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MG MG Y Type - Exhaust gas analyser.

YA 3348 has a rebuilt motor. points ignition, still positive earth - and I've been " fine tuning" the carb setup as part of my ongoing learning experience. Hence lots of questions from time to time.
I followed the excellent notes from Dave Dubois and Skip Burns, plus comments in the archives, re. the importance of correct ignition timing, points gap etc first, and then setting float level, jet height, needle types and so on. The result was pretty good for an amateur - the rebuilt carb performing as it should throughout the rev range. I plugged in a Colortune, and I must admit I was underwhelmed by the "bunson blue" it showed me - more like insipid pale blue, and hard to see. So I borrowed the attached old-school exhaust gas analyser. What a useful device - accurate indication of air/fuel ratio, plus things like CO if you want. My afr readings at this stage are : at idle, 13.6 and at 2000 rpm (but car stationary), 13.2. Apparently I should be aiming for leaner, at around 14 to 1, so will keep fiddling. It will be interesting to do a road speed afr test tomorrow.
The other thing I checked was compression, which was a bit disappointing at 115psi, but exactly the same on all four. I believe 115 is "a bit low, but acceptable"?
I'll be interested in anyone else's experience with Colortune and/or exhaust gas analyzers - neither of which I had used before the YA restoration.
John.


J P Hall

Hi John
I wouldn't be going any leaner
For a good strong reliable idle you need 12.5 (5%CO)and no leaner than 13. The 2000 rpm thing in neutral means nothing really but 2000 - 3000 cruising on the road is the one you're after and should be around 14.5 but no leaner than 15. it'll be interesting to see what you have there
It pays to recalibrate that gauge quite regularly while testing, they're a bit prone to wandering related to battery voltage if you're using it with the car's battery---A seperate battery for the gauge is ideal.
What fuel are you on----% ethanol

willy
William Revit

I bought a couple of old analysers over the years, but they would never hold their calibration and rather hard to read while driving.

A few years ago I bought a new one from Raytech Automotive in Adelaide. Super tool and no issues apart from one dud sensor.

Best results are achieved with the sensor as close to manifold as possible through a welded in plug - sniffer tube in the exhaust can be misleading, and less than ideal.

All my classic MG's (M-Type, YA, YT, Midget) are now running the factory "lean" needles. I could spend more time with the Lambdaking and refine it even more on them all, but there are too many other projects to work on in the shed.

Safety Fast

Tony Slattery
The Classic Workshop
Black Mountain


A L SLATTERY

Lovely piece of kit, Tony, but I think it's a bit above my budget. $300+ if I remember? I'm sure you and Willy are right about the inadequacies of the old-school analogue type, but I'm enjoying the ride at the moment. Had another go with the Colortune today, nice warm engine, car willing to purr along at 55, but on the idle test I still couldn't get the highly desirable bunsen blue. So I think I'll go by what the gas analyser tells me hopefully later today, and make afr adjustments from that. As I said originally, this is pretty much fine-tuning, based on more than the ever-popular "lift the piston a whisker" test. Shall keep in touch - thanks Tony and Willy for chiming in. John.
J P Hall

Cheers Tony
Yep same here with the analysers--I ended up making up a board for mine with a zenner diode and heat sink to stabilize the voltage out on the road and since then it's been magic
Like you i've found the further up the pipe the better, specially on modified cars with a bit of exhaust reversion at lower speeds
Surprises me you're on lean needles, i've found the opposite with the crappy dosed up fuel we get now
Cheers
willy
William Revit

John
Are you still on a single carb setup or twins
What needle/spring combo are you running with
William Revit

Willy, single H2 carb, light spring (blue marker paint), currently using the ES needle, and running on ULP 91. At this stage I haven't been putting in Valvesaver, as I'm not sure the xpag needs it? Interested in your & Tony's thoughts on that and on my 115 compression (above); and also why you favour the standard needle - I have one I can put in for comparison.
Today, I spent more time swapping between the old-school manual method of setting the afr by adjusting the jet nut and testing with the side of the screwdriver, and plugging in the gas analyzer. Quite honestly, the analyzer tried my patience too much - erratic/confusing readings, recalibrating every time I wanted it to work ... in the end I went back to the best "manual" setting, best idle, best road test, and left it at that. The analyzer says about 12.8 at idle, which I know you reckon is a bit lean, but I'll leave it for now, run the car around a bit and then have another play. I think I've exhausted my ability and my patience!
John.

J P Hall

12.8's ok, we can live with that, you were on 13.6 earlier
I'd imagine if you poked the colour tune in now it'd be a healthy blue
The reason i asked about fuel type was that if you have fuel with a dose of ethanol you need the colour to be just going from blue to a bit of yellow
As far as preference of needles, I'm not against the "lean" needle as long as the mixture comes out right on the car being tested ,the gas analyser is the only real way of knowing if that's right or not.
You can swap needles around all day and the idle mixture on a gunsen won't be any different, most needles are the same in the idle position, it's further out the needle under driving conditions where the mixture changes with different needles. If it's pulling ok on your 'lean' needles then it 'should' be ok, but a good hard drive up a longish hill somewhere then pull a sparkplug out on the side of the road at the top of the hill and see what he looks like.
115psi compression sounds good to me, they're only 7.25:1 compression so you're right up there-
Now--with valveguard, you'll get plenty of opinions but really what happens is the lead that used to be in fuel got into the cast iron of the head/seats and lubed it against wear---If you had a car that had done a heap of miles on leaded petrol the seats were sort of conditioned and didn't really need any aditives for unleaded-BUT if the head gets reco'd and the seats are cut destroying that protective layer from the past then there will be a need to run some lube(valveguard)
I've got interested now so will look into the differences between the ES and std needle just because
AND to check on that damper spring ,I didn't think Y's ran a spring
William Revit

Re.the needles then Willy, the thing I notice at the moment is, she's willing enough pulling through 1st, 2nd and into 3rd, but in top I feel she's missing that last bit of oomph - although when she does get there, she'll sit on 50 and at a push ease up to 60. Would the standard needle make a difference up at those dizzy heights?
The head was not recoed, skimmed or anything.
Thanks always. John.
J P Hall

Crikey--ES is a very lean needle compared to std.FI
So MGY std needle is FI then Rich is DK but hardly any different to the std FI except a tiddle up high in the revs -the lean needle is listed as EF which again is similar to std but leans off a bit from half way onwards
I can't find any reference to a damper spring for MGY and the parts list doesn't show one--therefore the only reason you're getting away with the very lean ES needle is because having a damper spring in there is enough to richen the mixture during acceleration BUT there's a danger doing this because once the piston reaches the top of it's travel on full throttle /full load it'll still be way lean compared to std
Did your car have a damper spring originally ---I'm trying to get my head around this
Who recommended the blue spring/ES needle ? Is it a known combo
William Revit

Oh lord, sorry Willy - I have the EF needle, not ES. My bad. From your comments, could the EF be hurting me in top gear? Yes, this car came to me in 1984 with the damper spring, but now you've got me thinking. It's certainly not a heavy brass piston, but I wonder is there actually a known weight of piston below which a spring is advisable?
John.
PS - I'm only up because of the cricket!
J P Hall

I'd be popping the std FI in for a try and see if it's stronger in 4th gear--If it is better,then give it a try without the spring--If it then develops a slight stumble flat spot on light/medium acceleration then poke the spring back in
i'm up watching the Femes
William Revit

I'll do both of those things tomorrow, Willy. Great to have your experience.
Would that be the football femmes?
John.
J P Hall

nah--bikes
William Revit

Willy,

Your knowledge of all things post-war MG continues to totally astound me! Thanks for your invaluable advice, and supplying rare parts, in restoring YB0952. Constance proudly displays your MG Car Club Tasmania transfer!

Regrettably, poor health, and advanced age, force me to offer YB0952 and my late 1967 lhd MGB GT for sale. I have owned numerous MG's since my freshman year in engineering college (gatech.edu). I have restored both of these cars to as original factory condition as possible with my 50+ years experience.

The cars are located in beautiful, historic Williamsburg, Virginia. Limited photos are available upon request. Dealers, catfish, or bargain hunters please need not apply.

Rocky
TC2482
YB0952
67 MGB GT

Rodney C "Rocky" von Dullen

Aah- Rocky - That's not the news we wanted to hear.
I hope your health picks up.
Whoever buys your cars will be lucky people, I really like the look of Constance, it's a magic car. It'll be hard for you to let go when the time comes.
Cheers mate, and stay safe
willy
William Revit

So: the spring is the lightest one (blue paint); the old H2 carb doesn't have a lifter pin, so screwdriver it is. The needle I'm using for road testing is the lean EF, but I'm also keen to try the standard FI, for comparison. I have the jet at 65 thou below the bridge, and the fuel level is another 65 thou below that. All subject to change of course in the light of driving experience or better advice!
Today's job is reassembly of the weird and wonderful bits of the fuel filler cap, before more road testing.
John.
J P Hall

Sorry Willy (and anyone else interested) - I put my last update on the other thread, Carburettor Problem. As my school reports usually said - "could do well if paid more attention".
John.
J P Hall

This thread was discussed between 25/07/2023 and 17/08/2023

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