1275 with 1098 Ex/Inlet & Carb!

Post by **dunketh** » Thu Jun 22, 2006 1:22 pm

Wow!
You'd be amazed how well this combination works... well I was anyway.

After fitting a 1275 a few months ago I never managed to get the supplied twin carbs working properly.
I balanced them and set the mixture correctly but still they gave me problems and guzzled fuel. (using a hose and a colortune plug no less!)
They were fitted using the Midget 'log' manifold, the exhaust being the standard morrie one with the inlet hacked off!

Anyway, I put it down to old age and lack of use (sat idle in a breakers for years) and fitted my old Minor manifold and carb.
I fitted the old HS2 minor carb with one of the needles from the twin carbs - just as an experiment and Voila!

The old dear has never run so well!
Obviously I know fuelling is an issue with such a small carb and it does run out of puff at around 75mph but the low down and mid range torque is excellent. I can tackle proper summits in 3rd and even 4th gears!

I was just wandering if anyone else had had similar experiences?

Before anyone asks, I did check both the timing and dwell angle and its all fine. I think one or both of the twin carbs were knackered. I'm off to find a single HS4 now!

BTW: For anyone who remembers seeing my 'fitting guide' my '2-bolt' flywheel did come loose whilst tuning and I did get it drilled for 6 holes and bolted it up properly. (lucky escape there!

)

Post by **rayofleamington** » Thu Jun 22, 2006 1:35 pm

I was just wondering if anyone else had had similar experiences?

Yes - I had both similar experiences - gave up with twin carbs on log manifold and at another time I ran MG-1275 engine in a Minor with standard Minor carb and exhaust - it had loads of torque and could tackle hills at 80 instead of 60.
If yours is running out of steam at higher revs it'll either be fuel or timing advance.

BTW: For anyone who remembers seeing my 'fitting guide' my '2-bolt' flywheel did come loose

Well, like we said at the time - they wouldn't have gone from 4 to 6 bolts for no reason. You are very lucky that the flywheel didn't sheer the bolts when it started to flap about (you could have ended up needing a new gearbox as well).

Post by **dunketh** » Thu Jun 22, 2006 1:43 pm

Well, like we said at the time - they wouldn't have gone from 4 to 6 bolts for no reason. You are very lucky that the flywheel didn't sheer the bolts when it started to flap about (you could have ended up needing a new gearbox as well).

I was lucky it started making anoise when tuning, rather than on an A road somewhere. I don't fancy having my face remodelled by Mr Fly and his nasty teeth!

Post by **Innovator** » Thu Jun 22, 2006 3:56 pm

If you fitted a single 1 1/2" HIF with decent exhaust manifold and system with a rolling road tune up by an operator who knows what he is doing then you will be really amazed!

Trying to get the correct mixture on carbs on your own is hardly ever succesful.

John

Post by **dunketh** » Thu Jun 22, 2006 5:12 pm

That sounds like the way forward, its a 2-3 hour drive to the 'local' RR though.

Trying to get the correct mixture on carbs on your own is hardly ever succesful.

I use the old 'lift the damper and see what happens' trick. I've found this gets it almost right then I tinker with it after driving.

Post by **bmcecosse** » Thu Jun 22, 2006 5:47 pm

Yes - colourtune is waste of time. Lift the pin works. twin carbs main advantage is that the manifold is NOT exhaust heated ! The engine only breathes through one cylinder at a time - so feeding with one or two carbs of the same size makes very little difference - but NOT heating the inlet with red hot exhaust makes a big difference. Obviously with 1275 when it gets going it hits the limit of what a 1.25" carb can pass - but up to that point - you will have almost all the benefits of the bigger engine. If 75mph is enough for you- then stick with it!! Ideally the 1275 engine wants a 44 HIF SU - on an MG Metro inlet manifold - but this will mostly increase the top-end power - and the fuel consumption. So if it suits you (sir) as it is - leave well alone and enjoy the extra low revs torque of the bigger engine.

Post by **Kevin** » Thu Jun 22, 2006 6:56 pm

I'm off to find a single HS4 now!

I have one for £5 if you want it.

Post by **Shawn** » Fri Jun 23, 2006 4:36 am

I've got a Mini HS4 on my otherwise standard 948cc; the difference was remarkable, even with the standard exhaust. No way I'd go back - extractors are next!

Post by **Innovator** » Fri Jun 23, 2006 6:01 am

The problem with tuning carbs at home is that you can not check the mixture at higher engine revs under load. All the methods (colour tune etc) set the mixture at tick over. They will do a reasonable job of this, but you do not drive around at tickover.

The only practicle way to tune at home is with a good lamda sensor or by reading the plugs after a steady run and then by cutting the engine and stopping.

I would also dispute the fact that fitting a bigger carb will increase the fuel comsumption. A bigger carb will make the engine more effecient, an engine that is more effecient will use less fuel. However if you always use the extra power (tempting) then the fuel comsumption will rise.

I have said it before and will no doubt say it again. A rolling road tune up is part of the whole modifying process. If you do not get a rolling road tune up done you are not utilising all of the modification work you have done. I would go as far as saying that I would prefer a standrd Minor to one with lots of modifications and no rolling road.

Trust me..........if you modify your engine then a rolling road tune up is essential. If you are not prepared to get a rolling road tune up then leave things standard.

Example: My first 1275cc engine I thought was quick. It had 3 branch, tweaked head, different cam, single HIF and K&N. I took it to the rolling road and power went from 33 at the wheels to 55bhp. Now that is a huge increase. The car started better, ran smooth and went like a rocket.

I had a £200 1984 1100cc Polo completly stock. I took it to the rolling road and came away with another 10bhp, thats a huge percentage increase.

If you need help finding a good rolling road get onto the Mini forums or advice on how to prepare your car for a rolling road tune up let me know.

John

Post by **dunketh** » Fri Jun 23, 2006 12:11 pm

I have one for £5 if you want it.

What engine did this come off?

I'm only guessing that the HS4 will be ample, infact 'ideal' for the 1275 engine. I'm after one thats already setup for a 1275 though so I don't have to change the needles.

wrt a Lamda sensor... could I just cut the boss and sensor out of a modern manifod and weld it into the minor one?
Also - am I right in assuming that I can connect this to an AFR meter in the car so I can actually see what the fuelling's doing while driving?

Cheers
jon

Post by **Innovator** » Fri Jun 23, 2006 12:26 pm

The chances of getting a carb that is set up perfectly for your engine is very slim.

Yes you could get a lamda sensor and an AFR meter, then drive and tune. The best way is to get a needle that is too lean then file it to suit in the correct area for the revs you are driving at. The chances of getting a needle off the shelf is again quite slim.

A rolling road tune up would be quicker and easier though.......

John

Post by **bmcecosse** » Fri Jun 23, 2006 1:54 pm

Bigger carb does increase fuel consumption - because it allows more power to be developed - which if used - uses more fuel!! Of course - if you don't use the extra power - then it will be much of a muchness - the engine will be no more 'efficient' - it's still being starved of air by the throttle - doesn't matter if big or small carb.

Post by **Innovator** » Wed Jun 28, 2006 8:11 pm

BMCecosse....Yes you are correct, a big carb at low throttle is like a small carb at wider throttle. An engine will only take as much air as it needs.

The only exception is if the engine is for example off cam (an extreme case!)

John