Ok. Lets talk.
My mission has always been for you to have a good driving experience.
I would also prefer to educate. Ideally I prefer you to fix your own stuff, and just let me help here or there.
Not singling anyone out, and I absolutely do not want to seem like I’m pointing a finger at my customers and saying, “It is your problem.”
But……
I have gotten many messages over the years, that start with, “Hey bud, carbs do blah blah blah, I’m ready to tear carbs apart, what do I change?”
Or someone asking to send carbs back due to a running issue. It’s rare, probably 2 out of 100, but still, worth addressing.
The carburetor is the last thing installed on an engine, but when an issue arises, it is often the first to be blamed.
This is coupled with many vintage car owners that are not really mechanics. Heck, your new stroker engine with the big cam might be your first real old car.
If I learned anything from my 1.5 decades at sea as an Engineer, I never assume anything will function unless the whole system is within spec.
I also will not be talking about carbs here….. this is about the supporting cast, which is critical to function. I’m often amazed at how little a person knows about the system, yet they are often willing to dig into a random part.
Assume nothing, prove everything.
So lets do a run down:
Engine runs badly. Lets assume the engine itself is sound, compression is good, no intake manifold leaks, valve issues, etc
Let us not forget AirFuelSpark is the name…
-Air
Seems easy right? Of course the engine has air. But not so easy. Is the filter clean? Can it flow sufficient air? Intake leaks? Pcv functional? Does Pcv stay in correct position for idle speeds? Do we have a vacuum leak in distributor advance?
These are all important. I’m reminded of the customer who was pulling his hair out over his brand new massive stroker running rich. Months of effort testing, trying to find the issue, tearing into everything only to find the A/F ratio jumped two points when he took off the air filter. $18 air filter was killing a $20,000 engine.
No carburetor will work if choked down, or a vacuum leak is present. The carb is designed for smooth flow with NO unmetered air leaks.
If using a stock PCV, your large roller cam may not make enough vac at idle to keep it closed. Perhaps a restriction needs to be in the line (works well actually to stuff a small hose in a larger hose).
-Fuel
Is the fuel old? Is the fuel from a known good source? Does the pump provide 5psi at idle AND at speed? Is the filter adequate?
I can’t tell you how often when a person asks about a rich condition, they are not aware of the fuel pressure. These carbs were designed with a very small window of pressure tolerance. I test at 5psi. If you are running 7psi, you may have a problem.
I will stress good filters again. The only thing keeping your carbs from dumping fuel everywhere from a tiny tiny piece of trash is that filter.
I prefer a 100micron filter, electric fuel pump, regulator, 10micron filter, carbs. Obviously with a restoration, you need to use an original appearing pump, but be sure to check check and double check filtration and pressure.
As is often the case in this era, new does NOT equal good, or often even functional. Check and know.
-Spark
Is the coil good? Is the ignition box good? Is timing correct? Does timing advance correctly? Are the wires showing little resistance? Are the plugs good and of the proper range?
Have you actually tested these things?
Don’t say “It’s new.” That literally means nothing to me. Actually test EVERYTHING. Bonus points for having extra known good parts.
I actually keep a known good coil, ignition box, and distributor on hand in my shop. I’ve seen parts that test good but often break down with heat, or seem to fail in use. Being able to swap parts around is a huge benefit when chasing gremlins.
This is also why all of my vehicles have the same running gear and share parts.
Timing is so critical and often overlooked. Knowing your initial and total is not enough. How does the curve look? Is it smooth? Does it jump around? What does it do?
I will mention one other thing to be aware of.
-Heat
Gasoline boils. This makes vapor pressure often well beyond what a needle and seat can handle. Do everything in your power to keep all fuel as cool as possible from the tank all the way to the carb.
So there.
If all of the above is perfect, maybe the carb really is the issue. Heck, I make mistakes, and happy to own up if/when I do.
If ANY of the things listed above are out of range, NOTHING you or I do to the carburetors will fix the issue. All of the above needs to be perfect or the whole system is out of range.
A tiny vacuum leak, or irregular fuel pressure, or boiling fuel cause a myriad of issues. There is no band aid, only fixing the issue.
But there is a valid reason I test almost every carb that leaves the shop. I know that on a baseline engine that I am VERY familiar with, the carbs perform as they should.
Shipping and install can jar them, parts can get broken and bent, new carb parts may fail… but I am assured that the carbs worked well for me on my engine.
You may need minor adjustments (ok, you really should make minor adjustments), but they should work as designed.
Welp, thanks for listening, please share this info, feel free to add info in the comments, tip your waiter 15%, have a cold one for me, call your mom she worries, do not look directly in laser with remaining eye, and most of all, happy motoring.
Drew Pojedinec
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