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270 Offy
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The Offy's documentation invites the builder to experiment with alternative solutions to the engine's induction system. In addition to Ron's four carb setup, I've seen an online photo of another builder's single Perry carb sitting on a custom tubular manifold.
However, I very much like the looks of Ron's four carb design even though it will likely require a lot of effort to get properly working. It was designed to be reminiscent of an injection system that was available on the full-size engine. Ron used four air bleed carburetors similar to the latest tiny O.S. nitro units available in hobby stores. One of the problems with these simple commercial carbs is that each requires the addition of a remote needle valve and a pair of interconnecting hoses to become functional.
I spent several days trying to come up with a design based upon four O.S. 15LA's that didn't require a mass of out-of-scale Tygon tubing. The carb's form factor, however, made it difficult to hide the needle valves within a believable looking assembly. I eventually decided to start from scratch and design my own quad carb body with an integrated fuel bowl where I hid the needle valves. It's still something of a work in progress and will likely continue to evolve as I make parts for it. I already machined the airhorns to break up the monotony of sitting in front of a computer for the last two weeks. They were the only parts of the design that didn't seem to change daily.
Currently, the fuel bowl has a capacity of about 1.5 ounces. I also completed a design based upon a larger bowl that would have allowed me to eliminate the fuel tank altogether. After a few days of looking at it, though, I realized it was overwhelming the appearance of the engine.
I've included a number of Solidworks renderings of my current design along with notes detailing some of its features and dimensions. - Terry
However, I very much like the looks of Ron's four carb design even though it will likely require a lot of effort to get properly working. It was designed to be reminiscent of an injection system that was available on the full-size engine. Ron used four air bleed carburetors similar to the latest tiny O.S. nitro units available in hobby stores. One of the problems with these simple commercial carbs is that each requires the addition of a remote needle valve and a pair of interconnecting hoses to become functional.
I spent several days trying to come up with a design based upon four O.S. 15LA's that didn't require a mass of out-of-scale Tygon tubing. The carb's form factor, however, made it difficult to hide the needle valves within a believable looking assembly. I eventually decided to start from scratch and design my own quad carb body with an integrated fuel bowl where I hid the needle valves. It's still something of a work in progress and will likely continue to evolve as I make parts for it. I already machined the airhorns to break up the monotony of sitting in front of a computer for the last two weeks. They were the only parts of the design that didn't seem to change daily.
Currently, the fuel bowl has a capacity of about 1.5 ounces. I also completed a design based upon a larger bowl that would have allowed me to eliminate the fuel tank altogether. After a few days of looking at it, though, I realized it was overwhelming the appearance of the engine.
I've included a number of Solidworks renderings of my current design along with notes detailing some of its features and dimensions. - Terry
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Does this mean that the carbs are the last parts needed to finish?
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Very nice. How will you adjust the needle of each cylinder?
I still have the crankshaft, rods, pistons, and rings to do.
That, I'm not sure of (yet).
Hi Terry,
I always look forward to your posts as I learn so much from them and appreciate you taking the time to document things as you do.
Could you explain how you made the airhorns for the carbs? They seem like a simple part but I can't figure out how they are made. Are you using form tools for the outside and inside? Are the tools the same radius for the outside of the airhorn and the inside and one is just convex and one concave? How do you keep a consistent thickness to the walls?
Sorry for all the questions!
Thanks!
Shawn
I always look forward to your posts as I learn so much from them and appreciate you taking the time to document things as you do.
Could you explain how you made the airhorns for the carbs? They seem like a simple part but I can't figure out how they are made. Are you using form tools for the outside and inside? Are the tools the same radius for the outside of the airhorn and the inside and one is just convex and one concave? How do you keep a consistent thickness to the walls?
Sorry for all the questions!
Thanks!
Shawn
Shawn,
I turned them on my little Wabeco CNC lathe. I used two operations - the first with a 35 deg rhombic insert to turn the outside contour. A hole was then drilled through the center of the blank so a second operation using a boring bar could turn the interior. I designed the Horns in SolidWorks to have a similar thickness throughout except for the outside rim which I made with a circular cross section. The curve was a spline that I played with until the shape looked good to me. I know you were hoping to hear about some clever manual operation that I had managed to come up with, but I'm sorry to say I'm not that clever. - Terry
Terry
Thanks for the quick response! You're willingness to answer questions and share your knowledge is very much appreciated. I'm sure there are many people here like me who are learning so much from people like you.
I made a airhorn once but it was just a simple one with straight, angeled sides. Thanks for sharing how you made yours!
Shawn
Thanks for the quick response! You're willingness to answer questions and share your knowledge is very much appreciated. I'm sure there are many people here like me who are learning so much from people like you.
I made a airhorn once but it was just a simple one with straight, angeled sides. Thanks for sharing how you made yours!
Shawn
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Perhaps a small tube into each manifold that you can measure vacuum at and cap off after. The old school way of tuning multiple carbs.
lohring
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Model boaters have used a flow meter to adjust needles. The pressure comes from a standard propane cylinder feeding into the fuel line. The needle flow rate is read on the Dwyer ball gauge with the pressure adjusted on the propane bottle to a standard on the Magnehelic gauge. We usually used 40" of water. This was most helpful for setting a new engine to numbers from similar engines and for adjusting needles for air density changes during the day where the correct setting is approximately known.
In your case you might find the flow at the recommended needle setting of a suitable OS carb, then set your needles a little richer. It will be easy to equalize the flows from each carb despite manufacturing variations. Below are a couple of pictures. It's easy to build. You can probably find a model boater with one for sale for under $100.
Lohring Miller
BaronJ
Grumpy Old Git.
Nice idea. I just added them to the design. As you say, they certainly can't hurt, and they'll make the induction system look more techie. - thanks. - Terry
thefishhunter
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Wow, a chance to add something to one of Terry’s builds (I usually just lurk in awe 
if you add the manifold ports, then a set of carbs sticks (4 mercury filled tubes similar to the single one shown above.) are what we have always used to sync motorcycle carbs. They are relatively cheap, hang from handlebars or frames and allow you to get the manifold vacuum signal to be equalized at all 4 carbs.
This is used two ways, you can just cycle the starter (no ignition) and measure the vacuum to set the throttle blade position is equalized.
Secondarily, with the engine running, you can adjust your needle balance at idle to again get an equal reading, again to get the engine to “balance” across all 4 carbs.
You can get a set at just about any motorcycle shop or online.
if you add the manifold ports, then a set of carbs sticks (4 mercury filled tubes similar to the single one shown above.) are what we have always used to sync motorcycle carbs. They are relatively cheap, hang from handlebars or frames and allow you to get the manifold vacuum signal to be equalized at all 4 carbs.
This is used two ways, you can just cycle the starter (no ignition) and measure the vacuum to set the throttle blade position is equalized.
Secondarily, with the engine running, you can adjust your needle balance at idle to again get an equal reading, again to get the engine to “balance” across all 4 carbs.
You can get a set at just about any motorcycle shop or online.
Just a note on how I adjust my carbs. I remove the exhaust manifold and look directly into the combustion chambers while the engine is running. I adjust each needle for a nice blue flame at the exit of each combustion chamber. I watch for a consistent flame throughout the throttle range. Easy and I get consistent results.
I once tried to tune a motorcycle carb using that method.
It took months for my eyebrows to grow back.
It took months for my eyebrows to grow back.
BaronJ
Grumpy Old Git.
Hi Peter, Guys,
Somewhere kicking about I have a kit of parts which includes a clear glass topped spark plug and a bottle of cleaning fluid. I can't recall the name at the moment.
But you took a plug out and replaced it with the glass one and tuned the carburetor for a nice blue flame in the glass plug. I recall that it was very good with SU and Webber carbs.
Somewhere kicking about I have a kit of parts which includes a clear glass topped spark plug and a bottle of cleaning fluid. I can't recall the name at the moment.
But you took a plug out and replaced it with the glass one and tuned the carburetor for a nice blue flame in the glass plug. I recall that it was very good with SU and Webber carbs.
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That is called colourtune
Michael
Michael
I have a colourtune plug, but it comes with dire warnings about using it at anything other than tickover, so I don't see the point.
