Intake leak symptoms (20-30 kicks when normally 1 or 2) made me pull my carb again last night. It's pissing with rain here so I didn't mind. I suspected my o-ring seal was good as I only went through this about 500 miles ago and it had been starting great up till recently. Air test and the old bubbles confirmed my manifold was holding great and I checked thoroughly! But when I was putting my carb on last time, I was noticed that the manifold flange on the body had become distorted over time maybe someone before me had been overtightening with thick gaskets? Anyhow, I didn't need a straight edge to see how the ears around threaded holes had been pulled in and now I'm betting thats where this leak is coming from.
My questions are this: what is the accepted repair for this? (or is there one). I can think of two possibilities... 1 sanding/ machining flat again. Will this only make it more prone to distortion in the future? 2 attempting to press them back to flat, but I am not confident this could be done without some type of damage to either the throttle shaft or body itself.
Also, exactly why CAN'T you pressure test through the carb intake? There must be a reason cause I never hear of anyone doing it. This would be a surefire way to test the carb to manifold seal. I am merely a student in the school of M74B who hasn't got to that chapter yet, please bare with me.
Post by kevsett on Nov 3, 2006, 11:24pm
Much brighter minds are out there than mine but I've battled this for a number of years and can tell you that machining/sanding down the carb intake flange is the way to go. And don't forget the intake manifold flange, at least check it.
And I've read on here several times that there are problems doing the leak pressure test with the carb mounted. Those that have heeded this advice usually get the seal quickly and so I see no reason to do otherwise. Those that didn't, well let's just say no one has come forward in the past 3 or so years that I know of.
Just my two pennies.
]Intake leak symptoms (20-30 kicks when normally 1 or 2) made me pull my carb again last night. It's pissing with rain here so I didn't mind. I suspected my o-ring seal was good as I only went through this about 500 miles ago and it had been starting great up till recently. Air test and the old bubbles confirmed my manifold was holding great and I checked thoroughly! But when I was putting my carb on last time, I was noticed that the manifold flange on the body had become distorted over time maybe someone before me had been overtightening with thick gaskets? Anyhow, I didn't need a straight edge to see how the ears around threaded holes had been pulled in and now I'm betting thats where this leak is coming from.
My questions are this: what is the accepted repair for this? (or is there one). I can think of two possibilities... 1 sanding/ machining flat again. Will this only make it more prone to distortion in the future? 2 attempting to press them back to flat, but I am not confident this could be done without some type of damage to either the throttle shaft or body itself.
Also, exactly why CAN'T you pressure test through the carb intake? There must be a reason cause I never hear of anyone doing it. This would be a surefire way to test the carb to manifold seal. I am merely a student in the school of M74B who hasn't got to that chapter yet, please bare with me.
Post by cotten on Nov 4, 2006, 2:33am
Pressuretesting is never performed with the carb installed.
...Cotten
Post by panfreak on Nov 4, 2006, 5:50am
Cotten,
You have said this before so I know you have good reason. Could you elaborate for a student exactly what I would be wrecking by trying this method?
Kevsett,
Cotten didn't say NOT to sand it down, and he has probably been dealing with these carbs since I was pedaling my mustang around in the driveway... well, maybe not quite that long, but nevertheless it looks like me, some wet/dry and a flat surface are gonna battle it out with my carb this weekend unless someone else steps up and has a horror story. Thanks,
Post by cotten on Nov 5, 2006, 2:30pm
It's not that pressurizing the carb hurts it. It's in the way of seeing the entire circumference of the nuts/clamps. And it gets gas all over when it blows out the vents.
All manifold flanges distort.
Four-bolt flanges distort most often because the stacked gaskets and phenolic spacer allow the fastener stress to bend upon them.
The idle gallery in the casting stiffens only one spot, holding that area back as the four "ears" of the flanges are pulled into the spacer.
So yes, they must all be ground flat again, both carb and manifold.
Sanding works, but use a coarse grit. I use a "Polish Mill" (i.e.: A large grinding wheel layed upon its side. It sits still while you run around it with the work.)
....Cotten
Post by panfreak on Nov 6, 2006, 3:48am
Thank you both for your replies.
I opted to use a sheet of coarse wet/dry sandpaper on the concrete shop floor. Backyard machining at it's best, but I was nevertheless cautious and as precise as I could be by checking frequently with a straight edge. Actually, it didn't take much to get it flat again. I had more of a problem upon reassembly getting the fuel line not to leak.... Anyhow, I just finished firing it up: FIRST KICK I might add! Things are back to normal. Funny how you really get a feel for your machine and when something isn't quite right you know it.
I read last night in Palmers book (pg 251) where they describe using a brass punch to straighten ears. I didn't have the balls for that. Could you imagine the worst case scenario?
Post by cotten on Nov 6, 2006, 3:25pm
Worst possible scenario?
I imagine those are the ones where the bike was dropped on them.
It is not unusual to have to massage the ears back a touch with a wooden mallet, and sometimes the flange gets thin from grinding out everything from hacksaw gouges to initials stamped in them, but I've never found one that was un-saveable....yet.
....Cotten
