Random misfire

Discussion in 'Mech Tech' started by Marty SmartyCat, Jan 22, 2023.

  1. Marty SmartyCat

    Marty SmartyCat Supporter

    Got a bit of a weird one at the moment.

    Background - 1700 Type 4 with twin Weber ICT's. Carbs were rebuilt back in April 2022 by Way Out Westie so it has the suggested jets, emulsion tubes etc. for my engine. After the rebuild, engine ran really well all through the summer. It's got Pertronix Flame Thrower electronic ignition.

    The last few months, not really used the van much.

    The last few times I've driven it, I get random misfire when cruising on light throttle (around 50mph).

    I've checked the following today: -
    • Timing is spot on
    • Tick over is around 900rpm
    • Carbs are balanced
    • At idle, van ticks over nice and smoothly
    • When stationary, no issues when moving through the rev range
    • Under throttle load, van pulls strongly all through the gears with no hesitation or misfiring
    It's just when running at very light throttle in 2nd, 3rd or 4th, it will randomly misfire - but not every ignition cycle.

    Anybody got any thoughts or idea?
     
  2. Marty SmartyCat

    Marty SmartyCat Supporter

    I should add, the last couple of journeys have been 70-100 miles each. I'm using E10 fuel with fuel conditioner.
     
  3. Check your idle jets aren't bunged up with gunge :hattip:
     
  4. mikedjames

    mikedjames Supporter

    One strange one I have seen a couple of times is an interaction between a Facet style pulsing fuel pump and the Pertronix module. At certain engine speeds, the voltage dips from the pump drawing power from the T4's ignition wiring are big enough to disturb the "adaptive dwell" of the Pertronix.. presents as a slight misfire ..as the beats of the pump beat against the pulses of the ignition.. I saw it on an oscilloscope watching the ignition .. fun at Techenders.

    The fix seems to be to power the fuel pump via fused wiring and an ignition switched relay directly from the battery.
    This reduces the voltage drop .

    (Or put the Pertronix to one side and use a contact breaker and points , or the much older, far simpler and less temperamental Ignitor ONE)
     
    Soggz and redgaz like this.
  5. Marty SmartyCat

    Marty SmartyCat Supporter

    Good shout - will check that next weekend.
     
  6. Marty SmartyCat

    Marty SmartyCat Supporter

    Fuel pump is on a switched relay already - but thanks for the suggestion.
     
  7. Stick some points back in to test, if you've got some.

    My suspicion would be anything from Pertroxix with "Flamethrower" written on it. Have you got one of their coils?
     
    redgaz likes this.
  8. mikedjames

    mikedjames Supporter

    And stupid things like the spark plug lead is half fallen off the spark plug so it misfires occasionally, or flash over on the coil from time to time, if the centre lead is loose..
     
  9. Zed

    Zed Gradually getting grumpier

    Or a cracked plug cap - pesky long things on Type-4.
     
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  10. Marty SmartyCat

    Marty SmartyCat Supporter

    not got one of their coils. old dizzy is shot, hence the Pertronix (less than 12 months old)
     
  11. Marty SmartyCat

    Marty SmartyCat Supporter

    Plug leads and caps all look good - less than 12 months old.
     
    Zed likes this.
  12. Soggz

    Soggz An inquisitive supporter

    Old school points never failed my old 1700, never had any trouble like you describe. Just check them every couple of weeks. Wish I still had my old 1700. More reliable imho.
     
  13. Marty SmartyCat

    Marty SmartyCat Supporter

    Checked them out - nothing obvious but gave them a blast with air for good measure and wired the outside. Will try the van the weekend.
     
    F_Pantos likes this.
  14. mikedjames

    mikedjames Supporter

    I wonder if the carbs are actually too well insulated from the manifolds for this weather - after you spend ages fitting fibre spacer gaskets to avoid heat soak issues on a hot day, you are cooling down the carburettors in cold weather.

    Remember the stock Solex dual carburettor system for the T4 engine also uses a hot air feed, similar to the T1. And it is quite cold at the moment.

    At part throttle which represents high manifold vacuum, you are rapidly cooling the air going through the carburettors as the air expands below the throttle plate. So this could be a subtler version of the issue with centremount carburettors with inadequate heat. .
     
  15. Marty SmartyCat

    Marty SmartyCat Supporter

    Think I may have found the problem. The left hand carb responds when the idle mixture screw is turned. The right hand carb does nothing when the screw is turned...which makes me suspicious
     
    Last edited: Jan 28, 2023
    Zed and Lasty like this.
  16. Marty SmartyCat

    Marty SmartyCat Supporter

    Well, swapped the idle mixture screws between the carbs and no difference so that rules out the mixture screw.

    Took the carb off and squirted carb cleaner into where the idle mixture screw is fitted as well as giving the idle jet a clean and squirt of carb cleaner.

    Checked carb balance and all good.

    Checked all plugs were clean and gapped correctly.

    Still can't get rid of the random misfire...
     
  17. Perhaps dodgy o-rings on the idle screws :thumbsup:
     
    Last edited: Feb 5, 2023
  18. mikedjames

    mikedjames Supporter

    Swapping the screws just shows it's not the screws.. if the seat that the screw fits into is damaged the fault will stay with the carb.
    Also never assume that because recently rebuilt , there isn't something wrong with the needle valves. or the idle jets. a bit of dirt disturbed by you refitting hoses either raising fuel level one side or totally blocking the idle jet ( most likely, take the carbs out and clean the jets ..I can usually sight through the net on my progressive and see a blockage against a bright light)
    If you take the tops off the carbs in situ, you should see the fuel sitting in the float chambers at the same level both sides..
     
  19. Marty SmartyCat

    Marty SmartyCat Supporter

    The coil isn't Pertronix as far as a I know.

    My original distributor was worn out - hence the Pertronix electronic ignition. Happy to buy a spare distributor to rule out the Pertronix - any suggestions or recommendations (that aren't overly expensive as it may be kept as a spare)?
     
  20. Poptop2

    Poptop2 Administrator

    Have you checked the exhaust and manifold are sitting correctly and have no air leaks?
     
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