Skorpion 660 Lightweight Build

[droy]

Nice work on the 400 wheel - curious is the center line of the sprocket to center line of the wheel the same as the stock MZ? That the trick if it is.

BTW - put a Skorpion Replica wheel on the scale ( with rotor, disk, crush and spacers) last night - 19.7 pounds. That 1.6 lb from the 400 wheel is a nice weight savings.

[jpmsteadi]

The center line of the sprocket is really really close to lining up with the stock wheel. I measured the wheel's center line vs the MZ and it is close enough to be within the error of my measuring methods. In test mounting it and looking at the chain, I made the spacers to move the wheel 1mm towards the chain side to have better alignment. Wasn't bad before by any means, but is near perfect now. If someone has a better methodology for ensuring it is a perfect chainline I am down for learning; this is my first real motorcycle project that involves custom stuff, not just bolting on things that other people figured out.

The weight savings should be nice. Modern casting techniques have come such a long way from the 90s. Even the cheaper bikes like the Ninja 400 have become crazy light.

[Puffs]

On alignment; not much help, but you'll see misalignment on the sprocket when it's worn (wear on one side).

For now all you can do is check with a ruler, or maybe a laser? After making sure wheel alignment/tracking is correct.

[jpmsteadi]

So I finally got the MZ back on the road this evening. The new spark box seems to be working well. Certainly a hotter spark, which is leading to it wanting more fuel. Wouldn't even run well without choke on. Very interesting. I suppose a weaker spark lead to less complete combustion, so it needed less fuel to get a good air fuel mixture. I will go back to starting with the MXrob dr650 jetting and start from there again.


The rear wheel and brake seems to feel well. Haven't ridden it hard enough to really know a difference, but it certainly still feels light (though coming from the buell it is at least 125 lbs lighter and I can't tell an additonal 7 or so)

[Puffs]

The way I understand how it works with sparks is a bit different. If the spark is strong enough to ignite the mixture, once it has been ignited around the spark plug, the flame front does all the work. It expands swiftly throughout the combustion volume, independent of how that flame front was started.

A strong spark is desirable as it causes the mixture to ignite under a wider range of conditions: a more stable ignition. But I can't quite see how a stronger spark would need a richer fuel/air ratio.

Hold on - are you sure your spark is stronger? A richer mixture ignites more easily (= why you need a choke), so a weaker spark might need a richer mixture to ignite. R-plug/non R-plug? Try a non-R one.

[jpmsteadi]

I will try a non resistance plug. That makes some sense to me.

I understand that a hotter spark will allow for a wider range of combustion. It is very strange that I would need to be so far off with jetting just by changing the box. Maybe the advance has something to do with it too? if the spark is more advanced it needs more fuel cause it is able to burn more efficiently? sorta a dart board on that theory.

[breakwellmz]

Hi.

There have been quite a lot of posts in the past on here about non standard ECUs have you seen them? Yours is one you set up (in terms of advance curves) yourself i assume?
Have you a manufacturers link to it you can post please?

Cheers

[Puffs]

I didn't even know these 660's with a carburettor had an Engine Control Unit... At least my 600 didn't, just a CDI control unit.

But yes, advance (curve) & mixture will have to be matched to each other. Sorry, I hadn't realised what your new 'spark box' actually did, I only reacted to it giving a stronger spark. The strength of the spark should not really influence the required mixture (unless it gets real weak), but the timing will. The velocity of the flame front depends a.o. on the temperature & pressure of the mixture, and also the oxygen/fuel ratio I'd guess.

Your new 'spark box' might need quite a bit more setting & adjustment then, I hope it makes the engine perform better; good luck!

[jpmsteadi]

Ahhhhh, yes. It is a complete new CDI box. So a hotter spark and completely adjustable timing. Not only that, but I have the throttle position sensor from the FCR carb hooked into the CDI and doing a two axis chart for advance vs TPS.

I will say with the choke on the bike runs REALLY well. Much crisper on the thottle response and revs much quicker, as well as feeling like a bit more power all over.

Alas I have been super busy and it has been raining, so I haven't had a chance to play with different jets, but I am going to go back to the base MX-rob and adjust from there.

The spark box is this: https://www.ignitech.cz/en/vyrobky/dccd ... 1_race.htm

I bought it through Bill in Germany.

[breakwellmz]

Hi.
Bill Jurgenson then, a proper font of knowledge. He used to post on here a lot but not for some time now.
If it arrives with the standard MZ timing as a baseline that would help.
Lots to fiddle with and you still have to find time to fit work in!
Cheers.

[Puffs]

Thanks for that link jpmsteadi. A CDI control unit (not quite an ECU), which includes a quickshift UP function. Interesting! But you might need a dyno bank to fully optimise the advance map (timing vs. RPM & TPS) & mixture.
Depending on the mapping there will be some, but I wouldn't expect too much variation in mixture, as there is an optimum in oxygen/hydrocarbons ratio. Too much oxygen may eat your piston & too much fuel will reduce the temperature & efficiency (& cause unburnt excess).

I interpreted 'Certainly a hotter spark' as meaning a spark with a higher temperature, as caused by a higher voltage...

[jpmsteadi]

Getting the carb dialed in. I put the carb with the air cut valve in to get rid of the overrun popping. Works so well, much quieter.

The fueling is getting dialed in. mostly richer in the area where the timing increased a ton, which makes sense while the main jet is just slightly richer. Rode around about 50 miles today, feels real smooth. Ordered another needle to try, in between the ones I have right now to dial in the part throttle area.

[jpmsteadi]

This morning I got the skorpion on the scale again to see what the new rear wheel, axle, li ion battery, and new slimmed down wiring harness got me to. Currently as it is sits (probably just under 1/4 tank of gas) is 338.8 lbs. I was hoping to get under 350 with this build, and it seems I have achieved that. Though, I think I can shave off a touch more with some subframe cutting, loosing the center stand mounts on the frame, and fabricating other aluminum or lighter steel parts for the rest of the bike. Been riding it around a decent amount with the carb getting better and better. Its super fun. SO light and powerwise feels squirty. Not overly powerful, but really fun in the city. I ordered some relativly cheap clip ons to play around with seating position. Could be fun to play with a bit more aggressive feel. I am sure it is going to be uncomfortable, but maybe fun for 30-60 mins at a time!

[droy]

Good News on the weight - I shooting for 300 ib dry on my track day Skorpion.

[Puffs]

Good going! That'll be about 330lbs without fuel then, thus saving over 50lbs (?).

It all depends what you want from a bike, and for me light is also quite important. For long trips you'd prefer something else, but a bike with that weight in the 60hp bracket should be good fun on twisty roads!