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MG MGB Technical - Tuning HIF4

Two questions.

1973 MBG Roadster......HIF4 carbs.....polution control components have been removed by a previous owner.

First, when doing the rpm test and lifting the carb pistons I got a 1 to 2 second increase of about 300 to 400 rpm. Then the rpm rate dropped back down to where it had been before the lift. Is this what the various tuning instructions are describing for this proceedure and is that about the correct rpm increase for the "lift the piston" test?

Second, I just realized I forgot to top up the oil in the pistons prior to doing the tuning. I had the carbs out to put in rubber tipped needles and had them upside down on the bench. Will that loss of oil affect my tuning session? IE, do I have to set up and do it again?

Thanks,
BH
BH Davis

Ummmmmmmmmmm.......guess it didn't matter that much. Car is running GGGRRREEEEEEAAAAAAAAAAATTTTTTTTTTTTT!!!!!!

BH
BH Davis

Oil in the dashpots (or not) only affects running, not tuning. 1 to 2 secs is longer than I'm used to, normally it is only momentary i.e. significantly less than a second. If it goes on longer than that it indicates too rich, and it's possible other factors are causing it to drop again. You can check (in fact you should always do this to tune your ear in before tuning the carbs) by weakening the mixture until you get a recognisable *drop* in rpms i.e. obviously weak, then richen it until the revs rise and stay risen i.e. obviously rich. Between these two points is close to the ideal. I say to 'tune your ear' because my HIFs are difficult to detect the actual correct point, significantly more so than my HSs.
Paul Hunt 2

Paul,

Thanks, that exactly the feedback I'd hoped for. Since I got nowhere trying to do it by ear or by having a mirror on the drivers seat so I could see the tach, I went out and bought a tachometer to connect to the ignition system.

I may have been overestimating the rise time....maybe I'll hook it up and take another look.

Thanks again for the lesson!
BH
BH Davis

When tuning HIF's it is important that the carbs are up to engine bay temp. After initial tuning go for a 3 mile drive & then retune the mixture. They have a bimetalic strip on the needle adjustment and lean off the mixture when hot. Only lift the piston 1mm when testing. If you have a cold witer climate then use 20/50 oil in winter - it gives better throttle response cos a light oil allows theiston to lift faster which makes the mixture lean.
Garth
Garth Bagnall

The methods descripted above are the way to do it, but I always check the spark plugs too. It's best to drive for a few minutes before looking at your spark plugs. When the're nice coffee brown/light brown your mixture is perfect. When the're almost white (In the worst case they look like glass) your mixture is too lean and black means too rich.

It's best to keep your mixture slight too rich, too lean can damage your cilinder head seriously. You can get burned valves for instance.

Good luck!
Rainier
Rainier K

This thread was discussed between 27/07/2007 and 07/08/2007

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