Moss Motors, Ltd

Recently the idle on my '71 Midget has fallen low and is very rough when the engine is warmed up.  With this problem I have noticed two thing to go with it.  

My fuel pump has always been slow (1/sec) thump thump thump.  Now, when the engine is warm, the pump has a rapid (several/sec) tap tap tap.  

The second thing is that as the engine warms the tachometer begins to erratically spike upwards.  When it started, I began diagnosis by separating, cleaning, and reconnecting with dielectric grease, every connection I could find.  I looked most closely at the grounds.  I also checked the points and there connections.  So far everything looks good.  I am not sure where to turn next.  Any thoughts? 

Norman 

N & D, I don't have any thoughts on the tach, but it sounds like something is amiss in the carb float chamber. Possibly the needle valve is allowing too much fuel in and the overly rich mixture is on the verge of flooding. That would be especially noticeable when the engine has warmed up. I'd have a look at the needle valve and see if it's shutting off the fuelsupply from the pump once the proper level is reached. It might even be the float itself gone bad and not rising to the occassion.

 

The combination with the tach also hints at the coil going bad with heat.

 

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Bayless
Never express yourself more clearly than you can think
'48 Prefect
'67 Sprite (project)

Folks-

All posts are good but I was thinking condenser. Also, the quicker fuel pump sound could actually be a rattle caused by the rough running.

Don't know for sure of course.

LCJUTILA 

As the coil is much easier to replace than tearing into the carbs as poolboy suggests, I will start there.  The points and condenser are less than a  year old, (I was looking at this cars file) so hopefully it is not that as I would hope it would not go bad in a year.  But then again, I have seen stranger things in my life.  

"tearing into the carbs"; hardly. If you have SU carbs, let's see; there are about 3 screws to remove to examine the floats and the needle valve.

Agreed Poolboy.

Don't even have to take 'em off the car.

LCJUTILA 

My worry is that I will damage one of the gaskets.  With the coil, I can just swap it out with my wife's car.  If the problem follows the coil into her car, then the cause is 100% confirmed.