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270 Offy

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mayhugh1

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Thanks all for your comments. I'd never heard of color tuning before. It sounds like what Ron did. - Terry
 

johwen

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That, I'm not sure of (yet).
Johwen Here You can use a tachometer to adjust each needle to maximum revs at fixed throttle setting or a vacuum gauge but you need to have a balance pipe or drilling on the engine side of the throttles to balance the vacuum across all cylinders once again adjust each to reach highest reading. Hope this helps John
 

mayhugh1

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Construction of the Offy's induction system began with the machining of its two main components - the quad carb body and the fuel bowl. Just after machining the carb body, I became concerned that the throttle barrels were probably uncovering the air bleed passages a little too early. So, I plugged them and their adjustment screw holes and drilled four new sets on the opposite sides of the throttles. This also resulted in a change in the direction of operation of the throttle which wasn't necessarily a problem. But, I never really liked my push-pull throttle design with its awkward two-axis motion, and so I'll likely replace it with a rotating lever.

In my earlier post I forgot to include a cross-sectional CAD view through the center of the carb body showing details of the Venturis. I've included one here that also shows the updated air bleeds. The continuation of the Venturis using tapered bores through the throttle barrels is a feature lifted from one of George Britnell's carburetor designs. None of the simple commercial air bleed carbs that I've seen use it, probably due to the additional manufacturing cost. The manifold vacuum taps suggested by David Sage were also added to the final part - Terry

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gbritnell

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To use a term taken from my kid's language 'Sweet'
gbritnell
 

Art K

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Reminds me of a pair of Weber carbs.
Art
 

mayhugh1

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Each needle valve assembly includes a jet with a .022" dia. orifice and a .030" diameter adjustable needle. In addition to metering fuel into the engine, the four assemblies also secure the carb bowl to the carb body. The tops of the jets are sealed to the carb body by a Teflon bowl gasket, and their bottoms are o-ring sealed to the inside floor of the bowl. Nuts threaded onto the bottoms of the assemblies protruding through the bowl will compress the o-rings and then hold the bowl tightly against the bowl gasket for a (hopefully) leakproof 'four barrel' carburetor.

Both halves of each needle valve assembly were lathe-turned from phosphor bronze, and all features associated with the needles themselves were turned in the same setup to insure concentricity. A sensitive drill attachment in the tailstock was used for the .022" diameter flow passages. The adjustable bodies are threaded 10-56.

The orientations of the pickup tubes are limited by the space available inside the bowl. Their hole locations were determined while the jets were threaded in place and, after drilling, the jets were numbered with their locations in the carb body. An alignment fixture was used to soft-solder the pick-up tubes to the jet bodies.

The adjustable needles were designed around .030" diameter sewing needles that were epoxied inside their own threaded bodies. The carb adjustments will be done through the bottom of the carb bowl while o-rings seal the needles to the jets. In order to provide a consistent starting point for the four needle adjustments and to prevent damage to the seats, a slip-on Delrin collar limits the travel of the needle into the jet.

During construction, with the needle bodies fully threaded into the jets and tightened against the collars, the sewing needles were slipped through the needle bodies and seated in the jets before being epoxied in place. As such, these collars provide safe indicators of when the needles are fully seated. Marks engraved on the sides of the needle bodies will give visual indication of their settings with respect to their fully closed positions. Finally, the needles were match-numbered with their jets. - Terry
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mayhugh1

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The throttle barrels were machined from 5/16" drill rod, and their bores inside the aluminum carb body were lapped with Timesaver for a smooth sliding fit. Grub screws in the carb body ride in grooves machined into the barrels and retain them inside the bores during rotation. The barrels' Venturis were machined from both sides using an 1/8" ball end mill and a tapered boring program running on myTormach.

The throttle arms were machined from 303 stainless. The clevises rotate on threaded studs Loctite'd in their ends. Instead of the push-pull throttle control in my original design, I simply extended one of the arms for a rotary control.

I'm currently gathering the materials (and knowledge) for a last minute decision to color anodize the carb bowl. If it turns out well and I get a reasonable color match to the red magneto housing, my next step will be to tackle an oil/fuel combination tank that I'll also anodize before I dispose of the chemicals. - Terry
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dsage

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Terry:
Just a word of caution on anodizing. I would be careful anodizing something that has already been sand blasted. Try a piece of sand blasted scrap first of the same alloy. (Which I have no doubt you'll do).
Also I've found if you want a good color match between pieces do them all together in the same batch. In my experience (lots of it) the chances of getting the colors exactly the same between two batches is difficult. There are a lot of variables to be juggled in anodizing and getting them exactly the same between batches is difficult unless you have a professional setup. (temp/time/acid strength/ current density/ color bath concentration and ph /dipping time/ sealing time and temp - to name a few).
Black being the exception. Black is usually black but it can go bad too. Some colors are very difficult - red being one of them.
I'd hate to see your wonderful and beautiful workmanship spoiled by an anodizing job gone bad. And it can go very bad for no apparent reason.

Caswell Plating has a lot of good information and reliable supplies.
 

mayhugh1

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Terry:
Just a word of caution on anodizing. I would be careful anodizing something that has already been sand blasted. Try a piece of sand blasted scrap first of the same alloy. (Which I have no doubt you'll do).
Also I've found if you want a good color match between pieces do them all together in the same batch. In my experience (lots of it) the chances of getting the colors exactly the same between two batches is difficult. There are a lot of variables to be juggled in anodizing and getting them exactly the same between batches is difficult unless you have a professional setup. (temp/time/acid strength/ current density/ color bath concentration and ph /dipping time/ sealing time and temp - to name a few).
Black being the exception. Black is usually black but it can go bad too. Some colors are very difficult - red being one of them.
I'd hate to see your wonderful and beautiful workmanship spoiled by an anodizing job gone bad. And it can go very bad for no apparent reason.

Caswell Plating has a lot of good information and reliable supplies.
Thanks for the advice Dave. I actually never bead blasted the bowl that I'm planning to anodize. It was polished, but not buffed to a high luster like the air horns were. Unfortunately, I had already Loctite'd the brass inlet/outlet tubes, and since I don't know how they'll affect the process, I just finished drilling them out. I'll likely do some test parts as you suggest, but if you have any other tips, I'd be glad to hear. So far, my knowledge comes from a couple Youtube videos. - Terry
 

dsage

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Sorry I was going by post 347 that shows the bowl (as well as the body). It looked blasted as well. Bad angle I guess. Anodizing covers NOTHING. So the starting finish must be as good as what you're looking to end with.
Good you drilled the brass out. It would have messed up the whole process and possibly dissolved the brass.
There are a lot of Youtube videos on anodizing. Some not so good.
As mentioned you might want to look through the Caswell information (reliable). They tout the LCD (low current density) process. i.e lower current longer time. They have a Forum on there somewhere.
There are too many tips to go over here. For sure use an adjustable constant current supply. Not a battery or battery charger. You need to be able to calculate and control the current. It's key to the process.
There is a formula for the time required to anodize vs the current and the thickness of desired anodized layer. A quick search found it here but there are likely better resources.
But it's well described on the Caswell Plating anodizing forum. Search for "720 rule".
I usually calculate current / time for 0.75mils of anodizing layer. Any less and it might not take the dye well. Up to 1mil is ok. Nothing to be gained (IMO) to go any more. More takes more time then you get into temperature problems with the bath. If the temp of the bath goes much above 80deg F the anodizing may be removed as fast as it is developed. Best to go low current longer time.
As mentioned do all the pieces in one setup if you're going for other than black.
By the looks of it you might need a pretty large tank to hang everything and to avoid the bath getting too hot. Knowing you, you can probably calculate the wattage (current voltage) on the tank and the temp rise and how much it can self cool. :)
A few of other points.
Agitate the bath. Bubbles are produced that can stick to the parts and stop the current flow / anodizing. I use a plastic paint sturrer on an aluminum shaft driven by an electric screwdriver and hang that off to the side of the tank. Crude but it works. Some tanks have pumps and coolers.
Never hang anything but aluminum or titanium in the tank.
I use #11 dead soft aluminum wire to hang parts and I can usually find a threaded #6 or #8 hole in the part to force screw it into. It's soft enough ti does not damage the trhread. You can wrap the wire around the part but it will leave a non-anodized mark where it touches - and those types of connections usually always fail. Aluminum MIG wire seems to burn up before you're done. Wrong allow I guess.
There's no way to start over. The anodizing is very hard and can only be removed my lye or another caustic substance. Then you're in real trouble.

I'm sure you'll figure it out. But to be honest it can be risky on a perfect part.
Have you considered powder coating?? A lot less variables to manage.
:)
 
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mayhugh1

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The carburetor was finished up by color anodizing its bowl to match the engine's red magneto. Since my total knowledge of DIY anodizing comes from a couple Youtube videos and Dave's comments above, I was prepared for the possibility that I might end up machining a whole new part. Basically, the home version of color anodizing aluminum involves electrolytically depositing a one mil thick layer of porous oxide on a well-cleaned part. After a brief soak in dye, the part's surfaces are sealed using boiling water.

The surfaces of the 6061 aluminum carburetor bowl were finished and cleaned before receiving a final three minute dip in sodium hydroxide (diluted solution of drain cleaner). This last cleaning step etched the part's surface to a frosty gray appearance - a fairly nice finish on its own. Similar to nickel plating, though, the anodized layer will be too thin to noticeably change the appearance of a part's surface. After a rinse in distilled water, the carb bowl was hung in a small plastic container filled with electrolyte.

A proper electrolyte is a 15% solution of sulphuric acid. Battery acid (30%) purchased from a local auto parts store was diluted with distilled water. Acid concentrations greater than 15% risk the oxide being dissolved as fast as it's being deposited, and so more isn't better.

The carb bowl, which will become the anode in the process, was connected to the positive terminal of a low voltage power supply. To create a cathode, thin sheets of lead were formed around the inside wall of the plastic container. A proper power source is one capable of constant current operation since the resistance of its load will change as the non-conductive oxide layer builds up around the part. I used what I had on hand, though, which was an unregulated variable voltage supply capable of several amps and voltages up to some 18 volts. I used an operating point of 2 amps which required about 12 driving volts. The voltage setting had to be tweaked after several minutes into the process in order to maintain the 2 amps. The 2 amp operating point came from a Youtube video demonstrating the anodizing of a part similar in size to my own. Two amps was also close to the value calculated using the '720 rule' recommended by Dave.

Some of the water in the electrolyte will be decomposed into its hydrogen and oxygen constituents during anodizing. Oxygen will collect around the positively charged part while hydrogen collects around the lead cathode. A rough visual check should show twice as many hydrogen bubbles compared with oxygen bubbles.

For a hanger, I initially used a piece of 3/32" titanium welding rod bent into a hook through one of the holes in the end of the bowl. After a few minutes, the anodizing current abruptly dropped to zero. I replaced the titanium hanger with a similar piece of (4043) aluminum rod but got the same result. I thought my 60 year old power supply was teetering on failure, but I eventually discovered the electrical connections between between the hangers and the part were failing due to oxide build up on the hangers. One of the two holes in the bowl happened to be threaded for a 6-32 hose barb. After running a die over the end of an eighth inch aluminum rod, it was screwed into the hole and used as a hanger with no further problems. Dave had warned about this very issue. Strangely, even though the hangers in the Youtube videos looked a lot less robust than either of my initial attempts, there was no mention of any difficulties with them.

About 60 minutes later the part was removed from the electrolyte, rinsed in distilled water, and immediately immersed in a jar containing the dye which had been warmed to 140F. I used 'scarlet red' Rit dye obtained from a local fabric shop, but dyes especially formulated for use with metal are also available. The color's darkness increases with the part's time in the dye, and after about two minutes it was close to the color of the magneto. After withdrawal, it was rinsed one last time and then dropped into a pan of boiling water for about 20 minutes. This closed the pores of the oxide and sealed the color into the part's surface.

I was concerned that the ends of the bowl might not anodize since they had no direct exposure to the cathode. That turned out to be a non-issue, and I was surprised to see that even the inside of the bowl came out as nice as the outside. - Terry

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dsage

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Phew !! (sp?)
Am I glad you had success. I was truly worried for your part. There are a lot of pitfalls in the process and I'm glad none caught you up too badly. I'm also glad the current disconnect was recoverable. Sometimes I purposly put a #4, 6 or 8-32 threaded hole in a hidden spot just for the purposes of anodizing.
I had no doubt that you'd research it and would have success. Good job.
The bowl looks beautiful.
I will add one thing. I've found the Rit dyes to fade over time especially if you put them in the sun. Shouldn't be a problem for you. I did a long term test by masking off half of a red anodized piece with black electrical tape and putting it in a window for (??) a long time. Strangely the commercial red dye I did the same test on actually got darker (much darker). I guess you only did the one piece. A good thing because I mentioned there could be a color difference between pieces done in separate batches. I figured this all out when I did several gauge rings for my friends off-shore race boat. They were red and turned out really nice. But he sold the boat a couple of years later so I never saw what happened to the color after sitting in the sun most of the time.
Great job.
 

mayhugh1

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I found this item on Ebay. I've seen them come up before, but they're usually sold before I can post a URL. Evidently, a die cast manufacturer (GMP) started producing and selling these 1/6 replicas of the Offy 270 in 2002. Production continued through 2010 and although I've never seen one, there should be a lot of them around. It seems it was one of their first products, and they made lots more. These Offys probably don't have functioning internals, but their exteriors look very realistic. - Terry
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