Morris Minor Owners Club

Christmas is coming, the goose is getting fat... and I need a new carburettor

What would be the best one for me to ask Santa for in regards to decent performance but that's not going drain this poor student's bank account with the amount of fuel it uses?

Stick with an SU, HIF38 or 44 depending on what engine you have, plus inlet manifold, plus exhaust system.
There is a direct correllation between fuel consumption and the right foot.

mike.perry wrote:Stick with an SU, HIF38 or 44 depending on what engine you have, plus inlet manifold, plus exhaust system

It's a standard 1098

mike.perry wrote:There is a direct correllation between fuel consumption and the right foot.

An HIF 38 on an MG Metro inlet manifold will give a moderate improvement in power - and shouldn't harm the fuel economy if driven carefully.. I have a feeling the very BEST economy will be with an 848 engine, a 4.22 final drive - and an H1 SU !! I'm working up a thread on fuel economy figures gleaned from the original Autocar/Motor road tests.
http://cgi.ebay.co.uk/Single-HIF38-1-2- ... 3362bd823a
http://cgi.ebay.co.uk/ORIGINAL-MINI-MAN ... 588b1093a3

I'll definitely keep an eye on those. What are filter will I need to go with the carb? Will the standard one fit?

No - I tried all manner of tricks with the original filter - even an empty case caused hopeless running. A taper cone filter of the K&N type (but copies are fine) is the way to go.

Don't suppose you've got a link as I don't know what I'm looking for

And also... What to do with the exhaust manifold/exhaust? This list for Santa is getting longer

Just cut the old inlet section off and throw it away........ Yes - there is a little extra power (and noise) to be had from a fancy exhaust manifold and larger bore system - but the cost is prohibitive and certainly not justified by the small power gain.
This is the idea for a filter - but FAR too much money! You can get one for much less - search around.
http://cgi.ebay.com/ebaymotors/K-N-Raci ... ccessories
This is fine - http://cgi.ebay.co.uk/Classic-Mini-Cone ... 20afa2d4ff

I used an alloy trumpet that mated to a 1.5 SU and only needwed to file the airbox seating flange a little to fit a standard 1098 filter can.

Yes - but the limited airflow into the standard can is the problem!

BMC,

Awww, I was watching that manifold! Can't compete against Matt...him's a local lad after all! Guess I'll have to keep watching


Matt,

Worth biding your time on fleabay...I got a '92 HIF38 in perfect working order for ....£1.20 in the last few weeks ... and that auction also included a Mini manifold (cast iron combined exhaust/inlet), mini downpipe and an air filter!! Bargain!

I also got a spacer (in with a load of others for exhausts and different carbs) for 99p! Now I'm looking for an abutment plate.

Yes - but the limited airflow into the standard can is the problem

Forgot to mention that I also opened out the intake on the can!

I'm using a marina air filter on my 1098 with an 1 1/2" HS4. It's been on the car ever since I bought it. The engine was modded 20 something years ago. Is this the best arrangement?

Cheers

John

i've got a HS4 on my 1098 too, but using a K&N filter. if you change the needle and spring accordingly then it's pretty good. If you went any bigger you would loose air speed (due to greater area). technically the HS2 (standard) is alright as the smaller diameter makes air speed high, but when you want the volume at high revs, the HS2 becomes restrictive. the HS4 has a bit better flow at top end.

Technically the HIF38 is the same diameter as the HS4, with the added benefit of the central float chamber reducing fuel surge under hard cornering or braking. on my minor, it can cut out when i do an emergency stop, as the fuel flies forwards and starves the engine.

In my opinion, if you get the right needle and spring, and a K&N filter you'll be onto a winner. Minits Minis east of birmingham are good for telling you what needle and spring to use for your setup, even taking into account mods to your engine, eg head tuning or cam shaft.

The whole advantage of the SU carb is that it has a VARIABLE choke area - so the 'airspeed' idea is really not relevant.......It sets the flow over the jet automatically by lifting the piston - the amount of lift being controlled by the air speed - and the spring above. A single 1.5" SU will be fine on a 1098 - even BMC eventually realised that and fitted that size carb to the 998 Mini for many years. You can never go too large on the airfilter - the bigger the area = the better!

i take on board what you say about choke area, but after that the cross sectional area increases therefore the velocity must decrease accordingly. or so my fluid dynamics teacher tells me

Only if the volume of air passing through it remains the same... which it doesn't

Velocity = Volumetric Flow Rate / Area

Whilst the volumetric flow rate is not necessarily constant, it does not automatically follow that the velocity is constant either; in practice both are likely to vary and mass flow rate is possibly more useful to consider.

Casting my brain back (too) many years to doing this stuff as a student...For constant temperature and pressure, the laws of fluid dynamics dictate that as the cross sectional area decreases(for the choke area):
1. velocity must increase - conservation of mass (mass of a closed system will remain constant over time); and
2. pressure must decrease - conservation of energy (energy in an isolated system remains constant over time)

The converse will happen as the cross sectional area increases (at the velocities we're talking of here)

Given turbulence in the choke entry and exit areas, heat differentials along the path, etc., there are too many variables and you'd be here for a month of Sundays working out an accurate prediction of velocity ... but the laws still apply.

The critical section is velocity across the jet bridge - after that - increased area will of course = lower velocity, but also reduced drag on the flow. Obviously a HUGE carb won't work well on a small engine (unless revved to the skies) - but within reason - a larger SU carb will be fine - and handy when an upgrade to larger engine comes along!

bmcecosse wrote:Obviously a HUGE carb won't work well on a small engine

that's what i was getting at when you said 'bigger the better', even if my logic in getting there was a little flawed

i've got a HS4 on my 1098 and my dad has always said 'you dont need a big carb, a HS2' is plenty' which led to our own internal 'more size less speed' argument!

i've been told a 1 3/4 carb (HIF44 or HS6) gives problems of 'over-fuelling' on a 1098. is that just a case of poor needle selection/availability?