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MG MGB Technical - su hd6 carburetter

A carburetter, not on a MG car, but on a british car though (Rover P4 100)

As great tech knowledge is available here, I dare to ask!

It looses its damper oil (ATF oil filled in) quite rapid...
How is this possible: the tube on the piston is closed and only open on top; it is not leaking (have the carb dismantled at the moment)..
Could only imagine it get sucked away upwards and passing the piston then gets burnt in the engine...
What are your ideas??

On opening the carb I found a black smeary coating on the piston and in the housing, but there only
on the part to the aircleaner entrance, *not* in the direction of the throttle valve..
Is this normal debris or is the engine running too rich (for a long time)?

The jet bearing nut was very loose..
It should be fix isn't it?

The jet-needle has some minor 'raw places': it should be renewed? with the jet-bearing??

The throttle axis has some radial play (about 0.5 mm.)
It's better to replace to stop false air being drawn in?

Hope you can give me some replies!

(wished I had a MGB too ;-) )

rob

rob

Rob,

I wouldn't dare attempt to answer most of your inquiry (maybe one of the guys over there will volunteer). The HD6 should, however, be similar with respect to the piston assembly to the HS4s, HIF4s, HS6's, and HIF6s described throughout this BBS. ATF (if you're describing "Automatic Transmission Fluid"), is considered too thin/runny for the MGB application. Our application uses something on the order of 20 wt (I use 20w50) motor oil. I should think, given what I've seen of the HD series in my carb book, that it should have the same requirement. So, for the loss of fluid, you might consider a heavier oil. I'll finish this off with the disclaimer that I know nothing of your setup otherwise, so you'll want to consider this "advice" within that context. (Maybe your owner's manual states that ATF is to be used.)

Hope it all works out.

Barry
Barry Kindig

Barry,

Yes, it is described in the manual this carb. should be used with SAE 20 damper oil.
Alternatively SAE 20W-50 engine oil is widely used.
My suggestion for ATF (automatic trans. fluid) was given by a professional restorer of old Jaguar and Rover cars; they used this to fill the damper-channel.

My question was aiming at the *way* my oil disappears.. It will not be that volatile, and as leaking doesn't seem to be the case, I was wondereing: "where goes the stuff??"

As there is a blackish hue alongside the piston maybe it evaporates/get sucked away and is burnt?

Maybe others have thoughts/experiences too?

rob
NL
rob

Rob

Black deposits are normal, after considerable use, but you will get better performance and economy if you clean piston ,chamber and the inside of the carburetter body so that the piston moves freely with no hint of friction

raw edges on your needle sounds like the jet is not centred.this can be corrected by loosening the jet securing nut and moving the jet slightly until the piston will fall freely onto the bridge of the carb .then tighten and make sure it still falls freely, if not , repeat until it does

the needle may need replacement - but in fixed needle carbs the jet is much less likely to do so

is your economy poor - if so a needle may be the answer

the spindle play at 0.5 mm does not sound abnormal - there is always some play, and when they are worn it is very obvious and much much more than 0.5 mm
Chris storey

Chris,
thank you for your useful input!
The raw edges on the needle were minimal, with some polishing paste between my fingers rubbed these disappeared: needle is very smooth now.. I think I leave it and give it a try!
The jet securing nut was loose, in my manual it is not stated clearly that it should be tight, but you say
it must be tightened!?
Will remain with the throttle axis 'as is';
Seems I only will need a gasket for floatchamber to lid, and a rubber ring for the slow-running valve.. :-)

With the opened floatchamber, I found the 'fork' actuating the float-valve was bend very much downwards.
I tried with a 11 mm diameter rod between 'fork' and edge of the floatchamber-lid (as in the manual), and I had to bend up that fork about 8 or 9 mm.'s!!

Hope the jet is not getting to much fuel now!
Will try some measuring to see if the fuel level at 'float valve closing point' is about equal with the upper
edge of the jet...

Hope you will comment my thoughts again!

rob
NL
rob

Dear Rob,

The needle is a very accurately manufactured fuel metering device which, when matched with an unworn jet, provides just the right ammount of fuel for your engine at any given moment. Sooner rather than later, you are going to need to replace these.

Yes, the securing nut should be nipped up tight. But you should centre the jet while doing this.
Take out the damper, remove the air filters so you can lift the piston in the dome.
Lift the piston and let it fall back. You should hear a clear "tap" as it reaches the bottom.
Screw the jet adjusting nut (the smaller inner) in to the top (gently)
Tighten the nut finger tight.(the larger of the two).
Lift the piston again and listen for the "tap"
Tighten the jet retaining nut a bit
Repeat until the nut is tight.
If the "tap" can't be heard, you will have to try again.
Unscrew the jet adjusting nut two whole turns to get the basic adjustment.

It sounds like the float chamber should be OK now.

Your throttle bearings do sound tired. For a nifty way of doing the throttle bearings, have a look at the Chicagoland MG-Club website.

If you're somewhere near Den Haag give me a ring on 070 340 4769.

Mike
Mike

Rob;

All the above is spot on advice. Just for your info, the twin HD6 carbs are also used on 1959-61 Big Healeys. I have them on my 61 3000 Mk1 Healey. Same basic concept as the HS4 & HS6's.

Pete
Pete Haburt

Thanks to all who answered on my HD6 question!

rob
rob

This thread was discussed between 13/04/2002 and 17/04/2002

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