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MG MGB Technical - Gravel Pit Fix

FWIW, a quick and dirty gravel pit fix that worked for me, offered in case anyone ever finds themselves in similar straits.

I had to get my triple carb MGC through local emissions testing, so I sent it to a local mg specialist with a reputation for being able to tune them to do this (I had no time to do it, nor �sniffer� to test it to see whether mods were heading it in the right direction.

They got it through, sure enough, but they also whipped out my old .100� SU jets (probably a good thing as the plastic tubes get brittle with age) and replaced them with .090� jets with stock early MGB needles. Ran clean, but way lean above 2000 rpm, to the degree that it would miss � poor drivability (it has stock compression but headers and a mild cam). I was reluctant to buy yet another set of brand new jets and needles, so I thought about it and did the following.

I managed to find a metric drill bit that equated to .101�. The depth of the metered orifice in an SU jet is only about .200� deep, and the rest is much larger. I removed the dashpots and pistons one at a time, positioned a shop vacuum in the mouth of the carb, put a bit of grease on the bit (probably a waste of time as the fuel would wash it off, but a good belt and suspenders approach, I thought), and simply drilled the jet.

Why, you ask? Because normally changing jets requires removal of the carbs � the Phillips screw that holds the jet body to the choke adjuster is all but inaccessible unless you are triple jointed.

Gave each jet a quick suck to hopefully remove any lurking swarf (that�s brass filings, not the Klingon), popped the .100� needles in the pistons and fired it up. I�ll need to adjust everything, but it pulls right up the way it always did. Mission accomplished.

So if you ever need to do such a strange chore, fear not � it would seem that quick and dirty sometimes works.
Bill Spohn

Good move, Bill! If anyone else is looking for a bit, the number size "39" is 0.09950", and may be easier to find. Cheers - - Alec

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Alec Darnall

Bill,

Obviously the scheme has worked for you. However, if the carbs were in the car and therefore you used a hand-drill, then how accurate do you think the final size was?

Neil
Neil

Neil, it doesn't really matter.

The steps between needle data points are typically in the order of around .010" difference anyway. I have often 'fudged' mixture setting with the trick of simply recessing the needle a bit in the piston, richening the whole range when I didn't have richer needles available at the race track.

If the diameter was off a thou or two, it would simply mean that the mixture adjustment for that carb would be slightly different, up or down, from the others.

In fact (and because I like to know such things too) I experimented with a sacrificial old .090" jet and drilled it by hand in a vise - and then measured it at the drill size plus or minus .001".

I think that's close enough, don't you? :-)
Bill Spohn

That is indeed pretty close!

Neil
Neil

This thread was discussed on 16/05/2006

MG MGB Technical index

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