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  1. #1
    Join Date
    Jun 2007
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    Lanarkshire
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    43
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    202

    Carb tuning or Exhaust drawing air?

    Hi Chaps

    Quick question for the engine and carb guys.

    On lift off anywhere above about 2500 rpm I'm getting a loud popping from exhaust. When I say loud, I mean like a rifle shot on gear changes or lift off. Overrun (after the initial excitement) isn't too bad, in fact barely noticeable pops and rumbles.
    The big bangs happen as soon as I come off the throttle unless is am very very gentle. At standstill it can pretty much shoot flames on demand. Whilst its all very exciting, I could do to tone it down a bit if possible. So getting down to the technicalities, I understand it could be one of two things (feel free to correct me if I am wrong)

    1. Exhaust drawing air, likely at point headers meet manifold
    2. Carb set-up

    I'm very reluctant to adjust the carb as the engine is making good power and torque all the way across the rev range 2000-6500k, no idle issues, no hesitation, albeit it nearly makes my eyes water with the fuel (vapour) if the bonnet is up. Previous owner had engine built and dyno'd, albeit this was a few years ago, it has only covered around 2000 miles. Its safe to say the exhausts have minimal, if any sound deadening.

    Engine is a 383 Chevy with Street Avenger Carb.

    Thoughts and suggestions for easy checks welcome.
    Andy
    Dax- 350 Chevy, Holley Carb - Loads of smiles to the gallon SOLD
    AK- 383 Chevy - Even more smiles to even more gallons

  2. #2
    Join Date
    Sep 2010
    Location
    Nova Scotia
    Posts
    903
    I had that problem before. It was from your reason one. Loose manifold to head gasket.
    John

    “A prudent person profits from personal experience, a wise one from the experience of others.”

  3. #3
    Join Date
    Oct 2018
    Location
    Shipston-On-Stour
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    188
    Most likely the exhaust manifold as John says. Can be hard to hear/see, better to feel with your hand (before it gets hot). On another thread last week a couple of us were discussing the compressible and re-usable gaskets you can get, like these https://www.summitracing.com/parts/mrg-7412g (this particular set is for an FE). Very good.
    Chris
    Dax with 1964 Dual Quad 427 and Toploader box

  4. #4
    Join Date
    Apr 2004
    Location
    East Leake, Nottinghamshire, England.
    Age
    55
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    9,553
    Quote Originally Posted by ChrisP View Post
    Most likely the exhaust manifold as John says. Can be hard to hear/see, better to feel with your hand (before it gets hot). On another thread last week a couple of us were discussing the compressible and re-usable gaskets you can get, like these https://www.summitracing.com/parts/mrg-7412g (this particular set is for an FE). Very good.
    Chris
    Don't waste your money on gaskets, I just use high temp silicon which a lot of people use (it wasn't my idea), been using it for over 12 years now after changing loads of inlet gaskets and replacing and re-tightening the bolts. Never had to re-tighten a bolt since and never had a leake either, not touched them in years now.
    Martin

    351W 416.7 HP and 466.8 ft-lbs

    Running Mega Squirt and Edis 8


  5. #5
    Join Date
    Jun 2007
    Location
    Lanarkshire
    Age
    43
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    202
    Thanks Gents

    Will have a nosey at the weekend and report back...
    Andy
    Dax- 350 Chevy, Holley Carb - Loads of smiles to the gallon SOLD
    AK- 383 Chevy - Even more smiles to even more gallons

  6. #6
    Join Date
    Jun 2007
    Location
    Lanarkshire
    Age
    43
    Posts
    202
    Quick update. Some of the manifold bolts could have been tighter but weren't exactly loose. Tightened them up and it made no difference. Looks like next step is gasket or high temp silicone.

    I assume there won't be enough play to do this without removing the side pipes...
    Andy
    Dax- 350 Chevy, Holley Carb - Loads of smiles to the gallon SOLD
    AK- 383 Chevy - Even more smiles to even more gallons

  7. #7
    Join Date
    Oct 2018
    Location
    Shipston-On-Stour
    Posts
    188
    Quote Originally Posted by Andy76 View Post
    Quick update. Some of the manifold bolts could have been tighter but weren't exactly loose. Tightened them up and it made no difference. Looks like next step is gasket or high temp silicone.

    I assume there won't be enough play to do this without removing the side pipes...
    Depends on your set up. Luckily, I can do mine without taking the side pipes off with a bit of twisting and sliding! Can you feel exhaust gas escaping when the engine is running? It would be good to be sure that is the problem before going to the trouble of taking them off. I usually run my hand around before the engine gets too hot. It’s a lot easier to feel than see.

  8. #8
    Join Date
    Aug 2016
    Location
    WELLINGBOROUGH
    Posts
    119
    Very Interesting thread, but I don't understand the physics of the problem.

    I get that gas (burnt fuel residue and some unburnt fuel) is escaping from between manifold and header.

    Is the popping and banging (unburnt fuel igniting) going on inside or outside the exhaust system?

    If it is inside where is the oxygen coming from to burn the fuel?

    If it is outside the exhaust system it is igniting fuel in the engine bay (dangerous) and pumping exhaust gas into the engine bay. Also what is the ignition source once the gases have left the manifold?

    I probably have it all upside down but I just can't see it.

    sceptic

  9. #9
    Join Date
    Feb 2011
    Location
    Northampton
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    4,239
    The theory is that the exhaust gas is carrying a lot of unburnt fuel - well over the stoichiometric ratio, so it doesn't easily ignite.
    The exhaust gasses passing through the exhaust draw in fresh oxygen by the venturi effect, which lowers the air/fuel ratio to a point where the mixture will spontaneously ignite in the exhaust due to the latent heat present in the metal

    At low revs the exhaust gas velocity isn't high enough to cause the venturi effect so you can feel the gas come out, but at higher revs it's actually sucking more than it's blowing.

    The alternative could also be seriously retarded ignition timing causing the plug to fire with the exhaust valve open, causing the same thing to happen; but given how basic the ignition timing is on our cars I'd expect it to run like a bag of nails as well if that were the case
    My DeDion build diary..
    Hon Sec of the Digidash branch of the Unpopular Kit Car Design club

  10. #10
    Join Date
    Feb 2007
    Location
    West Sussex
    Posts
    8,968
    Quote Originally Posted by scepticwoody View Post
    Very Interesting thread, but I don't understand the physics of the problem.

    I get that gas (burnt fuel residue and some unburnt fuel) is escaping from between manifold and header.

    Is the popping and banging (unburnt fuel igniting) going on inside or outside the exhaust system?

    If it is inside where is the oxygen coming from to burn the fuel?

    If it is outside the exhaust system it is igniting fuel in the engine bay (dangerous) and pumping exhaust gas into the engine bay. Also what is the ignition source once the gases have left the manifold?

    I probably have it all upside down but I just can't see it.

    sceptic
    Yup I also noticed the OP saying that lifting the bonnet made his eyes water.
    That doesn't sound right to me. My big ol' FE might stink a bit at the sidpeipe exit, but you cant smell anything when I lift thee bonnet, or not that I've noticed.
    Are Chevy's usually like this?
    Crendon Chassis No.49
    Huddart FE428 + toploader

    Not listed in the Shelby Register.

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