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MG Midget and Sprite Technical - How to reduce the height of an SU?

I've fitted an HIF44 to my 1275 Sprite. I've bought a flat topped damper, number LZX1389, and the carb just touches my bonnet.
I intend raising the bonnet slightly, so that it aligns better with the rest of the bodywork. I remember that somebody clever (Guy?)posted about washers in the correct places. But I'd like a bit more leeway.

If I remove the upper chamber from the carb body, what is the best way of gaining a few more millimeters clearance?
Can I be as crude as filing down the threaded portion?
What about the steel piston that sticks out of the top of the piston? That looks like it will be quite hard.
The damper rod looks like it is a press fit in the cap. I assume that I can pull it out and shorten that a little then recreate the ears before pressing it back into the cap?

Ta, Mark
M Crossley

Mark: have a troll thro the Archives. There are lots of threads on this one including a suggestion from Lawrence to taper the spacer between the carb and the manifold to tilt it down. Some have machined (filed?) the faces of the manifold where it fits on the head to create an angled face to tilt the whole assembly down a few degrees. That does complicate the fitments to go on the studs to clamp everything firm. Perhaps this is where Guy's washers go.
One stumbling block which you may have already encountered but seems to be one that many miss - make sure that you can get full throttle. I understand that the regular accelerator pedal and cable attachment doesn't have enough throw to turn the butterfly through its full range of movement. Again... there are threads in the archive.
G Williams (Graeme)

you can fit a shorter dashpot and plunger onto your carb, think they come from a TR7 or somesuch, again it's in the archive, probably in same thread.
David Smith

If I was talking of washers, it was to adjust the alignment of the rear edge of the bonnet to match the scuttle. You use washers between the hinge bracket and the bonnet - under the rear two fixing bolts and it will raise the bonnet; under the front two will lower it. Under all 4 cancels out and it doesn't change! Its all to do with the angle that the hinge pivots through.

Nothing to do with HIF carb heights, although if your bonnet is very low at the rear edge, you may gain a millimetre or two! My HIF didn't need lowering as I have Titan manifold - one of the rare ones designed specifically for the Spridget. Smirk!
Guy W

I chopped a few mm off the top of the dashpot tube on my HS6... I figured out how much I could safely lose by lifting the piston body until it no longer reduced the cross-sectional area looking into the carb, and then measuring (not very carefully!) how much higher it could be pushed up (with carb fully assembled).

So was able to remove around 3mm without affecting the carb operation.

BTW - the bonnet is convex (looking from outside), and so there's a little more room underneath than might appear; use plasticine or similar to find out how much clearance you have... but don't slam the bonnet shut unless you are 100% sure.

A
Anthony Cutler

Yeah ...like Guy said, raise the bonnet 1st as he described

I want to say the an HS is shorter then a HIF...might take a measure from the bolts that attach the carb to the manifold to the top of the carbs, for a comparison

Prop
Prop and the Blackhole Midget

I did exactly the same as Anthony way back around the lare '90s early 2000, -- I probably saw his mention of it in the live or archives and copied it, cheers Anthony --- and I also shaped the spacer to a wedge as graeme describes.

You can see it in this picture.

Lawrence Slater

Mark, watch out for the LZX1389 damper, it has an extra valve that slows the return of the piston. Different from the original domed top one. I used an LZX1944 which works for me.

The lower piston chamber was fitted to the Dolomite Sprint with twin HS6's.

Also with a flat top damper, you may need to cut about 6mm or 7mm off the top of the piston tube to get full lift of the piston at full throttle. The domed top damper allowed the piston to rise inside.

A Titan manifold would sort your angle problems.

Regards Tony
Tony G

Thanks for the help chaps. The manifold I have was sold as Spridget compliant, and it almost is. It's all assembled , and I didn't really want to dismantle everything for machining.
I'll realign the bonnet which will gain me 2-3mm, swap the damper caps to gain a few more without compatibility issues with the damper rods themselves; check for piston clearance,and then think about machining the dashpot, piston tube etc
Anthony/Tony G, was the piston tube difficult to cut?

Regards Mark
M Crossley

Hi Mark, no the tube was not difficult to cut, just marked with tape and used a new blade, ground off flat with a dremmel. Cut as little off as you can, as I had an issue with light damper oil being sucked out until I changed the oil to standard 20/50.
Tony
Tony G

Mark,
If you are that close to adequate clearance, then cutting the phenolic spacer as Lawrence has done (see his photo) is a quick, easy and low-risk way of achieving that little bit of extra clearance. And if you do make a mistake, a new spacer is a whole lot cheaper than a new carb!
Guy W

Tony/Mark
(just a by the by) Rod and I have been using "90 weight gear oil" in the dashpots as recommended by John Twist and we're both pleased with the results

at 1:55 here - http://www.youtube.com/watch?v=v9QI3NlvwiY&feature=plcp

(actually Rod used 80w/90)

table below to show the numbers in relation to different types of oils

Nigel Atkins

Mark. You should also note that just creating clearance when the bonnet is shut, won't be enough.

Startin the engine, unless you have rock solid engine mounts, can be enough to slam the dashpot into the bonnet if it's close enough. Even accelerating/decelerating can do it.

Another thing you can do(try) is to lower the engine a little, by sloting the engine mount brackets. I seem to reacall I did this too.

Lawrence Slater

You could also try some new engine mounts. Paradoxically the modern ones seem to be thinner i.e. lower that the original Rover ones. I know because I changed from aftermarket mounts giving plant of clearance to genuine Rover mounts and got a dent in the bonnet from my HIF44. Because of the angle, a reduction of a 1/4 inch or so of thickness drops the engine by around 1/2 inch (just make sure the crank pully remains above the cross member).
Chris H (1970 Midget 1275)

The dash pot off a Dolomite Sprint is shorter than a normal HS6
K Hughes

We cut a wedge out of the manifold and changed the mounting angle that way.

We also have put a restraint on the engine mount, it allows for the normal range of engine vibration but prevents anything excessive. The restraint had the happy side effect of limiting carb travel upwards over bumps, though it was done mainly because we got tired of breaking mounts during autotest reverse to forward flick turns (aka J turns but done properly it's a straight line not a J) when the big clutch dump after getting first gear coincides with and engine mount already sufferening from engine inertial loads as the car flicks round.

The restraint is simply a bit of flat plate bolted to the rear of the engine mount bracket, with a 1/2" hole at the top. Through the hole protrudes a 5/16" bolt which is secured firmly to the engine front plate. If the bolt is aligned with the centre of the 1/2" hole then engine movement of up to 3/32" (2.5mm if you insist) can take place without restraint. Anything more than that and it's held back. It's been 100% successful, no contact in normal use, no more broken engine mounts or panelbeaters bills for straightening a carb-contact nipple out of the bonnet.

Paul Walbran

I decided that if I shaved a new angle onto my particular existing manifold there wouldn't be a lot of metal left on the mount flanges.
I also decided that chopping into my HIF could land me in a world of hassle.

So I cheated. This nice manifold fits with 3/4" to spare.



M Crossley

It also gives a lot more room around the manifold studs.

Thanks for all the input chaps,
Mark

M Crossley

Looks neat enough but I bet you don't go very fast!
Guy W

filter looks like it's seen better days
Nigel Atkins

Whose brand of manifold is it?
Lawrence Slater

Hi Lawrence,
It's a Maniflow, L159SP. SP is a Sprite variant of a steel Mini manifold. It probably won't be asked any hard questions til I swap motors, but at least it fits, it matches the ports on my Calver head, and yours truly can't bu**er up the carb with a hacksaw.

Mk
M Crossley

Lawrence need to learn to read upside-down!
Guy W

I'll have to turn the screen upside down then. ;)
Lawrence Slater

This thread was discussed between 02/08/2013 and 19/08/2013

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