Results 1 to 12 of 12

Thread: Misfire at high revs

  1. #1
    Senior Member 4tecdog's Avatar
    Join Date
    Oct 2015
    Location
    Wales uk
    Posts
    95
    Blog Entries
    1
    Chats
    0

    Misfire at high revs

    Can anyone put some though to an issue that has developed in this warmer weather (may be a coincidence and not related at all) My Bj8 has started missing above 3500 revs under hard acceleration also with an occasional backfire in the exhaust. The car will drive perfectly up to these revs even under heavy load in top climbing a steep gradient it will not miss at all. As soon as I slip out of overdrive and give it some beans it feels as if it is being held back and sometimes wakes the dead with that backfire.
    The coil is fairly new with petrinox ignition, running on twin SU carbs with a tubular manifold and side exhaust. The valve clearances are ok and the plugs seem to be burning the correct colour if a little on the lean side.

  2. #2
    Jedi Trainee
    Join Date
    Jun 2014
    Location
    NEPA
    Posts
    207
    Chats
    0

    Re: Misfire at high revs

    I would look into the lean condition if You feel good about all the ignition aspects. Fuel flow, jet settings etc.

  3. #3
    Darth Vader John Turney's Avatar
    Join Date
    May 2002
    Location
    Concord, CA
    Posts
    2,583
    Chats
    0

    Re: Misfire at high revs

    The backfire suggests to me that you may have a manifold leak.
    John, BN4

  4. #4
    Darth Vader
    Join Date
    Feb 2007
    Location
    California USA
    Posts
    2,945
    Chats
    0

    Re: Misfire at high revs

    Had similar symptoms from my BJ8 years ago, caused by a 'leaky' distributor cap. Run the engine in a dark--but well-ventilated garage--and look for any arcing around the cap or on on the secondary wires. Leaky cap was Bosch, replaced with a genuine--this was before counterfeits became common--Lucas cap and problem was gone (and has been for many miles).

    When I got my overhauled engine installed it had a nasty backfire due to the #6 wire making contact with the heater hose shutoff valve and causing that cylinder to misfire.

  5. #5
    Yoda Randy Forbes's Avatar
    Join Date
    Feb 2003
    Location
    SW Florida (Sarasota/Bradenton area)
    Posts
    4,150
    Chats
    0

    Re: Misfire at high revs

    I thought this would be easy to answer, until you mentioned Pertronix; had it been a points/condenser ignition, the block wearing down, or a faulty condenser would've been a sure-fire diagnosis.

    Back-firing under load is almost always indication of an overly lean condition, so I'd verify that your fuel pump is delivering sufficient quantities of fuel (partially blocked pipe or filter, as well as the pump itself). Unless someone has been playing with the carbs, they shouldn't have gone off just like that; can that be ruled out...?
    http://www.britishcarforum.com/bcf/image.php
    57 Healey BN6L-942 Wine Red/Honey Tan
    99 BMW M Rdstr Cosmos Black Eurosport Twinscrew Supercharger
    01 BMW M Rdstr Steel Gray Performance Center (factory) Delivery
    11 X5 35i Sport Deep Sea Blue Metallic Wife's Turbo Hauler

  6. #6
    Senior Member 4tecdog's Avatar
    Join Date
    Oct 2015
    Location
    Wales uk
    Posts
    95
    Blog Entries
    1
    Chats
    0

    Re: Misfire at high revs

    Thanks for your suggestions I will be doing more diagnosis this evening, the distributor cap (Lucas) rotor and leads were changed the same time as the coil, the carbs have not been touched except for checking dash pot oil, fuel pump was replaced some eighteen months ago for an SU electronic type, not to say there is a fuel restriction somewhere which to my way of thinking could be the cause. Back later with any findings.

  7. #7
    Jedi Hopeful
    Join Date
    Dec 2010
    Location
    West Bloomfield, Michigan
    Posts
    134
    Chats
    0

    Re: Misfire at high revs

    How old is the fuel in the tank?

  8. #8
    Senior Member 4tecdog's Avatar
    Join Date
    Oct 2015
    Location
    Wales uk
    Posts
    95
    Blog Entries
    1
    Chats
    0

    Re: Misfire at high revs

    Fuel is fresh did 120 miles in her yesterday and refuelled shell optimax. Fuel tank is old though so who knows could have picked some crud up .

  9. #9
    Senior Member 4tecdog's Avatar
    Join Date
    Oct 2015
    Location
    Wales uk
    Posts
    95
    Blog Entries
    1
    Chats
    0

    Re: Misfire at high revs

    Should have said V power don't know where optimax came from some Marine engine I suspect, anyway looked at distributor cap rerouted HT leads as no six was a bit close to heater valve and rechecked plugs was susprised to note the engine has recently been running a little on the rich side except cylinder no 1 which seems a perfect colour. Not much in it though. Checked the choke cables and Vaccum pipe both ok. Went for a fairly fast run did not get any backfire however does still seem to hold back a bit at high revs not in overdrive. Once in overdrive would not know there is a problem as she is doing well over our speed limit at 2700 revs. Could it be the ignition is not advancing for some reason causing a flat spot.

  10. #10
    Jedi Knight
    Join Date
    Mar 2003
    Location
    Hampshire in the UK
    Posts
    1,494
    Chats
    0

    Re: Misfire at high revs

    It is possibly fuel related - my similar problem was on a BJ7, different carbs of course, but one of the jets was blocked at the connection into the float chamber great economy but lousy top end speed could not get above 60mph.



    Bob

  11. #11

    Re: Misfire at high revs

    Also check the rotor arm. If there is any movement it can start 'floating'. I had the a similar problem at the top end of the rev range. I fitted a red rotor from the Distributor Doctor and that cured it. I would recommend getting one of these rotors anyway, even if it doesn't cure this particular problem. Excellent quality.

  12. #12
    Senior Member
    Join Date
    Aug 2016
    Location
    Greensburg, PA
    Posts
    62
    Chats
    0

    Re: Misfire at high revs

    How old is your fuel pump? I was having similar issues on my E-Type and when the 50+ year-old pump was replaced with a new one last year the misfire went away.

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •