Help with Derbi SM 50

[Rcannon]

I thought about using the mini forum, but this beast is not exactly a mini.

Please help me get this beast jetted!

http://www.thecycleshed.com/smotard.htm

The bike belongs to a friend. She is quite small and wanted something to play around on. Being a typical motorcyclist, stock was not enough!

The bike now has a 74cc high performance kit (Head, cylinder, piston, pipe silencer) and a POS!!!! Dell'orto carb measuring a huge 19mm.

The kit came with a 95 main jet. The jet is so far off I cannot tell if it is rich or lean. The plug is BLACK, but the bike will not rev with a load on it. On the stand it is ok, but not with my fat ass driving it.

I dropped down to the 75 main that was stock in the original 14mm carb.. I am still getting a black plug, but it will now rev to 8000. I am thinking of trying the optional 71 jet. Still, how could the larger piston require leaner jetting especially after adding a larger carb???? Apparently this kits works well at sea level with a 95 main. The bike also uses oil injection , so I have no idea what ratio it is running at.

Buy some more jets, right??? WRONG!!!! The closest main is in oregon. They will cost about 10.00 each, if available.

This is one of the coolest bikes I have ever seen. I would really like to get this running for her. Any and all advise would be appreciated.

Studboy, I fully expect you to participate in the jetting experience. I will buy the pepsi if you will pry yourself off of the pocket bike.

Rich and Eric, I hope you will consider helping me. Should you decline, I will be forced to send the engine to you. I will then call hourly and ask to pick your brain, is it done yet, can I bore it to 82cc's, or , is a turbo available.


Overall this bike is cool, except for the fact it is spanish. It would be way easier to find bultaco parts.

 

[Studboy]

I'm not sure how long you will have the bike, but I could probably stop by on Wednesday or Thursday and take a look at it. And yes, I will bring the pocket bike. :)

I know how you're feeling, sometimes things get so out of order that you don't know where to start and what is wrong. That is the story of the RMX project.

Give me a call.

 

[Rcannon]

That sounds great. I borrowed some jets from a friend who has Lem's.

These exotic bikes are great...so long as you dont need a part for them.

 

[reelrazor]

I dropped down to the 75 main that was stock in the original 14mm carb.. I am still getting a black plug, but it will now rev to 8000. I am thinking of trying the optional 71 jet. Still, how could the larger piston require leaner jetting especially after adding a larger carb????

The jets control the RATIO of fuel to be added to the air passing through the carb's venturi. You would THINK the larger displacemnt would need more fuel, but you need to remember that it is also drawing more air as well. Especially with that larger carb.

I would however pull the emulsion tube out of the carb and ensure that the air bleed passages are clear and that the emulsion tube bleed holes are clear. You may have a false rich condition from the fuel not getting atomised. I have seen some of the euro kits carbs like that have some residual cosmoline(or some kind of corrosion inhibitor) in the passages.

With consistently black plugs I can pretty much guarantee that you aren't lean.

 

[Jaybird]

You may want to take to the guy who owns the website you listed in your thread. Call them up and ask for "Shed". He is probably the most knowlegable Derbi guy I know.
If he isn't covered up with work and will come to the phone, you may ask him about the posibility of disabling the injection system or any other tricks.
The Cycle Shed
812) 346-8620

 

[Rcannon]

Thanks a millon! I love the clogged carb idea. I never imagines it coming to me clogged.

I will call our contact here if that does not do anything. DRN continues to impress me. I had no clue anyone woudl know what a Derbi was.

One thing still puzzles me. A friend has a LEM 50 for his son to ride. It uses the same 19mm carb. The bike is a rocketship. I notice it makes a lot more noise than the Derbi.

I wonder how restrictive my high performance pipe and silencer are? The only identification on the hi-po kit are stickers that say "MK".

Thinking this over more, this is a job for Studboy. He loves the projects. I remember his project rmx having a plugged silencer.....hum....

 

[Studboy]

How loud a bike is definately doesn't mean that's how fast it is.
Restriction doesn't always equal less noise.

You should definately clean out the carb good. I've had my fair share of carb passages getting clogged by air filter oil and giving a false rich condition. IE Won't rev fast or clean, loss of power, fouling plugs when you normally wouldn't.

 

[Rich Rohrich]

Some of the Superbikers2 guys ride these little rockets. Check the forums over at DieselJo , I'm sure you'll find someone who has the same kit on their bike.

http://www.dieseljo.com/forum/viewforum.php?f=2&sid=1c59b2401260354888d3cf4fdde80c39

 

[Rcannon]

I will try the carb cleaning. That is making more and more sence the more I think about it. Rich, I appreciate the forum help.

 

[Rcannon]

The bike is working now. I cleaned the carb out. As I was doing that I remember a comment the owner made while dropping it off....."I cleaned the airfilter..."

I pulled the filter out. There was at least a full pint of airfilter oil in the filter. Maybe more. I cleaned it out and the bige began workign again.

Thanks for all the help!