302 help, backfires on warmup

Discussion in 'Technical' started by Bad1974maverick, Jun 29, 2014.

  1. Bad1974maverick

    Bad1974maverick Member

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    Hey guys my name is simon and I have a 74 Grabber 302. about 4 weeks ago I was driving it and is would sputter just a little bit but go away. when I start it it runs great for about 15 minutes or until it warms up then the carb begins to sputter and fuel will pop up out the top, you can put the pedal to the floor and it wont go over 15 MH. Thinking that the older holley 2100 2bbl had a float stuck or needed rebuilt I temporarily replaced it with an edelbrock intake and 600 cfm carb I already had handy. took it out yesterday and drove great then after 10 minutes or so when the engine warmed up, it started backfiring and spitting trickles of gas out of the top just like before. I have checked for vacuum leaks replaced rubber plugs on lines, I don't hear a leak, I'm at a loss on this. Any thoughts? looked under the car because it is a 74 and didn't see any cats on it. Thanks for any help guys.
     
  2. Bryant

    Bryant forgot more than learned

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    unclear about a few things.
    when you say fuel sputters out the top. are you seeing liquid fuel come out of the bowl vent or a spray mist coming out of the venturies?
    i have several thoughts on this.
    the distributor may be loose and slowly changing timing, the fuel pressure could be rising, or the floats could be sinking.
     
  3. Bad1974maverick

    Bad1974maverick Member

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    The fuel that comes out of the top of the carb seems more like liquid. The fuel pressure rising or floats sinking seems possible but on two different carbs and intake setups?
     
  4. Bryant

    Bryant forgot more than learned

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    you have the same fuel pump on both carbs. but rising fuel pressure would give you a definite fluid poring out the top of the carb. so its sounding more like you may have some sort of valve problem, like an intake valve some how sticking open or not seating properly and causing a small back fire out the intake. also a really lean fuel mixture can can back fire out the carb.
     
  5. Bad1974maverick

    Bad1974maverick Member

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    a lean out on two different carbs on the same engine I can see that happening, ill check it. But the sticking valve, Why does it run so great until warm up?
     
  6. Bryant

    Bryant forgot more than learned

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    when the motor warms up the metal expands. this could change the shape of the valves, valve seats, valve guides, ect.
    this is all speculation.
     
  7. Bad1974maverick

    Bad1974maverick Member

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    could it be a problem with my vacuum advance and timing?
     
  8. Bryant

    Bryant forgot more than learned

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    nothing with the vacuum advance should cause that. if the timing it self has moved far enough it can cause back firing.
     
  9. simple man

    simple man Member

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    What ignition system are you using? If it's a Duraspark II, the module may be bad causing poor ignition once the engine has run for awhile. If this is the case, you will see some fuel spitting back out the carb throat because the intake pulses are now not even. If you have a timing light, hook it up on the coil wire when the problem occurs and see if the ignition pulses are even. If the ignition pulses become erratic when the engine is warm, the module could be suspect. :)
     
    Last edited: Jun 29, 2014
  10. Bad1974maverick

    Bad1974maverick Member

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    simple man I'm not sure whats on it ignition wise. It's stock for 74. Vacuum advance, coil. It might have duraspark or points I started with the carb cause it acted like the float was sticking and it an old rebuild. I changed the coil out of my 289 Ranchero and it continued to do the same thing.
     
  11. Bad1974maverick

    Bad1974maverick Member

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    The electric choke isn't hooked up but the plate is open and it still does it. The choke on the 2100 holley was open too but was doing the same thing.
     
  12. Bad1974maverick

    Bad1974maverick Member

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    Ok guys My friend and I got up this morning and began a deeper investigation. I noticed something. The ignition box bolted the the driver side apron under the horn, labeled motorcraft, has a clear looking gel all around the underneath it on the fender apron and the rad. core support. I seem to remember my dad on his 79 truck it doing that and the gel cooling in the ignition box leaked that when it came up to temp started acting up.
     
  13. Bad1974maverick

    Bad1974maverick Member

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    027_original.jpg 028_original.jpg I replaced the ignition control module that had leaked all it's cooling gel out the back thinking it might have been the problem, it it still persists. Could it be in the distributor? if it's an ignition problem? Given the fact that it's had two carbs, two intakes and the problem was the same, other than a fuel pump, fuel wouldn't be the issue. When up to temp the carb shoots out white puffs of fuel like a cloud and its coming out of the the two the choke plate is over.
     
  14. werner

    werner Member

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    what kind of carb is it holly is know blow its power valve and can cause this. it hard to to tell with out hearing it run. well if you have points get rid of them,it like the other member said timing changes when the go bad and the they do sell a kit to replace the piont, i had dual point and found a guy that mad an hei for ford motors and yes it looks like gm one but it mad for ford. i would up grade if you are running point they suck and you will get more power hi system.
     
  15. Bad1974maverick

    Bad1974maverick Member

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    I took the holly 2bbl off and the intake and replaced it with a Edelbrock alum intake and 600 cfm carb. The car doesn't have points and I replaced the module which if wasn't bad still needed replaced (see above). I can't for the life of me figure out why it will run great for about 15-20 minutes and then lose all power and start backfiring out of the carb!? Has anyone else had this happen?
     

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