Fuel Mixture

[KevJ+2]

Hi All,
I have now started my engine after a full rebuild and it literally started on the button - fantastic!
After setting the timing by strobe and balancing the carbs by synchro balancer, I've moved onto the fuel mixture which seems rich.
Before starting, I set the mixture adjusting screws one turn out (Weber 40 DCOE31) but after checking the plugs, they were very black and the exhaust was putting out quite a bit of smoke.
I've now tried color tuning it but to get to blue on idle, I have to screw the mix screws almost right in. Is this okay or should I try something else? The plugs are certainly a better colour and the exhaust less smokey.
All in all though, happy as a pig in s**t

[wotsisname]

Hi Kev,
It does sound like you have done all the right things.. bet you are Mr Happy !

What spec is your engine built to ? eg SE, big valve, etc. and what was it originally ?
anything done to the carbs ?
- I doubt I'll know the answer to your question.. but the above will probably help those that will

[Elanman68]

Doing the same on my S4 at the moment. The workshop manual states that the idle mixture screws should be closed fully and opened to 3/4 of a turn and then adjusted in turn to give the correct mixture (the manual rev drop method or my preferred Colour Tune method). If you've got a nice blue flame on each cylinder and the plugs are the right colour then I suggest you've cracked it and the fact that the idle mixture screws are barely open isn't an issue?

[billwill]

Are they Webers or are they Dellortos?

(some ? ) Dellortos have Idle-bypass screws to balance the two throats of the carb. In reality they should be closed on one throat and possibly a little open on the other throat. If they have been balanced with both a little open that would account for the ideal idle-mixture screw being right down near the bottom.

<later>
ooops I see you said they are Webers.. Oh Well might as well leave my comment here for the edification of Dellorto owners..

[KevJ+2]

Thanks all,
The engine is standard not 130, with standard cams.
I refurbished the carbs with Weber kits so no new jets, only new float valves (1.75) and new mix screws.
Mains are 115
Starter jets are F5/100
Slow running jets are 45 F8
Number 4 seems worst as it is still rich with the mix screw right in.
Idling seems uneven and slow to calm down but maybe this due to the uneven fuel mix.
Kev.

[bill308]

Hi Kev.

Your first post indicated carb balancing by synchro balancer. What is this device?

Over the years I've used four methods. My ranking of balancing devices, from best to worst is:

1. 4-bank manometer system (looks at all 4-cylinders continuously and simultaneously, very sensitive, no calibration required, allows dynamic viewing during transients and off idle operation, requires vacuum taps in the carb or intake runners, relative column heights reflect flow in each cylinder, hands free operation to make immediate adjustments and evaluate results)
2. SK Synchrometer (1-cylinder at a time, yields a quantifiable flow rate, not sensitive to orientation, has to be held)
3. Unisynch (1-cylinder at a time, sensitive to float column orientation, obstructs air flow path, not very repeatable, has to be held and manipulated)
4. hose to ear judging 'hiss' volume (1-cylinder at a time, very subjective, has to be held in consistent location and orientation, no measureable data)

I think you still have a idle, balance issue, because you state,"Idling seems uneven and slow to calm down but maybe this due to the uneven fuel mix".

The other thing to look at, is whether the timing advance is working properly and the throttle linkage, in its entirety, moves freely. Failure to return to a stable idle timing can cause a slow return to idle.

What is your idle speed?

When does the timing start to advance/retard and is it smooth?

I like the use of a Colortune for idle mixture adjustment. I'm just not sure what the actual mixture is from the color. In my mind (and we use a 10% alcohol/gas mixture), bunson blue may be a bit on the lean side for idle conditions. I think you want to be close to the blue/white or blue orange color. I've never seen a color chart correlating color to actual A/F mixture.

Some things to consider:

Most power and fastest burn is at an A/F ratio of about 12.3:1. This is about 6% CO. Ferrari specifies 3% CO at idle for my 308, basically an 8-cylinder twink. Note that this is fairly rich mixture and I have not correlated it with the Colortune (on my agenda).

Stoichiometric for pure gas is A/F=14.7; safe full power 12.5, cruise (my estimate) 12.5-13.5

Stoichiometric for E10 (my gas) is A/F=14.1, safe full power is 12.0, cruise (my estimate) 12.0 -13.0, so I should run a little richer based upon fuel types.

Bill

[ort]

is no. 4 cyl pulling more or less air than the other cyls. if so it could indicate a twisted spindle on that carb.
are the butterfly's central on the spindles?
on type 31 the 1st progression hole is very close to the idle position of the butterfly, is it uncovered allowing fuel to flow from it.

[KevJ+2]

Thanks Bill for such a comprehensive reply.
The synchro tool (without going in the garage to see) is a simple variable rate cone with a tube going to a 'gauge' of about 6 divisions with a float or pea inside - very simple. I have also tried with ear to pipe and the balance dosn't seem a million miles out.
Idle speed is about 950 rpm
My ignition is about 12 degrees advance at idle (not yet set any higher)
The distributor is new TTR electronic / new coil, leads & ngk plugs.
One query I do have is running a new Facet electric fuel pump (low pressure, but between 2&4 psi) this could be putting too much fuel into the carbs and I'll have to check this out.
I also have to ask why number 4 is still rich leaving a black plug.
Is it normal to remove the plugs and see wet piston crowns? Does the fuel still trickle in on shut down?
Today I'm over the moon that the engine is up and running but by tomorrow, the carb niggles are going to bring me down to earth......such is lotus life
Kev.

[KevJ+2]

Ort, all I can say is that when reassembled, the shafts and butterflies were true and the first holes were covered. So shouldn't be that.

[Craven]

Hi,
I thought it was well understood that the 31?s run hopelessly rich at tick over, the progression i.e. the first hole is just simply too far forward. Lotus tried to get over this by drilling a small hole in the butterfly to retard the progression but this is not very effective.
Reason why 18?s are so desirable. I will now duck below the parapet.
FWIW.
Ron.

[bill308]

Hi Kev.

You used a Unisynch to balance. See my previous comments (2)

The reason I ask which carbs you have is wanted to see if you have an 18, 31, or 131. An air flow mismatch within a carb (twisted spindle) can be tuned out in most cases with an idle balance adjustment, as found on a 131.

Balance would likely improve with a better measuring instrument.

The thing about butterfly geometry, it controls 2-things, airflow to the cylinder and (emulsified) fuel flow.

The important event for fuel flow is that the first hole uncover at the same time as the 3-corresponding cylinders. It's the edge of the hole, relative to being uncovered, that is important.

Insufficient air flow, to properly meter the intake charge, can cause misfiring of a cylinder. If the butterfly is a little too closed, there is not enough air flow to meter and atomize the fuel. The the cylinder will run likely lean, causing misfiring. A look with the Colortune will uncover this malfunction. If the flashes are erratic, you may have an insufficient flow problem at idle.

Bill

[KevJ+2]

I'm thinking of swapping the jets around as a process of elimination, are there any thoughts out there about this or what else to try? I forgot to mention that I have no vacuum take offs as I have no servo and electric pop up lights.
Kev.

[billwill]

I'm thinking of swapping the jets around as a process of elimination, are there any thoughts out there about this or what else to try? I forgot to mention that I have no vacuum take offs as I have no servo and electric pop up lights.
Kev.

But you have blocked the vacuum take-off on the inlet manifold ?

[KevJ+2]

Yes, I should have added that they are.