Carb Diaphragms Leaking Around the Edges. Easy fix.

[Orange4]

As I was cleaning my Carbs ('78 Standard) I did the test before disassembly in which you block the oval opening in the intake of the carb body and push up the piston. All but one of the pistons dropped quickly which, according to the information on this site, indicate damage to the diaphragms. I found an issue which seems to explain their malfunction.

On disassembly, I found the diaphragms to be in perfect shape. Not a single pinhole. So I took a look at the carb body and noticed that the flange which mates with the diaphragm cover was poorly finished at the factory, leaving some flash which would interfere with the cap when tightened down. This would leave the diaphragm loose in it's groove and allow air to pass.

My fix was to file down the flash and run a small bead of RTV around the groove just before assembly. Now the pistons fall nice and slowly as they should. No new diaphragms needed.

[Schming]

Yep, I did a similar treatment to my carbs. I smoothed the sharp corner on the inside, where the diaphragm seems to wear out, as that area is like a dull knife on the rubber that is constantly working back and forth on it.

I also used a coating of silicone grease, around the circumference of the body, where the lip of the diaphragm sits.

I want my new diaphragms to last 60 years, 'cause I know the engine will.

[Orange4]

Silicone grease is probably a better solution. I would have used it if I had it.

I didn't think of the inner lip/rubber interference but I can still do it on my '79 when I get to those carbs.

Cool.

[James England]

An excellent post. In my carb rebuild tonight, I found one slide dropping far faster than the other three. I'd already checked all diaphragms with a torch and they are absolutely perfect with no pinholes or tears or worn creases. "Strange", I thought..... Then I checked in and read your post. I found tiny burrs on the top of the main carb casting..casting defects. I applied a thin smear of silicone gasket compound to the carb body and the top lid. The slide immediately performed like all the others......

Very helpful, that one, so thank you for posting...

[Orange4]

Thank you, kind sir. I am very happy to report that I reinstalled them on the bike tonight. i followed the cleaning and pre-setting instructions and it fired up nicely. After fiddling a bit with the idle speed and the idle mixture on #4 it's running like a champ. Have not taken it on a ride but it revs up smoothly and instantly with no hesitation. Idles in good sinc. Still have to tune the idle mix but so far so good.

Virtual high five ||||/

[Orange4]

I should say, my symptoms were:

Relatively good idle but only with the choke pulled.

Massive stumbling and stalling and sputtering when attempting to rev the engine.

Visible differences in the slide's movement when applying throttle.

What I found during the rebuild:

Tight screws removed after spaying penetrating oil overnight and turning with a with a ratcheting mini wrench and new bits. Tapped the bits into the screw heads with a small hammer before applying heavy pressure to the bit and turning carefully. Tapping the bit in was the real screw saver and really worked well.

Idle screws ranged from 3/4 to 2+3/4 turns from seated. (Turned out 1+1/2 turns as pre-adjustment)

Butterflies out of sync. Used a single piece of thin copper wire stripped from a 14 gauge stranded automotive wire

Casting flash caused the diaphragm's rims too be too loosely held in their grooves by the carb caps. My solution was to file the tops flat and run a small bead of black RTV around the grooves before assembly

Diaphragm springs slightly mismatched in length. Stretched them to match at about 5mm longer than the longest one.

One float out of adjustment by 1mm and partly covered in a flaky whitish film which I wire brushed off with a brass brush.

One pilot jet screw driver slot was partially stripped but a freshly sharpened and fitted screw driver got it out fairly easily. Whew!

Otherwise pretty clean on the inside with no signs of rust. The outside of the carbs and the throats were pretty grotty though.