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Weber 32DFM


Duckpit

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Well my Lotus is nearly restored. The engine runs very nicely but it is running too rich so much so that when it is restarted after a day or so it misses & needs a plug cleaning.

The mixture screw is right in but still a bit too rich. 

The car hasn't run since 1987 but in the 90's it was rebuilt it had a BCF2 cam in the xflow and headwork & balancing.

I have used an overhaul kit on the carb

Any ideas on how to weaken the mixture. I assume that the jet is OK as it used to run with it, I have done some research and suggestions apart from the jet are the fuel level too high in the float and the power valve diaphram.

Even before I used the overhaul kit it was running too rich

Any ideas ?

 

 

 

 

 

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I can't give you a definitive answer, but here are my thoughts.

The DFM is a road spec carb, but it has been paired with a BCF2 cam profile. This is a mild profile, but nevertheless it has significantly more duration than the standard cam which means that at very low revs (in this case, idle) it doesn't fill the cylinders well (this is why it "comes on the cam" higher up the rev range than a standard cam). To provide enough air for the engine to idle, you therefore have to open the throttle more than you would for a std cam, but this can have the unfortunate effect of moving the edge of the throttle butterfly too far away from the zone where the mixture screw has any effect. In essence (no French pun intended), the fuel outlet that is fed via the idle mixture screw should, at idle, sit very close to the edge of the butterfly where there is high air velocity and therefore a strong low pressure area to suck fuel into the barrel of the carb; you adjust the quantity of this fuel with the mixture screw. If you have to open the butterfly significantly to maintain enough airflow for idle, this particular fuel outlet no longer sees such a low pressure and little fuel will flow.

The result is that you are unable to adjust the idle mixture and the fuel that is getting through is likely to be coming instead through the progression holes directly from the slow run jet. Unfortunately, you cannot adjust this amount of fuel without changing the slow run jet and if you attempt to do this in order to get a smooth idle it will be disastrous for fueling a little further up the rev range. It is not for nothing that carbs have separate slow run and idle mixture arrangements.

Typically you will see sky high hydrocarbon emissions if you analyse the exhaust gas at idle. But if you fit a pair of DCOEs you suddenly find that the car will idle as sweetly as a nut and have vastly lower emissions.

This sort of engine spec was common in the 1960s through to the 1980s, but in those days no-one cared about emissions and a horribly rough idle was how you "proved" to your mates that you had a tuned engine. These days, we know that a tuned engine doesn't need to be a pig at low revs, but in those days it was received wisdom that such was the case. I actually remember receiving a complaint in the 1970s that an engine I had built was too drivable to have been "properly" tuned.

Certainly, my experience has always been that fitting even a mildly uprated cam to an engine running on a road type carb gives symptoms similar to those you describe. It is not helped by the fact that older type profiles such as the BCF2 tend to be of the long duration, low lift type whereas more modern cams have generally changed the philosophy to short(ish) duration and high lift. It is the long duration that causes the engine to fill its cylinders poorly at low revs.

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On reflection having worked on the car yesterday, we are missing the point. The engine is running rich so much so that the plugs get really wet and spark seems to become intermittent it seems to be flooding and running rough, once plugs cleaned up all OK. The engine when running is really nice although a bit rich. Colortune is showing blue bordering on yellow with the needle right in.

So could it be the float level to high, the wrong jet or power valve diaphram knackered or something else. The engine has been rebuit but not run for 20 years

 

Thanks 

 

Nick

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Thanks Bob

 

I have spoken to Webcon - they are a really helpful company. They have told me how to check for flooding & if it is flooding then either the pump is working at too higher psi, the float chamber is too full or float damaged.

If its not flooding then its jets

What is perhaps more important is that there is a company called Carburettor Hospital who can supply parts for obsolete carbs like mine.

Nick

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