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lancemb

Excessive fuel pressure with rebuilt fuel pumps?

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Remove the airhorn from the carb and turn it upside down with the needle and float still in place. Blow into the fuel supply fitting. See what happens.

Bernie

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It took me a long time to figure that trick out on my own, Bernie.

Definitely take the top off first though. It tastes funny and makes your tummy hurt if you don't.

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Ok, Bernie I'll bite.  What does the "book" say and what point are you trying to make blowing into an upside down airhorn section other than demonstrate the integrity of the float valve??  :huh:

Two years ago I was troubleshooting a fuel delivery problem I had in 95* + and elevation of 6,000 ft + (it was vapor lock that was not resolved by my electric pump after extended use).  Anyhow pressure tests showed:

1) electric pump only before the mechanical pump = 9 psi (Airtex 5-9psi pump)

2) mechanical pump only at the carb = 5psi

3)electric pump only (but pumping through the mechanical pump) at the carb, engine not running = 5psi

4)electric pump with mechanical and engine running =5 psi

So the mechanical pump will regulate pressure with the valves and the diaphragm with the spring under it.

1955 had two pumps with the later rated at a higher pressure and the only internal difference is the size of the valves and a different spring assembly under the diaphragm (same diaphragm used)

The stroke volume is limited by the cam that the pump rides on.

The pressure would be regulated by the valves and the spring under the diaphragm.

More to  come after I test an upside down airhorn with a pressure gauge (where I have we heard that before  :rolleyes: )

Willie

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The needle valve should make a nice tight seal. Most have a Viton tip, but even the brass seals well.

I read the book in 1959 and maybe three times since, not counting a couple of days ago. The basics plus the intervening years of actually working on the stuff kind of develops an intuitiveness. 20 years ago, when I did the AFB on my Riviera I had a bunch of parts from a 1958 Lincoln AFB in the pie tin. They were a little different but if they looked bigger or better I put those in. Wait until the next owner finds those!

 

I don't do any of the things "real" hobbyists do; lay the cars up in winter, stabilize the fuel, fight the dreaded vapor lock, switch to radial tires, use zinc additive oil, search for boat gas pumps, or run a degausing line stem to stern. Yeah, I blow in carburetor passages and cut the tags off mattresses, but the stuff works (and you know I wrote "stuff" but wasn't thinking it.).

 

All you need is a few special tools and a good helper.

MikuniFantasiesAdHiRes.jpg

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So they took the carb apart and said it looked pretty varnished. Not too surprising. I'll probably know more next week!

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{Remove air horn, intact. Invert and pressurize fuel feed passage in carb orally.  Breathe through nose ONLY!}

 

Even with a good lever arm for mechanical advantage, I don't really see the float (in the above scenario) being heavy enouth to keep the needle in place when the passage is orally pressurized more than a "sigh".

 

The Carter AFB and AVS Strip Kits had a few different needle/seat assemblies, with different orifice sizes.  An "oversize" needle/seat orifice would slow down the fuel's speed in the line, allowing it to accumulate more under hood heat, which might cause other issues of vehicle performance at lower rpms and fuel needs.

 

Hope y'all weren't doing that with leaded fuel!

 

NTX5467

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Maximum fuel pressure is determined by the diaphram SPRING, the other components only determine flow rates.

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{Remove air horn, intact. Invert and pressurize fuel feed passage in carb orally. Breathe through nose ONLY!}

Even with a good lever arm for mechanical advantage, I don't really see the float (in the above scenario) being heavy enouth to keep the needle in place when the passage is orally pressurized more than a "sigh".

The Carter AFB and AVS Strip Kits had a few different needle/seat assemblies, with different orifice sizes. An "oversize" needle/seat orifice would slow down the fuel's speed in the line, allowing it to accumulate more under hood heat, which might cause other issues of vehicle performance at lower rpms and fuel needs.

Hope y'all weren't doing that with leaded fuel!

NTX5467

It actually does work, Willis.

Think about it. It only has to resist to less than 9 lbs. max.

Realistically more like 4-9 like my electric.

Edited by buick5563 (see edit history)

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I wear Diesel jeans when I blow in carburetor air horns, not those little shorts.

three-little-pigs-5.gif Bernie

That's good. You probably wouldn't look as good in them.

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Dang it, had pinned this one for What's Happening on the Forum but the word count got exceeded about 10 posts ago....

Looking forward to the outcome though...

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I'm wondering how many have flipped and air horn over and blew into the fuel fitting. They'd get a pretty good lesson in spirometry.

Bernie

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5563, to me, I can do what I feel is a good check of how the needle/seat might be by actually looking at the contact pattern on the needle's "tapered area" and then where it touches the "seat".  Looking for consistency of sealing area and integrigy thereof.  Works for the Viton-tips, too.  Of course, the float level adjustment would need to be correct, too.

 

I don't recall any of my automotive "mentors" EVER advocating doing what Y'all are allegedly doing with a carburetor!!  Even the "older-timers" with a strong shade tree!

 

(Learn something new every day!!)

 

NTX5467

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5563, to me, I can do what I feel is a good check of how the needle/seat might be by actually looking at the contact pattern on the needle's "tapered area" and then where it touches the "seat".  Looking for consistency of sealing area and integrigy thereof.  Works for the Viton-tips, too.  Of course, the float level adjustment would need to be correct, too.

 

 

(Learn something new every day!!)

 

NTX5467

Willis, good theorizing but not very useful on new needle and seat. :)

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Well it's official! A good cleaning and rebuild of the carburetor and the fuel pump works fine. It was reading just under 7 pounds. Well I grew up working on fuel injection systems of the 1980's and early 90's so carburetors are something I am not well informed on yet I guess as I have never seen this where a car runs seemingly okay on one fuel pump and then needs a carb rebuild on another, but I guess the other one was just weak. It's crazy to me that I had this experience before and had a different outcome, and that threw me off. I hope this thread helps somebody else or there!

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Dang it, had pinned this one for What's Happening on the Forum but the word count got exceeded about 10 posts ago....

Looking forward to the outcome though...

Perhaps you can edit a few posts out that wouldn't change the important points of the discussion?

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