Carb rebuild tips

[rolesyboy]

Just in the midst of building a pair of SU's. Are there any modifications or minor alterations that I can make to improve/refine them? Only easy stuff that can be done by hand (as opposed to getting machining done)
I have deburred the casting edges but thought I might as awell ask before I carry on. Cheers. Mark

[dklawson]

Cosmetically you could polish the outside surfaces. That won't really require machine tools.

The only DIY improvement I can think of that would not mandate machine tools would be to narrow the butterfly shafts where they pass through the carb throat. That can be done with a file and great care.

[carbon]

Just in the midst of building a pair of SU's. Are there any modifications or minor alterations that I can make to improve/refine them? Only easy stuff that can be done by hand (as opposed to getting machining done)
I have deburred the casting edges but thought I might as awell ask before I carry on. Cheers. Mark

There's a whole load you can do to improve carb flow. The biggest benefits from doing this are with HS2s, and it is all well detailed in Vizard's blue bible, but be aware that if you do this properly it will require different needle. Also with twin carbs the BMC inlet manifold benefits from close attention, there are a couple of sharp edges that really benefit from being radiused.

['S'-type]

Clive Trickey's 'More Mini Tuning' covers some basic DIY mods to help the SU to flow such as DIY quick lift piston, radiusing the rear side lower edge of the piston, or cutting off the top of the piston tube to allow the bottom to completely clear the choke tube on full lift. Also has some tweaks to the butterfly.

This requires very basic tools, (drill, dremel, file, wet/dry paper).

I'm not sure I would do this to a £200-£300 set of H4s, but you can have fun experimenting with the plentiful HS carbs.

[mini63]

Face the flanges on a sheet of glass and some wet n dry/kero.
Do a piston drop check
Radius the leading edge of the piston
Snip off and file flush the butterfly disc screws....loctite these in place.
Use grose float needle n seats
Tune with SAE30 oil only
Radius the inlet manifold balance tube edges

[dklawson]

Use grose float needle n seats

I am in the anit-Grose Jet camp but the decision is yours. If you choose to use Grose Jets, be sure to soak them in carb cleaner and blow them out with compressed air prior to installation. They are supposedly shipped with a coating to protect the steel balls while in storage. Failure to remove the coating can result in float valves that either stick open, closed, or leak.

[ianh1968]

I had never heard of "Grose Jets", but the word "ball" here prompts me to
highlight a recent post of mine... I just looked up "Grose" and this looks to
be exactly what I had previously bought. Why do we always think that
different/fancy-looking will always be better?

The original post's topic was oil-pressure balls instead of the standard plungers,
But:

I've never had a problem with my balls... (In 'A' Series engines)

However... When I fitted a "ball-type-super-delux-uprated-better" fuel float bowl
needle valve replacement a while back, this exhibited the exact same symptoms
of "they cannot always seat fully on them".

My drive is on a slope and I'd filled up the tank the night before. Reversed the
car out onto the road the next morning and noticed a massive "slick". On opening
the bonnet, I found "thin oil" firing out of the transmission breather...

About a gallon of petrol had found its way to the sump - Not good!

This component came from a major supplier of Mini parts in England and I am
wondering if the design of this product has been changed as it is clearly flawed.

I now have a solenoid fitted in the fuel line which will stop this ever happening
again, although I have reverted to the traditional/uprated type now.

Any comments, Rich?

... accordingly, I am also in the anti-Grose camp!

Ian

[dklawson]

Grose Jet may be an archaic term. It was the brand name used by the original inventor but I believe they have sold their interest in the product.

In the image below, the traditional Grose Jet is on the left. I'm not sure what the valve on the right is.

[mini63]

Ok sorry to upset people with needle seat suggestion.
I had good results using the grose float seats in Jaguars using CD strombergs.
I used them if i was chasing a heat sink problem too on HS2/HS4s.
Either way just my two cents worth.

[ianh1968]

Ok sorry to upset people with needle seat suggestion.

I am not upset, and I am sure that Doug isn't either...
Neither of us has said "Don't use Grose Jets" - We have just added our own experiences/thoughts to the discussion!

Either way just my two cents worth.

...exactly! (Only I'd say my two pence worth...)

There is no right or wrong answer here and I am sure that Mark will
be able to make his own mind up on what to do with his carbs.

Something that I did not mention was what I do with the butterfly assembly.
I hacksaw the complete top section off and then attach the butterfly disk
using M4 button-head allen screws with green loctite. I also streamline the
remaining bottom section and taper off the appropriate edges of the disk.

Does anyone know what the "correct" thread for the retaining screws is?
I use M4 because it's easy to get screws in this thread at short notice.

Ian

[dklawson]

I am definitely NOT upset, nor would I flame anyone for using Grose Jets. As Ian said, I was just sharing my experience with the product and in my case that experience was not good.

I thought the thread in the butterfly shaft was 8-32. That's nominally .164" major diameter and 32 threads per inch. The "common" M4 will be nominally .157" on the OD with .0276" thread pitch (36.3 threads per inch). Therefore, you can probably get the M4 to fit in the tapped hole, especially with so few threads to engage.

[carbon]

For HS2 butterfly screws I am pretty sure the thread is 1/8 Whitworth, which has 40tpi.

[dklawson]

I stand corrected.

I found the thread linked below that says they are 5BA. That would match the 1/8" diameter Carbon measured. 5BA would be .126" diameter and 43 threads per inch. There is also an obscure inch thread close to that, 5-40 which is .125" diameter and 40 threads per inch. The only place I have seen that screw size used was when I worked for an old textile equipment company.

http://www.mg-cars.net/mga-bbs/help-su- ... 312597.htm