Both the Mikuni and the SU carbies have slides.
Although both cylindrical there is quite a difference.
The Mikunis have a wedge cut out of them and the
way I see it, it would make for a better flow through the venturi.
But then, maybe flow through the carby isn't everything, the
SU jet bridge has sharp edges and grooves on both sides of it.
Even so the Mikunis have their truncated slides for reason.

So has anyone tried truncating their SU slides?
I'd like to hear from anyone that has an opinion on this.

People have modified SU slides and jet bridges for years. For road use it's a waste of time, doesn't work.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

What is the effect on a road car?
If it's idle and low speed that's a problem, why
does it seem to work effectively on motor bikes?
Not trying to be smart here, just trying to understand.

I used to run Amal Monoblocs on a Mini years ago and had BSA bikes too.

If you gasflow the jet bridge area of an SU it mucks up the way fuel is lifted from the jet, particularly at part throttle.
It is detailed in one of Vizard's books but it creates more harm than good.

OTOH I have an HS6 here modified by Graham Russell. He mills the approach side of the bridge even squarer, for better performance.

The other thing with a car is you can nail the throttle quicker with your foot than you can turn a twistgrip on a bike. So transition off idle is more important.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Wouldn't be interesting to make up a perspex body
and push smoke through it and see exactly what happens?
I'd also reckon there would be a bit of air leak between the body
and slide even on a new SU.

Thanks for your reply, Doc.

the lip in the SU helps creat the low pressure needed to lift the fuel from the jet,,, stuffing with that part in an attempt to smooth it out or radiuse it or round it off is detrimental to performance

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No offence intended here but--> anyone writing a book about minis 30 years ago may not have experienced such worn or stuffed-with components as we are finding these days.

You should put your heart & soul into everything you do.

It's been a long long time since I've seen a Mikuni close up.
Do they have a jet bridge similar to the SU?

When you think about it ,the Skinner bros got it spot on back in 1908. Their carbs have been copied since then, and they have not changed much. Most of the bike carbs with the plastic slide and diaphram still have a sharp rise to the bridge, even the CR slide throttle carbs have a raised lip to create the lowest air pressure possible over the jet..
The little amount of clearance around the slide on a butterfly control carb is doing jack. Even the CR have a reasonable amount of clearance, but they are race carbys

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Research is the difference between speculation and investment. Anyone who copys some one else will always be second
www.minisprintgt.com

I've always been a fan of the SU. When I was a kid
I'd seen this HR Holden with triple SUs and no carb
since has looked sweeter. Many years later I acquired
a M2 Jag and began to really appreciate them.
Another significant difference is the Mikuni is controlled
by the slide where the SU has a butterfly.
So that makes me think that throttle response for the
SU would be slower because the slide floats upwards
as it responds to vacuum.
Does the Mikuni truncated slide have any effect on
throttle response?

When SU 's were first made they were designed with a ram tube and a filter on the end. It was not good for dusty areas, so they fitted air filters to the front of the carby. This meant that as the piston lifted due to differental air pressure the atmospheric side now is influenced by a fluctuating air pressure. They still worked,but the biggest problem is to make sure that the piston lifts to it's maximun or the air flow is restricted.

A slide works better as the air flow is kept directly around the jet area. I have seen SU's that were modified for race cars that had the cable connected directly to the piston, and the butterfly removed same as a Amil carby. Definately no good for road cars.
There are some very interesting articals on the net about Su's and other slide/constant flow carbs.

_________________
Research is the difference between speculation and investment. Anyone who copys some one else will always be second
www.minisprintgt.com

So David if i read you correctly, If the ram tube was
placed in a plenum chamber and the air cleaner attached to that,
that should resolve the fluctuating air pressure?
Actually I think I've seen a picture some where of set up like
that

The original set-up was a ram tube bolted to the front of the carby that lined up with the throat. The air supply holes to the piston were open and not filtered.
This is a modification I do on the same idea. The ramtubes fit inside the air filter,but the atnos side of the piston is connected to a separate filter. This keeps constant 14.7psi on the atmos side of piston.



They help to stabilise the piston and stops the flutter caused by fluctuating air pressure.
Some of the early pommy cars that had blow thru charging on SU's had a separated pressure line to the holes. The carbs also had a fuel en-riching device as well for start-up

_________________
Research is the difference between speculation and investment. Anyone who copys some one else will always be second
www.minisprintgt.com

Clearly I don't understand these carbs as much as I should,:oops:
so I took myself out to the shed to have a look.
Yes, yes , I see now. The vent or intake that allows
atmospheric pressure or to be more precise, the differential
pressure, to lift the slide is just above the tickler.
OK to you blokes this is very basic.
So David, Is this right? Those ram tubes isolate
the vent so it is less affected by the engine intake pulses.
My monitor isn't the best so the photo is a little hard see.
Or as you said, a separate tube could be attached and vented
to the atmosphere.
Sometime ago I was talking to a Charger guy and he was saying
it was very difficult to get triple SUs to work on a hi po hemi because
of pulses caused by the cam.
I wonder if the solution is as simple as above?
When would the air flutter effect on the piston slide
most likely to occur?
I suppose this would depend on revs and the cam?

David, if you're around I like to learn a little more.
I have a few old and broken SUs in a box and I have
noticed the slide on some have a single 4mm? hole
on the side and others have two small holes on the base.
I imagine this would make the slide more sensitive at lower revs.
Am I right?
Back to the truncated Mikuni slide. Why do they cut the wedge
out of the slide?

Am I the only one that's wondered about
these things?