Classic Alfa Forums

Hi guys,

I have something that puzzles me, and I dont want to destroy a carburator body to find out whats going on....


So , on the carburator, with top removed so you can see inside the chamber, you see a little hole that allows the fuel to be sucked by the pump thats on the outside of the body, and then allows the fuel to be pumped out through the jets...


That little hole on several of my carburators is clogged. So fuel doesnt travel down.

Questions:

1 - Given the fuel chamber is full, with pump cover and difragm off, should the fuel drip out of that hole ?

2 - Is there like a little bearing ball inside that hole that goes up and down depending if the difragm is pumping or sucking ?

3 - There's clearly something in there but its sealed from the top. Can that thing be removed somehow and can I find a replacement ?



I hope you guys figured out what I'm talking about.

the acceleration pump has a ball bearing inside the pipewrok somewhere within the carb body, to act as a one way valve so that fuel is pushed out the jet and not back into the carb body.
i've known them to be stuck.
blast of air with some carb cleaner down the opening usually moves it.
best to do it whilst off the car though incase anything drops down the throat and into the engine...

thanks mate, I tried that

Had no luck

If you happen to know, does that ball move up and down or from side to side ? I'd like to see if I can help it with a paper clip or something

i looked at a schematic of the IDF but couldn't spot the ball, so wondering if i'm confusing carb designs....

the mechanism will have a one way valve of some kind within it so that fuel can't flow back into the body instead of out the jet so a blast of compressed air down the inlet hole for the acceleration pump should clear it.

It doesnt mate.
Plus I've looked closer, cant see any balls in there either.
All I see is a pin that goes straight down.

Cant see how that stops fuel from getting back to the top :/

must be some kind of one way valve?
when the throttle is pressed down very quickly the rushing air causes the mixture to go weak which needs richening up.
the acceleration pump does this by the little diaphragm pushing it's little pocket of fuel up through the tubes and out the jet above the butterfly.
if it had no non return valve then it'd just push it back into the float chamber.

if you take off the diaphragm does fuel come through into the diaphragm chamber from the main float chamber?

on one of the carbs it bearly drips, on the other , nothing

should flow freely mate, could it be that old fuel has got in there are turned to gel blocking up the pipework?
can you get some carb cleaner or thinners or something into the inlet hole from the float chamber?

I have mate, left carburator cleaner over night, blew air in the hole from the top and from the side, nothing happens, not even bubbling the carb cleaner in the chambers.

It's properly clogged...

Anyway, I think I'll just drill it out, see what the hell goes on in there, at least maybe next time I'll know how it works so I can fix it

thread resurrection - hope you don’t mind. First of all engine is 1.7 8V with twin 40 Weber IDF
Car runs great, pulls cleanly from low rpm and revs generally cleanly with no splutters or stutters other than being a bit of a pig when cold.
However, on motorway on a steady cruise I get a strange pulsing sensation. This seems to be when cruising at about 3000rpm on very light throttle - apply even a slight bit of throttle and it disappears. Carbs were running a bit rich (have since adjusted) but were recently cleaned plus I balanced them too, they were perfectly matched. Tickover when warm is pretty good, the odd little miss, but I recall this being normal on the boxers.
Any thoughts on where I should be looking? A bit of research indicates this is about the point it goes from idle to main jets. What should the jet size be?
Also last night I elected to check mixture screws and they were all over the place - from under one turn out from seated on cylinder 4 to nearly 5 on cylinder 1!! I set all to 2 1/8th as a baseline for now, will check all air bypass screws are shut too just to be sure.

Hi, not an expert on IDFs (more DRLA fir me) but principle should be the same. If cruising, then likely you are past the progression holes point (which smoothly manages change from idle jets to mains) - but worth checking those holes are clear. After that, I’d suggest checking the main jets - are they the right spec size or have they been changed? Best to check with a jet of jet gauges (eBay quite cheap and very useful) - the amount of times I find jets that are not the size they say they are. If that’s all good then look for air leaks in vacuum piping or around the carb. Also, could it be ignition issue - I had a similar issue years ago with a Sunbeam Ti that kept missing at cruising- turned out to be a broken wire from the coil to the distributor.