Hi Junky. Thanks for the post! I just looked at the updraft carb bolts of my '28 to fully understand your question. Turns out my bolts were loose, so, thanks to you, I found this and tightened them up! 😉

I've read elsewhere that the fastest penetrating oil is a mixture of automatic transmission fluid and acetone (50/50). I like and use JB Blaster.

As to how the oil actually gets to the tightened threads, here's how I would approach it:

1. Take a small wire brush and try to clean out the bottom of the bolt holes from the underside of the carb's mounting face. The idea is to clean out crud that covers the end of the bolts, thus exposing them for oil penetration. Maybe squirt some WD40 into those bolt holes too. It is a good 'cleaning' spray.

2. Then,  with a penetrating oil in a spray can (like JB Blaster) that has a flexible hose on the nozzle,  spray some on the underside of the bolt holes in the bottom of the carb attachment surface. Also, put some penetrating oil around the bolt heads.

3. Tap on the heads of the bolts a little with a small hammer (to jar the threads) and then let the oil soak for a few hours.

4. Try, ever so gently, to TIGHTEN the bolts just a hair. This action starts the threads of the bolts to slide inside of the threads in the carb face.

5. If the bolts move a tiny bit, loosen them just a hair and work them back and forth. Occasionally, put some more penetrating oil on the heads of the bolts. As the bolts go back and forth, the oil should work it's way down the threads with gravity helping.

Let us know how you finally were able to get them out.

Cheers, Dean 

Thanks Dean.  Your finding of your loose bolts makes me feel better before I do my future attempt.  I've worked in the automotive environment 1969-77 (Dealerships) and my older brother, now retired started out as a bedroom/driveway mechanic (wack-o story) to eventually end up a GM service manager for many years.  During all of this...I have never heard of the 50/50 tranny fluid and acetone.  But with "my" following 37 years as a designer for professional engineers (architectural, mechanical, electrical, chemical), I can see that formula and its penetration.  Yes, the Blaster would seep in quicker (viscosity), but your mixture is also a great solution.

Yes, I agree.  Cleaning the exposed areas is always beneficial before the solution is applied.  And, with any thoughts of a possible seized bolt, one should always first try to tighten, slightly, then reverse to unlock.  And yes, additional solution applied often during the process of working the seized bolt back and forth to disengage.  And finally, once that "seized bolt" becomes disengaged, replace the bolt new (apply proper thread sealant for security and future removal ease).

Since my scenario has inverted bolts into tapped holes, once applying the first attempt as you outlined above, I will be saturating small pieces of cotton balls with the solution and pressing the saturated cotton balls against the bolt/hole engagement and securing the c-balls in place.  Days later would then be my first attempt of bolt removal.   I will update all of this when completed.  Hopefully all goes well.

P.S.  before I got back to your response to my main question posted, I called my brother who also recommended JB Blaster.

While we are talking about solutions for stuck bolts, I have another 'tip' to share.

A friend of mine, Andy Shyers, is the Teacher of the Auto Shop at our local high school. His 'go to' solution is to heat the area around the threads with a small propane torch and then melt some of the end of a candle (paraffin) where the bolt goes into its threads. The wax wicks its way into the threads and lubricates them so the bolt will turn the threads and not have the head break off.

If course, this is NOT a solution for anywhere around a carb where there is gasoline nearby. ;-0

When I rebuilt my '28 engine in Lurch (see link below) in 2017, the rusty bolts holding the thermostat cover on broke in the middle leaving a piece of the bolt sticking out of the block. I heated the block around the broken bolt stem, applied some candle wax, and then grabbed the bolt stem with vice grip pliers. The bolts unscrewed nicely from the block! 😉 

Since these particular bolts are exposed to the radiator fluid inside the thermostat cover, when I put the engine back together I used 3/8-16 brass bolts instead of steel and painted the bolt heads semi-gloss black so they wouldn't draw attention.

Cheers, Dean

PB is good.   I’ve had really good results using a 50/50 mix of acetone and automatic transmission fluid.