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TR2/3/3A problem just rebuilt SU carbs

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I have rebushed many SU carbs with Delrin. I made the bushings from solid rod stock. The only problem was that if bushing was too tight of a fit in the carb body, it would cause the throttle to stick when the car was up to temp, shut off, then restarted. The solution was to relax the fit of the OD of the bushing in the carb body. In link to the Thos Bryant article, I think mentioned .010" interference, in my experience, that was way too much. Other than that problem, Delrin seems to be a great material for the bushings.
Berry
 
So they would stick even after line-reaming to .3125"? Did you happen to try .3135"?
 
TR3driver said:
So they would stick even after line-reaming to .3125"? Did you happen to try .3135"?
Click to expand...
For some reason I found that reducing the OD of the bushing to make it a light press fit had more effect on the problem than increasing the ID. Also, if I remember correctly, the throttle shafts are .310", so reaming the bushing to .3125" should provide enough clearance.
Another also, I found that using a lathe to be more accurate than a drill press in keeping the holes in line. Chuck the piloted counterbore (what I use) in the headstock and push the carb with a center in the tailstock. I saw that method in an old Moss TR3 catalog.
Berry
 
titanic said:
I have rebushed many SU carbs with Delrin. I made the bushings from solid rod stock. The only problem was that if bushing was too tight of a fit in the carb body, it would cause the throttle to stick when the car was up to temp, shut off, then restarted. The solution was to relax the fit of the OD of the bushing in the carb body. In link to the Thos Bryant article, I think mentioned .010" interference, in my experience, that was way too much. Other than that problem, Delrin seems to be a great material for the bushings.
Berry
Click to expand...

My experience is quite the opposite. Unless there is a significant press fit between the bushing and the carburetor body, then the bushing will spin during the reaming process, and it will not be possible to get a proper fit between the shaft and the bushing. Once the ID is properly reamed to 0.3125, it really doesn't matter how much press fit there initially was; the bushing thickness, and the inside clearance, will remain at a constant value, regardless of the amount of press fit on the OD. That is, if they don't bind when the engine is cold, they aren't going to bind when the engine is hot, either. That's my experience, based on more than 2500 Delrin bushings installed over the past 27 years.

There are numerous reasons why shafts might bind in Delrin bushings, but I've never seen a single one caused by there being too much press fit on the OD. Too little press fit, yes; that can allow the bushing to spin, resulting in binding caused by slight eccentricity in the bushing.

Tom Bryant
 
Thanks, Tom.
 
Tom-The problem I experienced was that the binding problem occurred when the car was up to temp., shut off, then restarted a short time later. No binding when cold or running at normal temp. It resulted in head scratching and several "come backs". I found that if the bushing was about .003" interference fit in the carb body(and lightly staking the end of the carb boss to prevent rotation), the problem didn't occur. I can't claim to match the volume of your experience (50-100 carb sets here), but that was my experience. Probably not relevant, but I used a 13/32" piloted counterbore, which is large enough to remove the factory bushing. End of story.

Berry
 
TR3driver said:
Thanks, Tom.
Click to expand...

You're welcome.

This morning, I had some peculiar binding problems with three Type HS4 carb bodies that I was rebushing for a guy in Australia. One of the throttle shafts was binding in the new Delrin bushing. No matter how hard I looked, I could find nothing wrong with either the shaft or the bushing. I ended up replacing the bushing. I also had binding problems that were caused, at least in part, by the brand-new throttle plates that he provided for me to install. Rather than re-invent the wheel, I'll just copy and paste from the e-mail that I sent to him less than one hour ago:

As for the throttle plates:

The first carburetor I did was the one without a vacuum advance hose nipple. I randomly grabbed a throttle shaft and one of the new throttle plates and installed them. Everything seemed okay, so I moved on to the second one.

The second one I did was the one with a cap on the vacuum advance nipple. That one didn't go as smoothly. The first problem I had was that I could only barely, with considerable effort, get the shaft into one of the two new Delrin bushings. Most peculiar. I checked the size of the shaft, and it was perfect. I checked the size of the bushing, and as nearly as I could tell, it also was perfect. My 5/16" hand reamer turned freely in the bushing, so it should have been proper size. After about a half hour of messing with it and trying to find the problem, and failing (because the size seemed to be perfect) I finally removed the bushing and installed another one. Reamed it out, and this time all was good. Except:

The new throttle plate that I installed didn't close well, and when I tightened it down, the shaft warped and was binding a bit in the bushings. After messing around with that for a while, I checked the thickness of the new throttle plate and compared it to the old ones. It was thinner. So I put in an old throttle plate that was slightly thicker. The binding problem improved but didn't go away. So I put in the thickest throttle plate, and all was well.

So, I moved on to the third carburetor, the one with the vacuum advance fitting that does not have a cap on it. This one was somewhat the same story as with the second carburetor; when I installed the new throttle plate and tightened the screws, the shaft warped and bound slightly in the bushings, except the binding wasn't as severe. I ended up installing a used throttle plate that was a smidgen thicker, and the problem went away.

Tom Bryant
 
titanic said:
Tom-The problem I experienced was that the binding problem occurred when the car was up to temp., shut off, then restarted a short time later. (snip) End of story.

Berry
Click to expand...

Thanks, Berry. That was a peculiar problem, for sure....

As you might infer from my previous post, binding problems can be difficult to track down. I never did find the actual cause of the binding on that second (of three) carburetors. Both the shaft and the bushing seemed to be perfect. I gave up and replaced the bushing with another one "just like" it, using the same tools, techniques, and tolerances as used the first time, and the problem went away. I have no explanation.

Added 12/20/2019:
After thinking about this for a day, I do now have somewhat of an explanation. When using a reamer in Delrin bushings, if you sneak up on the size from below, when you get to that largest reamer, if it's only a smidgen larger than the last previous one, it may not actually do much, if any, cutting; rather, the Delrin will sort of move away from the blade of the reamer and not actually get cut, so the hole remains the same size as the next smaller reamer, or nearly so. When I replaced that "peculiar" Delrin bushing, I reamed it in one fell swoop, using my finish hand reamer, so the hole might well have been cut just a smidgen larger than the hole left behind (by that exact same reamer) in the "peculiar" bushing.

Tom
 
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