Section : Engine

Engine, cooling system, fuel system, exhaust system.

Slow cranking on rebuilt engine

Posted By : David Christmas, 31-Jan-2007, 03:26pm

I have rebuilt the engine on my MK11 (rebore, pistons, rings, etc) and it will crank at a reasonable speed on the key, but then crank speed drops off with in two revolutions. At first I thought it might be the bendix on the starter motor, so I went and got another exchange unit, fitted it and the same result occurred. The battery is new, but I also swapped it with the one on my TR6, with the same results. I noticed the Earth strap from the front of the engine was quite thin, so I bypassed it with a jump lead to a good earth point, still the same result, although the jump lead did hot up. The engine did fire and ran very smoothly and the dynamo was charging fine, so I now know it works. I appriciate that the engine will be tight, but it doesn't explain to me why it will crank initially at an acceptable speed the drop off so fast. Doe's anyone have any ideas were to look next?

Followups :

Slow cranking on rebuilt engine

Posted By : Michael Davis, 31-Jan-2007, 03:37pm

I'd think you're probably looking in the right place with the engine earthing. I get similar symptoms on my racecar sometimes (but not always) and whilst I've never quite got to the bottom of it I've managed to improve the situation by remaking all the earth connections in the past.
- Slow cranking on rebuilt engine
  
  Posted By : Ian Mulford, 31-Jan-2007, 04:30pm
  
  I agree, make sure all connections in the circuit are clean and good. Battery to solenoid, solenoid to starter, engine to earth and body to battery. I've probably missed one out but clean and check them all.
  - Slow cranking on rebuilt engine
    
    Posted By : George Ralph, 31-Jan-2007, 05:04pm
    
    I had a similar problem, sometimes it cranked OK, sometimes it cranked slow, sometimes it just clicked. DId all the things suggested here without fixing it. Then I decided to take a look at the shiney new solenoid. WHen the motor cranked slow, the solenoid was getting hotter and hotter! I put the old rusty original solenoid back on and everything was fine! I Don't use a solenoid now 'cos I've got a fancy hi-torque starter with integral solenoid that spins so fast you get a decent oil pressure reading!
  - Slow cranking on rebuilt engine
    
    Posted By : Ian Osprey, 31-Jan-2007, 05:08pm
    
    Sounds like the earth to me too.
    To help confirm it, try taking your jump lead from the battery earth terminal directly onto the engine block. The gearbox to engine flange is a handy point close to the starter.
    - Slow cranking on rebuilt engine
      
      Posted By : Peter & Mick Lewis, 31-Jan-2007, 09:15pm
      
      David you mention benidx and getting an exchange unit,, was that complete starter or just the pinion you changed,,
      just a thought you can get this if one field winding is failed.. flies up then powers down like a flat battery as you describe.. if recon only had a bench test it could be as bad as the one you took off...
      ours now has a geared high torque motor, it so fast you dont need a battery,, best mod
      Ive done ...peter
Slow cranking on rebuilt engine

Posted By : David Christmas, 01-Feb-2007, 10:54pm

Thanks to everyone who responded, got the taps and sandpaper ready to clean up the earths! if that's no good, then an uprated starter looks on the cards.
- Slow cranking on rebuilt engine
  
  Posted By : Peter & Mick Lewis, 02-Feb-2007, 01:11pm
  
  one small point ,, we fitted earth links to all the body sections to improver volt drops over the car , one area that always
  amazed me is that the battery is earthed to the front body tub,,, most of which is on rubber and rusty mounts
  we added an earth lead from battery/baulkhead to chassis and then on to the cly block to make it bullet proof
  
  peter/mick
  - Slow cranking on rebuilt engine
    
    Posted By : Barry Huffer, 02-Feb-2007, 01:53pm
    
    Another good and traditional place is from the front engine plate to the steering mount.
- Slow cranking on rebuilt engine
  
  Posted By : Chris Taylor, 02-Feb-2007, 04:47pm
  
  Should be no need for an uprated starter. Standard Lucas one can spin the engine over quite well enough if it's in good fettle, and a reconditioned one is much cheaper, and a good secondhand one cheaper still.
  Worth checking old one to see if brushes have worn, or the pressure spring on one of the brushes has broken. This could give the symptoms you desribe, and is a simple job to repair, and cheapest of all!