Backfire

Discussion in 'Mech Tech' started by Barry Haynes, Nov 14, 2017.

  1. Help
    The trouble I was having was the carbs were overfilling and flowing into the sump,so I changed the float valves and took each jet out and cleaned and blew air through,
    I didn’t touch the timing or anything else and there are no air leaks at the manifold, now the van starts and ticks over lovely , but as soon as I open each throttle by hand ( I’ve got the linkage disconnected at the mo) there is a almighty back fire through the actual carbs, left one worse than the right, could it be the mixture???
     
    jivedubbin likes this.
  2. I think back firing/popping through the carbs means the mixture is weak, what carbs are they?
     
  3. jivedubbin

    jivedubbin Moderator

    engine size,and carb make that always gets asked
     
  4. They are twin Weber 34 icts, I’ve had them on van for about a year, thinking about it, if the carbs were over filling that would make them rich now I’ve checked the float levels and changed the float valves I’ve altered the mixture to lean am I right in thinking that?
     
  5. Flakey

    Flakey Sponsor

    Are you opening them both exactly together?
     
    paradox likes this.
  6. T4 2-0 litre engine, twin Weber 34icts
     
  7. No, I’ve disconnected the linkage but I did it by hand and it still happens, it is ok on tick over
     
    snotty likes this.
  8. Fish

    Fish Administrator

    Did you check the valves and find out if you have solid state or Hydraulic lifters?... Hydraulic lifters can lock up and cause symptoms like this.
     
  9. If you open one carb and not the other one half of the engine will try and accelerate, but not the other...

    Hence bangs coughs and spits - fire out the top is likely as well.

    Easy fix, just reconnect then linkage so they both open and you're laughing.
     
    Flakey, nicktuft, Dubs and 2 others like this.
  10. No I didn’t but the van was set up on a rolling road and was running sweet, I was driving back from the Brighton Breeze when it started backfiring, I rang Pete the rolling road fella and he said it sounded like it was over filling and I should tap the carbs which did the trick, it happened a few more time so that’s why I changed the float valves, the only difference is before it was backfiring through the exhaust and now it’s backfiring through the carb
     
  11. It was tuned with leaky rich carbs . Now its clean and lean. Back to square 1 (ish)

    Either that and all the petrol flowing around has got in the oil and the engine is mucked up usually thats siezing up though..

    Backfires can also be timing, missing sparks or cam/valve issues. Or it just likes backfiring because its now lean on the overrun.
    I have exactly that... a gentle crackle going downhill , throttle closed no fuel and incredibly lean.
     
    Fish and Barry Haynes like this.
  12. That’s what I was thinking, is it right if I screw in the mixture screws all the way in, then unscrew them 21/2 turns this gives me a starting point, then adjust each one until the engine is just about to die then screw in a little ? If that makes sense
     
    snotty likes this.
  13. ^this, possibly. If the mixture has now become too lean, you may/will get backfiring through the carbs if you snap the throttle open. Spectacular, isn't it :eek:? I managed to remove several eyebrows setting my Dells up.

    So...sounds like you're ok, just need to adjust the idle mixture screws on the ICTs :thumbsup:
     
    Barry Haynes likes this.
  14. Yes, if that's the procedure for ICTs. Turn the screws slowly a bit at a time, and wait for it to settle. You may then have to adjust your idle speed on the linkage.

    The fact that one side of the engine will be trying to run faster than the other doesn't seem to be an issue.
     
  15. I had a few backfires today.......................
    ........................I think it may have been the curry I had last night!:D
     
  16. jivedubbin

    jivedubbin Moderator

    shut the door on your way out
     
  17. I'm using my sons iPad, what's this man Utd Marmite at the bottom and the dogging in my taxi ha ha ha :eek: first time I've seen it,how long has that been on there? :p:p:p
     
    Dicky, philntfc, Flakey and 1 other person like this.
  18. Should I connect the linkage first?
     
  19. Ages :). I know who I’d suspect...
     
  20. I had a set of the idf knockoffs ( the hpmx empi ones) on a t25, surprisingly good. But I did find if you tried opening just one side flames and a brief portal to hell opened up, regardless of how well tuned they were.....
     
    Barry Haynes and snotty like this.

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