Mixture adjustment

[10kph]

Last week week I had the Plus 2 starting on 3 cylinders because it had been moved in and out of the garage and not had a good run on the road to clear its chest. Instead of dabbing water onto the exhaust manifold to see which cylinder was not firing I put a new toy into action. I recently purchased a hand held infrared temperature scanner to check the temperature when heating acylinder head before removing and installing new guides and seats. Just the tool for checking the exhaust manifold temperature and suer enough it showed number 3 as the misfire.
What I did notice was that the other 3 cylinders had readings from 200 to 350 degrees. After cleaning the plug and taking the car for a blast down the road I checked the exhaust temperatures again after idling for 5 mins they were all different by 50 to 100 degrees this can only be due to mixture so with screwdriver in hand I turned the mixture screws one at a time and sure enough the temprature on that cylinde can be made to rise or fall.
When I say turn the mixture screws I mean less than 1/8 of a turn will move the temperature 50 degrees, one cylinder showed 150 degrees and no sound of misfire but increasing the mixture slightly brought it up to 300.A higher temperature must be more efficient and this was increasing the mixture ie. richer
The measurement I took were at tickover with all the carb airflows balanced and resulted in a super smooth engine . My next check will be on my 6 cylinder Bristol engine with 3 downdraught carbs
Tony

[mbell]

Interesting. Do you happen to know if you have the fine or course threaded idle adjustment screws?

I've been seting mine with fine thread to the nearest half turn with a color tune but sounds like might be worth revisiting this with some finer adjustment.

[JonB]

On my Plus 2 with Dellortos those screws only set the idle mixture. When the throttle opens, the acceleration pump and main jets come into play. The pump can be adjusted but to alter the "throttle open" mixture you need to change jets. I've been meaning to see what jets I have fitted. When I used the ColourTune to adjust the idle mix last weekend, I noticed that higher engine speeds gave a rich mixture (yellow flame).

[MarkDa]

The thermometer sounds like a really neat bit of kit.
It would be interesting to know if it is more sensitive than Colourtune which I've found to be pretty good.
In the limit one needs to get the correct rather than just equal mixture. So it may need to be a combination of both?

[Chancer]

.A higher temperature must be more efficient and this was increasing the mixture ie. richer

You might want to reflect some on those assumptions Tony.

[alan.barker]

I thought the engine runs hotter if weaker mixture
Alan

[10kph]

Hello Jonb and Alan,
The dellortos I adjusted were on a 72 Plus 2 with the idle mixture screws standing up with coil springs. The later ones from 73 had the adjusting screws in the same position but in and below the alloy housing , this is probably the finer thread screw and a finer set screw on the lefthand carb for overall idle speed.
True a weaker mixture runs hotter but say the weaker mixture is only burning at say 80% efficiency and undetectable to the ear then making it richer brought it upto 100%.
Thinking more about it today I recon that this is the equivalent to a lambda sensor for carbs because it is making each cylinder produce the same thermal efficiency and is not like a co meter which tests the exhaust gas for an overall average but could be used in conjunction to give an overall better balance.
The readings and adjustments are only for tickover say 900 rpm and will have nothing to do with mid or high speed ranges

[Chancer]

On twin sidedraughts whilst I adjust the mixtures at tickover and take comparative cylinder readings be they vacuum, header temp, colortune etc I am not adjusting the idle mixture per s? but the throttle response, pick-up and general engine response and driveability over 80% of the rev range and probably 95% of the vehicles operating time.

That is to say that I will compromise the idle mixture for the benefit of the above especially where flat spots occur, the balancing and mixture adjustment is done at tickover but its not the tickover I am setting.

I think that you are sending yourself down a blind alley, an engine iwith a hotter exhaust temperature is less thermally efficient, going for the hottest should not be the aim, you might just as well retard the ignition.

Granted a misfiring engine will have a cold(er) header temp and have close to zero thermal efficiency on that cylinder.

I have like you used my IR thermometer to check between headers but found that it was only of use in the first minute or so of running before it went off the scale so only good as a comparator or fault finding, you dont set mixtures on an engine that has not reached operating temperature.

[denicholls2]

Not a Weber expert and Webers are definitely special in many ways, but in defense of Tony, if the cylinder is not firing or barely firing at idle then badnesses are likely happening that will also affect it off-idle. In any event, making all the cylinders pull about the same at idle is probably not a bad thing, though I hear you on the issue of whether top temperature is the perfect goal.

My understanding of most carburetors is that although the idle mixture becomes a minor contributor at road speed, it is still a contributor. This may be where Webers differ with the behavior of their progression holes. Setting my ignorance aside for a moment, let's say that due to disparate flow volumes, idle mixture remains a 10% contributor to total road-speed mixture. If that 10% is fuel-starved to 0% or over-enriched to 20%, then the total mixture in that cylinder is still off by the idle mixture's contribution ratio from ideal. Is it not? The point is that using the same jets in all cylinders means that the jet contribution is fixed across the cylinders and the idle contribution represents the only fine-tuning capability of the total mixture, richer or leaner.

If so, Tony's goal is spot-on even if his method requires minor empirical adjustment. And it's a bit easier than drilling out the header to put a wide-band CO sensor in each tube, where we'd then need to argue the impact on flow characteristics of the sensor tip.

[Chancer]

The last engine I tuned witn weber carbs did not come off the idle and on to the main jet circuit until 4500rpm, one of the aux vent locating bolts fell out and the venturi turned blocking off the main jet flow, it ran fine on a partial throttle up to and above the motorway speed limit.

But I think that you may be right, the idle trimming adjustment may not have any effect on the progression circuit, I'm too rusty to recall.

[Donels]

I think Tony has hit on a cracking method to get the the idle mixture similar on all 4 cylinders. What it doesn't do is tell you that it's the correct mixture, so maybe a combination of Colortune and the IR sensor would be ideal.