Section : Carburettors

Running problems at 3000rpm Please help

Posted By : Stuart Evans, 21-Apr-2006, 07:09pm

Hi All

Can some one help me? My 1974 mkIV has developed a miss fire at around 3000 rpm when being driven, and has become very hard to start when cold, but once it's started it's starts with the first turn of the key and when reved it's fine and smooth.
At first i thought the problem was the vac advance but on testing the plate in the dizzy moved freely when i sucked on the pipe and i got suction from the carbs, the timming is set as it says in the book at 6 BTDC.
I have fitted new plugs, leads and coil and it has electronic ignition (which was on the car purchased it)
So what i am left with is the carbs could these be the problem?
I thinking of buying a referb kit anyway
But is there anything else it could be?
Any help would be very much appicated.

Cheers
Stuart

Followups :

Running problems at 3000rpm Please help

Posted By : Nigel Gibbins, 21-Apr-2006, 07:45pm

This problem seem to be raised on the MB at regular intervals. You should check the archives as I can't remember what the solution was...
Running problems at 3000rpm Please help

Posted By : Steve Cureton, 21-Apr-2006, 08:40pm

Have you looked at the colour of the spark plugs to check the mixture is correct?
- Running problems at 3000rpm Please help
  
  Posted By : Stuart Evans, 21-Apr-2006, 09:23pm
  
  I think the the set ups of the carbs is all wrong i am going to have a look tomorrow.
  I have got a good strong spark at the plugs.
  I have fitted a s/s exhaust and manifold and the carbs had k&n filters on when i got the beast.
  I've been doing some research on the net and i have read about richer and weaker jets and different stations on the jet, could be that the jet is running to weak/rich at around the 3000 rpm mark?
  - Running problems at 3000rpm Please help
    
    Posted By : Chris Taylor, 22-Apr-2006, 10:19am
    
    If you have K&N filters with SU's (but not Strombergs!) you need richer needles.
    If you have a free-flow exhaust manifold, you need different needles.
    SO I conclude that you need different metering needles! The ID is stamped on the shank of the needle (sadly means you've got to take them out).
    Not sure what the correct needles should be for your spec, but if you have a look at previous posts on this board you'll get the info.
    It's not just a matter of adjusting the mixture strength (that just gets it right at idle; with the coorect needles that means it's right for the whole rev range ASSUMING the needles are the correct profile for the engine spec involved).
    Your previous owner MAY have fitted the right needles, but no guarantees of course.........
    - Running problems at 3000rpm Please help
      
      Posted By : Stuart Evans, 22-Apr-2006, 02:18pm
      
      Right
      
      Looked at carbs this morning, my starting problem is the jet sticking in the front carb (not moving down when choke is on) so i need to fix that.
      
      As for the miss fire at around 3000rpm upwards it looks like i might need richer needle valves and stronger springs because of my mods
      
      Would HS4 carbs be a suitable upgrade (watching some on ebay at the moment) ?
      
      Does anyone know of any suitable spring and needle combinations for 1300 mkIV with HS2 aud624 carbs with K&N filters and s/s free flowing manifold and exhuast?
      
      I have read that AAQ needles and yellow springs would be a good starting point does this sound about right?
      
      Stuart
      - Running problems at 3000rpm Please help
        
        Posted By : Mark Astley, 24-Apr-2006, 05:31pm
        
        Hi,
        I have a 1300 mkIV the same as you, I upped my needles to AAQ and changed the springs to yellows having suffered poor performance like yourself. I already had the extractor manifold and free flow exhaust but when I added the K&Ns the standard needles couldn't fuel fast enough to cope with the rgeater air volume flowing at higher revs. It was never rolling roaded with this setup but the plugs always looked fine and it went much much better at the top end.
        
        Yours
        Mark
        
        Running problems at 3000rpm Please help
        
        Posted By : Stuart Evans, 24-Apr-2006, 11:07pm
        
        Thanks for that mark, i have ordered the needles and springs and once fitted i will you let know the results.
        
        Cheers
        Stuart
        
        Running problems at 3000rpm Please help
        
        Posted By : Stuart Evans, 05-May-2006, 08:36pm
        
        I would like to thank you all for your advice,
        In the end i upgraded the carbs to HS4 carbs from a 1500 and it now runs a 100 times better it pulls all the way through the rev range in all gears and it starts first time
        I will get the carbs set up on a rolling road once i finish the last few jobs for mot retest time
        
        Thanks Stuart