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MG Midget and Sprite Technical - HIF piston drop

Hi, got the manifold, heatshield, carb and throttle cable attached tonight so good progress (choke cable too short so on the lookout).

I filled the dashpot up with su damper oil as per manual and I am finding that after raising the piston a few times the oil has emptied itself and I can feel a difference in resistance when pushing it up.

Also how long should it take the piston to drop, its currently taking about 4-5 seconds (red spring fitted) and I compared it to the twins I took off and they seem to drop in 1-2 seconds.

Thanks
Mark
Mark Whitmore

Mark,

have a look at this
http://www.sucarb.co.uk/TechnicalDetail.aspx?id=71

you can only tell the condition of the piston and chmber with it off the carb body

Mike
M J Pearson

Should have added that I've also fitted a service kit and had the carb in pieces to clean and inspect. Before I filled the dashpot with oil it dropped fairly quickly with no hesitation/sticking.

One more question regarding K&N filters, are they ready for use straight out the box or do they need oiling first? I've always had 2nd hand ones so this is new to me!!
Mark
Mark Whitmore

If the damper valve assembly was dismantled, it may be assembled backwards. That would account for both the oil being pumped out and the excessive drop/easy rise problem. It is a type of one-way valve.

The proper drop test is done with no spring and no oil in the damper, or damper out. Holes in piston plugged.

K&N will be pink to red if oiled, white means no oil.

FRM
FR Millmore

Yep FR is right as I too have seen these reversed, but I am not sure what you are saying. If the damper pipe is filled to the top then fuid/oil will be slightly lost in the tube. You can for example fill the tube with oill to about a 1/4 and then raise the piston. If you do not have enough oil in the tube the piston initially will be easy to push up and then upon reaching the oil the effort required to raise it will increase by a good amount. So wih oil in the tube raising the pison should require a reasonable amount of consistant effort. When released even with oil in the tube the piston should fall smoothly with a nice clunk as it hits the bottom.

Yep white is un oiled and red oiled. I have never bought a K and N that came oiled.
Robert (Bob) Midget Turbo

The filter is a reddy colour, which is why I asked. Wouldnt hurt to clean and re-oil just to be sure, so I'll do that.

The damper wasn't taken apart so unless someone else did? Again I'll check

Got the engine running today and it seems to rev quite freely although it did stumble a couple of times, I havent had chance to fine tune it yet and its still set two turns down from the bridge.

1275 +30
LCB, RC40
MG Metro head
Titan, HIF44, K&N
Unsure of cam, probably standard (no noticable coming onto cam)
Red spring, BDL needle

Thanks for the advice so far

Mark
Mark Whitmore

Mark if the damper is faulty then you will have a problem when you plant the throttle. The damper is there simply to to stop the opening up of the carburettor to quickly when the throttle pedal is opened. With the SU we do not have a throttle pump so the piston rises slowly and does not allow the mixture to become weak when accelerating. Unlike the weber type carb that gives instant response to the throttle, which is why performance enthusiasts use the Weber in preference to the SU.

Before you do anything simply set the mixture on the SU it takes seonds and then other issues can be addressed.
Robert (Bob) Midget Turbo

Some K&N do come oiled.

The damper has a brass bit pressed on the shaft, like hollow rivet with the stem down.
Then a brass washer, then the damper piston, then a circlip.

The stem mounted piece is chamfered on the lower face OD of the head, matching a chamfer in the washer ID. The washer is also chamfered on the bottom face OD, with a matching chamfer in the piston ID.
All this acts as a check valve and restrictor. Haven't figured out the permutations of mixing up the pieces. But if the action is backward, oil would go up through the check as the piston rises, and then (oil) not come down again, so it would be spilled over the top of the center tube.

There are different dampers, giving more or less "losrt motion" before the damping starts, and there mayt be deviations from the description. I know that late HIF have things we never saw here.

FRM
FR Millmore

This thread was discussed between 22/07/2009 and 23/07/2009

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