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Thread: Carb Backfire... Help!

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    Carb Backfire... Help!

    Hi guys

    Cars been running fine up until starting it tonight, there's now a squeaking and backfire (throws the petrol out the carb) on cylinder 2. Never done this before and its even popping out the exhaust on idle (Sounds more clear with headphones)

    Here's a video so hope someone can help:
    VID_20131104_222317_zps19696f77.mp4 Video by liquidd_monkeyy | Photobucket

    Left heavy carbon almost oil like deposits in the backbox after a few minutes of running very rough

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    'VXR' Member Taurus's Avatar
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    Start with checking the timing.
    1972 Viva restoration thread - [url]https://www.thecorsa.co.uk/projects-builds/138-1972-vauxhall-viva.html#post1534[/url]

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    Quote Originally Posted by Taurus View Post
    Start with checking the timing.
    Will do thanks Taurus, Could the belt have slipped?

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    Poorly valves?

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    'VXR' Member Taurus's Avatar
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    Quote Originally Posted by Sam_Sri View Post
    Will do thanks Taurus, Could the belt have slipped?
    Possibly - anything to do with backfires, especially through the carbs, then it's always worth checking the timing is OK before progressing further.

    Quote Originally Posted by P1nky View Post
    Poorly valves?
    That's a possibility of course - hopefully not. If the timing has slipped far enough then it can happen. Or they can just stick or burn out. If the timing is spot on then the next step is to check the compressions. As ever, take it one step at a time.
    1972 Viva restoration thread - [url]https://www.thecorsa.co.uk/projects-builds/138-1972-vauxhall-viva.html#post1534[/url]

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    Thanks for the advice guys, I'll start with checking the timing tomorrow evening then go from there.

    Hopefully its not head related as I've only just rebuilt it.

    I find it odd that one day the cars fine and next time I start it, its like this, would have expected it to happen whilst driving or something.

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    Iirc youre on bike carbs arent you? Ive seen them stick before

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    Quote Originally Posted by P1nky View Post
    Iirc youre on bike carbs arent you? Ive seen them stick before
    Sure am mate, yeah? Thanks for the info I'll add that to my list to check.

    Found a total Vauxhall PDF which shows how to fully strip and clean bike carbs as I've only partially stripped and cleaned them before, so am going to try that after checking the above suggestions

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    id put money on it being the belt slipped a tooth or two. normaly happen on first crank. check the belt tentioner hasnt slackened off.
    Take a look, Its old and rusty[URL="https://www.thecorsa.co.uk/projects-builds/182-zetec-powered-mk1-fiesta.html"]
    https://www.thecorsa.co.uk/projects-builds/182-zetec-powered-mk1-fiesta.html[/URL]

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    Timing will be checked tomorrow as i'm missing the socket i need to turn it over by hand and will be doing a compression test at the same time.

    Last night i re-fitted the carbs, checked the plugs which looked awful, checked ht leads etc and when i started it, it run like normal! apart from a very unhealthy fuel/oil smell. Took it for a drive and it ran strong!

    BUT.... Started up today and its doing the same as the first video but worse and gets better as the engine warms up (less carb backfire) but still runs lumpy.

    Plugs...



    I think the oil on this one is due to a rocker cover leak but will double check tomorrow.. Just to note this one looks the worst but is cylinder 4 plug but its cylinder 2 carb which is having the issue:



    Video after start up:

    VID_20131105_190650_zpsa0ee7026.mp4 Video by liquidd_monkeyy | Photobucket

    After 15mins running:

    VID_20131105_192006_zpsa84ea064.mp4 Video by liquidd_monkeyy | Photobucket

    After a drive round the block: (bangs are fireworks)

    VID_20131105_193106_zpsc0b51ade.mp4 Video by liquidd_monkeyy | Photobucket

    But obviously back to square 1 now. I've got HT Racing at brands hatch lined up to map the ECU on the dyno but i need this sorted first and the carbs balanced.

    Any help is appreciated

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    When were the plugs last pulled to check them?
    Is it just one cylinder that has oily deposits as bad as that?
    Were the valve stem oil seals and guides done when the head was rebuilt, or was it pressure tested?

    It looks like it's burning oil in at least one cylinder.

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    Quote Originally Posted by Hardcore View Post
    When were the plugs last pulled to check them?
    Is it just one cylinder that has oily deposits as bad as that?
    Were the valve stem oil seals and guides done when the head was rebuilt, or was it pressure tested?

    It looks like it's burning oil in at least one cylinder.
    Probably about 4 weeks ago and they just showed signs of slight over fuelling. And yes just the one cylinder but it's not the one that I'm getting backfire off.

    Stem seals were replaced but not the guides, head was machined and tested and came back fine

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    Diesel lover Hardcore's Avatar
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    When you done the sump gasket was there any smell of petrol from the oil?

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    'VXR' Member Taurus's Avatar
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    Hmm - to be honest I'd check the timing and compressions before doing anything else. You need to know those are good first of all.
    1972 Viva restoration thread - [url]https://www.thecorsa.co.uk/projects-builds/138-1972-vauxhall-viva.html#post1534[/url]

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    I didn't notice a distinctive smell of petrol tbh seemed all normal when we did it.

    Sure thing I'll report back tomorrow evening and go from there

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    Compression test complete. Came back between 200-230psi over all 4 cylinders

    Timing was spot on as well.

    This narrows it down to being a carb/ignition issue...

    Noticed the fuel wasn't actually spitting out of the trumpet but it appears its coming from behind it as shown circled in red below.







    The screw thread thing (not sure what it is) seems to be at an angle but im not sure whether this would affect it? As you can see in the pictures theres a fair bit of fuel around that area which was very fresh

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    Most of the other things that i can think of, off the top of my head that could cause the backfire would be linked to lean running as well, but the plugs show it's running anything but lean.
    Hopefully someone with more experience with carbs will work out what's wrong.

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    Yeah i know I'm quite confused :/ must be something to do with that one carb... All fails I'll take them to a local bike shop or even sent up north to boggs brothers

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    Asked my dad there. He has some experience with bike carbs, but he said he's never seen a plug like that before in 40 years of working on bikes.

    He asked if you blocked the overflow pipes on your float bowls at the bottom of the carbs?
    He said the backfire looks to be caused by running too rich, but the rich running being a massive understatement. He said if the ignition timing is ok, then the fuel jets are far too big (if all plugs are wet like that)

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    Diesel lover Hardcore's Avatar
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    Bearing in mind he's used to dealing with these things when FPR's and ecu controlled ignition systems aren't in use

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