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  1. #21
    Join Date
    Aug 2007
    Location
    Bournemouth
    Posts
    341
    If this is a gradual problem then it will be down to something constantly changing with use, ie, ignition timing and/or carb set up. If your venturi chokes are too small or the jet is too big then that could induce backfiring as well. But check the float height adjustment too. Too much fuel will cause backfire as in your case, especially in the overrun.
    If this is an inherent problem then check your cam timing set up.
    Unless you have soot residue around the exhaust manifold mating to the head, I would dismiss that as the problem.

    David.
    Life is driving the Cobra. The rest of the time is just waiting.

    Wessex region caretaker rep.

  2. #22
    Join Date
    Jun 2007
    Location
    Lanarkshire
    Age
    43
    Posts
    203
    Quote Originally Posted by Darnpistonbroke View Post
    If this is a gradual problem then it will be down to something constantly changing with use, ie, ignition timing and/or carb set up. If your venturi chokes are too small or the jet is too big then that could induce backfiring as well. But check the float height adjustment too. Too much fuel will cause backfire as in your case, especially in the overrun.
    If this is an inherent problem then check your cam timing set up.
    Unless you have soot residue around the exhaust manifold mating to the head, I would dismiss that as the problem.

    David.
    The floats are fine as I checked them previously after reading the Holley manual so my thinking now is the overfuelling which in my simple non carb mind comes down to jets.

    The guy up here that does my mates mustang said its not really his specialist. His recommendation was John Sleath, unfortunately he is in Doncaster which isn't round the corner.... I was a bit surprised he didn't offer anyone closer or at least the right side of the wall.

    Thanks for the input from all so far
    Andy
    Dax- 350 Chevy, Holley Carb - Loads of smiles to the gallon SOLD
    AK- 383 Chevy - Even more smiles to even more gallons

  3. #23
    Join Date
    Jun 2007
    Location
    Lanarkshire
    Age
    43
    Posts
    203
    OK chaps. It's been a wee while so up date as promised. No real news or progress but some satisfaction and item 2 can now be ruled out.

    I had wanted the carb looked at because I have had a wee bit of a doubt about it. Spoke to a few people up here but no names came up and the people I spoke to weren't confident given the modified set-up. John Sleath seemed the closest name that kept coming up. As he advised before I went, if its the side pipes to headers drawing air the problem would persist.

    So, the problem still persists although slightly differently now. As theres no signs of manifold leak, the next place to look would be the side pipes to headers interface.

    The trip wasn't in vain though as a few changes and tweaks were made. Being the first time I've had a car on a dyno it was quite an experience, in fact best day out this year! The engine made good power and despite, some popping, banging and flames being ejected from the side pipes, a clean bill of health with some advice from a thoroughly nice gentleman.

    So next stop is to remove the sidepipes and see what can be done. I've ordered up some copper based sealant so will report back in due course.

    Cheers
    Andy
    Dax- 350 Chevy, Holley Carb - Loads of smiles to the gallon SOLD
    AK- 383 Chevy - Even more smiles to even more gallons

  4. #24
    Join Date
    Oct 2018
    Location
    Shipston-On-Stour
    Posts
    200
    Popping, banging and flames! Sounds like a proper cobra!!!! Seems you are getting there. Nice the carb didn’t need loads of fiddling.

  5. #25
    Join Date
    Jun 2007
    Location
    Lanarkshire
    Age
    43
    Posts
    203
    OK. Situation improved to a satisfactory point for now.

    I've had the carb looked at and fuelling idle jet changed, timing checked and advance curve adjusted so its still running well (as per previous post). This made no difference to the initial pop on lift off.

    Since then, I've had the sidepipes off and as best I could applied the copper sealant. Its had limited success and the worst of the initial pop is reduced most of the time. I suspect I needed considerably more sealant and even at that getting a seal will be difficult as the pipes wiggle into place.

    Pipes came off nice and easy as they were just dry fitted and didn't even have to jack the car up. The copper sealant proved to be a messy affair!

    So still getting the lesser pops and bangs on overrun and a lesser amount of the really loud cracks on initial lift off which was the aim of the exercise. All in all, it would be nice to tone it done just a little further but for the time being it probably not worth the effort. Will run it a while and see how it goes.

    Don't worry it still sounds like a Cobra and ear plugs still required!

    Cheers
    Andy
    Andy
    Dax- 350 Chevy, Holley Carb - Loads of smiles to the gallon SOLD
    AK- 383 Chevy - Even more smiles to even more gallons

  6. #26
    Join Date
    Jan 2006
    Location
    Chelmsford Essex
    Posts
    1,273
    I had a quick read through and looks like you have done most of the main checks. I trust you have had a vacuum gauge when you set up the carb and timing ? A good solid needle will eliminate most of the induction side.

    As you say the sealing of the sidepipes seams to of resolved some popping so you could still be drawing air in on the exhaust. I would look at the underside of the headers and any tail-tail signs of black exhaust soot leak marks of exhaust .
    You might be able to hear the buffing sound blowing past or even feel hot gasses with your had at any joints. With so much noise it is difficult to locate.
    Just some thoughts.
    Dax 351 Cleveland T5 2.88 PL

  7. #27
    Join Date
    Oct 2018
    Location
    Shipston-On-Stour
    Posts
    200
    Quote Originally Posted by Andy76 View Post
    OK. Situation improved to a satisfactory point for now.

    I've had the carb looked at and fuelling idle jet changed, timing checked and advance curve adjusted so its still running well (as per previous post). This made no difference to the initial pop on lift off.

    Since then, I've had the sidepipes off and as best I could applied the copper sealant. Its had limited success and the worst of the initial pop is reduced most of the time. I suspect I needed considerably more sealant and even at that getting a seal will be difficult as the pipes wiggle into place.

    Pipes came off nice and easy as they were just dry fitted and didn't even have to jack the car up. The copper sealant proved to be a messy affair!

    So still getting the lesser pops and bangs on overrun and a lesser amount of the really loud cracks on initial lift off which was the aim of the exercise. All in all, it would be nice to tone it done just a little further but for the time being it probably not worth the effort. Will run it a while and see how it goes.

    Don't worry it still sounds like a Cobra and ear plugs still required!

    Cheers
    Andy
    All good fun. Mine was doing it today! I know what the problem is, bit of damage on the heads from racing days which I have to repair every so often. Just let’s a tiny bit of air in and away it goes. Am in the process of shipping some same number heads in from the states to replace them. Happen to have a very rare set of heads on mine and am looking to keep it the same but without the popping (although I quite enjoy it especially followed by a jet of flame at night!!!!).

  8. #28
    Join Date
    Jun 2007
    Location
    Lanarkshire
    Age
    43
    Posts
    203
    Quote Originally Posted by jon1d View Post
    I had a quick read through and looks like you have done most of the main checks. I trust you have had a vacuum gauge when you set up the carb and timing ? A good solid needle will eliminate most of the induction side.

    As you say the sealing of the sidepipes seams to of resolved some popping so you could still be drawing air in on the exhaust. I would look at the underside of the headers and any tail-tail signs of black exhaust soot leak marks of exhaust .
    You might be able to hear the buffing sound blowing past or even feel hot gasses with your had at any joints. With so much noise it is difficult to locate.
    Just some thoughts.
    I think i've satisfied myself there aren't any leaks at the manifold. More sealant in the slip joints would have been the answer, i just ran a single bead around the inside of the side pipe slip joint which obviously will have smeared as I refitted the pipes so air will still be passing. I didn't want to over do it and eradicate all the excitement
    Andy
    Dax- 350 Chevy, Holley Carb - Loads of smiles to the gallon SOLD
    AK- 383 Chevy - Even more smiles to even more gallons

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