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MG MGB Technical - HIF advice

I've just put a pair on my 1995cc GT having stripped and cleaned them. Trying to get the set-up roughly right by looking for the highest rpm at idle I ended with one jet well above the bridge (2-3mm) and the other well below (2mm) and lots of run on. Which carb is suspect? There are slight air leaks around the throttle shafts but would that cause such a difference? If so is there a quick way of tuning until unitl I can get the bushes done? Just setting the jets the same height and praying perhaps?

Thanks,

Steve
Steve Postins

My throttle spindles were worn and it caused all sorts of weird settings both for the jets (both very rich) and the timing 31 BTDC vs 11 BTDC spec. Sounds a bit odd having one jet low and the other high though. I assume you've blanaced them?
Richard Evans

Follow Richards advice and check the air flow balance between the two carbs. I have found that this will cause the problem you described. Good luck. Ernie
Ernie C.

Thanks guys. I did balance the two using a meter before setting the mix, so I'm wondering if it's not the throttle shafts, if the float bowls are flooding or something else. I guess I'm dubious about the slight air leak being the major factor as I've got HS4s on a Roadster which have really sloppy throttle shafts but they tune up pretty well.
It would be useful to know which jet sounds within bounds for an HIF as I'll concentrate my time on the other.

Regards,

Steve
Steve Postins

Don't the HIFs have that valve in the throttle plate? If so, then maybe the one is sticking open and upsetting the mixture, as well as the bushes. According to the various tuning books these can be soldered shut. Their original purpose is to allow a small amount of air through so that when you shut the throttles suddenly from high revs it helps avoid neat petrol condensing on the manifold walls, or something to that effect.
FWIW
Martin

Steve,

Just set the two jets the same height - about 60 thou ( or 2 full turns of the screws ) down from level with the bridge. Then only move them together, the same amount the same direction.

The balance tube will actually allow the engine to run with just one carb! (albeit badly)
Chris at Octarine Services

Steve

It seems to me the jet adjustment is just a fine tune of the float setting. I believe that your floats are set differently causing a different position of the jet heigth.

The needle valves in the HIF have a spring to absorb some of the vibration. When adjusting the float, do NOT turn the carb upside down and let the weight of the float compress the spring and give a false float heigth.

To set the float, set the carb on its side, with a small hose blow into the fuel inlet and adjust the float level given in the manual to just close the fuel inlet. The specified float height should occur when the needle valve just closes.

When the fuel valve has a stiff spring, this is not necessary since the weight of the float will not compress the spring. But some rebuilt kits have softer springs, and this is the proper way to get the correct float height.

werner
werner haussmann

Werner

Your comment about the spring on the fuel needle valve has me intrigued. The HIF's on my 73B don't have any springs in that position - the only spring is the one above the main needle which permits it to self center in the jet.

Regards,

Barry
73B
B.J. Quartermaine

Barry

The spring inside the needle valve is in item 32 PN 3-565 in the Victori British catalog pg 123 MG.51 edition. This item is made of a float needle, and float needle seat.

To see the difference from the old and 'some' of the newer valves, push on the place where the float pushes up to close the fuel valve. It is spring loaded. To compare two needle valves, push one against the other. The weaker spring will push all the way in before the 'stiffer' one moves. I found the originals to be much stiffer. (I have not bought any in the last couple of years).

By the way the Gross jet does not have this spring loaded affect. I prefer the originals.

Werner
werner haussmann

Werner

Many thanks for the information - I was not aware of this hidden component. I rebuilt my HIF's last year with rebuild kits when I installed a new engine and seem to remember that the float needle valve and seat were separate items. I adjusted the float setting with the carb inverted per Haynes Sec 24, Fig 3.24 and the engine seemed to be run OK. For info, I later changed from ABD to AAA needles as I felt that the mixture was lean following installation of K&N filters.

Barry
B.J. Quartermaine

I suspect Chris has solved this for me. I was a bit to quick with reading the destructions and set one carb then the other rather than doing them in tandem. I'll set to next weekend, but I will have a good look at the floats now the carbs are off again.

Could you describe at what point on the float to measure the stated 1mm (+/- 0.5mm) gap? I've turned the Haynes schematic every which way and it just doesn't look like my float. The best guess I can make is the point on the float immediately opposite the inlet needle, where the plastic is slightly dished.

Thanks,

Steve
Steve Postins

Steve,

Yes you are right, measure in the centre where the float dishes.
Chris Betson

This thread was discussed between 13/09/2003 and 15/09/2003

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