I have been chasing a cold start problem which became a non-start problem.
Having eliminated everything that it could be, including "playing" with a different carb, I decided to replace the original 26IMB after giving it a stripdown and "clean". I inserted the apostrophes because it defininitely wasn't dirty.
I am familiar with the stripdown and usually find that the auxilliary venturi (the tube with "wings" that fits in the throat of the carb) is a very tight fit and needs gently tappng out. In this case it simply fell out and there is some wiggle in its fit which amounts to about 0.07mm at the most...ie a tiny amount, but noticeable. The interference fit seems likely to be something this device relies on for efficient running. My solution was to insert several carefully trimmed layers of aluminium foil between the blank side and the slot in the body until I got a close fit. When the foil fails and then enters the engine I am hoping it will vaporise quite easily. I noted that on the 28IMB there appears to be a sprung steel device to keep this item from rocking about. .
It's pretty certain that both the carb body and the sliding venturi are worn and so a new carb will be the only long-term solution to this
Has anyone else had this issue and got a solution to it.
Might it affect cold-starting....my original problem?

My engine is running great now except for a niggling point on fairly high revs where there is a slight hesitation. I feel that I've eliminated so many minor issues of late that this loose venturi seems the most logical potential cause.
MAL_1771 by Peter Thompson, on Flickr
MAL_1776 by Peter Thompson, on Flickr
There are some suggestions of what to do about this when related to more exotic Webers, so I borrowed some of that technique and applied it to the humble 26IMB.
The straighforward solution is to apply epoxy adhesive to the flute of the venturi and then peen the edge of the carb body to help pinch it in. The latter part seemed a bit tricky as the metal it is made of is quite brittle. But the adhesive is an easy fix.
MAL_1783 by Peter Thompson, on Flickr
I also placed a couple of very slight smears on the side with the orifice but did this very precisely to avoid blocking anything.
The venturi is normally a push-fit, but to show how loose it is, it slid into place, with the adhesive under its own weight.
So a wait of 24 hours to see if it does the trick, but failing this I will need to use a different carb.

I am having the same issue with slight visitation at higher revs following buying a refurbished 26IMB from Mr. Fiat here. If your solution works I may give it a try.

Luxe said:

If your solution works I may give it a try.

Click to expand...

I know next to nothing about carburation (including how to spell it), but I think the principle is that it uses all those complex holes and passages to add proportionately more fuel with changes in engine vacuum. So depending on the rev range that the problem occurs at, it would be easy for an expert to surmise which part of the carb is most likely to be slightly blocked.
In my case I have jumped to this assumption because the carb was recently cleaned and was clean already. The wobbly venturi was the only obvious fault.
I will be going on a 150 mile spin tomorrow so will give the verdict after that.
For today, all I can say is that when I needed to move it out of the garage it was very keen to start from cold, which is an improvement over recent performance.

You may remember that last year I had a large collection of original used 500 Weber carbs to deal with. The Giardiniara ones went to Australia and I started on the earlier 26IMB 4 carbs from the 500D. Shame I never took pics but the wear on some of those tubes was incredible and had to be scrapped. So well worth checking for any movement in that department.