Linkert M74B

[T. Cotten]

Cotton I did the leak test by spraying soapy water all around the manifold and fittings no bubbles, is that what you call the bubble test?

Did you use a constant, regulated air supply of no more than 15psi, Laurie?

....Cotten

[Laurie]

But you did not bubble-test, Laurie?



A bubble-test would eliminate the need for assumptions.



Please review http://virtualindian.org/11techleaktest.html

(Even the Seal-Lock product has its limitations, so most use original JBWeld, although it should cure a couple of weeks at least for fuel resistance.)

.....Cotten
PS: Please note the safety warning about using ether.

(I worked with a chemist badly scarred when it ignited off of a steam radiator; Hard to believe, but believe me, the scars were real.)

Cotton I did the leak test by spraying soapy water all around the manifold and fittings no bubbles, is that what you call the bubble test?

[T. Cotten]

...after no seen leaks during leak test thought I'd try spraying the inlet nipples to heads area with aerostart, (ether based) thought it would pick up at idle if sucking air, but no change.
Even tried another 74B which was rebuilt years ago for my '65 and never run, same problem!

But you did not bubble-test, Laurie?

...So it starts easily, Idles ok and revs cleanly but after a 5mile run the front cylinder plug is fouled and the rear plug just a little lean, all this with the low speed needle 5 turns out and the high speed needle shut off, 1/2 turn out or 1 or 2 turns out no difference
So am i right to assume that the rear nipple must be sucking air keeping that cylinder lean?...

A bubble-test would eliminate the need for assumptions.

...what would you use to seal the nipple threads if thats the case?
Looks like thats where I'm heading next, unscrew nipples and re seal?....

Please review http://virtualindian.org/11techleaktest.html

(Even the Seal-Lock product has its limitations, so most use original JBWeld, although it should cure a couple of weeks at least for fuel resistance.)

.....Cotten
PS: Please note the safety warning about using ether.

(I worked with a chemist badly scarred when it ignited off of a steam radiator; Hard to believe, but believe me, the scars were real.)

[Laurie]

So Cotton
Where do they usually leak from?and why? bad fitment or to big a gap for the o rings?

Cheers & Beers

Ok finally got back to the Knuckle, sorted a few more issues out some relevant to issues but some not.
found I had a bad plug lead front cylinder so replaced and tested both, all good
Tried another known coil, no change
found faulty fuel rod, replaced with Carl's Peek tipped rod, all good
after no seen leaks during leak test thought I'd try spraying the inlet nipples to heads area with aerostart, (ether based) thought it would pick up at idle if sucking air, but no change.
Even tried another 74B which was rebuilt years ago for my '65 and never run, same problem!

So it starts easily, Idles ok and revs cleanly but after a 5mile run the front cylinder plug is fouled and the rear plug just a little lean, all this with the low speed needle 5 turns out and the high speed needle shut off, 1/2 turn out or 1 or 2 turns out no difference
So am i right to assume that the rear nipple must be sucking air keeping that cylinder lean?
what would you use to seal the nipple threads if thats the case?
Looks like thats where I'm heading next, unscrew nipples and re seal?
all help greatly appreciated
Cheers & Beers
Laurie

[T. Cotten]

No the opposite, I mean it is seasoned. Now once they are straighten out, it'll stay in formation longer than it did way back when it was new...

A tiny bit, Duke.

But they can still warp over and over and over.

My inspection is the same as the remedy: I swipe them criss-cross on a very large stone.

POLSHMIL.jpg

When both carb and manifold are ground, they almost 'wring' to zero daylight.
(First pic I found that shows my criss-cross cross-hatch)...

BOREGRND.jpg


Just one installation, and there's daylight. Metal moves.

....Cotten

[dukekleman]

No the opposite, I mean it is seasoned. Now once they are straighten out, it'll stay in formation longer than it did way back when it was new.

Hope this helps,
Duke

[T. Cotten]

All metals and plastics change with heat, torque and vibration.
Cryogenics is another process used.
Once those parts have all been seasoned, they tend to hold their dimensions much longer. Unless you sand to much off and thin it down...

You mean, Duke,...

Our half-century and older hardware ain't seasoned?

Even if it was kept outside?

....Cotten
PS: We all agree, seasoned or not, metal moves!

[dukekleman]

All metals and plastics change with heat, torque and vibration.
Cryogenics is another process used.
Once those parts have all been seasoned, they tend to hold their dimensions much longer. Unless you sand to much off and thin it down...

[T. Cotten]

It's the heat and torque combination that I mentioned earlier that does all the damage.
The torque applied correct or incorrect combined with the heat realign the metallurgy and cause issues, just like all other components in the engine and transmission. The term "green" , "seasoned" often get thrown into the conversation.
Once the initial run in has occurred , you'll always notice it doesn't occur like it did to the extent.

Hope this helps,
Duke

We are talking cast steel and cast iron manifolds, Duke,...

Plus insulating gaskets to the bronze carb flanges.

Do you really think a couple of hundred degrees makes the flanges warp?
I will give vibration some credit!
But its the gaskets that give the flanges room to move with fastener stress.

(Bronze actually becomes more malleable when chilled, and 'brittle' when heated.)

....Cotten
PS: Aluminum manifolds warp the same way.

PPS: Shouldn't the carb/manifold junction be cooled by the incoming air, plus vaporizing fuel?
(I've heard legends that they could ice up!)

[dukekleman]

It's the heat and torque combination that I mentioned earlier that does all the damage.
The torque applied correct or incorrect combined with the heat realign the metallurgy and cause issues, just like all other components in the engine and transmission. The term "green" , "seasoned" often get thrown into the conversation.
Once the initial run in has occurred , you'll always notice it doesn't occur like it did to the extent.

Hope this helps,
Duke

[T. Cotten]

Cotton,
Heat Transfer/soak from it's encapsulated environment.
most damaging rises seen after shutdown.

If your carb and manifold get hot enough to distort, Duke,...

They are on fire!

....Cotten
PS: Fastener stress alone is enough.

[dukekleman]

Cotton,
Heat Transfer/soak from it's encapsulated environment.
most damaging rises seen after shutdown.

[T. Cotten]

Yeah, along those lines.
Certainly over-torquing and heat are the culprit causing major failures.

Duke

Where's the heat come from on a carb flange, Duke?

...Cotten

[dukekleman]

Yeah, along those lines.
Certainly over-torquing and heat are the culprit causing major failures.

Duke

[T. Cotten]

Like this, Duke?

FLNGDST3.jpg

flngprs3.JPG FLNGPRSS.jpg

I found that 13 ft-lbs is enough to distort the flanges:
FLNGTST.jpg

One bright customer zipped two of his carbs on with an impact, and even broke the castings at the bushings!
JWCRACK.jpg

....Cotten
PS: A late note on the importance of assembly torque:
When I honed bores clean, it was always torqued to a plate. If a disc is cut to that exact bore, it will not install to zero daylight unless the plate is re-applied, as if installed to a manifold!

OEM discs seem to have been produced intentionally sloppy and undersized.