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Old 04-18-2021, 08:07 AM   #1 (permalink)
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Simson S51

As I've not only got a car, but also a light motorcycle, I'll give it a thread as well.
Many of you may never have heared of that particular model as it was never sold in the US for political reasons.
It was made in the GDR and is still rather popular here in germany as it has a few nice advantages over its modern counterparts:
1. it is legaly a 50ccm "scooter", but is allowed to go 60 km/h (38 mph) and is a foot shifted motorcycle
2. It only goes 60 km/h uphill with a passenger, usualy they go at least 70 km/h (44 mph)
3. The aftermarket and spare parts availability and prices are insane.
Every part is still in production, often by several different manufacturers.
4. Despite beeing an aircooled 2-stroke, it's very fuel efficient.
5. No TÜV

I'd like to improve on point 4 as I'll have to overhaul the engine soon anyway.
As of now it has taller gearing (17:31 instead of 15:34) than stock, electronic ignition and a lightweight rider, leading to higher topspeed as well as lower fuel consumption.

Possible modifications I'm considering:
-raising the compression ratio
The M541 engine was designed to run on VK78, wich is 78 RON.
As the lowest octane fuel I can find is 95 RON, I can increase compression with no issues and likely still have room to adjust the timing.
-carburettor
I will need to tune it regardless, so I'll do my best to make it run efficient without getting lean.
-gearbox
I am considering to swap the 4-speed for a 5-speed.
I'm allready at the limit with the sprockets, I've fit the largest posible front sprocket and a smaller rear one from a different model, wich I had to modify.
Plus I'd prefer a little more spread, 1st gear is now about as tall as 2nd gear used to be...
-tires
It is currently running tube type bias ply tires, not exactly known for their low rolling resistance.
When they are worn out I will likely mount radials.
-aerodynamics
I'd realy like to optimise that aspect.
However it will likely get complicated to do so and require complicated fiberglass fabrication.

My end goal is to break the magical 1L/100 km (235 mpg) sound barrier as well as the 100 km/h (62 mph) barrier.
While this may sound overly ambitious, keep in mind the manual mentions a fuel economy of 1,8L/100 km at about 40 km/h and my topspeed allready exceeds 80 km/h.

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Old 04-18-2021, 08:20 PM   #2 (permalink)
It's all about Diesel
 
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I'm aware of some MZ-Simson motorcycles being exported to Latin America, including some which were assembled in Brazil.
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Old 03-08-2022, 03:55 PM   #3 (permalink)
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It's been a while since I last posted here, but I got my Simson out of hibernation and she's running great again after I cleaned her carb.
I just love the simplicity of it, took me only about 10 minutes, a 10 mm spanner, a flathead screwdriver, a bit of brake cleaner and some air from my compressor.
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Old 03-08-2022, 08:45 PM   #4 (permalink)
It's all about Diesel
 
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Brake cleaner? I remember cleaning carburettors with kerosene instead.
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Old 03-09-2022, 02:08 AM   #5 (permalink)
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Quote:
Originally Posted by cRiPpLe_rOoStEr View Post
Brake cleaner? I remember cleaning carburettors with kerosene instead.
I used what I had on hand, and it worked well enough for this rather simple carburettor.
In the end, most of the actual cleaning was archived with compressed air carrying all the stuff dissolved by the brake cleaner away.
Especialy great if a jet is clogged as unlike cleaning it with wire, you don't risk damaging the jet.
It's also very fast, all of this took me about 10 minutes maximum.

And if she starts right up at the 3rd kick after months on hibernation with old gasoiline and in cold temperatures, I can't have done too bad of a job...
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Old 03-10-2022, 01:06 AM   #6 (permalink)
It's all about Diesel
 
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Quote:
Originally Posted by Autobahnschleicher View Post
I used what I had on hand, and it worked well enough for this rather simple carburettor.
Of course. BTW at least until 2011 it used to be much easier to find kerosene at fuel stations in my country, at least the ones either at a roadside or near the access to a major road, much cheaper than the bottled kerosene available at some supermarkets...


Quote:
And if she starts right up at the 3rd kick after months on hibernation with old gasoiline and in cold temperatures, I can't have done too bad of a job...
If I could kick-start it that fast with my awful knees, I'm sure I could say you did a great job
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Old 03-19-2022, 02:48 PM   #7 (permalink)
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Aerospyder - '00 Toyota MR2 Spyder
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90 day: 76.59 mpg (US)
Thanks: 25
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Ok, had an "oh sh*t" moment recently:
There is a fuel filter between carburettor and fuel tank.
For purpose of cleaning, it can be opened.
During a recent ride to a train station, it split open and spilled fuel all over the engine.
Not only is this rather bad for fuel economy, but I could also have ended up in a fireball.
I'm glad it just ended up cleaning my engine...
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Old 03-19-2022, 04:01 PM   #8 (permalink)
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Probably cooled it off a bit as well.
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Old 03-19-2022, 04:05 PM   #9 (permalink)
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Aerospyder - '00 Toyota MR2 Spyder
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Thanks: 25
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Quote:
Originally Posted by freebeard View Post
Probably cooled it off a bit as well.
Well, before I noticed it also ran pretty badly at anything but full power because of all the gasoline fumes getting sucked into the intake.
Also I ended up smelling like fuel for the rest of the day as it spilled onto my pants as well.
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Old 03-21-2022, 07:24 PM   #10 (permalink)
It's all about Diesel
 
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Quote:
Originally Posted by Autobahnschleicher View Post
There is a fuel filter between carburettor and fuel tank.
For purpose of cleaning, it can be opened.
Not an unusual feature on motorcycles anyway, yet I rarely see people opening them for cleaning.

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