Tuning carburetors...
 

Seems to be witchcraft! (well at least to me)


I wish I could plug my laptop in and mess with the software, can't find the USB or serial port! (ok, bad joke...)


Long story short - looking at a project which has an engine with a carb... Guy who currently owns the project had the carb rebuilt, and reintstalled, but never had it tuned, and is currently non-op.


I looked at the carb, and have no idea what is going on there, so I assume it has something to do with magic and witchcraft!

More seriously, can someone give me a crash course on tuning these? Or can you point me in the direction of figuring out how to do it...

TIA - Justin

To start, --=--
 

What kind of engine, make of car, brand of carb, original like Rochester etc or aftermarket like Holley double pumper, etc, and how many barrels, like 2bbl, 4bbl, 3-deuces etc.

Knowing that would help on here.

Carb:


I would usually explain it to females, like this:

You know those pefume atomizers on the counters of perfume sections in Macy's, Nordstrom's, places like that?

Where you squeeze the rubber bulb on the bottle and a mixture of air and perfume gets spritzed out?

A carburetor works sort of the same way, except what gets spritzed out is a mixture of gasoline and air, and it's adjustable constantly so you can alter the mixture or air and fuel how you want to make it lean or rich, dependng on whether you need accelleration or fuel economy more at the moment..

The spark plug ignites in sequence to the other spark plugs when its spark explodes the mixture of fuel and gas and moves the pistons up and down to create power.

I then ask them to visualize a kitchen hand held manual egg beater to visualize the twisting power is transferred to the driving wheels to make the car work.

That *sometimes* gives them the general idea. \o/

As Jim B said, we need more information to be helpful. Carb design varied over the years from very simple to pretty complex. Make, model and year of the car, and, if it is the oem carb or not.

What kind of carburetor is it?

You only need to do a little reading and learn about the different systems of the carburetor and how they effect performance/drivability/economy.

There are variations, but most all carburetors have:

A fuel bowl system with a float and needle and seat to maintain a constant fuel level. Setting the float level properly is the first step in calibration.

A main system consisting of a main jet and venturi. As the fuel speeds up to make it through the venturi it creates a vacuum that draws fuel through the main jet.

A power enrichment system of some description. This allows for a richer fuel mixture as the vacuum goes down. The lowered vacuum means an opening throttle and added cylinder pressure requiring a richer mixture.

An accelerator pump system. This is to squirt raw fuel proportionately as the throttle is opened so as not to introduce a lean condition from a rapidly opening throttle with no additional fuel to maintain proper mixture.


All of these systems are incorporated in one form or another in most every auto carburetor. In small engines they usually get much simpler. With some foreign engines, you will have a different way of achieving it in slide type carburetors.

When dialing in a carburetor, most of these systems interact. Always make ONE CHANGE AT A TIME in the calibration process. If you change more than one thing at a time, you can quickly get way out in the weeds and not know what's going on.

Even though it's brand specific, the best book I've seen for learning the fundamentals is an HP book on Holley Carburetors first published about 1970 or so. Even if you never put your hands on a Holley Carburetor, this book would be an excellent source for learning the fundamentals of carburetion.

One other thing. Many, many years ago I read a book on auto maintenance. When it came to the chapter on carburetion, there was one paragraph which said something like "Carburetor is a French word that means DON'T MESS WITH IT.":laughing: Don't pay any attention to that chapter. Learn how a carburetor works and make it perfect!

Hope this helps.

Multi carbs are a nightmare to tune. I used to have a color tune till some one borrowed it.
Best find a good mechanic who is Roy's age at least & let them play with it.
Tuning a carb properly is about as hard as finding a good woman !! ~ once you get it right look after it !!

Yes, multi carbs are definitely not for the novice, but with a good understanding and some experience, they're really no problem.

Some advice I read 40 years ago about SU curbs may apply here. SU carbs never are in perfect tune--so they invite tinkering. OTOH, it is difficult to mess them so badly that they will totally fail to run. The perfect carb for tinkerers.

Yes the Solex and SU are a different animal. They are relatively simple to deal with. I never read that they are never in tune, but I can believe it. Messing with springs and needles is pretty easy and easy to test.

The thing that might make Solexes or SU's a challenge for a beginner is that there are typically more than one of them.

If it was properly rebuilt, the float setting should be OK and the only 'tuning' required should be the 'cold' (choke on) idle speed, the 'hot' speed, and (maybe) the hot idle mixture.

Once the fuel pump has filled the float bowl, and assuming the choke blade closes, the engine should at least try to start.

With a cold engine, and ignition off, step on the gas pedal once and then look down into the carburetor barrel to see if the choke plate is not 'blocking' the throat. If it is, you should be able to crank away, possibly while adding a small bit of accelerator pedal to get it going. It may require some light pedal press, but it should run.

Then, it is a matter of finding the fast idle screw and setting it so the engine starts and runs by itself when cold.

Hot idle is adjusted once the choke plate has opened. Again, it is a matter of finding the hot idle screw and setting it so the engine starts and runs by itself when hot.

If there is a mixture screw, usually 1-1/2 or 2 turns out from lightly seated is a good place to start, with small adjustments until the engine is running at its smoothest.

A carburetor is only magic and witchcraft if you have never played with it. Then, it become fun!

[QUOTE=Jim H;3059827]If it was properly rebuilt, the float setting should be OK and the only 'tuning' required should be the 'cold' (choke on) idle speed, the 'hot' speed, and (maybe) the hot idle mixture.

Once the fuel pump has filled the float bowl, and assuming the choke blade closes, the engine should at least try to start.

With a cold engine, and ignition off, step on the gas pedal once and then look down into the carburetor barrel to see if the choke plate is not 'blocking' the throat. If it is, you should be able to crank away, possibly while adding a small bit of accelerator pedal to get it going. It may require some light pedal press, but it should run.

Then, it is a matter of finding the fast idle screw and setting it so the engine starts and runs by itself when cold.

Hot idle is adjusted once the choke plate has opened. Again, it is a matter of finding the hot idle screw and setting it so the engine starts and runs by itself when hot.

If there is a mixture screw, usually 1-1/2 or 2 turns out from lightly seated is a good place to start, with small adjustments until the engine is running at its smoothest.

A carburetor is only magic and witchcraft if you have never played with it. Then, it become fun![/QUOTE]



I agree. It is fun.

Your post reminded me that I failed to list the idle system in my post.

Can't help but notice that the tech talk here on carbs come from folks likely to be in their 50s and 60s and up,

And iwrock is in his early 20s.


heh :P:cool:

Anyone have something to say with respect to causation and correlation? \o/

[QUOTE=Air&Road;3059832] [QUOTE=Jim H;3059827]If it was properly rebuilt, the float setting should be OK and the only 'tuning' required should be the 'cold' (choke on) idle speed, the 'hot' speed, and (maybe) the hot idle mixture.

Once the fuel pump has filled the float bowl, and assuming the choke blade closes, the engine should at least try to start.

With a cold engine, and ignition off, step on the gas pedal once and then look down into the carburetor barrel to see if the choke plate is not 'blocking' the throat. If it is, you should be able to crank away, possibly while adding a small bit of accelerator pedal to get it going. It may require some light pedal press, but it should run.

Then, it is a matter of finding the fast idle screw and setting it so the engine starts and runs by itself when cold.

Hot idle is adjusted once the choke plate has opened. Again, it is a matter of finding the hot idle screw and setting it so the engine starts and runs by itself when hot.

If there is a mixture screw, usually 1-1/2 or 2 turns out from lightly seated is a good place to start, with small adjustments until the engine is running at its smoothest.

A carburetor is only magic and witchcraft if you have never played with it. Then, it become fun![/QUOTE]


But...nonetheless -

Your primer,Larry, was superb and concise! I loved it.

The others including Jim H.'s I liked too...

And knowing iwrock personally, and how smart and car savvy he is, I can't help but wonder just a LEETLE bit, whether he is having us all on.

Nah.

He'd NEVER do that..

Yes Jim. Among most of today's techs, they can draw codes, troubleshoot circuits, and analyze all the latest automotive technology. I remember in a column Jay Leno wrote, he spoke to several young techs whose worst fear was that someone would bring in a car with points and a carburetor.

As far as symptoms and causes go, they are probably too numerous to go into in any volume outside of a text book. There are only a few subsystems in the carburetor, but there are different combinations of things that can happen and possible multiple symptoms to be encountered due to certain failures of a single system.

That said, if someone understands how the different systems operate and their effect on engine operation, they should be ready to troubleshoot most commonly encountered situations.

Some common ones might be:

o An engine that idles and cruises normally, but stumbles when cruising and hitting the throttle. This would likely be the accelerator pump, but it also could be, or be exacerbated by, a generally lean condition. A generally lean condition could be any number of things such as low float level, clogged or too small main jets, non functioning power system.

o Engine not idling is not necessarily a problem with the idle system in the carburetor, it would probably more likely be a vacuum leak below the throttle plate. In an older vehicle (how many carbureted engines aren't older?) it can be worn out throttle shafts, something often overlooked.

o A lack of power. This could be any number of things certainly not limited to the carburetor. On a carbureted engine, timing problems including centrifugal and vacuum advance could seem like a carburetor problem. Never overlook the fact that problems CAN be caused by things other than the carburetor. A malfunctioning power system in the carburetor will usually result in an engine that feels like the accelerator pedal is connected to the carburetor throttle arm by a rubber band. It has no added fuel enrichment to add power when the throttle is opened significantly farther.

o Poor cold engine operation. I failed to list the choke system originally. A misadjusted choke, obviously can make for a hard starting engine that runs normally after warmed up. The opposite problem to be experienced from a misadjusted choke would be when it is adjusted such that it doesn't open when warm. This will make for sooty black spark plugs and a stop at every other corner for a tank of gasoline. It has more serious consequences if not corrected soon, because it is allowing raw fuel to wash the oil off the cylinder walls and dilute the engine oil.

o Flooding. It is not uncommon with carburetors for the needle and seat to stick open causing serious flooding. If you suspect flooding, remove the carb and invert it. You should not be able to blow into the fuel inlet. If you can, trash or something is causing the needle & seat to be open. On some older carburetors that have neoprene needles, after sitting, the needle will sometimes stick against the seat, preventing any fuel from flowing into the carburetor, which is of course the opposite of flooding, but it should be covered when thinking of needle & seat problems.


Again, to be proficient at troubleshooting carburetors, learn about the different subsystems and what they do. As the famous Physics Professor Julius Sumner Miller constantly repeated when teaching Physics; "it's what you UNDERSTAND that counts."

The engine in question is a Chrysler Marine Hemi.


Dont know what type of carb is on there, but was told it is the original carb.

It's most likely a Holley. Now you're TALKIN'. These carburetors are relatively simple with great replacement and tuning parts availability. On the front air horn there will be a number that starts with "LIST." Get that number and post it and I'll see if it shows up in some of my documentation. There are several configurations of the Holley four barrels, but they are all extremely similar.

If it's indeed a Holley, you DEFINITELY need to find the HP Holley Carb book.