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Thinking out loud and trying to understand.

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JPSmit

JPSmit

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Happy Monday friends, it was a bit of a cool weekend here but I did get some time with the Midget.

Some of you will recall that I have really struggled for a year now to get it running right. Last fall I removed the plugs only to find them completely sooted up, I bought new ones - and they sooted up in about 4 blocks of driving. So, too rich. Turned the nut anti-clockwise (from top) on SU two flats. (BTW I have a single HS4 MGB SU) - still sooty. Another flat and a half and finally the plugs seem a better colour. My question is simple - how can it have fallen out of tune so much? The nut was tight enough to need a wrench so it can't have vibrated. Yes, I currently have the air cleaner off - but to my mind that could only make it leaner. So, what could make the car run so rich all of a sudden? Oh, and I believe the choke is properly turning off.

thoughts? thanks all!
 
Not much other than the choke not going fully "home" with an SU would be a cause for 'spontaneous enrichment' IMO. What are you using for damper oil?

Float level could be a factor, but I would expect you would see fuel overflow if so.
 
Leaking vacuum hoses?
 
Another vote for choke not going the off position completely. Other than the mixture screws not being correct, the choke is the only thing that would seem like a logical culprit.
 
DrEntropy said:
I should think that would lean the mixture.
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Ok, I guess I was thinking of modern cars. Vacuum leaks can cause the computer to adjust the mixture to compensate for more air (leaks).
 
Gliderman8 said:
Ok, I guess I was thinking of modern cars. Vacuum leaks can cause the computer to adjust the mixture to compensate for more air (leaks).
Click to expand...
Also if it was a modern car a bad Mass Air Flow sensor could cause it.
 
What??? Waitaminnit!

You're sayin' SU carbs ain't MODERN?!?
 
You young whip-snappers - it is M O D E R N!

curmudgeon.jpeg
 
Thanks - may have to look back at the choke. No vacuum at all. damper oil is IIRC ATF.
 
Check carb diaphram could be brittle frm the alcohol fuel.
 
LarryK said:
Check carb diaphram could be brittle frm the alcohol fuel.
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Is there a diaphragm on an SU? unlikely though - I don't use ethanol fuel and it was rebuilt not to awfully long ago.
 
My 57, 58 Jags do, both my old spits did and 1 Land Rover. All my Strombergs did.
 
No rubber diaphragm in the SU's after 1960 or so. The earlier ones had one below the chamber, as part of the jet adjustment assembly. The HS4 from an MGB has the piston as their only "diaphragm". Stromberg replaced the piston & chamber with that poxy rubber thing.

The illustration in the early 2.4, 3.4 &3.8 litre Mk-2 Jag workshop manual, part #12 is the jet assembly with a rubber diaphragm:

SU4.JPG
 
Are the needles centered, and set at right height. Is the needle set screw tight? I know you probably checked these and sync of throttles. It's the little things that get you. Is spark hot enough? Too much plug gap? If it worked before, has to be something easy.
 
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