Tuning issue- seeking thoughts

Tom in TN

Tom in TN

Senior Member
Hey y’all.
I’ve got a situation with Ms Mayberry (rebuilt 272, stock everything) that I would like to hear your best guesses as to what the heck the deal is.
My wife and I decided to make a run down to Jiffy Burger in Manchester, TN, last Saturday. So we jump into the ’55 and head down Highway 41.
She cranked fine, idled good, and accelerated as normal, but at cruising speed, say 50mph, the car seemed to be starving, as if it was wanting more fuel. Not actually missing, but like “gimme more gas”.
Oh, it still had passing power and pulled off from stop lights just fine.
We made it to Jiffy Burger and had a yummy meal.
So, on the way back, it seemed to be doing the same thing…like horses wanting to run, but can’t get turned loose…
So, and this is the interesting part…I decided to play ‘what would this do’ from the driver’s seat and I pulled the choke out about one-quarter of the way…. And wow, what a difference. The engine ‘woke up’ is the best description. It started feeling like the way it is supposed to run.
Now, my logical thought is ‘maybe some trash in a carb passage’ even though it’s a new tank and I run 2 fuel filters…or is this how an engine reacts when the carb jets are too small?
A few months ago, I had changed my carb jets from a #50 to a #49.
I have sooty pipes and the plugs showed a little bit of ‘excessive fuel’ coloring, so I dropped the jet size…and I cannot honestly say that we have had it out on the open road since I did.
What do y’all think? Did I lower them and create my own problem?
Go back to the #50’s and it will be fine again??
What else could pulling the choke out a little and it start running fine, point to?

Thanks y’all.
 
You didn't say how you ran the car before changing the jets, sooty pipes before could mean just poking around the block speeds. Put the 50's back in and take it for a road trip at highway speeds, put your foot in it, in other words, blow the cobs out it.
 
It does sound like a cruise circuit problem, but a single # jet size is unlikely to cause this, unless the carb jetting was already cut really close to begin with. It is possible though. You'd notice this particularly in cold weather. I'd probably look first at a restricted air cleaner or possibly a fuel pump issue.

Look carefully at the spark plugs - the "base ring" is the key for the clues to jetting.
 
Thanks guys..all good ideas...I'll keep you posted...

ps . we ran it all over the area...not just puttering around the block...
 
A restricted air cleaner would make it run rich, so pulling out the choke would make the problem worse. If the fuel pump were weak, pulling out the choke would only help momentarily. In addition to reinstalling the #50 jets, check the float level as well as float drop.
 
Let's see a good clear sharp close up picture of a spark plug or two?

"Lean surge" or misfire at cruise is most definitely a signal of not enough jet but still find it tough to believe that a single step down would make a dramatic change generally speaking. Typically most carburetors are setup so far on the rich side of things a single step change wouldn't even be detectable.

Adding choke as a remedy definitely points to a lean condition though. I wonder if there is a piece of crud in one of the jets, something like that.
 
You run two separate fuel filters? How are they placed?

First you need a vacuum gauge to help diagnose (vacuum and fuel pressure/volume). If it indicates a lean condition, the following is the safest way to find it, other than a smoke machine -

Note using propane, it is best to slowly open the valve and point the pickup hose to the carb/TB opening and try and stabilize the curb idle, You then go though the method shown on the video to hopefully find the leakage source.

- https://www.classiccarrestorationclub.com/video/locating-car-vacuum-leak-014793/#

CAUTION - Using CARB CLEANER or STARTING FLUID to diagnose can lead to facial hair loss and dirty undies ...

Is this a car or truck?
 
Ole Don said:
Old gas?
Click to expand...

Most likely -

if you’re driving an older vehicle, the ethanol may be causing problems. Alcohol is a highly corrosive substance that’s hard on plastic, rubber, and even some metals, and over time it will eat away at your vehicle’s most vulnerable components. Additionally, alcohol in an old engine can start to dislodge the build-up of carbon - (and varnish in tank and lines) - deposits from inside the engine. That may sound like a good thing, but that buildup can actually move into places where it chokes the engine’s fuel supply.

And finally, when an ethanol/gasoline mixture sits stagnant in a gas tank for a long time, it can separate due to a process called phase separation. Phase separation, in short, happens when water finds its way into your gas tank and mixes with the ethanol. Ethanol will absorb and hold water until it reaches a saturation point, at which time the water and ethanol separate. This leaves a layer of alcohol and water at the bottom of your gas tank, which blocks the actual gasoline from entering the engine.
Click to expand...

SOURCE - https://blog.raleighclassic.com/is-it-safe-to-use-ethanol-in-my-older-engine/
 
Couple of items,
I do have a vacuum gauge and will be using it.
It's a car.
I use nothing but ethanol-Free gas in my old cars...gas is fresh, well as of 3 weeks ago.
Fuel filters WERE in series - I say were because I have removed one.
Pertronix electronics is now installed. Initial start-up, timing check/set will be Wednesday night.
New plugs will be going in Friday night.
The #50 jets have been reinstalled.
Checked the vac hoses and the only one that might have been leaking was the hard line going into the back of the carb (for dist advance) and it doesn't seem to tighten up. The fitting is tight, but the line can just slide in and out.
That's where I'm at 9.13.2021. Getting her ready for the Habitat for Humanity poker run on Saturday, 9.18.21

Thankyou for ALL the suggestions- they DO help!
 
Tom in TN said:
Fuel filters WERE in series - I say were because I have removed one.
Click to expand...

Why the two filters? Were both placed between the mechanical pump and carb?

Checked the vac hoses and the only one that might have been leaking was the hard line going into the back of the carb (for dist advance) and it doesn't seem to tighten up. The fitting is tight, but the line can just slide in and out.
Click to expand...

There is one vacuum leak.
 
2 filters because I had an original "renewed" tank start to fall apart and my system was full of gunk...yes they were between the pump and carb. Got a new tank and was just paranoid...filters are clean, so only need one anyway..

OK - will fix fitting leak not entirely sure how, but lemme look at it.
 
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