ESE's works engine tuner

[41juergen]

The 3MA10 cylinders from the MJ 1990 3MA3 also have low timings (192/126), but a wider C-port and a more rectangular A-port what makes it difficult to get the triangle shape of the aux export and so the required blowdown..

[wobbly]

A wider C port is not optimal ,and means the B rear wall cant be moved around as much.
The big issue though is the dumb rectangular Aux port - even looks like the useless 4DP.
You would have to weld up 1/2 that port to get the triangular shape needed to prevent vertical short circuiting , and reduce the effect of port linking
to the A via the small end hole.

[diesel pig]

The 3MA cylinder I have looks like just like the one 41juergen posted. I think you have helped make up my mind, The 3XV seems to be the one to use.

[lohring]

Re spark plug projection: Removing the plug washer is a trick used in karting to get the spark closer to the center of the combustion chamber. I have no data on how much if any this helps.

Lohring Miller

[wobbly]

Plug projection - yes and no. I spent hours setting up the long version R7282A -105 used in Honda's with under plug deto sensor to remove the extra 3mm of thread
to have the electrodes well into the combustion chamber middle. Made no difference at all in a KZ.
But Frits has done a similar thing by removing the bare unthreaded end of a conventional plug , and reports that its as good as the all conquering R7376 for way less dollars.

[Orengo98]

A Bultaco head, isn't it? All Bultacos I've worked on had long reach spark plugs so I wonder what type yours is.
Anyway, never fit a short reach plug in a long reach head or vice versa. And if your head was modified from a short to a long reach plug, chances are that it was not the only modification, so you may forget about the manual in this respect.

Hi Frits!

Thanks for the reply. Yes, It's an early Bultaco head yes! It has that typical "trench" combustion chamber.

You're right, all Bultacos have long reach Spark plug, except the 100cc and 74cc models.
The bike I'm rebuilding is a Lobito AK 100, a factory racer based on the Lobito trail bike, to compete against Hodakas in the 100cc class in the US.
It seems to be stock, I can't see any modification to it. I've asked a friend that has the same model and he sent a picture, and the same happens:



So it must be this way from the factory, but I don't know why. I've tried using a long reach spark plug and protrudes around 1mm:



What would you do? With this being a 100cc engine with a 14:1 head, using one plug or the other is going to affect the compression ratio considerably also.

Thanks in advance!

Re spark plug projection: Removing the plug washer is a trick used in karting to get the spark closer to the center of the combustion chamber. I have no data on how much if any this helps.

Lohring Miller

Hi Lohring!

Thanks for your comment. Funny considering this cylinder and head where designed initially for the 100cc direct drive kart class.
The picture above is using the plug washer and a long reach plug. It protrudes about a mm.