carb fire

Discussion in 'Tech/Mech General' started by johng, Mar 8, 2008.

  1. johng

    johng Read Only Funster

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    all you mechanical whizzes out there ;)

    my 5.7 chevy has a rochester 4brl carb, this has had an internal fire at some point in the past. The float is half melted, and the metal cylinders/framework that surround jets in the primary throttles are melted away.. :Sad:

    The engine has single point LPG... upstream of the carb..

    any ideas as to what causes backfiring appreciated (I can only think plug leads arcing, dist. cap same)

    thanks
    John
     
  2. Geo

    Geo Trader - Funster

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    Are you sure the engine is or has back fired, ie poping and banging when running, or are you asuming it has, hence the fire in the carb
    A back fire in the true sense of the word is when fuel and a source of ignition appear together at the wrong time inside the engine, your referance to cap and leads would cause a misfire, the unburnt fuel could then result in a backfire, incorrect ignition timing is the most common cause of backfireing along with carbon build up on high milage engines,also poorly seated or damaged valves or valve timing errors the list goes on im afraid try to give a more presice description of your symtoms aprat from just a melted carb, wich may have been caused by over fueling and or dirty oily air filter and burnt after the engine was switched off
    Geo
     
  3. johng

    johng Read Only Funster

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    Geo thanks..

    erm.. there are no other symtoms to speak of really.. I just want an idea of what may have caused it, any connection to LPG, and what to do next.. a large tank of fuel and LPG at the back of van is a bit scary :(

    I did a compression test last week, and all 8 were equal and good...

    the van was running well enough before I decided to strip the carb, but I have no point of reference with a chevy engine really :( there has never been a backfire or noticeable misfiring in the short time I have driven the vehicle..

    can I ask how overfueling or a bad AF could cause a fire?

    Damage appears to include ,float, filler block, power piston assm, melting of alloy of main body and air horn...

    timing is controlled by a vacumn advance but I have not had the chance to check this much.

    The LPG point was made because, when i have looked at LPG fitting before, the question of plastic manifold and sequential systems (and backfiring) keeps raising it's head.. so what is the link between LPG and backfiring if any.. ?

    I brought a carb repair kit, but it will be wasted on this carb, there is too much damage for it ever to be efficient. so now a second hand rebuildable carb seems the cheapest way out of this money pit I am in.. anyone got one ;) ? (boats use them too it seems)

    John
     
    Last edited: Mar 8, 2008
  4. johng

    johng Read Only Funster

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    some pics..
     

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  5. Geo

    Geo Trader - Funster

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    Hi John carb engines are know to "Spit" every once in a while and a carb thats not set up properly can soak air filter and when engine spits it sets light to the excess fuel, same aplies to a dirty air filter that is virtualy oil soaked, it is highly unlikely this fire ocured whilst running, due to the inrush of air a sustained fire long enough to cause the damage you described is remote, as the fire would be dragged into a cylinder away from the carb The most likely scenario is a small backfire as you switched of and walked away the fire was allowed to continue unoticed till it ran out of combustable material, hence the amount of damage
    as you have been running on LPG the plugs will appear very clean anyway,so there will be no clues there
    I would be looking at checking ignition timing
    Geo
     
  6. Lindy-C

    Lindy-C Deleted User

    JC would know but he's still in the States :RollEyes:
     
  7. johng

    johng Read Only Funster

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    Thanks Geo

    I have only done 70 miles on 3 different journeys (Mainly petrol).. so I wonder how long the carb has been in this state.. how much power I may lost with it like this, how it ever ran !:Smile:

    J
     
  8. johng

    johng Read Only Funster

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    JC would know but he's still in the States

    thanks LC
    can he smuggle a carb into his suitcase ;)
     
  9. Lindy-C

    Lindy-C Deleted User

    Probably could have done John, but I have no means of contacting him as I have no idea where he'll be.
    Could put one in the next shipment though - if you can't find a Rochester, surely a Holley would work? I know thats what he did with one of our old busses years ago.....from memory the Holley was a better unit :Smile:
     
  10. johng

    johng Read Only Funster

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    hmm ;)

    I have this nice rebuild kit now, and finally got my head around how the roch works today .. ;)

    grrr bloody RV
     
  11. Lindy-C

    Lindy-C Deleted User

    You could always flog it and buy a tent! :Rofl1:
     
  12. American Dream

    American Dream Read Only Funster

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    These engines WILL backfire in the carb if given too much Gas at startup, although it sounds like yours ie an extreme case.

    Has the unit got an air filter fitted?

    I have a 440 charger block 7.2 v8 and it doed the same on startup.

    These engines need a lot of fuel in the system when starting, especially from cold.

    If it spits at startup just keep the accelerator down and any potential fire blowback gets sucked into the engine.
     
    Last edited: Mar 8, 2008
  13. johng

    johng Read Only Funster

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    AD.. hmm

    It starts with some difficulty..and needs to be warm before I can drive it really.. One reason I decided to strip the carb was that I noticed the primary choke flap had been forced permanently open with some flat string.. this was (one reason) why it took a fair while to start and needed gas... quite why it was forced open and not temp/vac controlled I have not found out yet

    IT has an LPG mixer (sprint australia) in place of what I imagine was a stock air filter housing.. and because this the air filter is bigger circumference and not as tall, and is 'open'

    pic follows

    Has anyone got an old rochester lying about ? for a boat even...

    John
     
  14. American Dream

    American Dream Read Only Funster

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    My Primary choke had been removed!!!

    I invested in another Carb to use as a donor and replaced the missing items.

    Why people have to pr.t about with vehicles I don't know.

    There is a rumour This results in better fuel economy but IMHO if the engine is a Pig to start, what's the point.:Doh:

    PS Mine's driveable from cold and idles well even when cold now.

    Tweaked the idle mixture screws and she now runs well.
     
    Last edited: Mar 10, 2008
  15. johng

    johng Read Only Funster

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    yikes..

    I dont suppose you still have the old carcass, for a crate of ale ?
     
  16. American Dream

    American Dream Read Only Funster

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    I do but it is 99% complete and I am keeping it as a spare as getting rare to find.

    Trawl ebay.That's where I found mine.:thumb:

    It would be too big for yours. 625 CFM Carb.
     
  17. johng

    johng Read Only Funster

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    ok :) there was a chap in Gloucester selling one, BIN £50 ? some water damage though, should be cleanable..

    these second hand units are only $20 on ebay .com :(
     
  18. American Dream

    American Dream Read Only Funster

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    Yep.Just bear in mind the carriage charges and import duty on top of that IF the sender is prepared to ship to the UK.

    There are Rebuilt carb suppliers over there but mind you you use.
     
  19. johng

    johng Read Only Funster

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  20. pappajohn

    pappajohn Funster Life Member

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    eeerhem!! sorry, but is this guy for real ?.:Doh:
    if i knew it was on fire i wouldnt be waiting to see if 'it'll go out on its own'.:Eeek:
    i'd be stood on the bumper p***ing on it if thats what it took.
     
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