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Re: Weber synchronisation problems.

PostPosted: Mon May 13, 2019 7:36 pm
by Craven
If Lotus fitment the throttle plates will have small ~ 1mm hole drilled in the lower half.
I gave up on 31’s years ago, the progression is way too advanced for a Twincam.

Re: Weber synchronisation problems.

PostPosted: Mon May 13, 2019 8:29 pm
by G4ILN
I was able to extract the brass covers with the aid of a pair of 90 degree circlip pliers, They're in a fit state to be reused.

Two of the leather washers were still soft, supple and wet with oil. The other two were hard and dry, but not hard enough to snap. If I don't have any new ones in my spares box I may well make my own. I notice that in illustrations the washers are bell shaped, as indeed mine are, whereas replacement items appear to be flat. Do they assume a bell shape in service?

The bearings are in good shape. I flushed them out with WD40 and will regrease them, which should help with the sealing.

Given the limited amount of use this car sees, I cannot justify the cost of replacement carbs or even new bearings with integral seals.


Re: Weber synchronisation problems.

PostPosted: Sat May 18, 2019 5:01 pm
by Grizzly
Did you check the vacuum over a few rpm's? (1000,2000 and 3000 for example)

Re: Weber synchronisation problems.

PostPosted: Sat Jun 01, 2019 7:10 pm
by G4ILN

As suggested by Grizzly I made my own sealing washers using 2mm thick leather soaked in mink oil for a week before being cut to size. Before fitting these I rinsed the old grease out of the spindle bearings and repacked them. Everything was checked on reassembly including float height. I checked this by rolling a twist drill of the correct diameter between the floats and the cover gasket.

When refitting the carbs to the engine I paid particular attention to the 'O' rings, carefully checking the gap in the Thackery washers along with up and down movement of the carbs.

There was no difference at all though. A steady flow meter reading on cylinders 1 and 2, but an oscillating reading on cylinders 3 and 4. I did find that I could greatly reduce the amount of oscillation by adjusting the mixture screws, but could not eliminate it completely. All the mixture screws are effective, adjusting the mixture from weak to rich. Setting them up for best tick over is difficult though as the revs waver by about 50 RPM and the plugs tend to foul.

Timing was checked with a strobe. Rock steady on 12 degs BTDC at 1000 RPM, advancing when the engine is revved.

Brake servo and headlight reservoir vacuum take offs blanked off during tests

"Manifold" vacuum checked on cylinders 1 and 4 using the brake servo and headlight reservoir connections. To get a steady reading' rather than an oscillating one on the vacuum gauge, the connecting tube must be severely restricted or a reservoir used. I opted for the former. Both cylinders had a steady reading of 10 inches of HG. I've never seen any official figures quoted, but I believe this OK for a single cylinder "manifold". I've seen figures of 25 inches of HG quoted, but it turned out this was actually 25 cms (ie 10 inches) of HG.

I don't think a different type of flow meter will help, as the reading on cylinders 1 and 2 is fine.

I'll see how the car performs.

Re: Weber synchronisation problems.

PostPosted: Sun Jun 02, 2019 9:37 am
by JonB
If you are tweaking the mixture you need some means of seeing the effect. I use a Gunson's ColourTune for this. There is nothing worse than altering these settings blind - you have almost zero chance of getting it right.

One of the best tools I have, next to the timing strobe and carb balancer.

So.. to get my Big Valve / Dellorto engine purring, I set the idle mixture on each cylinder with the ColourTune, then balance, then check and set the mixture again, then check the timing.

Re: Weber synchronisation problems.

PostPosted: Sun Jun 02, 2019 10:15 am
by G4ILN
I have a ColourTune, but I've found it to be next to useless where the Lotus is concerned. It only fires the mixture intermittently, presumably because the spark gap is so far removed from the combustion chamber.

What is does demonstrate is that the mixture needs to be rich (orange flame) for the engine to idle at all. weakening the mixture cuts the cylinder.

Re: Weber synchronisation problems.

PostPosted: Sun Jun 02, 2019 11:56 am
by Craven
Don’t think it’s been mentioned here but have you check out the choke mechanism, Air or fuel bleeding in from there will effect the mixture. Make sure the pistons are returning and seated correctly when closed. Operating quadrant can stick and hold the pistons off slightly, from memory I think you can get at and remove cover plate with carbs in place.
Didn’t think to swop left too right when you had them apart I guess.

Re: Weber synchronisation problems.

PostPosted: Sun Jun 02, 2019 12:53 pm
by G4ILN
Yes I have checked the correct operation of the chokes, which I never use. There is slack in the cable to ensure that the choke pistons are fully home.

Two stomps on the throttle to operate the acceleration pumps and the car starts first time from cold. It runs well too, apart from the fact that a recently fitted "upgraded" engine mount with zero miles on it split in two giving a juddery start. That's tomorrow's job.

I like things you can measure and overall I'm most impressed with the vacuum gauge as means of setting the mixture strength. I need to get a larger diameter one though to detect small changes in the reading.

Thanks for all your replies.

Re: Weber synchronisation problems.

PostPosted: Sun Jun 02, 2019 2:46 pm
by Grizzly
Did you check the throttle plates were correctly aligned? If you balance it at 1000 rpm does it stay in balance at 3000 (use the idle screw to adjust the revs don't just open the throttle and wing it)

I have been working on mine to try and get it running the best i can so i fitted a new set of Throttle shafts and throttle plates (no 1mm holes drilled) after a good couple of hours shimming the plates to the correct position's i didn't get allot of bounce pre Shaft's / Plates with the SK Synchrometer but the bounce i did have went rock solid, i then balanced the two carbs as best i could, drilled .5 holes in the two inner plates to get them identically balanced then went back and readjusted both carbs at 1000 rpm and 3000 rpm.