with a used chinese clone I had sitting on the shelf, collecting dust. Had to fix the keyway on the crank shaft first(welded it and made it round again). It mounted the same, but was a tight fit with the propane carb. Wound up modifying how the propane mixer(?) mounted, so it would fit and be accessible:
I got really curious about how these things work, so I pulled the one off my bigger engine and mucked around with it.
There are two hoses going to the mixer. I figured one was for delivery, and one was for vacuum. I was wrong! We'll get to that in a minute...
Looking through the carb, there's an extra port:
http://ecomodder.com/forum/attachment.php?attachmentid=23043&stc=1&d=15118646 47[/IMG]
Strangely, it's just
before the venturi. The far end of the port is right before or right at the start of the venturi, unlike the port for the gasoline, which is right in the middle of it.
Initially this made sense, if the mixer metered propane based on the vacuum it would pick up from the other hose on the bottom of the float bowl; it would just be there to deliver the fuel, which is under (minor) pressure...so it wouldn't need vacuum to suck it through.
Delivery port, on the outside. Not much to say here at the moment:
Looking at it from the back:
Yellow: The gasoline shut-off solenoid; does not affect propane use.
Lower Yellow(my bad, didn't change color): Low-pressure Propane/natural gas intake port.
Blue: Propane delivery pipe.
Red: The second hose.
Following the second hose to where it connects to the mixer, there's a big screw on the other side:
Figuring the second hose needed vacuum on it to make the mixer deliver propane, I put a bit of vacuum on it...and nothing happened. It held the vacuum, nice and tight...put it back up, took off the screw...yup, it goes to the float bowl alright. But there's no openings in the inside where the hose connects to the mixer...
It's
just a drain screw!! Make sense, since the other drain screw on the float bowl is hard to get at with the mixer in the way. Convenient. I'm surprised they bothered.
That threw my theory out the window.
Turns out, it's all up to there being vacuum on that port by the venturi. Put just pressure on the mixer's intake port...nothing flows. (that's good - safe) Put just the tiniest bit of vacuum on the delivery hose, and propane flows. That's it.
Metering must be dependent on how much vacuum there is on that brass port, which presumably needs to be a specific size to get the mix right.
There's also the propane/natural gas selection switch(red), which has two different size orifices through it(green):
Compared to this:
http://ecomodder.com/forum/attachmen...1&d=1511864706
Why? Density of the two different gasses might have something to do with it, but mostly because they are delivered at different pressures. ~0.5 PSI for propane, and ~0.25 PSI for natural gas.
So, to summarize how it presumably works...vacuum is created as the air flows past the port/venturi, which in turn activates the mixer, allowing propane to flow. The stronger the vacuum, the more propane is let flow. No vacuum, no flow.
Hence why you have to shut off the choke completely to run it on propane...it will "flood" instantly. With the combustion chamber filled with propane, completely starved of air, it isn't going to fire!
And also hence why starting it with an electric start is effortless(gets steady vacuum going, so propane will flow), and pull-start is best done by forcing it to deliver a bit of propane first(by pushing the button in the middle of the mixer for a moment).
...All this was because I wanted to figure out if I could adapt it to work on other carbs and such. I think I could, with some careful planning.
Alright. Now, from some reading, it seems that you want to advance your timing when running propane, because it burns slower than gasoline, supposedly. If you don't, you risk burning your exhaust valve. Makes sense...I'd better see about advancing the timing on these small engines.
More reading said the opposite...yes, more advance than gasoline at low RPM, but
less at higher RPM.
Sooooo....Which is it?
Well, what few charts for timing I could find for propane show to stop it around 25 degrees, while gasoline can continue to be advanced further(and further yet if you run high octane), as RPMs rise.
Well, my RPMs aren't going to rise, so that's a moot point. And 25 degrees is, apparently, the more common ignition timing for these GX clones(Honda runs 20 and 25 on the GX200 and GX160, respectively, but the clones tend towards the higher of the two in spite of displacement). I should be where I need it to be already.
So...screw it. Maybe it'll run rich, or the timing is too retarded(it certainly is!) and I'll burn the exhaust valve. Maybe it run lean and melt my piston. Maybe it won't do either. The heck with it. I only paid $35 for the engine anyway; it can be my guinea pig. I'll find out on it before I find out (The hard way) on my generator.
So, I figure the spark plug should tell me a bit about how things are inside. I'd only run the pressure washer for a minute or so, but it ran like a champ and worked hard while it was on.
Propane pressure washer spark plug, Honda clone:
Nice and clean! But not crazy/too hot clean(white). Running on propane cleaned it up. I don't have a before shot, but I have a couple of shots from similar engines that were running on gasoline:
An actual Honda:
And the Mitsubishi:
Now, to be fair, the Honda was probably run with no real load on it last time it ran, and probably didn't get up to operating temperature, which might have a lot to do with it looking rich. The mitsibushi on the other hand was used in the exact same circumstances as the propane, and was fully warmed up, so it has a similar disposition. But while the electrode is about as clean, there's notably chunkier carbon around the rest of it.
There's barely enough carbon around the base of the propane's spark plug to leave a mark on your finger. And the story is the same inside the exhaust port on the engine, and on the exhaust valve...no "chunky" carbon. This engine was "well used", but looking at the carbon on/in it, after running propane, it looks almost new. (I also ran it in my shop with just the window open, and it didn't noticeably haze up the air with it's exhaust). Presumably it isn't running remotely rich, or there would be more soot/carbon.
Now I just need to come up with an excuse to use it. What should I turn in to my first victim?
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