Carb Rebuild Mystery Quiz

OK Folks,

I had an issue with my XJ650 (1981, Hitachi) carbs which took me a while to figure out and decided to post what I discovered here for anyone else's benefit to avoid the time I spent on this "issue" with the diaphrams and the vacum pistons...

I'll make this a quiz:

I've followed all carb cleaning and polishing instructions zealously (total break down of all 4 carbs, etc.) -- and have totally reassembled the carbs -- (with many new parts, all clunking OK, etc. -- but did not replace vacum pistons or diaphrams.)

Carbs back onto the rack and was lifting/dropping each plunger alone and two-togther to compare speeds and sounds trying to ensure everything was healthy and moving swiftly and freely...

On carb #1, just after the plunger falls (and clicks at the bottom of the fall), there was a "wheez" or the sound of "air" slowly escaping from somewhere. This "wheez" or "air leak" sound would last for perhaps 2 seconds after the plunger fell... I thought "Where the heck is air trapped, and from which passage is it finding its way out slowly?"

If air was trapped somewhere:

- upper diaphram?
- squeezing slowly around the piston from below the diaphram?
- space below the hat?
- needle compressing air into the bowl?

In any case I thought "darn -- some passage is not totally clean!!!" (Which means I have to break all this work down again!!)

...

I could drop a hint here, but will wait for a few guesses -- What was the sound?

 

+Between the diaphragm and the carb body, the air vents out the oval port.
Put your thumb over the oval port and the piston plunger drops much slower.

 

VERY logical guess -- but - Not the issue

 

What happens when you Pinched the Diaphragm between Carb hat and body for 400 Alex?

 

air slowly entering your lungs because you held your breath in anticipation of your clunk test results

 

THAT'S IT -- I was waiting to exhale! -- NOT, but funny as hell!!

The carbs were FULLY assembled -- clunk is done with hat's off

 

It was the diaphram spring...

On some of the carbs, for some reason, when the piston snaps to the bottom it sends a reverberation up the diaphram-piston spring (just under the hat) -- and to my ear it sounded like a residual, pressure-based air leak from inside the carb "somewhere".

I didn't discover it till I tapped the carb-body (lightly) with the plastic end of a screw driver handle and reproduced the sound on carb #1 (in this case).

Anyway, I wasted time disassembling #1 re-cleaning the piston bore and pressure-testing passages, etc - NOT necessary -

Thought this explanation might save somone time in the future.

Thanks,

 

Did you carefully inspect the diaphragms to ensure you don't have a pinhole somewhere?

Not disagreeing with the "singing spring" just checking.

 

Thanks Fitz

I spent a lot of time reviewing them and putting Chacal's "rnr" on them - I couldn't see any pin holes via the "back-light" procedure

Thanks

 

This is not about faith.

The slides are lifted by pressure differential.
The clunk test confirms many things. (friction, seal and air entry)

A properly cleaned and bench synced carb rail will start any functional engine.

 

Amen! Oh I'm sorry...

 

But wait - the Clunk Test is done with the "hats" off and springs out. Any other testing is just messing around to check overall functionality.

this "thumb test" of mine proves that you didn't fold over the diaphragm during installation, like I did once. Doing that (folding) basically de-activates that carb; it idles, but doesn't run.

 

I'll do it - thanks