Weber 40 IDF problem

[Kev Rooney]

I've recently help to finish a long term project that uses a Rover V8 engine with 268 degree Crane cam , ported big valve SD1 heads and a Janspeed manifold using 4 x 40 IDFs ( as fitted to 1800 /2000 Fiats sports ) .Timing set at 10'btdc . The hope was that the Webers would be pretty much ballpark on the jetting.

Unfortunately they are eye wateringly rich at idle .

The air bleed screws are all adjusted and the carbs balanced, as a set, for airflow . However the engine will only idle with 1/8th turn on the mixture screws any more and the fuel fumes are choking and it will not run.

My first move was checking that the idle enrichment choke system was clean and seated correctly and no obvious problems in the circuit.

I then dropped the idle jets from .60 to .45 and this allowed the mixture screws to be turned out 1/4 before the problems began again.Based on this information it appeared that the entire circuit was rich .

Next the main jets were dropped from 145 to 120 but this has made no difference at all to the mixture settings ??

EDIT : I forgot to say that the fuel supply is regulated and , on the advice of someone who previously ran similar set ups on Cobras ,set a 2 psi . Each carburettor has it's own fuel supply from a shared fuel block.

I'm more used to dealing with twinchokes and may be overlooking something blindingly obvious so any help is appreciated ,

thanks in advance
Kev

[Guy Croft]

Kev, hi

This should NOT be happening! The idle mixture screw response should be progressive. I know you are pretty expert so I won't dwell at this time on excess fuel pressure, fuel dripping into barrels (float settings, pump jet actuating levers under carb jammed on manifold) and suggest that it could be primary pipe interference or statci back pressure in the ex system.

You should - in the first instance - do a cylinder by cylinder diagnostic with a Colortune (see links - Gunson) and have a look at the combustion flame. You may well find intermittent - erratic firing on one or more cylinders indicating that gas and a pressure wave from another cylinder which has ended its exhausting cycle is coming back up the pipe into that cylinder during its overlap phase. In that case it's impossible to set the idle mixtures.

It can also be caused by excessive exhaust system back pressure - ie: static pressure betw engine and muffler way too high. You can measure with a manometer or pressure gauge with the take-off pipe fitted so that it doesn't protrude into the gas stream ie: just measures static pressure at pipe wall without picking up the 'pitot' pressure. You would be looking for under 3.5psi at idle.

It goes without saying that over-rich to that extent can bore wash and damage the rings so don't drive the thing under load.

The only other observation I'd make at this time is that if the throttle plates are too_far_open at idle the carbs will pull fuel from the progression drillings and the idle mixture screw will be completely ineffective at leaning it out...

GC

[Kev Rooney]

Many thanks Guy. I did think you'd got it first go as the engine is in a dual purpose street/ dragstrip car and uses headers that double as silencers by using an insert . Unfortunately removing the baffle plates made no difference . I didn't fit the Webers so I'm going back to basics by removing them and take your advice by ensuring the throttle fully closes .