MM tuning

Post by **RobThomas** » Wed Aug 19, 2009 6:02 pm

I'm now fairly well set up for tuning engines. I've had a crack at our MM and have changed the rather dilapidated MM distributor for a Mini Cooper S dissy 23D4 (Lucas part 40819) which has no vacuum advance unit and appears cosmetically similar.

Using a quite clever piece of kit I have determined that the EK needle gives a very rich idle (AFR 12.0) and it has a rich acceleration below 2000rpm at full throttle and then runs up to 14.4 AFR at the higher end. Cruising keeps it at about 14 to 14.5 which is about ideal, if still a little rich. This is all a little odd in that the carb height adjustment is right at the top of the travel so it would suggest some needle wear and jet wear.

Once I've played with a new needle and jet I will look into modifying a later Mini electronic Lucas 65DM4 distributor to work on the MM. No more points, condenser or wobble to worry about. I have all the parts to make about a dozen units. They'll also help out on the fuel economy quite a lot.

More to follow over the next few months!

Post by **mike.perry** » Wed Aug 19, 2009 9:09 pm

The EK needle is unusual in that it is parallel from stage 8 to stage 12.
I have fitted the M9 rich needle in my carb which has the same profile as the EK from stage1 to stage 7 then keeps tapering to stage 12, using the theory that for cruising and gentle acceleration it would perform the same as the EK but would give the engine a richer mixture for hard acceleration and faster cruising.

I would suggest that you do have wear in the needle or jet since all .090 needles have a stage 1 dia. of between .088 and .090 and the EK needle is .089 so in theory you should be able to adjust the carb to the correct tickover on any needle. It is only when you open the throttle that the mixture becomes rich or lean depending on the needle.

For many years I ran the engine with a 25D4 dizzy for the purely scientific reason that the original DKYH4A dizzy was knackered and the 25D4 fitted. I tried it both with and without the vacuum advance (on a SII H1 carb with hor. float chamber) but the jury is still out as to which worked better. The vernier adjustment was definately useful. I bought a new DK dizzy for too much money at the NEC Classic Car Show last year and fitted that, which looked more authentic and did not appear to have a detrimental effect on the performance although after 10 years hard use since its last rebuild it was getting past its best.

I have finally thrashed the nuts off the engine and it is awaiting a rebuild

Post by **alanworland** » Wed Aug 19, 2009 9:33 pm

Most interesting! I have run a 40819 dizzy for years (about 70k miles with still no shake in the bearing assy) I seem to remember finding out that the advance curve of this dizzy is the same as the original MM?
To the untrained eye it doesn't look too disimilar from the 'proper one'!
I ran the EK needle on the single carb setup and then on my later twin carbs, with both setups I found that the correct running mixture (by plug colour) would give a rich tickover.
I now have a pair of M9 needles and it was possible to setup the correct running mixture and achieve a far more satisfactory tickover.
All my experimenting and running have been carried out with ignition timing at exactly TDC, I keep saying that I will try some advanced settings just to see the results - but haven't got round to it yet!
Any idea what is your timing set at for your normal running?

Post by **mike.perry** » Wed Aug 19, 2009 9:42 pm

My timing was normally set scientifically at fastest tickover, then tweaked a bit. On the drive on the Series MM Rally it was tweaked a lot several times to try to keep the load off the big ends.

Post by **alanworland** » Wed Aug 19, 2009 10:01 pm

Any idea what that was in terms of degrees?
I changed my crank shells, as a precaution, just before Christmas after about 75/80k miles and found they were just starting to break up with the leaded bearing surface coming away from the shell!
I think I changed them just in time as much longer I am sure would have taken its toll on the crank.

Post by **mike.perry** » Wed Aug 19, 2009 10:18 pm

I am afraid that I have no Idea what the degrees were, especially as you cannot see the timing mark on the crankcase with a water pump fitted. I will try to be a bit more scientific when I have the engine rebuilt.
What are the vernier lines in degs. on the 25D4?
Anyone know a good s/v engine builder?

Post by **RobThomas** » Thu Aug 20, 2009 10:14 am

The 40819 has the same max advance on the basepalte as the MM and it brings the advance in quickly so that you have high advance early, which to my mind is good if you aren't using post-war 80 octane pool petrol! I started at TDC with no pinking. The other way to set it up is to run at 4000-ish rpm and adjust the dissy to give the max vacuum and then wind it back to drop 1/2 PSI. Good logic behind that method.

My aeroplane uses hall-effect magnetic pickups to time each pulse and then works out an advance based on the rpm seen from the time between pulses. Absolute timing accuracy and no moving parts. Simples!

Alan. Your experience with the EK sounds just like mine. M9 sounds like a good option. I'll have to find my old needle books.

I'll be back from Manchester/Egypt in a few days and will report further.

Cheers

Post by **mike.perry** » Thu Aug 20, 2009 11:19 am

I would like to keep the original dizzy and if possible modify the innards to get better performance. The MM should certainly be tuned for the best power delivery at lower revs. As I have said before, the piston speeds at 4000 rpm with a 90mm stroke are fast enough for comfort.

Post by **alanworland** » Thu Aug 20, 2009 8:08 pm

That's what we need Mike, tyre shredding torque!!!!!

Post by **mike.perry** » Thu Aug 20, 2009 10:49 pm

I've surprised a few other drivers with how quickly I can get my MM off the lights! Low down power is ideal for hill climbing and towing the trailer.

Post by **RobThomas** » Sun Aug 23, 2009 10:26 am

Watch those big end bolts!!!

Hey, wasn't someone looking into better rod bolts?

Post by **mike.perry** » Sun Aug 23, 2009 11:37 am

Provided the big ends are torqued up to Series MM figs, 27 lbs ft and mains to 42 lbs ft and not to A Series figs there should be no problem, besides there is a mountain of con rods with big end bolts available.

Post by **alanworland** » Tue Aug 25, 2009 8:59 pm

My original bolts have been used 3 times now, and they seem fine!
The original torque figures seem very low compared to other engines, and I think some guys 'give them a bit more' just in case! which has the opposite effect of helping!

Post by **mike.perry** » Tue Aug 25, 2009 11:17 pm

Its all to do with the metal. Any more than 27lbs ft and the bolts start to stretch.
It didn't help matters that a certain well known purveyor of Morris Minor parts listed the Series II torques for the MM. Since I pointed out the error they seem to have removed it from their website.