MZ/MuZ Riders

First run today after some modifications and adjustments: synchronised carbs with adjusted float bowl height, lightened flywheel assembly and some minor other tweaks

A few days ago I finally found enough courage for pulling the flywheel. I stressed the Kedo puller to the max and -with a bang- the rotor/flywheel assembly came loose. And with a bang my forehead hit the fueltank. Ouch.

Original

Original

Minus 309 grams



Boxing day was used for some hours well spent at my dad's lathe. I removed 5 mm (3/16") from the center piece radius, leaving the shoulder for easy centering of the assembly. The flange with the one-way bearing is a pain. Even with a new carbide chisel it took ages removing a substantial amount of material. So after removing 3 mm (1/8") I called it a day. I aimed for the "Bill Jurgenson" aluminum flange design, but that would take ages. Still, in total 309 grams (0,68 pound) is removed.

Unfortunately I forgot to take pictures before assembling

Today I hit the starter button to determine any differences and checking the RAIN switch :

https://youtu.be/5UsaYGW9u1U

The rain switch retards the ignition curve by 10 degrees overall. The rpm drop at idle is significant when changing from 29 to 19 degrees advance around 1,400 rpm.

Although I checked the float bowl height and adjusted it according the Mikuni specs it still sounds rich when idling, even with 17,5 pilot jets and the pilot screw less than 1/2 turn out. My Innovate MTX-L AFR meter disintegrated a long time ago so at the moment it is guessing...

After some issues with a vague spongy brake lever feel I decided to swap the front disc for a brand new Brembo Serie Oro disc brake and installed new Brembo "SA" brake pads. With a pricetag of only € 159,- I ordered the disc directly from Italy based Carpimoto.

Spiced it up a little with some grade 5 titanium bolts and secured it with Goodridge locking wire.

The former HTR disc is definitely lighter (over a pound!)

But worn askew due to the large holes near the rotor. Pity, it looked ok.

Already the lever feel has been improved. The semi sintered pads should enhance the initial response as wel, so I'm curious what the experience will be after bedding pads and disc.

Great looking bike. I was wondering if you could give me a few pointers on the wheel conversion. I have a freshly powder coated set of SZR wheels ready to go on my project Skorpion but have been holding off until I could get the last few parts that I need. From what I read, the SZR axle and some shimming on the brake rotor is all you need up front. The rear requires the caliper carrier plate from the SZR as well but I can't find one. I think I'll need to fabricate a 15mm axle and put bushings in the chain tensioners to bring them down to 15mm. Does that sound right or did you covert the SZR wheels and spacers to 17mm?

Any help would be greatly appreciated.

Just read some of your blog regarding the Mikuni carbs. How do you like the 130/135 main jets with 17.5 pilots jets? I also have Mikuni flat slides on my bike which has a similar engine build; 11:1, 686cc, Megacycle X4 cam, +1mm SS valves, Kibblewhite springs, ported head, full race exhaust, except I opted for the TM 34/38 which has a 38mm carb for the right side intake. Bike runs great until I get to about 8000rpm where it starts to lose ignition. Don't know whether it's getting rich and putting out the flame or if I have something else wrong. My carbs came with 130/140 mains and 20 pilots.

at 8000 rpm it may just be running out of time to get a fuel charge in there and get it to burn

my rt 125 will scream right up to red line but just seems to sort of hit a flat spot at 10,000

and above 8000 i keep wondering will the frame and tank stop the piston if it comes loose on the up stroke

dave

@edf:
The SZR conversion needed a different approach front and rear. Front:



Top axle is the original MuZ Grimeca. The lower drawing is the (original) SZR axle conversion. Red is removed material (bolt head), yellow is new parts. The greenish bushing is an original SZR bushing slighty turned down for proper alignment. The right yellow in the drawing part is a 25 mm aluminum bushing shrink-fitted around the 17 mm SZR axle. It still slided around the axle when installing the axle. So a put a 5mm steel pin through the bushing and shaft. The left thin bushing in the drawing is pressed into the right fork leg. All these parts came from a scrap metal bin...

As you can see the tacho drive isn't used here...

Rear axle:
I made a complete new 15 mm axle (chromo vanadium alloy) exept the right part which is 17 mm.

Only three pictures per post... Bushing left chain tensioner:

Oh, these markings are caused by three (3!) loose sprocket (grimeca) carrier bolts. The bolt holes were completely elongated after 20 k miles and ever since I secure every fastener.

About shimming the brake caliper at the front: the caliper is indeed slightly outwards, but within limits according to Brembo. It is about 0,5 mm. To be honest, I've never contemplated it. The SZR brake carrier plate is the easiest way to go, but I stumbled across this picture:

Really nice Harris F1 carrier!

Carbs:
I bought the Mikuni's 34/34 mm before I found the tuned engine. Otherwise I probably would have ended up with the 34/38 like yours. I must say I can live with this setup The 130/135 mains are great, even at lowish < 50F outside. No dying out at high revs at all up to 8,400 rpm (even hit 132 mph@ 7,900 rpm and nice whitish spark plug afterwards). For the 38 mm side I would switch to a 10% bigger jet from 135; 145-150 would do. If you're using the original coil that may be the problem. But that's guessing; I swapped it straight for a huge Silent Hektik Solocoil (no longer available) with over 120mJ true power.

@edfmaniac: regarding your >8,000 rpm problem, to me it seems more likely it is running lean instead of running rich. Whenever I'm encountering carburetor issues I'm working from idle up to full throttle. Carb settings are (generally speaking) not related to rpm, only to throttle opening. You can even run your carbs properly without a main jet up to 3/4 throttle. Although, a heavily tuned engine has inherent flat spots due to valve timing and detrimental intake/exhaust effects. No carb or even EFI can fix this. So, according to this simple logic, your rpm related dying should also occur at lower rpm if related to the carb settings. The coil, if iffy, should fire the weakest spark at max torque rpm. Bottom line: high rpm alone doesn't induce rich or lean running by itself. A few other possibilities:
-petcock fuel obstruction
-fuel tank vent clogged

My blog Muz Manx is now in english... at least the most recent posts are. I'm translating older posts as well, but it takes time.

So, from now on it is more than just pictures... and hopefully it is interesting to read as well

Yesterday I attended a race licence course with my MuZ ManX. Although it was cold and raining constantly it was much fun. Trying to keep up with zx10r's and BMW 1000rr's with rain tires and traction control was a challenge, but my Skorpion did well, until the head gasket failed during the third session One of the left head bolts had come loose:




Luckily no further damage was done. It was a pity, I therefore missed the most important sessions, so no licence this year.

All bolts were torqued according to spec, although I have failed re-torqueing the bolts after the first 200Km... due to the labour involved tilting the engine to remove the valve cover. On top of my to do list is removing the frame crossmember for easy maintenance.

I took this gasket failure as an opportunity to remove the auto decomp. I know information is somewhere in the archives around here (something to do with Bill Jurgenson and steel balls ), but the search function didn't give any proper results. In the end I came up with a solution myself using bearing balls and aluminum rivets.

If you ever feel like those fixes aren't solid, I discovered another one that's pretty bullet proof. Just thread hole where the plunger that opens the valve at low rpm is and put a set screw in it to hold the decomp rotor/cam in place. Done!

Edfmaniac, I read about that solution in a Raptor forum thread. A proper and simple solution, with the mentioned disadvantage of creating threads in the plunger hole. The main reason for me not using this solution: removing the decomp cam ensures unrestricted oil flow to the most heavily loaded bearing.

These plugs are tight:




I will test my solution using a Cannondale fork pump, which goes up to 300 psi:



If the plugs can withstand this pressure I'm sure it will hold. Hopefully the Hotcams stage 3 cam allows the starter motor cranking the engine. If not a Megacycle x4 cam should do the trick. Something to do with valve timing overlap decreasing compression at low rpm... Edfmaniac, what's your opinion?

Really enjoyed your blog Hajok.

I like that you are going to test it.

I run the X4 in my engine and it turns over easily with the decomp system disabled.

The valve timing overlap of the X4 cam is about 5,5 degrees more than my Hotcams Stage 3:



If the X4 is no problem my engine should be cranked as well.

Today I had the guts to remove the frame crossmember.



This removal should give no noticable detrimental effects. We'll see, the worst case scenario is welding the crossmember back in place

Testing the plugged camshaft proved to be difficult. With tape and tie wraps the oil passages are closed, the maximum pressure was around 6 bar (87 psi).



I have confidence the plugs will hold.