Monday, 19 March 2007

100E Misfire

My Dellow has a 100e with Aquaplane head and Shorrock blower and i am plagued by a misfire under load when the engine is hot. failed to get off the line at the MG Spring trial.
Engine strts cold Ok and accelerated with no problem until hot then misfires when you really need it.
Have changed coil, condenser dist cap, plugs -tried NGK B6ES and B8ES .
Sometimes engine cuts out when giving slight acceleration from rest then will not restart until you let it cool down for 15 mins. Any ideas before the Lands End?


  1. hi if you have a SU carb fitted make sure the dash pot has oill in it and the piston is free to move up and down

  2. Hi.  This sounds like a classic case of exhaust valve recession to me.  Are you sure that the valve clearances are set correctly?  I would imagine that with the extra heat being generated via the supercharger that the valves are expanding and taking up all the available clearance and so causing a misfire as the valves aren't seating properly.  I don't know how the clearances are set on a 100E engine with an aquaplane head but maybe as an experiment you could just try opening up the clearances to the maximum allowable and see if it makes any difference.  Best of luck.

  3. Bad petrol, must be the lamda sensor! All that Silicone. Seriously rich mixture? Choke presumably returning jet up against the adjusting nut, needle flush with bottom of piston, something that has happened suddenly or a gradual deteriation? Foat level OK , bowl  not punctured. Points not shorting out on advance or retard (loud bangs in Exhaust usually when it happened to me once). Water in float bowl - severe cutting out. Filter OK.

  4. Sorry that despite my helping you at the trial with reconnecting your ignition to the fuel pump wiring that it was still misfiring, I wonder if there is an earth connection problem, and/or that the coil connections need reversing. Perhaps the ignition timing is too far advanced.
    On another issue did you have a look at the steering ball joints.

  5. One other idea from experience. Just how hot is it getting under the bonnet ? and does it refuse or is difficult to start and misfires if you've just stopped for petrol, after you come out from paying ? Reason I ask is because this is a classic heat soak problem prevalent in the Liege. The fuel in the float chamber vapourises and it can be difficult to start or you get it started but as soon as you try to drive it, it farts & bangs and misfires for half a mile or so until fresh cool petrol makes it's way through the system. It can be "helped" by pulling the choke out a bit until it picks up again, but be careful then not to flood it. I presume you are on "suck through" the carb into the blower ? Is this a new'ish problem ? Had you changed anything recently before it started to manifest itself ? I suppose it could just be the petrol pump, is yours mechanical or electrical ?   Best of luck, SimonR

  6. My blower and float chamber stick out of the bonnet and I have had this set up for 20 years with no problem.
    Have taken the inlet side apart in case air was getting sucked in now today will take pressure side apart in case there is a leak in one of my soldered joints.
    You can do 5 miles of hard accelertaion up a hill, blower at 5Lb boost temp at 85-90C, come down the other side on overun put your foot down and misfire which will not vlear until you let it cool down.

    have checjed timoing with a strobe next will check valve clearances or put it on a rolling road.

  7. Not caused trouble for twenty years! I presume the coil to distributor wire has been changed, had an intermittent fault there once, the coil to battery was hot wired? so can't be that. Sounds like plug oiling almost but they have been changed and cooling down would make that worse and the smoke would be obvious! Can't be icing as all conditions have been experienced this winter! Really like to know what it is, tell us please.

  8. Hi, Had you considered petrol vapourisation ? What sort of day was it upon the MG Trial ?  This problem can manifest itself as a result of the slightest change, be it fuel pipe route or changing the size of sump guard.  The symptoms are as discribed. Lucky if it was this simple. Regards Jonathan

  9. I have had similar problems with a sidevalve and it was either fuel vaporisation due to a poor position of the pump and the petrol pipe being two close to the engine (remember there are two sorts of SU pump one is a suck as used on a Morris Minor mounted at the front of the car and one is a blow.Mini, MGB etc mounted by the tank)
    On my car the previous owner had mounted a "blow" pump at the front of the car and it would not suck enough fuel.

    One other problem unique to the 100E is that the condensor is inside the distributer and can get cooked by the engine heat so will work when cold but will not work when hot I mounbted the condensor on the coil on the Buckler and it cured the problem.

    I have assumed that the coil is new as an old coil can cause the same symptoms when it gets hot - they short out inside bur work again when cold.


  10. How is the misfire?  Would be very interested to hear how you managed to cure it.   Regards, Andy.