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TB Casting Flaw?


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I cleaned the big brass screws and the bores today, and noticed a little bit of O-ring stuck to the side of the right TB bore. Using a very fine pick, I pulled it out, and discovered this little hole in the wall. Has anyone ever seen anything like this:




It looks like a casting flaw, and since the O-ring is below the hole, I assume it's doing no harm, although the roughness tends to chew up the O-ring on the BBS. I could clean the hole out with some acetone, then fill it with a tiny bit of JB Weld, as they claim "when fully cured, J-B Weld is completely resistant to water, gasoline, and about every other petroleum product or automotive chemical." A piece of vinyl film or Teflon tape wrapped around the smooth part of the BBS would guarantee a smooth patch while the JB Weld cures, and would pull off cleanly, as neither sticks to epoxy.


Historically, the right TB screw has always needed to be turned in ~1/4 turn farther than the left to get them balanced at idle.


What say you?

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Afternoon Selden



If you can get the BBS screw back in without damaging

the "O" ring then that hole shouldn't cause you any

issues as that hole is rearward of the throttle plate

(low pressure side of the throttle plate)

so about all it should do is leak a little dirty air

into the TB with the throttle plate open above idle.


If you can repair it with JB (or Devcon) & not make a

mess inside the TB bore then that would be nice.


I think that it were mine & would just chuck the BBS

screw in my lathe & add a second very thin "O" ring

just outboard of that hole so it wouldn't leak any dirty air

into the engine & leave the hole as is.


OR, just install the BBS & adjust the idle TB balance then

smear a little silicone sealer on the BBS screw & work some

in between screw head & TB casting (just enough to seal the

BBS screw to outside air)

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I would take that immediately back to the closest dealership and demand a replacement due to a manufacturers defect! :rofl:


On a serious note, I wouldn't worry about it. How many miles on that RT now? I would think you have more leaks in those throttle bodies from general wear than anything having to do with something like. Interesting find though, can't say I've seen anything like it on a BMW

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I think I'm going to go the JB Weld route; I have repaired a lot of things this way, including when I was a teenager, a model airplane engine with a broken intake throat. It's really easy to get a good, smooth finish with a non-stick film. The void behind the hole is slightly larger than the hole itself, so the JB Weld in the void will be bigger than the opening, so there is no way it's going to work loose, and there is absolutely no mechanical stress on this area, so a filler seems like the perfect solution for this sort of problem.


I just stuffed in a dab of JB Weld using a bamboo skewer with a sharpened tip, wrapped the non-threaded section of the BBS with some Teflon tape, and screwed it in. By morning I should know how it works out. Worst case, I would need to use a little bit of 600 grit wet/dry paper to smooth out the bore, but that seems extremely doubtful.

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No surprises this morning. Fresh JB Weld flows pretty easily, so I expected to see that a little had oozed out of the hole, leaving a (smaller) void, and flowed down the side. A 3/8" wooden dowel, with wet or dry paper wrapped half way around, proved to be the perfect tool for cleaning up the small amount of residue. I put a little more JB Weld in the hole, then a piece of tape over it, and am waiting for it to set up, with the wooden dowel used to keep everything in place. I suspect that two shots of JB Weld will do the job, but if not, I'll keep at it until I am satisfied, as I'm in no hurry — keeping in mind the adage, "there is never enough time to do it right the first time, but always enough time to do it over."

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Just as heat make epoxy set up faster, already mixed stuff can be kept for many hours or even days in a deep freeze.


It would be something if we find more porosity in throttle bodies of other bikes though.

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JB Weld made a believer out of me on my old '93 R100RT. I was once over zealous in torquing the transmission drain plug and noticed some leaking around the extended boss that the plug screws into.


Removed the drain plug and noticed a hairline crack between the threads all the way to the outside area of the boss, which is where the oil was escaping to.


On a hunch I did the JB weld trick to the crack and it worked so well that even five years later it was still leak free.


The RT was a nice bike, but seriously down on power and that fairing....boy did that thing trap the heat. :(



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After the JB Weld set, and a little bit of very light sanding, the repair came out very nicely:




It wouldn't surprise me if this problem exists on other throttle bodies from this era, but most people don't look closely inside the air bleed bore to notice.

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Afternoon Selden


Just make sure that whatever part of that JB that remains inside the throttle body bore is SOLID enough that it won't ever break off & lodge between an intake valve & valve seat causing a burnt valve. Or bounce around in the combustion chamber & close up the spark plug electrodes.

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It's very solid. I cleaned the metal thoroughly, then blew it out. For the throttle body, the void in the casting is larger behind the hole than in the bore, so the cured JB Weld is basically keyed in place, and isn't going anywhere. The bore for the air bleed is surprisingly roughly machined, and some pits below the hole were also filled by the JB Weld, so it was already a rough surface, perfect for bonding.


I used to fly model airplanes when I was a kid, and accidentally broke off a significant portion of the air intake of the motor. I built the missing portion back up with JB Weld, sanded it flush, and it never came loose — despite the ravages of methanol, nitromethane, and castor oil, not to mention the vibration of a little 2-stroke single running at ~14,000 RPM.


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Great job. I'm willing to bet there are thousands of Boxer engines out there, possibly with that casting flaw that are running flawlessly for hundreds of thousands miles. But being anal, is priceless.

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I did a little more gentle wet sanding, so that only the area immediately around the hole now has any surplus JB Weld, with feathered edges. This not a part of the TB with tight tolerances, as the O-ring fills any imperfections. I doubt that I removed more than .001" of material.

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I finished major maintenance today, hooked up the old Twinmax, and fired up the engine. With both BBS at 1.25 turns out, almost perfect balance, and a very small tweak of the right screw was required to balance at idle. This was the easiest TB sync since I have owned the bike.


When changing multiple variables, it's always difficult to identify which one did what, but I'm 99% certain that closing that hole in the wall made things easier.

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