Calling all MB100 tuners and bucket engineers

[steve k]

I have got the opportunity to aquire an mb100 engine and was looking for a little tuning information. really want to suss out optimum carburation size, best port timings and reedblock modifications. any info on exhaust systems would be appreciated as well. pm me if anyone does not wish to share this info in a public forum. I understand these can be closely guarded secrets. any help and advice would be very much appreciated. btw I am currently reading 2 stroke tuning by graham bell and finding it quite interesting.

[F5 Dave]

Depends what you want to build.

For a starter an easy to ride kart track type bike I would start with a 28m flatslide. Easy to fit & tune, jets common.

Pull the flywheel off if it has a big flat iron ring around the outside, grind the rivets off & remove. Keep sparkplugs fresh & the ignition should be ok. Just.

With that inlet don’t make the port too wide. Much better to run a slot either side of the main one which will give heaps of area & support the piston ok.

Some people make up a odd Vblock arrangement so they can run a bigger reedbuck. Much easier to just hack the back 3/4" of the barrel off an old Yam (RD/DT/whatever, they pretty much all used the same block) & weld it onto the MB. Then grind the heck out of the MB till the Yam reedblock fits in. To prevent distorting the barrel it is best just to tack the corners of the block & fill with Devcon F (paykel engineering got mine in, it has been updated to a new number). Use Yam manifold, fits 28 carb fine.

Std steel reeds worked best at moderate revs for this set-up. I built mine for Peak power at 10,000rpm. Best to make the bike MX type power, you can add more later but it will be harder to ride. Follow the book for port timing but use minimum transfer timing suggested, from memory I used 128 which was too high for the revs & very fat pipe I used as it over scavenged. Another pipe it may be fine.

[steve k]

thanks for the info dave. would be looking to build it to suit the kart tracks. will keep you up to date with progress.

[nsrpaul]

you could also fit the 6 speed box from a mb 50 , should go straight in

[steve k]

you could also fit the 6 speed box from a mb 50 , should go straight in

Ive heard that from another source also. has anyone fitted the 6 speed box from a mb50 into the 100?

[F5 Dave]

Oh, forgot to type that. Yeah, my H100 (same as MB bottom end) I dropped one in it. My MB100 does too. Good luck finding one. I remember a friend boastnig how he'd spent 20 hours shimming one up to work. Maybe he was trying to keep spares plentiful by making people think they didn't drop right in.

MT50 (wasp trail version) will also fit, but only a 5 speed.

[Kickaha]

MT50 (wasp trail version) will also fit, but only a 5 speed.

There's one of those sitting on a bench at street and sport in Chch if you can't get a 6 speed

[nsrpaul]

marty in blehnhiem has access to one , f5 dave should have his contact details

[speedpro]

From experiance - initially don't bother replacing the original reed block. All you need to do is file the centre divider/support away and replace the reeds with a single piece of fibreglass, that was fine for 19.9hp on my sidecar. A 28mm carb is fine but I've used a 32mm Mikuni which was just sweet after I pissed round with all the jets. Go with the 28mm and use an RD350 carb mount with a couple of new holes drilled in it to line up with the MB mounting holes. Leave the rubber bits that stick out on the engine side and just jam the whole lot onto the original reed block. The sticky-outy bits will form a 1/2 reasonable transition from the round carb hole to the square reed hole.

As far as porting I wouldn't bother touching it, "maybe" widen the exhaust port a couple of mm each side and "maybe" put extra ports each side of the original piston port. I would make an alloy spacer about 1.2mm thick to fit under the barrel which will raise the porting enough for a start. Use 2 base gaskets as well.

You do need to raise the compression quite a bit and will need to have both the barrel and head machined. The original combustion chamber shape is crap. The reshaping and compression raising is fiddly work, be careful who you get to do it, if you aren't doing it yourself. The top of the barrel MUST be machined perfectly square to the bore or the squish will be wonky. Doing this can be a real juggle, getting the squish clearance, compression, and combustion chamber shape all just right. When you start getting the compression right up there any small changes make quite a differance to the actual compression. I run 15.2:1 compression but I really need to watercool the motor, or head at least, as it fades a bit as it heats up.

If you get enough horsepower that the clutch starts to slip you can space the springs but that can get a bit hard at the lever or what I've done is modified the clutch to take an extra pair of plates. Even with stock springs it handles 22.5hp no problems. You do need the MB50 gearbox, it drops straight in. I use Motul Transoil. Good luck finding one for sale.

MB100s are one of the best 2-stroke bucket engines you can buy. A good crank will handle 14,000rpm for about 6 years of bucket racing and even the stock piston is good for it as well though the rings are a bit fat.

[FROSTY:Me]

Maybee Im biased but why not buy a cosmeticly fucked fxr150?
seems to me they would be the weapon of choice -dont hey put out 15hp stock? --add a cam carb and pipe kit and it shoulf fly--and probably cheaper than developing a stroker to full potential.

[Gixxer 4 ever]

Maybee Im biased but why not buy a cosmeticly fucked fxr150?
seems to me they would be the weapon of choice -dont hey put out 15hp stock? --add a cam carb and pipe kit and it shoulf fly--and probably cheaper than developing a stroker to full potential.

Arn't buckets limited to 140cc?

Edit: Classes catered for here are:

Formula 4 - 51cc to 100cc 2 stroke or 140cc 4 stroke solo bikes.

Formula 5 - 0cc to 50cc 2 stroke or 100cc 4 stroke solo Bikes.

F4 Sidecars 0 to 100cc 2 stroke or 140cc 4 stroke

[Kickaha]

Arn't buckets limited to 140cc?

Edit: Classes catered for here are:

Formula 4 - 51cc to 100cc 2 stroke or 140cc 4 stroke solo bikes.

Formula 5 - 0cc to 50cc 2 stroke or 100cc 4 stroke solo Bikes.

F4 Sidecars 0 to 100cc 2 stroke or 140cc 4 stroke

As of July 1st the four stroke limit is 150cc

[speedpro]

Frosty is right. That is definitely the way to go , , , , unless you want to go REALLY fast, in which case you need a 100cc 2-stroke and a LOT of very careful work.

[steve k]

Frosty is right. That is definitely the way to go , , , , unless you want to go REALLY fast, in which case you need a 100cc 2-stroke and a LOT of very careful work.

hoping to go really fast!!! quite keen to go down the 2 stroke path. thanks for the info speedpro. much appreciated

[Brian d marge]

Frosty is right. That is definitely the way to go , , , , unless you want to go REALLY fast, in which case you need a 100cc 2-stroke and a LOT of very careful work.

I third that,,,,, A bit short of time to allow me to give an in depth answer ,,,but the long and short of it ,,,the 4 strokes are way more forgiving ,,,of the file

But ,,in a nut shell ( this is for a kart track which I assume is tight and twisty )
Flywheel weight. lighten flywheel but dont change dia ( this is a hard one as MX bike use different wieghts depenent upon track ...I use a big basket see photo _ My design _ cos I am a woos and need the smoother power delivery ....)

Fuel
91-93 unleaded..
Carb
26mm as u go up u will lose metering strength so it wont carb out of the corner cleanly ...28 would be a good comp...better to have a small carb working at max than a slow gas speed through a big carb ...

Reeds
thick reeds.... that taper ( only know mx so cant help much)

Compression 9 ish 10 ish and a large ish squish 60-65%

Pipe ...beyond me ..at this time ....But an MX pipe would work ,,,,, FHM ?? ( or is that a naughty mag ..) Fatty pipe ????

sliencer ...short/ small dia ( becarefull to short and the piston crown heats up and will melt)

Ports ...Just match them a small mismatch creates a disturbance in the force and can actually block ports ,,,
but Exhaust will be around 90 ISH ATDC transfers about 118 ( A BIG ISH )

Rod Ratios ..depends on circiuts ...about 3.7 ISH

Finally GEARING ,,,possibly the single most important change ..... Chose the part of the circiut where the is the most opp to over take ....and tune /gear for that .....remembering if you gear short and there is a big straight ...you will be flat out at 1/2 track and will be passed at 3/4 of the straight ... Unless you are up against a demon late braker who will run a slightly taller gearing braking later and harder ,,,than Us mere mortals ....


Four stroke tune ing is all about getting that torque ..... low down dirty rotten Torque ....

I like two smoke you can really get them to go well ...but they are SOOOO easy to FUGGG up on ,,,,,,

The priceless piece of Oponions is now trading of the futures market for 0.002 cents worth

Stephen