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Carburetor Problems On Older Cars



  • smreitersmreiter Posts: 1
    I'm scratching my head at this one. It's a ford auotlite carb, pretty simple, 2bl. . As far as I know, the engine is ok (no valve leakage). I have yet to set the timing but believe it's close enough right now (it starts).
    Anyway, when the car warms up and the choke begins to open, the engine will not idle. Nor will the idle screw adjustment work. There is simply no effect. If I close the choke, the car will idle (badly).
    I've replaced plug wires and gapped the plugs correctly. The car is a 1977 mustang ii. The orginal carb had this same issue, so I put on a different one. Same problem. Both are used carbs and could be bad I suppose. New power valve and new acc. pump. However, no fuel in the power valve bowl (don't know if there should be).

    there is a PC valve that could have a problem (right now I have unfiltered air goint into the engine. (I'll purchase a new valve)
    The egr is disconnected and plugged.

    Any suggestions would be most welcome.
  • zaken1zaken1 Posts: 556
    In my experience; it is highly unlikely that two carbs in a row will have the same problem. The leanness you are encountering can be caused by several different issues. The most basic prerequisite for a motor to run properly is good cylinder compression. You stated that there is no valve leakage; but I don't know how you determined that. There can be compression leakage from many other areas besides the valves; it can leak from worn piston rings, cylinder bores, and a warped or cracked head, or a bad head gasket. This is why a compression test on all the cylinders is the only reliable way to determine the engine's mechanical soundness. When evaluating the results of a compression test; it is necessary that all the cylinder pressures are higher than the manufacturer's minimum allowable limit; and also that there is not more than 15% difference between the pressure in any two cylinders. If both of these conditions are not met; it will not be possible to adjust the engine to run properly. An engine which fails a compression test will have to be rebuilt or replaced.

    But, on the brighter side; if your motor passes a compression test, I would expect the lean condition comes from a restricted fuel supply. There should be fuel visible in the accelerator pump well. So it seems to me that the fuel pump may have failed, or the fuel filter has become plugged. Try disconnecting the fuel line at the carb, and running it into a container which holds a quart or more. Crank the motor, and measure the fuel volume from the line in 15 seconds of cranking. There should be at least 8 ounces of fuel pumped in 15 seconds (a quart a minute). If you have a fuel pressure gauge; connect it to the end of the line, and crank the motor until the pressure stops rising. There should be at least 3psi (and very likely 4 or 5 psi). If the pump fails either one of these tests, and you can blow air through the fuel filter without feeling any significant restriction; then the fuel pump should be replaced.

    The fuel level in the carb float bowl will also be low if the float level is set too low. Sometimes people adjust the float level by pulling the float upward until it stops; without considering that, on those float valves which are spring loaded, the fuel flow will shut off long before the float stops moving. So if your carb has a spring loaded float valve (which Autolite carbs often do) try turning the carb upside down and measuring the float height when the float is only pressed closed by its own weight. That is the proper point at which it should be measured.

    There are separate fuel metering jets for the idle fuel and for the main fuel flow. If the tiny fuel passage in the idle jet is plugged; it will cause the leanness you experienced. A plugged main jet will also cause this condition; but the main jet orifice is considerably larger than the idle jet, so it is much less likely. And clogged internal fuel passages can also cause leanness. You could also completely unscrew the idle mixture screw, and spray carb cleaner into the orifice. Then put the screw back in place, turn it in until it is LIGHTLY seated, and back it out 1 1/2 to 2 1/2 turns. That should be a good starting point for the idle mixture screw setting.

    If there are any vacuum leaks at the choke pull off, power brake booster, distributor vacuum advance, or EGR controls; this will also excessively lean out the idle. And if the EGR vacuum hose has been plugged, but the EGR valve is still in place; that valve sometimes gets stuck partly open. And that will cause the condition you reported.
  • A bad PCV valve can cause the rough idle problem. Will the engine die if you cover the air horn completely with your hand? If not-look for a vacum leak.

    Do put a timing light on that engine.

    As for carb I agree with another reply that 2 carbs acting the same would be unlikely-but not impossible.

    Along with float setting-if your carb is the Ford type that the top is removed-the fuel level should also be checked with a pocket scale ruler -with the top off and engine running.There are specs on fuel levels in some manuals.

    But before anything check the compression and see if it is reasonable and fairly even. Or do a leakdown test if possible.


  • Mr_ShiftrightMr_Shiftright CaliforniaPosts: 44,413
    Carburetors are pretty dumb. They are at best compromises that try to meet every situation----so if anything at all is amiss, the effects are immediate. They have to be mounted just right and adjusted just right or they don't work well at all. They are fussy, and sensitive to tinkering, to climate, and to air filtration.


  • 0patience0patience Posts: 1,542
    edited January 2011
    In the "old days", a ton of problems were blamed on the carb that in reality were often something else. Among the most common were vacuum leaks.
    The vacuum leak was probably the one most mis-diagnosed problem on the older carb engines. Second to vacuum leaks was timing. Especially on the old points engines.

    But I agree that most carbs were susceptible to be extremely finicky about adjustments, especially on those that the people weren't certain what they were doing. Some of the older FoMoCo and Holley carbs were especially tempermental, as most guys I knew at the track that ran them, ended up bringing boxes of power valves for them.

    Some guys used to cuss Rochester "Quadra-junks", saying they were the most complicated, yet I found them to be the most forgiving.
  • isellhondasisellhondas Issaquah WashingtonPosts: 17,603
    Those Thermo Quads were a pain too along with Ford's Varible Venturi carbs.

    I think our Honda shop had maybe one tech left that could overhaul a carb.

    A lot of today's techs have never seen one!
  • animaljimanimaljim Posts: 3
    edited January 2011
    I never liked those carbs myself. But Rochester Quadra Jets are a great carb when working correct. They do have mutiple problems when aging. Nitrofill float soaking fuel--passage leaks-vacum motor failure, choke lock keeping secondarys from opening. When all is corrected they work great again.

    I modified many for drag racing and they run very strong.

    I guess I was lucky. In 1964 -the car dealer I worked for sent me to an advanced tune up one week class. The first 3 days were on the upcoming Quadrajet. So I had a heads up. At 69, I am prob. an minority now who knows how to rebuild and improve on one.

    First thing is to remove the choke secondary lock. I found countless cars even new back then that the secondaries never worked.

    The worst case I ever saw was a new 70s 454 El Camino. The choke lock out was hanging up-the factory had mounted the engine lift ring on the rear of the intake on top the vacum line that activated the Turbo' modulator, vacum adavnce and the choke motor on the carb.

    The dealer had replaced the smoked convertor once. No way they could have test drove that car.

    When I got done my customer almost craped his pants when we took it for ride and it all worked like it should. I also had turned the AC lid upside down so the engine could breathe. They moan real nice then! I also took 1/2 a turn off the rocker adjustments that gave it even more response and better mileage.

    I had couple of my own cars with Q-Jets. My 70 429 CJ Cyclone could run 120 on primarys. When you floored it -it was like 2 soft N2O hits when the secondaries opened and the Ram Air hit. If you drove it decent it got 17 MPG with a 350 gear. It would top at 150. The 3.50 gear was perfect to sqeeze most the HP out. The mileage was not 17mpg doing that! LOL!

    My other Q-Jet car was a 77 Olds. It ran real nice. I had to rebuild the car once in 160,000 miles.
  • zaken1zaken1 Posts: 556
    As another 69 year old with roots in 1960's drag racing; I came to appreciate the many different ways the fuel metering on Quadrajets could be dialed in. There was (and probably still is) a southern California specialty shop called The Carb Shop. They were really into quadrajet tuning; and sold a kit to modify the quadrajet float chamber cover to install an adjustable stop for the primary metering rods. They also sold different taper rods which worked better with the stop. The adjustable stop was used to set the mixture at heavy throttle; and the faster taper of the rods would bring the power in sooner under load. There were many things that could be done with air bleeds and the main discharge nozzles to alter their response to changes in venturi vacuum. But it was frustrating to tune the late 1970s quadrajets that came with design features which prevented richening up the secondary metering rods.
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