at idle rpm, the engine develops a specific amount of draw...fuel and air are drawn thru the idle circuit which is then made available to the cylinders...so far so good.... when the engine's cold, more fuel to air is required to compensate for poor atomization at low temp. so the choke butterfly closes off the venturi mouth and only permits a small amount of air to be drawn thru the carb body....the fast idle screw works directly on the throttle plate to ensure a very specific amount of air is drawn....it serves no function when the engine is warm So, the engine still wants air so draws it from wherever it can...in the case of a 34Pict 3, the small hole in the throttle plate and the small holes or ports just below the throttle plate...these 'ports' are linked to the air bypass screw (big) and volume control screw (little), which both work in tandem to regulate the air and fuel flowing thru the idle circuit ( via the idle jet) .The idle jet is fixed and can only pass a certain amount of fuel thru the orifice....the volume (and speed) of air 'drawing' on this jet determines the ratio of fuel to air available to the cylinders With the throttle wide open, no vacuum is pulled on the idle circuit, since air is drawn thru the venturi..this pulls fuel thru the main jet... air correction jet (s) account for variations in throttle position... just to complicate things further, there's also an accelerator pump mechanism which sprays neat fuel straight between the opening of the throttle/carb body, to provide a big fuel rich hit when you stomp on the pedal....without it, you'd get a very lean condition, just when you least need it.... This is why, if you can't seem to get the engine to idle smoothly when warmed up, (with all other things being set up correctly) it's usually down to air being drawn from places other than the idle circuitry (manifold boots, throttle spindle seals, cracked vac pipes, base gaskets, head gaskets etc.) The idle circuit governs running up to around 2000 rpm (higher than you'd think) so is critical for normal everyday driving, especially in traffic... if you can't get the carb to set up within the ranges outlined in Rob and Daves article, suspect an air leak or a dodgy float valve... as someone once said....a carb is a simply controlled fuel leak
Wow that is some explaination @vanorak pretty much just what I was looking for. The carb - perhaps especially the 34pict3 seems to be the most complex part of the engine and that goes a far way to explaining it. It also to my untrained eye appears to be one of the most exacting bits too - tiny jets, very small holes etc etc that could easily get knackered. I've got a few days off at the start of the week to have a good tinker. Will let you know the results. Once again massive thanks for going out of your way to explain it - thats far more detailed than the manuals I reckon
happy to help....there's loads of info on the 34 Pict 3 on the web. If you're removing the carb to clean it, be sure to buy a new base gasket, or a rebuild kit. These include various gaskets, a float needle valve plus washers, O rings for the by pass and volume control screws, and sometimes, the screws themselves. good vid here the guy's had a personality bypass, but it shows the various components and how they go back together...good tip re the accelerator pump spray jet ("the skwurtuur")
Nearly all the ones ive refurbed have needed new throttle spindle bushes The ones of ebay require further work to fit and work properly If theres corrosion in the float bowl you can be pretty sure the passages are furred up No amount of blasting with carb cleaner will clear this The rebuild kits are ******* poor nowadays aswell
Add some big globs of grease round your throttle spindle bushes If you can set the idle good for a while and then it goes all wrong after a few miles then you have found your air leak
Nice post from @vanorak, but the good news is that although the design of carbs is complex, you don't have to worry about it. There's not much that wears out on a carb and most problems are due to filth and air leaks elsewhere. A good strip and clean plus possibly a change of spindle bushes, and the thing will work as well as it did when it left the factory (although it may be grubbier on the outside).
Hi, the missus has gone out with her mate to watch 50 shades, so I'm free to carry on with carb refurbs. I've noticed that both carbs are missing their hot idle valves and was wondering a) are they a vital component and b) where can I buy them? They are solex 32-34 pdsit 2 & 3 for a 2l type4. Thanks in advance for your help.
When the throttle is fully closed you should not be able to move the throttle spindle around, as the throttle plate should be touching the sides of the carburettor throat all way round. If that photo is 'throttle closed' then you are sure that the electric choke is working properly? Because the choke also holds the throttle open using the stepped cam on the side of the carb. It is lifted up with a cold choke element and a tap on the throttle (why the start instructions tell you to press the accelerator before starting ) and the stepped cam will drop down when you open the throttle with a hot choke element. . Make sure you have left enough time for the engine choke element to heat up, and the throttle end stop is not still resting on the stepped cam If it is, it should eventually drop down when you blip the throttle momentarily. Then you can set the idle and mixture. My 34PICT-3 was in a bit of a state when I got my bus but eventually I worked out and fixed several things wrong with it - worn bushes, sticky stepped cam, broken heater element in the choke (a rivet needed hammering to reconnect the heater) . In the end I bought a Weber progressive as it was " Made In Spain " not somewhere in the Pearl River Delta and that took a lot more work to get right, jet changes and fabricating kit to fit it in properly.
