XS650 Carburetor Metalurgy ...or... "What is this mystery potmetal"

[TwoManyXS1Bs]

XS650 Carburetor Metalurgy ...or... "What is this mystery potmetal"

I've seen this question occasionally, "What casting metal is my carb?".
This can be important to know, if considering aggressive cleaning agents, or fixable repairs.
Without doing extensive chemical tests, it's possible to determine the likely metals in a Non-Destructive Test (NDT), by finding the metal's Specific Gravity.

40 miles north of here is the beginning of large areas of calcite, and mixed-in with all those calcite stones is the jewel of central Texas - Topaz. Topaz and calcite are virtually indistinguishable, near impossible to separate one from the other. However, Topaz has a slightly higher Specific Gravity compared to calcite. So, about a dozen years ago, I built a simple weighing system to help me sort and find precious stones.

The portable sealed field kit.


Opened up to reveal the precision digital mini weighing scale, specimen basket and water tub.


The specimen is placed in the basket, weight is recorded.
Then a lever is pulled, bringing the water tub up, submerging the specimen.


The specimen's submerged weight is recorded. Subtracting this weight from the specimen's weight gives the weight of the volume-displaced water.

Dividing the specimen weight by the displaced water weight gives the Specific Gravity.

Easy, huh?

 

[TwoManyXS1Bs]

So, we'll use this principle to find the Specific Gravity of the various carburetor castings.
These parts are too big and heavy for my field kit, so used different scales.
Note: This is my 71 XS1B carburetor. Yours may be different.

The Carburetor Body is suspended from a weighing scale, and the weight is recorded.


Dry weight of the Carburetor Body = 670g


Then, the Carburetor Body is submerged into a pail of water, and the weight is recorded.
(A couple drops of dishwashing soap added to the water reduces surface tension and improves wetability. The submerged part is also agitated to eliminate trapped bubbles.)


Submerged weight of the Carburetor Body = 570g

Displaced water weight = (670g - 570g) = 100g

Specific Gravity of the Carburetor Body = 670/100 = 6.7


Float Bowl

Dry weight of the Float Bowl = 93.7g
Submerged weight of the Float Bowl = 58.8g
Displaced water weight = (93.7g - 58.8g) = 34.9g
Specific Gravity of the Float Bowl = 93.7/34.9 = 2.7


Diaphragm cover

Dry weight of the Diaphragm cover = 50.4g
Submerged weight of the Diaphragm cover = 32.0g
Displaced water weight = (50.4g - 32.0g) = 18.4g
Specific Gravity of the Diaphragm cover = 50.4/18.4 = 2.7

Not shown: Starter/enrichner housing
Specific Gravity of the Starter/enrichner housing = 6.7

 

[TwoManyXS1Bs]

Summary of results:

Specific Gravity of the Carburetor Body = 6.7
Specific Gravity of the Float Bowl = 2.7
Specific Gravity of the Diaphragm cover = 2.7
Specific Gravity of the Starter/enrichner housing = 6.7

Using this chart, we can take a pretty good guess at determining the mystery metals.
Should be easy, since most carburetor castings are either Aluminum or Zinc alloys.



I would say that:

Carburetor Body = Zamak (zinc alloy)
Float Bowl = Aluminum
Diaphragm cover = Aluminum
Starter/enrichner housing = Zamak (zinc alloy)

Knowing the metals involved, better choices can now be made concerning:

- Aggressive (acidic or caustic) cleaning chemicals
- Repair methods (welding, gluing, cutting)
- Surface finishes (polishing, painting, pickling, anodizing)

Whutcha think?

 

[grizld1]

I think you do good work, and take nothing for granted, TwoMany! The scratch test isn't nearly as cool as your specific gravity method and would involve some risk of sample damage; but since calcite's a whole lot softer than topaz, wouldn't a scratch test be quicker and easier? Maybe for the carb materials too if you knew where zamak and aluminum fall on the Moh scale; just a matter of elimination, since we know that Mikuni used only aluminum and zinc alloys in carb bodies. Anyhow, thanks for a great read!

 

[angus67]

Isnt there steel inserts for the throttle shafts embedded in the carb body?
That's great to know how to do that. You made it very understandable. thanks.
Why would they use zamak for the carb piece? easier to cast? harder? cheaper?

 

[arcticXS]

Very good writeup Twomany!
I have noticed that while some VM carbs are fairly heavy, like the RD 350 VM28, and Suzuki T500 VM32, some snowmobile carbs are very light, even in the bigger bodied VM36.
I wonder why some engine manufacturers choose alloy Mikuni carbs, while others use zinc ?

 

[TwoManyXS1Bs]

...since calcite's a whole lot softer than topaz, wouldn't a scratch test be quicker and easier?

Griz! Shhhhh! Yer lettin' out our secrets!
Yes, but it takes a trained eye/hand. Difficult to explain.
Then, there's the secret 'third' test.
C'mon down sometime for a "scratch & sniff" party.

... Anyhow, thanks for a great read!

Yew betcha. I was hopin' that the kids who grew up on "Mister Wizard" would enjoy it.

Isnt there steel inserts for the throttle shafts embedded in the carb body?

Yes. Caught that, didja? I found them in my 71 carbs. Don't know about other carb years, though.
It could influence those cleaning chemical choices.
There's also small brass parts buried in those castings too.

Why would they use zamak for the carb piece? easier to cast? harder? cheaper?

Back then, the Zamak offered some superior benefits compared to other alloys of the time, like what you mentioned, plus reduced porosity and longer life of the casting molds. The websites that offer Zamak give more info. It gets deep...

 

[TwoManyXS1Bs]

Very good writeup Twomany!
I have noticed that while some VM carbs are fairly heavy, like the RD 350 VM28, and Suzuki T500 VM32, some snowmobile carbs are very light, even in the bigger bodied VM36.
I wonder why some engine manufacturers choose alloy Mikuni carbs, while others use zinc ?

Thanx, ArcticXS.

I wonder too. I recall porosity issues with some aluminum carbs of the '60-'70s, and some transitional alloy changes to improve pourability (silicon) for engine parts. Also notice that the VM carb bodies don't have as much geometric complexity. And, casting alloys have evolved since then. Given all that, I can see where the material choices are different nowadays...

 

[angus67]

soooo...wouldn't the inserts in your carb bodies, as well as the copper bb's that plug passage ends make the body heavier than it really is?
I have a later years set of 38's that the body looks to be cast with zinc in it. I can see a crystalline pattern in the metal. I assume its zamak?

 

[TwoManyXS1Bs]

soooo...wouldn't the inserts in your carb bodies, as well as the copper bb's that plug passage ends make the body heavier than it really is?

Yep. But a negligible amount. That thing weighs 1-1/2 pounds!

I have a later years set of 38's that the body looks to be cast with zinc in it. I can see a crystalline pattern in the metal. I assume its zamak?

Probably. If it weighs close to 1-1/2 pounds, I'd say it's Zamak.
An Aluminum body of equivalent size would weigh about 9.5 ounces...

 

[robinc]

Holy post count Batman.

I can't believe just over a year ago I first dipped my toe in the waters here with such trepidation.
And now I'm at my 500th post.

Wanted it to be in a thread of 2M's.

Seemed fitting that it should be in a Carburetor thread.
Thanks man!