....I broke the idle screw in a VHB carb. Looking for post-game analysis

[mabajada]

I confess that I broke/sheered the idle speed screw in my VHB29CS carb body.

I was rebuilding the carbs for a '74 Eldorado. I cleaned them (using simple green overnight), cleared out the passage, dried them and put them back together while I was waiting for some parts to arrive. They were jetted 52 instead of 45 and one had a bent needle.

After getting the parts I put one carb together fine. Second carb, I am unscrewing the idle mix screw (smaller) and it is getting tighter, if I screw it in, it remains tight. I try to oil it, and it still gets tighter as I am backing out. Could not figure out why this is happening. There is some aluminum material in the thread of the screw, which I clean and clear out.  I cannot really see the internal threads. Screw goes in and out okay now but a bit tight.

Same thing happens on the idle speed screw. Gets tighter as I unscrew it. Here is where I should have walked away and slept on it. I even thought about doing that, but I was greedy and wanting to get it all together. It gets so tight that I cannot screw it in or out. Just a little more and and I shear the screw.....

If I had a nice drill press or milling machine, maybe I could drill out that broken screw and repair the thread. More likely I am going to make a mess of it trying, so I think it is best to look for a new left carb body?  If you've got one kicking around let me know

So I am trying to figure why this happened. Why would the screw become more difficult to unscrew. I did not have a problem on disassembly or the first assembly. Could the internal threads have broken somehow and started to jam as I was unscrewing growing worse. Why both screws on one carb? I soaked one carb in 50/50 water/simplgreen and the affected carb in full strength simple green. I did rinse and dry them after soaking.

 Does one normally lube the idle screws prior to assembly. I did not so this, though I normally use anti seize when doing engine work.

[rodekyll]

My guess is either a chunk of crud in the threads bound them or threads were damaged somehow.  I'm not familiar with simple green as a carb cleaner, so I don't know if it could have gotten into the metal and swollen/distorted something. 

It might be fun trying to repair the carb body, but I'd rate it more as an arts-n-crafts exercise than a sure bet.  I think you're headed for a replacement.

[normzone]

Yeah, Simple Green is not sufficiently aggressive to injure the carb, so it was probably fated for thread failure.

I can't picture you cross threading it at installation, so likely a fragment of carb body thread was getting ready to separate and you were just lucky enough to be in the right place at the wrong time. Perhaps somebody previously stressed those threads.

[Kiwi_Roy]

Is it the screw that sets the idle speed by lifting the slide, if so you may be able to repair it.
File it flat, center punch then drill it thru with a small pilot drill. Increase to a drill about the same size as the hole, it may catch and run the screw out, what's to lose at this stage?

Since it's such an odd angle I would do it with a hand held electric drill, mount the carb solidly somehow.
If you stuff up the thread you should be able to bush it with something perhaps even JBWeld.

It's hard to see why it should seize like it did.
http://www.thisoldtractor.com/moto_guzzi_loopframe_carburetor_rebuild_-__vhb.html

[Peter from Sch'dy]

This from SG website:

Is it safe to use Simple Green All-Purpose Cleaner on aluminum?
When used with caution and according to the instructions, Simple Green All-Purpose Cleaner has been safely and successfully used to clean aluminum. Simple Green All-Purpose Cleaner, Crystal Simple Green Industrial Cleaner & Degreaser, and Simple Green Pressure Washer Concentrates have been used on aircraft, automotive, industrial and consumer aluminum items for over 20 years. However, caution and common sense must be used: aluminum is a soft metal that easily corrodes with unprotected exposure to water. The aqueous-base and alkalinity of Simple Green All-Purpose Cleaner can accelerate the corrosion process. Therefore, contact times for unprotected or unpainted aluminum surfaces should be kept as brief as the job will allow - never for more than 10 minutes. Large cleaning jobs should be conducted in smaller-area stages to achieve lower contact time. Rinsing after cleaning should always be extremely thorough - paying special attention to flush out cracks and crevices to remove all Simple Green product residues. Unfinished, uncoated or unpainted aluminum cleaned with Simple Green products should receive some sort of protectant after cleaning to prevent oxidation.


Best,
Peter

[Guzzer]

I would spray with penetrating oil--several times over the next day or so, and then drill a small pilot hole in the center.  Tap it with a center punch several times to shake things loose.  Then use reverse thread drills going up in size until you are getting too close to size of screw.  Then try a craftsman easy out.
I use a reverse thread set of drill bits from Harbor Freight but recommend something of higher quality if you can find them.

[mabajada]

Thanks.
I realised what else i did wrong.

I got fascinated with cleaning rusty parts with muriatic acid. And then neutralising with a baking soda solution. I did a trial with some rusty bolts, and threw in these idle screws as they were corroded. Its probable that i did not fully neutralize the acid. Then they were assembled into the carb, sat for a week. Thus reacting with the aluminum threads.

I'll have to look up how HCl reacts with aluminum

[pete mcgee]

I have seen common simple green cause serious corrosion in aircraft.
They also have an aircraft specific cleaner that doesnt cause corrosion that is the best cleaner I gave ever used.
Acid eh, well if it sat for a week that was never going to end well.
I would soak the area in a penetrant for a few days and try guzzers approach.
Good luck with it.

[mabajada]

What confused me was that as I screwed them in or out, they got progressively tighter. It was as if the threads were getting tighter locking the screw in. Perhaps the thread wall wa moving or sticking, increasing friction as I moved the screw. I feel a bit better figuring out what went wrong.

So, with the simple green, any recommendations for carb cleaners?

Also, anyone have a source for VHB carb bodies? other than continually checking in on the swap meet, and ebay?

[rodekyll]

I acid wash aluminum prior to paint/powder coating.  It is VERY interactive with the aluminum, etching and discoloring it rather quickly.  It gets HOT with contact and produces vapors that corrode any metals close and burn skin and eyes. 

[normzone]

Yeah, Simple Green is not sufficiently aggressive to injure the carb, so it was probably fated for thread failure.

Oh. So I was incorrect - and acid was involved as well.

Okay, I retract my previous statement, and replace it with " You are so cruel to your carbs - they gave up out of sadness "

 

[Don G]

mabajada: Contact Wolfgang @ Columbia Car & Cycle in Nakusp 250-265-4502 he is the Laverda go to guy, I am sure he can help you out with your dillema. DonG

[Farmer Dan]

I soak my carbs in "spic & span" floor cleaner. Works good and I have never had a problem with it.  I save the acid for my batteries.