16 replies to this topic

[thegreenhornet]

Hi everyone. My standard 998 Mini has a problem that I'm really struggling to solve.

The fuel mixture appears to be going lean under load, or at cruising speed. After a run around the ring road, the plugs are white. I have richened the carb at tickover - to the point where it can be a bit smoky and spluttery. If I drive in traffic or through town the plugs will be black, but then after another 3 or 4 miles on the dual carriageway they'll be white again. They are at least all even, so I'm not suspecting the head gasket.

I have changed all three gaskets on the intake manifold, the fuel filter, and I've even changed the main manifold gasket (which was a faff), and it seems no different. The vacuum advance unit was changed about 6 months or so ago, and the rubber joint things on the pipe don't seem split or perished. The breather hose is also new and is fitted tightly.

It's a standard 998 A+, with a standard air filter, HS4 carb with an AAC needle. The carb jet was replaced with a non-waxstat conversion last month. There's also plenty of 5w30 oil in the dashpot.

Any ideas about what else could be causing the problem? I suspect the car has always had the problem since I bought it a year ago, but never as drastic as it is recently (I just always had it very slightly rich at idle). I just don't want to damage anything if it's running too lean on motorway journeys...

I've attached a picture of how the plugs look when they're just starting to turn from black to white. As you can see, there's no brown deposits in between at all. After another 3 miles, these plugs would be white (though the base ring will stay black - it's the electrode and insulator that change).

Thanks!

[Gremlin]

The picture hasn't worked, 5w30 in the dashpot might be a bit thin, try something a bit thicker, I use 20w50 and find it good

[thegreenhornet]

This link should work for that picture:
http://i1313.photobu...zps68d39f43.jpg

Until last week the oil in the carb was 10w40, so I don't think that'd be it, really.

[Captain Mainwaring]

The picture hasn't worked, 5w30 in the dashpot might be a bit thin, try something a bit thicker, I use 20w50 and find it good

 

 

Won't make any difference - the oil is only to slow the piston movement, not to stop its ultimate position. Once the engine is at cruise the piston will sit at a position dependent on manifold vacuum, nothing else.

[Captain Mainwaring]

I'd look at jet and needle - spend a few quid on getting it RR'ed.

[Stu.]

Look into the floatbowl as a possible cause. The level of fuel in the bowl determines the level of fuel in the carb jet. Leaning / richening the mixture simply raises and lowers the jet, which when richening lowers the jet to expose more fuel in the carb, and if leaning reduces the exposure of fuel in the carb. If the float in the float bowl is incorrectly positioned it will cause either rich or lean running regardlsss of your attempts to adjust the mixture. Check the bowl lid / float clearence against Haynes Manual recomended gap. Also check the bowl lid flow valve to ensure its not sticking.

[thegreenhornet]

A rolling road would be very expensive (for me) when it should really be working with its current needle. Nothing has been added on or upgraded.

Can timing make the plugs go white? It doesn't knock at all, but in case it's a tiny bit retarded...

[thegreenhornet]

Thanks Stu, I'll take a look at that.

[crf150]

[crf150]

There is the picture of the plug to make it easier

[Gremlin]

The picture hasn't worked, 5w30 in the dashpot might be a bit thin, try something a bit thicker, I use 20w50 and find it good

 
 
Won't make any difference - the oil is only to slow the piston movement, not to stop its ultimate position. Once the engine is at cruise the piston will sit at a position dependent on manifold vacuum, nothing else.

The title includes lean under load, I know it's not going to stop his plugs going white but it will help get him up to cruising speed

[Stiggytoo]

As Stu said, take a look at the float level. Does it take slightly longer than you would expect to start up? This could be another symptom supporting this.

Also, if the needle is fairly recent, it could be the source of the problem. The newest ones are terrible and don't seem to match up to the correct sizes.  

[Captain Mainwaring]

 

 

The picture hasn't worked, 5w30 in the dashpot might be a bit thin, try something a bit thicker, I use 20w50 and find it good

 
 
Won't make any difference - the oil is only to slow the piston movement, not to stop its ultimate position. Once the engine is at cruise the piston will sit at a position dependent on manifold vacuum, nothing else.

The title includes lean under load, I know it's not going to stop his plugs going white but it will help get him up to cruising speed

 

 

 

The maximum rise time assuming full vacuum for the piston is around 7 seconds if I recall . That's assuming a piston at the lowest position with max vacuum. The function of the damping is not quite as you would think.

[Gremlin]

I know exactly how quickly the piston moves about, I can see it whilst driving about in mine, are you going to brooklands tomorrow?

[thegreenhornet]

I checked the float this morning and it seemed roughly correct - at least judging with an old Teenage Ninja Turtles 15cm ruler.

I went to see the guys at Surf Blue Garage about it as I needed a couple of other bits and they suggested that it might be the Maniflow exhaust, even though there's no change to the manifolds or intake. Originally I was advised that the pipe on its own would affect it enough to warrant changing the needle, but I'll give this a go.

I've bought an AAA needle to see how it runs with, so if it turns out to help then I can fine-tune it from there. HOPEFULLY this will help sort my poor little car out!