I have a pair of these carbs on my zetec silvertop and it is not running very well under acceleration or when cold. With the rubbish weather we have had lately it does not get much use when normally I would be out at least every other week.

When it is warm it ticks over OK though is very hesitant during acceleration at low revs. I think the electrical side looks OK and I reasonably matching air flow at tick over so the carbs are balanced.
What I have noticed when looking down through the carbs (at a safe distance) is that one set sees additional fuel injected and the other does not when the engine is reved.
Could this be the acceleration pump in the carbs and if so, how do I replace it or clean it?

[Edited on 16/12/21 by AlexXtreme]

Most likely a blocked passage on the accel pump, or could be a split diaphragm.

With the engine turned off, open the thgrottle sharply, you should see a jet of fuel spray from a small jet inside the inlet. On the side of the carb there is about an 1" square housing with a lever which is operated as you snap the throttle open. This is the accel pump,. remove 4 screws & carefully remove the diaphragm, IIRC there's a spring as well & sometimes ball valve, so take care! Check the diaphregm for damage & use compressed air to blow thro' the passage inside the pump to clear the jet in the carb, then retry.

You may need to spin the engine over or restart to top the carb up with fuel if much has been spilt

Rus,

Thanks for the comments.. I do not see any jet of fuel with the engine off (or on!).
So it sounds like the jets are blocked or the diaphragm is damaged. I will start by cleaning the jets.
If it is the diaphragm, is this an easy part to get and to replace?

[Edited on 16/12/21 by AlexXtreme]

It's been some while since I've even seen a DCOE carb & been having a think, I may be remembering the wrong setup, I think actually yours might be a piston pump rather than a diaphragm & it's possible it operates on only 1 of the chokes, but nevertheless you should certainly see a squirt of fuel when you snap the throttle open, you should be able to track the throttle lonkage to see what it's acytrually pushing to create that jet.

Other than that take a look for a parts diagram, or service manual online

https://www.carbparts.eu/exploded_view/WEBER_42_DCOE_8

Both carbs should be identical, so both will have accel pumps. 1 pump feeds both barrels.
Look at the diagram above. Pump jet is labelled 72. You can access this by removing one screw, labelled 74.
As a start take out the non functioning pump jet and see if it's blocked. Failing that take the carb top off and see if
the pump is operating. Parts 63 - 66. You should see that move up and down as you operate the throttle.

I've just stripped and refurbished a pair of 40s, so I'm fairly ok with how they work.

If its of any help with your rebuild, i have loads of Weber DCOE carb parts for sale


36 main jets from 100-150,
32 air correction jets from 140-210
12 idle jets (all in sets of 4)
Plus other spares, O ring plates, flexible stud mounting washer sets, pull off springs, etc. Jets alone would retail at over £200

£80 posted

Thanks for the advice. That website showing the exploded view of the carb is very useful!

I have taken the top off the carb and emptied out the carb of fuel. I could see no sign of any sludge in the bottom.

I could not see if the jets were blocked but cleaned them out with some compressed air followed by a squirt of brake cleaning fluid for good measure and they all seem clear.

How can I see find out if the pump is working as I am guessing this could be the culprit as neither of the jets were producing any fuel.
Is it easy to remove and check?

Does the pump move when you open the throttle? The pump is the bit that sticks up from the face of the carb with the top off
You should see it go up when you open the throttle. The pump is a brass plunger and there's nothing to fail as far as
I know, provide it's got all of the parts properly fitted. The pump rod, spring and plunger can be removed by prising out the brass
retaining plate that's pressed in to the body.
If the rod doesn't move then the operating lever may have become detached from the throttle spindle shaft. It's held by a roll pin and is accessed from the cover plate on the engine side of the carb, between the two barrels.

Have a look at https://youtu.be/c-YJdzhgrkw. There's a series of vids detailing strip and rebuild

Thanks for the advice JeffHs.. the pump seems to work OK. When it operates, I can see it pulling fuel from the float bowl.
I will lean out the other carb whilst I am at and reassumed and see if it was just blocked jets.
Also that is a good video and have saved it for further ref.

number-1, thanks for the offer but the engine worked fined before this latest issue and was tested on a rolling road at 165ish HP which was ok for me.