Diaphragm failure modes?

[Bagger John - #3785]

No, this ain't like "the rubber broke" but it was darn near as painful dealing with the aftermath. 

Went for a nice, leisurely ride to the middle of BFE yesterday afternoon with one of my riding cronies. After a water and BS stop, we fired up and continued our ride...except my Tourer didn't.

Out of fuel within a couple of miles. Had half a tank remaining (of an I/S tank, mind you).

AMA Roadside/flatbed called at 4:30PM. I didn't make it home until 10ish.

Today I began troubleshooting. First thing to look at was the petcock - and it wasn't holding vacuum when checked with a vacuum pump. Tank removal and inspection on the bench confirmed no fuel flow. Petcock disassembly revealed a slight irregularity in the diaphragm, so I rebuilt the thing and got fuel flowing once again.

The big question: I rebuilt this particular petcock 2k miles ago due to a low fuel flow condition, using the Honda kit. What would cause one to fail in such a short time - luck of the draw, QC issues, backfire up the vacuum line, aliens, ?

Tank is still off the bike, so any fail-proofing suggestions (short of replacing the petcock altogether with a Pingel) are welcome.

[98valk]

just a defective one.

I put a snubber in the vacuum line of mine. at 15k the original had failed. rebuilt with OEM and installed the snubber. 54k miles now, no problems. Thank God!!!

[longrider]

Although I've never had a diaphragm fail that quickly I've come to believe we are not taking enough care when attaching the internals together and causing a tear in there.  The rubber is very fragile

[98valk]

Although I've never had a diaphragm fail that quickly I've come to believe we are not taking enough care when attaching the internals together and causing a tear in there.  The rubber is very fragile

actually diaphragm is a piece of cloth covered on both sides with rubber, most likely Buna-N.  Now the carb-slide diaphragms are just Buna-N and no cloth.

[da prez]

  Possible fueled up with a fuel that contained more ethanol or had a addative that reacted to the diaphragm. We get in a lot of bikes (and out board motors) that have the old (now) fuel system with the rubber swollen and internals deterated. 

                                da prez

[DeathWishBikerDude]

See the Pingel thread.

[RonW]

anybody know the max pressure (hg) you should put on the vacuum line when testing fuel flow? Also, I'd like a longer vacuum line but what material is that tubing?

[Bagger John - #3785]

Although I've never had a diaphragm fail that quickly I've come to believe we are not taking enough care when attaching the internals together and causing a tear in there.  The rubber is very fragile

I do the surgeon bit when assembling these things. Gentle, no tools, minimal force.

And:

just a defective one.

I put a snubber in the vacuum line of mine. at 15k the original had failed. rebuilt with OEM and installed the snubber. 54k miles now, no problems. Thank God!!!

I had to go with Q-C-D (quick, cheap, dirty) as the bike was apart and I needed to be back on the road when I started this thread.

My local Autozone had a Dorman Vacuum Restrictor in stock (P/N 47311). It has a 0.010" orifice and the barbs are 3/16".

The vacuum fittings on the #6 intake runner and on the petcock are around 0.190", measured. So...the tubing must stretch a bit to go over the Dorman part. After I cut the OEM vacuum hose about 3" from where it attaches to the petcock fitting, I lubed the barbs up and was able to get the hose to fit over them with a little effort. Not a huge amount, mind you...but this thing isn't coming back out.

Tested with a gauge, all looks good. But...how prone is that hose to cracking with a little stretch?

[98valk]

Although I've never had a diaphragm fail that quickly I've come to believe we are not taking enough care when attaching the internals together and causing a tear in there.  The rubber is very fragile

I do the surgeon bit when assembling these things. Gentle, no tools, minimal force.

And:

just a defective one.

I put a snubber in the vacuum line of mine. at 15k the original had failed. rebuilt with OEM and installed the snubber. 54k miles now, no problems. Thank God!!!

I had to go with Q-C-D (quick, cheap, dirty) as the bike was apart and I needed to be back on the road when I started this thread.

My local Autozone had a Dorman Vacuum Restrictor in stock (P/N 47311). It has a 0.010" orifice and the barbs are 3/16".

The vacuum fittings on the #6 intake runner and on the petcock are around 0.190", measured. So...the tubing must stretch a bit to go over the Dorman part. After I cut the OEM vacuum hose about 3" from where it attaches to the petcock fitting, I lubed the barbs up and was able to get the hose to fit over them with a little effort. Not a huge amount, mind you...but this thing isn't coming back out.

Tested with a gauge, all looks good. But...how prone is that hose to cracking with a little stretch?

using a vacuum restrictor will not allow the diaphragm to fully open, resulting in lean conditions thereby providing a viewing window of the rod in the top of the piston. 

[longrider]

Reading your last post I assume you want to remove the vacuum pulse at the diaphragm.  This would be better done with a small home made canister placed in line

[Bagger John - #3785]

...using a vacuum restrictor will not allow the diaphragm to fully open, resulting in lean conditions thereby providing a viewing window of the rod in the top of the piston.  

Tested, it allows good fuel flow. Using a handheld pump and watching its gauge, the vacuum comes up smoothly and is maintained after pumping ceases - as is the flow.

The orifice isn't all that tiny, mind you - there's still ample draw through the tubing. It acts more like a damper (or snubber) than a limiter (restrictor). The idea being to minimize the effects of sharp vacuum pulses. Many of the mechanical carb-sync gauges I've encountered use a similar arrangement in the brass tubes which thread into the intake runners. You can still draw full vacuum, but the pulses are damped somewhat.

I'll take the bike out to a nearby freeway and do some full throttle work to see if I lose top-end power. Should this happen I'll remove the part and look at other options.

[Pete]

Make sure that the air vent on the petcock is not stopped up. Often I find that a dirt dauber packs them with dirt.

[98valk]

...using a vacuum restrictor will not allow the diaphragm to fully open, resulting in lean conditions thereby providing a viewing window of the rod in the top of the piston.  

Tested, it allows good fuel flow. Using a handheld pump and watching its gauge, the vacuum comes up smoothly and is maintained after pumping ceases - as is the flow.

The orifice isn't all that tiny, mind you - there's still ample draw through the tubing. It acts more like a damper (or snubber) than a limiter (restrictor). The idea being to minimize the effects of sharp vacuum pulses. Many of the mechanical carb-sync gauges I've encountered use a similar arrangement in the brass tubes which thread into the intake runners. You can still draw full vacuum, but the pulses are damped somewhat.

I'll take the bike out to a nearby freeway and do some full throttle work to see if I lose top-end power. Should this happen I'll remove the part and look at other options.

a snubber and restrictor are two different things.  snubber only reduces strong pulsations like the diaphragm sees during shifts.  restrictor reduces pressure/vacuum in the line, does nothing for pulsations for its not designed for that. Enjoy!

[Jruby38]

I ended the leaking POS OEM 15 years ago with a Pingle. Not a problem since. Buy once, cry once.

[Bagger John - #3785]

Did some high-speed (near 6-point) extended testing runs yesterday, mixed in with some 3K RPM roll-ons in top gear at freeway speeds.

It's not hurting for power.

Maximum vacuum can still be drawn through the system, just not at the same rate as with an unrestricted line. This leans more towards "damped" instead of "restricted", which was the intended result.

I'll do a plug read later to verify high-speed fueling.

Ultimately, the fix will be to convert all the petcocks to manual.