Moss Motors, Ltd

HI

998cc Mini Traveller.  Has been going like a boeing till now. Is a real cow to start.  Does not want to idle.  Has an intermittant misfire at low rpm - smooths out at higher revs - above 2000 r's.  The revs are also inconsistant - with holding the accelerator pedal constant the revs fluctuate - take your foot off and she dies.  Have checked ignition - strong consistent spark at each cylinder - has new points, condensor, cap and rotor.  Drinving a low revs is a no-no as the cars jerks and splutters.  Serious pinging under load and have checked the timing - it is spot on.  Also checked the carb dashpot - oil is right.

Any ideas??

Darrel,

If the ignition is truly ok, it sounds like you have indeed got a fueling problem. Strangely, it sounds from your post as though you have symptoms which could indicate either overfueling or underfueling!!  If it's belching black smoke, I'd check the float, float valve, and mixture control details. First, however, pull the sparking plugs out and see if they indicate lean or rich. It would also help to know exactly which model carburetter you have.

On the lean side, look for a vacuum leak or poor fuel delivery. If you need further help in how to diagnose each of these things, repost and we'll go from there. But try to supply as much information as you can, as that will make it all easier. Good luck.

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Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon

Hi

No blue/black smoke.  I have pulled the plugs to check - they look normal.  Have thought about the float but was unsure how to check this - first experience with SU's.  She has a single SU HS2.  Where could there be a vacuum leak?  How much should the fuel pump deliver and how would you test this?  This has been one thing I have considered.  Meant to mention in my first post - I did put on a K&N filter about 500 km's (about 300 mi) ago.  It has run ok up to now with this - but it has roused my suspicions as it is the only thing I have changed.  I did enrich the mixture on instruction from K&N - not very scientific I am afraid - 3 turns of the flat.  Not sure if this could be causing an issue.

Would appreacie info on the checks and settings I should look at.

Thanks 

 

 

is it fitted with a fixed jet or a waxstat type?

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Oz Cooper
Classic Mini Guru UK

Umm... what do you look for to know?

Darrel:

Umm... what do you look for to know?

fixed jet is like this...

 

and a waxstat looks like this...

fitted to the bottom of the carb.

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Oz Cooper
Classic Mini Guru UK

Thanks Oz.

Definitely a fixed jet

Hi

I have a mini 998cc and it had similar problems when I first got it and I solved the issue by spraying the distributor cap with silicone spray (which seals it) because the problem was water getting into the cap and shorting it out even though when you check it , it appears normal. You've probably already checked this but if not give it a try.  

Mini stig

Check the vacuum advance tube for good connections at both ends. A leak here will show up more at low revs.

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On their death bed, nobody ever said, "Gee I wish I'd spent more time at work!"

'68 Sprite 1275
'76 Midget 1500

Darrel,

"How much should the fuel pump deliver..."

That depends on the fuel pump that you have.  If it is a low pressure pump mounted in the engine compartment, it should deliver a minimum of 1.3 pints per minute.  If it has a high pressure pump mounted under the car near the fuel tank, it should deliver a minimum of 1.1 pints per minute (note this is the rates for SU fuel pumps and is the minimum for the car they are used in).

"...and how would you test this?" 

Disconnect the fuel line from the carburetor an direct it into a quart container that is  marked at a 1 pint level, turn the ignition on and time how long it takes to pump 1 pint of fuel - If the time is 55 seconds, the volume is 1.1 pints per minute, if it is 46 seconds, the volume is 1.3 pints per minute.  Any times less than the 46 seconds is good for either pump.

Something else to check for is that the piston in the carburetor is not sticking anywhere along its travel.  Remove the damper from the top of the vacuum chamber and then lift the piston with your finger through the throat of the carburetor.  If you feel any drag as the piston is being raised, either the jet is not centered, or there is dirt on the edge of the piston that fits inside the vacuum chamber or on the vacuum chamber wall itself.  This will have to be cleaned off both the edge of the piston and the wall of the piston, using a rag and some solvent, followed by wiping with a dry rag - do not use any abrasive on the edge of the piston or the vacuum chamber wall.  It the piston sticks at the lower quarter inch of its travel, you will need to center the jet using the procedure in your shop manual.

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Cheers,
Dave
http://homepages.donobi.net/sufuelpumps/