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Carburetor Problems On Older Cars

MrShift@EdmundsMrShift@Edmunds Posts: 43,644
This forum is for discussing the care and feeding (and fixing) of problems related to carburetors, those primitive and extinct devices found on most cars 15 years or older.


  • div2div2 Posts: 2,580
    I rebuilt a mid-seventies Quadrajet a couple of months ago. The Q-jet is actually a very elegant design; you can calibrate cruise, mid-range, and full throttle mixture to a very precise degree. The people who bad mouth them simply don't understand how and why the Q-jet operates the way it does.
  • MrShift@EdmundsMrShift@Edmunds Posts: 43,644
    Carburetors work quite well when properly set up and adjusted, which is like HARDLY EVER....congratulations on getting it right. I think the big fuel dragsters all use carbs don't they?

    Many the reason they were abandoned is that a computer can't control them very well---you have to use mechanico-electrico-vacuumo devices and these get all whacked out--not stable enough for emissions requirements.

    Remember those GM combo carbs, "throttle body injection" I think they were called? Or Jeep also had a electronic carb that was a nightmare.

    The best carbs are pre-emission types. British SU carbs are also brilliant if you understand them, and not at all troublesome.

    Old German Solexes and Zeniths however can be diabolical when used in tandem.
  • isellhondasisellhondas Issaquah WashingtonPosts: 17,350
    When I realize the average mechanic/technician today hardly knows what a carburator even is!

    Quadrajets came out in 1966 and I can remember how they were bad mouthed by a lot of "mechanics" who didn't understand them. Like div says, they were really well designed. The worst were Ford's Varible Venturi carbs and Chrysler's Thermo Quads.

    I used to enjoy watching a good "carburator guy" tear one apart and restore it to new.

    These guys are a disappearing breed.
  • MrShift@EdmundsMrShift@Edmunds Posts: 43,644
    What's even worse is many times people without experience will attempt to modify their carburetor systems without much forethought. You just can't bolt on a bigger carb without doing other things as well, like a good intake system.

    We were just working on a Porsche 914 that someone had stuffed a 3.0 liter carbed engine in. Ran like crap---had larger venturi carbs on it but nothing else added. So we switched to stock venturis, tuned the car and it ran about twice as well. So much money spent and wasted!!
  • isellhondasisellhondas Issaquah WashingtonPosts: 17,350
    There used to be a BLIND tune up guy at a Chevy dealer in So. Calif. A lot attendant would pull the cars in and out for him. He could overhaul a Quadrajet in record time.

    I refused to believe it until I saw him at work one day!
  • MrShift@EdmundsMrShift@Edmunds Posts: 43,644
    A lot of carb adjustments are done with a feeler gauge, so you don't need your eyes for that. Still, quite an achievement!
  • isellhondasisellhondas Issaquah WashingtonPosts: 17,350
    Think of the balls and springs etc he had to deal with and be sure not to lose.

    He had to ask somone to read the scope pattern for him if he couldn't figure out a miss or something. He was incredible!

    Remember Sun scopes? Talk about a thing of the past!
  • jsylvesterjsylvester Posts: 572
    I'm getting the carb on my 67 Galaxie worked on this summer. Let me tell you, finding a good carburator guy who is not charging an arm and a leg because most of his customers are wealthy musle-car owners is not so easy anymore.

    I use my mechanic mainly because his father is semi-retired, but does come in to do any of the carburator work they get. I've never seen top shelf older cars, but I've seen them working on 6 cylinder Mustangs, MGB's, Buicks, etc.
  • isellhondasisellhondas Issaquah WashingtonPosts: 17,350
    Trouble is, today's mechanics/technicians don't like to rebuild things. They just replace components in many cases.

    An old school guy would NEVER put an assembly line "rebuilt" carburetor, starter, alternator or anything else on a car. They wouldn't stake their reputation on the unknown quaity of a so called "rebuilt" part.

    Instead, they would carefully disassemble the part and with a great deal of pride, overhaul it to as new condition.

    I guess today's labor rates forbid this now?
  • stinkpot1stinkpot1 Posts: 11
    I have a 68 Ford Mustang w/289 2 bbl. mechanical fuel pump. It has recently developed an issue with fuel bubbling out of the carb all over the intake manifold. enough that it fills the valleys on the manifold. The fuel is coming out of the two upright tubes in front of the butterfly. It does not do this all the time, usually after you drive it a while, stop, and then go to start it back up. It has sat for 2-3 hours after driving and had this problem but shorter times also. It did this with both the replaced carb and the new one.

    I have replaced the carb. I have checked the float level, its ok. I have flushed out the radiator and engine. I have inslulated the fuel line from fuel pump to carb. I have changed the fuel filter. The fuel tank is not pressurizing. I am at a loss for what else it might be.

    The only engine work done recently was a mechanic attempted to adjust one of the hydraulic lifters due to a small tap.

    Can the mechanical fuel pump be overpresurizing? occasionally?
    What esle might be going on?
  • MrShift@EdmundsMrShift@Edmunds Posts: 43,644
    I suppose you could add a fuel pressure regulator and see what happens...the pressure could be enough to defeat the float valve.
  • bolivarbolivar Posts: 2,316
    -Any possibility some fool has added an electric fuel pump? And it is stuck 'on' all the time?

    -Does the fuel 'bubble' out when the car is running or stopped? If when running, even if you have a new carb, the float valve is not doing it's job. Have the carb rebuilt one more time, with new float valve and set the float to the right level.

    -I would think a 1968 would have a 'vented' fuel tank cap. Do you have the correct cap? If an unvented cap is on there, when the fuel heats up, the pressure might push through the carb when it is stopped.
  • stinkpot1stinkpot1 Posts: 11
    No electric fuel pump,
    It only bubbles out when running right after starting it up. It runs real rough. I have checked the float level and function several times now. It is fine.
    Fuel tank vent works fine.
    I noticed after shutting the car off the throat of the carb fills with vapor. So much that it looks like a witches cauldron.
    I had a thought on this tho. I may have gotten some fuel that had a high amount of alcohol. They are selling corn fuels here in Illinois and may have had a fuel mixup at the stations. That or some [non-permissible content removed] sugared me. Still have not ruled out the mechanical fuel pump.
  • bolivarbolivar Posts: 2,316
    Well, you are having fuel being pushed into the carb. The 'vapor' is just that, gasoline vapor. When it overflows the carb and goes down into the hot intake, it vaporizes and you see the vapor come back up through the carb to the outside.

    The float is either set too high, or the float valve is not shutting off the flow of fuel. I've never heard of a mechanical fuel pump overpowering the float valve. It might be happening, if someone put the wrong fuel pump on, one with a much higher pressure capability than the correct one. But I would think if the fuel pump is putting out that kind of pressure, the car would not run at idle or slow speeds, the fuel overflow, raw fuel running down into the intake, would flood it out.

    Alcohol might have eatten up the little rubber tip on the float valve that does all the work closing off the fuel. Take that carb off, turn it upside down, blow through the fuel line into the carb. If you can blow throught it, then the float valve is not working. (Holding it upside down should make the float close the valve.) If alcohol ate up something like the 'power piston' diaphram, I don't know what the symptoms would be. Tearing the carb down and looking is only way I know to find things 'eaten up'.
  • bolivarbolivar Posts: 2,316
    P.S. Sugar won't harm the car, other than stopping up fuel filters, etc. It doesn't dissolve to any entent in gasoline, it just sits there.

    Sugar in the gas is so old a tale it's not a urban ledgend, its an old wive's tale.
  • isellhondasisellhondas Issaquah WashingtonPosts: 17,350
    Do you mean the person in my frat house who sugared the motorcycle cop that night didn't do much damage? :)
  • warnywarny Posts: 1
    I had the exact same ploblem with a 66 Mustang 289. Totally rebuilt the carb but the problem remained. From a clue on another panel, I replaced the mechanical fuel pump and now no more problem. Seems that there was internal leakage that caused the pump to pump in spurts when starting from cold especially after setting for a couple of days. (When hot, it started fine.) Am guessing that the float was unable to cope with this pulsing action and the excess gas was blown out the relief hole.
  • oldharryoldharry Posts: 413
    There may be a clogged vent in the float bowl. If the air cannot get out, the float never rises, and the pump forces the fuel out where ever it can. The old Rochester 2 bbl's had a * TINY* notch below the cover gasket that vented into the ventry area.

  • Okay here is what I've got. I bought a 1965 Chevrolet Truck with a 327 corvette engine. It was running pretty good except for when you gave it gas it would sputter. It had the original Rochester carb on it (40 years old might I add). Anyway, I bought a new Edlebrock carb and put it on it. The only hookups on this truck to the carb is the throttle, gas line, and I guess the pcv valve is the big hose. The new carb has an electric choke so I hooked that up. The problem is the truck will not start. It will turn over but will not fire. I covered both the vacuum inlets in the front of the carb with the provided rubber stoppers. I also screwed in the hex head stopper screw into the hole in the back of the carb. As far as I can tell everything is hooked up correctly. I thought that I may have flooded it at first but, I waited for about 2 1/2 hours and then tried it again, but it still didn't crank. If anyone has any suggestions thanks.
  • MrShift@EdmundsMrShift@Edmunds Posts: 43,644
    Try to start it and check for a spark.

    Got a fat blue spark? Okay, then.....

    If it still doesn't start pull a plug and smell/look for gas.

    It is rare---nearly IMPOSSIBLE---for a car not to start at all due to a bad carburetor. It may run BADLY but it should always run.

    However, it could be flooding, that is dumping too much raw gas in there, and that douses the plugs like it was water. (stuck float?)

    If the plugs are wet, AND you're sure you have a spark, put in fresh plugs, then try to start the truck with the gas pedal pushed down AND staying down, and see what happens. If it starts up but refuses to run and you have lots of black smoke out the tailpipe, you've got some internal carb problem.
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