Writing something like this is like sending a message in a bottle. People may see it. People may read it, but most of the time you don't know if they do anything. In the 4 or so years since I first wrote these instructions only a handful of people have told me they straightened their Rochester castings. But they all said it helped their carburetor noticeably. So here it is again. If you are unable to see the images, please let me know and I'll fix that by re-writing and using the image hosting here on the forum:
The bad news is these carburetors will be warped. The good news is they can be straightened. The best news is I think you can do this yourself and I’ve tried to keep it as simple as possible. The only disclaimer is “this worked for me but because of all the variable factors I can’t promise it will work for anyone else nor take any responsibility if it doesn't.” Before doing anything, please understand the pot metal they’re made of is rather brittle and can be easily ruined (cracked) if treated wrongly. If that happens, there may not be a fix. If treated correctly and carefully, I’ve learned it is almost infinitely workable. Interestingly you can correct a situation of over or under bending as many times as it takes to suit you. Plus you’ll be dealing with metal heated to 400 degrees. Please use common sense and don’t burn yourself or your house or anything.
You can use your oven for this, but only if the parts have been cleaned well using a cleaner which is not a petroleum based cleaner. You do not need to bake these like a cake. All you need do is get them to 400 degrees and then turn off the heat and let them cool naturally. Honestly I do this outside on the gas grill. It is really hot here in Big D and I don’t like heating the kitchen up in the summer (which for us lasts about 300 days a year).
Now let’s talk also about cleaning. Some of the ones I’ve been dealing with were very nasty. Mud, grease, old gas, weasel spit, all sorts of things. Clean it before you do anything and clean it as well as you can. I’ve mentioned strong vinegar here before and you can use it (12% or greater). Remove as many steel parts as you can if you do this. Vinegar doesn’t seem to damage the pot metal if not immersed for very long (a few minutes) and does make it look nice and new again, but it may or may not cause corrosion on steel parts. Here is something else I’ve learned: you can use a mixture of common dishwashing soap (Dawn for example) and water. That might be an unexpected idea, but it works. Remove all things like steel balls, springs, needle/seat, float, everything but not the idle pick-up tube. Be careful with it as it sticks up. I put into a very old and no longer used 11 inch by 5 inch deep pan enough water to cover the parts and then I squirt maybe a quarter cup of soap in there. Put this on the gas grill and heat it. You don’t need to make it boil, but you can. I usually don't because I don’t want a mass of suds out there. Use an old toothbrush or hand brush to clean them with this method. Take your time. The days of dipping a carb into some horrid junk for 20 minutes and pulling out what looks like a new one are over and they are not coming back. I’ve also learned acetone on a Q-Tip (Cotton Bud if you’re in the UK or other parts of Europe) can help with stubborn things. If you need to remove choke and throttle valves, carefully file the non-slotted ends off of the screws first. Then unscrew. Replacement screws can still be bought. Ask me if you need a source.
Second, you’ll need a straightening jig. I used 3/8 inch steel plate cut 4” x 6” and bought it here in Dallas from Metal Supermarkets. There is a way to make the jig which would appeal more to the machinist. And then there is a way to make an almost comically simple jig. It would be easy for me to say you need to make an elaborate jig with this and that and such and there's a lot of magic involved with this and you'll probably never be able to do it...but it would not necessarily be the truth. When you get into this, you will learn straightening these castings is 90% intuition, common sense, inventiveness, whatever you want to call it combined with what I’m going to teach you and 10% equipment.
The things you will need for certain:
1. at least 1 piece of metal as described above
2. an electric drill and bits
3. a dial gauge. I prefer the mechanical type, but anything will work if it is accurate and well made so as to allow you to lock it with a screw. Mine looks like this:
https://photos.app.goo.gl/uWWrsrRHX9TGgYpD64. a feeler gauge with thickness down to .0015. AutoZone sells one for about $6 or $7 which is fine.
5. 4 screws 3/16” x about 5 inches long plus nuts and plain washers and 4 screws 1/8" x about 5 inches plus nuts and plain washers and screws
6. common hand tools like open end/closed end wrenches, screwdrivers, etc
Now, here is a view of the more complex jig:
https://photos.app.goo.gl/cMoZfUG8eQiqTx5d6And here is a view of the simple one:
https://photos.app.goo.gl/J8sRp3obxR9FB4RT6I’m assuming most of you will take my suggestion and start with the simple one. You can later make it as complex as you wish, and if you need to straighten Carter or other castings, you’ll need to make one sort of large hole in it anyhow. Using a piece of heavy cardstock or manila folder, cut out places for your float arms, your idle dip tube, your main nozzle and protruding vacuum passage nibs so that your air horn can fit flat on this cardstock and then mark the 4 screw holes. Cut or punch these holes out and mark them on your metal plate near one of the 4 inch edges as you can see I’ve done. Try to get them lined up as straight as possible with the carb holes. Drill 3/16” holes corresponding to those and that’s it.
Now, how to use it? Start with your air horn. Make certain the internal vent tube does not stick out above the carb opening. Trim it off level but slightly below the carb body if it does. Set the air horn upside down on the jig as you see below and insert the screws, attach washers and nuts:
https://photos.app.goo.gl/4kbvgrfFWtMWenVXABefore we begin, please remember the corners of this casting are what needs to be pulled downward toward the jig and that is what makes this simple approach work. To start, snug up the 4 nuts. Don’t tighten them, but just snug them up evenly finger tight so that the top of the air horn sits level on the jig surface and doesn't move around at this point. Now with your dial gauge, measure the distance from the top of the airhorn corner to the bottom of the jig (as you see here) at all 4 corners:
https://photos.app.goo.gl/1CATwjQ5AgRvgJJ6AThey should be very close if not the same distance. Make a note of the measurements and mark that down. Now move your dial gauge to the center of each of the 2 long sides as you see here and write those measurements down:
https://photos.app.goo.gl/gDkpKZFCiXxj7Svr8Now set your dial gauge to the height of the corners on one side and lock it in place. Using your feeler gauge measure the air gap at the center point on that side and write it down. The center point will be the low point in the arc. Repeat for the other long side. Now focus only on one long side...pick your favorite. Unlock your dial gauge, measure the distance to the jig at the corners and subtract .004” from each (as I said, the distance between each corner and the jig will probably be the same---at least my observation has taught me it is). Lock your dial gauge down reflecting this .004” reduction. Now with a screwdriver and a 3/8 inch wrench tighten each nut on that side a bit, moving back and forth between them. After you have made a half to 3/4 turn tight start checking each corner with your dial gauge while continuing to tighten slowly and evenly. Hopefully you remembered to lock it. Once that dial gauge fits nicely on each side, stop tightening, move to the other long side and repeat this procedure. Remember, we’re only going to correct these castings .004” at a time. Now about the shorter sides: they should bow slightly upward in the center. Don’t worry about this nor try to correct it...as you’ll see it will not be a problem at all. Now heat the casting and jig to 400 degrees and let it cool naturally. Keep repeating this, correcting .004” at a time until you get each corner of each side and the center of that side to be the same distance from the bottom of the jig. Important notes: 1. Don't be surprised if after your casting and jig cool down the nuts on those screws are loose...maybe just finger tight. You’ve relaxed that pot metal and there is no more stress pulling it out of shape now. 2. You’re pulling the edges downward by what you’ve done. This as you may notice will cause the edges to be slightly lower than the bore portion (inner circle). Given the nature of these carburetors and their challenges in sealing, this isn’t a bad thing. It forces the bore to seal more sincerely and this should help with vacuum losses at those 2 important passages.
The float bowl: you can straighten the bowl the same way but you’ll need 1/8” diameter screws, bolts and washers and be very careful not to damage the threads in the bowl. But...you may not need to straighten your bowl as much. It may not be as far off as the airhorn. Check it and see. The procedure is the same as above except you will place the bottom of the bowl on the jig and pull the 4 screw holes downward toward the jig. Remember you're pulling them down to straighten the thing.
Now you need to mate the 2 pieces together and make final corrections. Once you’ve straightened the air horn and the float bowl, you probably will discover when you mate them that although the long edges are straight, the air horn rocks from side to side on the bowl. Go ahead and install the 4 screws holding the air horn to the bowl and tighten one side until there is no clearance for the feeler gauge. Measure the gap on the other side using feeler gauges and write it down. It will look straight but will have a gap like this:
https://photos.app.goo.gl/wUGjw4i6Xoyjiswp6Subtract .004” from the measurement you got with your feeler, select the feeler gauge that matches that reduced amount and tighten that side slowly and evenly until there is no more clearance for the feeler gauge you’re using. Now heat the whole thing back to 400 degrees and let it cool. Repeat until you can’t slide the .0015 feeler gauge in there. When all that is done, you have a carb body which is as good as it is going to get. See what the one I've been working on looks like:
https://photos.app.goo.gl/enDB4Mv93rGUfrNz6And please save these instructions if this worked for you. Informed sources tell me this fix is good for about 4 or 5 years and then you'll get to do it again.
