Misbehaving Weber?

[richardcox_lotus]

Well done Ian, and a helpful write up. Let us know if there's any difference in on-road performance as well.

Regards
Richard

[mbell]

Yes good work finding and sorting that!

I suspect you'll find your carbs are quite a bit out of tune now you've removed that source of additional fuel! But hopefully tuning them shouldn't be too hard as they respond as they should to your changes!

[Ian T]

Gosh, it has been a year and I've progressed a bit. Thought I should come back and share my progress as it might help a few fellow tinkerers out.

Firstly, I have been experimenting more with fuel levels and feel that Keith Franck's advice (25mm below the milled top of the jet block) is the right fuel level. I've had good progression and pretty well eliminated the flat spot I had.

I've also acquired a second pair of carbs (40DCOE18s) that appear to have OE Lotus jet specs. They are a bit tatty on the outside but very clean inside, where it matters.

I bought a spare pair of carbs

My attention at the moment is on balance and I'll explain why.

Even with the fuel level correct, at very small throttle openings, I have been getting spitting back (lean misfire) on one barrel. This corresponds with the cylinder with the highest airflow when setting the balance via the throttle lever. On the original carbs, the imbalance is between barrels 3 and 4, on the 'new' set, it is between barrels 1 and 2 (2 being lean).

But how to correct it?

With the carbs off the car, emptied of fuel and thoroughly dried, I removed the auxiliary venturis and main venturis, unscrewed the idle adjust screw until the throttle plate was 'as closed as it can be' and looked down the barrel with a strong light behind the throttle butterfly.

Looking down the carbs, this is where I could see light around the butterfly unevenly against the bore

I could see that there was more light showing on one barrel than the other and unsurprisingly, it was barrel no2 serving cylinder no 2. My next move was to loosen the two staked brass screws that secure the butterfly to the throttle spindle and adjust both of them (mainly rotating them slightly (they have oversize holes hidden by the throttle spindle that can accomodate quite a lot of movement). After some trial and error, I came to the conclusion that they would never be a perfect seal but there was the same amount of light getting through with the butterflies closed.

How can I prove that I've made an improvement without fitting it and trying? This is where I think I've done something new. Remember I said I had thoroughly dried the carbs of fuel, inside and out?

I've drawn up and 3D printed an adapter that bolts to each carb in turn and connects the pair to a garage vacuum cleaner.

OpenSCAD design for the magic adapter. I have some learning to do as I couldn't see how to make the flange a perfect match for the carb, but the fit was perfect first time.

Magic adapter printed... got to love this machine!

The logic is that both barrels on the carb under test are now seeing the same vacuum input and I can now take meaningful measurements of airflow (using the sychrometer) with them off the car, saving about two hours of disassembly and reassembly each time.

Photos can do the talking...

Magic adapter in use

Here's the synchrometer I use - I just alternate it between the barrels

These are the 40DCOE18s, remember that the imbalanced pair was 1 and 2, which are pretty well perfectly overlaid in the attached plot, barrels 3 and 4 didn't show any perceptible imbalance on the car, so I have probably got them acceptably close. I'll still adjust 3 and 4 now to try to get them identical, in the pursuit of excellence!

Pretty darn close now, with barrels 1 and 2 pretty much perfectly overlaid. 3 and 4 seemed not to have an imbalance on the car, so perhaps I am close enough. The measurement range is 0 to 3 turns on the throttle idle adjustment screw, with zero being the point at which the lever starts to move

[john.p.clegg]

Nice one Ian, you seem to have it nailed , the best I could get was...

other than " setting" the spindle my next thought was to drill a micro hole in the butterfly and ream to suit...

John

[Ian T]

Thanks John. I'm not sure what this will look like on the car yet but I am hopeful.

I'm intrigued by your manometer balance gauge though, where do you connect it? The later carbs (I think) have a port on each barrel, or I suppose you have driled and tapped ports in the intake runners. I have a set of gauges but nowhere to plumb them in, hence buying the synchrometer.

I'm moderately excited that perhaps I'm getting there now. IF so, I feel the method would work for you as well.

[john.p.clegg]

Ian . yes , have drilled the inlet runners , and as a second thought , just using the tapered end from the vacuum cleaner pushed into the venturi , with a manometer fitted ??

John

P.S. Not sure what Vacuum the Cleaner is pulling ...

[Ian T]

Got it, thanks.

I will dig out my tinkering log book in a bit and see if I can relate the idle air flow from the engine at a specific opening (via the idle screw) and then I can tell you whether the vacuum cleaner is pulling more or less vacuum. While it has quite a suck to it, I would think the engine generates more vacuum, though in that situation I'd expect the difference I measure on bareels 3 and 4 to have been seen on the engine.

I can probably rig up a vacuum measurement if you have a reference value to compare against.

The reason for connecting up as I have is because it is the same as would be done on the car apart from guaranteeing I have the same vacuum at both barrels, I closed the idle fuel needle valves to eliminate them as a source of air.

Of course I have no way of knowing if each cylinder generates exactly the same vacuum, given that cyl1 feeds the headlamp lift system and cyl4 feeds the brake servo.

[Craven]

Hi,
On later versions of the DCOE carb?s the idle screws are fitted with small ? O ? rings for extra air leak control.
May be worth fitting if your 18?s don?t have them, not original fit.
Ron.

[Grizzly]

Ian T, Thanks for putting your findings on the forum, just checked both my Loti and all four Webers are leaking from the Pump jets.


Spent yesterday fixing and setting up my 2 seaters carbs. I was having a bit of a nightmare getting the carbs balanced, so i took them apart again only to notice two of the starter valves were not fully closing / sticking. Now that's fixed it's running better than ever.

Just one other thing of note, if you use Grinding paste on the demand valve seat the carb needs to have the Starter Plunger removed and not refitted until the carb has been deep cleaned as the paste does find it's way in there.