Engine difficult to start when hot

 In Engine General, MGA-MGB, Midget

If your engine won’t start easily when hot, and has good fuel delivery and good spark, it may be because heat soak is making the fuel expand and rise enough to dribble out the jet and flood the engine a bit.  Although we haven’t struck it with any of the Midgets, I have had this problem with my MGB. In that case, it was so bad that between 30 seconds and 20 minutes after switch-off it would not start with the starter regardless of throttle position and had to be pushed. Every time! (So parking was always on a handy hill wherever possible!) Outside of this time it always started instantly.

In this case it was exacerbated by a combination of big bore engine, very high compression ratio and mild street cam, together putting a lot of load on the starter – and so dropping the battery voltage significantly.

The solution was multipronged:

  • Dropped the float level a little lower than spec to reduce the amount of flooding.  (Having made this change I had to check that it didn’t it lean out under power of course)
  • Fitted a ballast resistor & ballasted coil so the coil now sees its full operating voltage when the starter is cranking
  • Fitted a geared starter to reduce the current drain and therefore system voltage drop.

This completely solved the problem. The engine still floods a little, but with the starter spinning the engine better and the system voltage staying higher and so allowing a fatter spark at the plugs the engine now fires after half a dozen revolutions at worst – usually it’s instant.

There are other causes of hot start difficulty, to check whether the above is relevant to any difficulty you might be experiencing I would suggest that next time it happens you whip off the dashpot & piston from one of the carbs & see if any fuel is dribbling from the jet or (if not) if the fuel level in the jet is at the top and about to overflow. This should confirm or otherwise whether your car is affected in the same way.

If it is, before spending any money  try the usual response to flooding of full throttle while cranking on the starter – if that overcomes it it’s a cheaper fix than what I had to do!  And check the float level in the carbs – it may be just that it is higher than it should be.

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