Motobecane 50V wont start, performs poorly

I just got a '79 50V in non working condition. The carb needed to be cleaned, as well as the gas tank, which I have done, though there might still be some rust in the gas tank. Well, after I did that, I tried starting it, and it started right up. It seemed to perform fine, but then it started running real rough, loosing power, and finally it stalled. And then it wouldnt start. This has happened several times, and as it is now, I can not get it started. It will start for like half a second and then stall out.

The muffler has a lot of carbon in it, but I cleaned the end piece with the tube. I cant find a wrench big enough to get the muffler off yet, so thats why the rest is not cleaned. The carb is spotless, the air filter is clean, the fuel filter is clean, and I get good gas flow to the carb. When I remove the spark plug, it sparks consistently IF Im touching it or if its touching the bike.

I really dont want to have to take the motor apart. Im thinking this could be a timing issue. Other than that yes the muffler should be cleaned more, though I believe I cleaned the most restrictive part already (the removeable piece). BTW, I did run the bike less than a mile with that end piece removed. Could that have caused any damage?

Well, any help greatly appreciated. I really want to get this bike running smooth and reliable. And then I want to make it go faster.

Re: Motobecane 50V wont start, performs poorly

If it performed fine when running, then your timing is fine.

you did not harm the engine by running a little with the straight pipe.

How is your compression? put your thumb over the sparkplug hole and turn over the engine. How does it feel? Can you hear the air all escaping from somewhere besides your thumb

is the decompressor valve properly seated? pedal the bike on the stand with the sparkplug in place. Is there alot more resistance (compression) when the lever is out?

get a big wrench, and remove the exhaust pipe. Put a piece of card board in front of the exhaust hole and try to start it. you should get a litle gas spitting out the exhaust hole. If no gas is coming out, then you still have a carb problem

Re: Motobecane 50V wont start, performs poorly

Sounds like a compression issue. In addition to what Dan said, you can examine the perimeter of the head gasket to see if there are any oily parts, which would indicate a leaky head gasket. Also, if you can get it to run on the kickstand, feel around for any air leaks. It's doubtful that it's a clogged exhaust port because mobys typically don't have this problem. Bad timing wouldn't explain why it would run well then die.

It could also be a bad condensor. With a bad condensor, your bike will run for a while then lose power and die. Check the compression first, then if you can't figure it out go ahead and order a new condensor (they're very cheap).

Re: Motobecane 50V wont start, performs poorly

Well, I got the exhaust off. It still doesnt want to start. When I bought it, I was told the compression was good, and the guy has or still does work for a reputable local motorsports company. I did try to test it by sticking my finger in it, but I dont think it was being pedaled enough. I didnt try that cardboard method to see if gas would spit out of the exhaust, but I will probably tomorrow. I was going to pull out the carb and try to clean it again. When I did the first time, all I did was remove it from the bike, spray the inside, and spray the holes I could see. It looked spotless, but I hear there can be deep channels in the carb which cant be seen, and it is tgerefore a good idea to soak the whole carb in carb cleaner, with all the soft parts removed. These Gurtners look like a pain to disassemble.

So at the moment I do not know exactly what a condenser is or where on the bike it is installed. But it sounds like I should at least do this anyway. Well, I'll post back when I have new info.


Chris P

Re: Motobecane 50V wont start, performs poorly

Do not soak it.

spray every hole, and make sure it squirts out somewhere else. watch out, it will get in your eyes.

remove the main jet for sure. do not remove the diffuser if it will not come out easily.

condenser is under the flywheel cover. nut is REVERSE THREAD!!!!!

Re: Motobecane 50V wont start, performs poorly

OK well I sprayed into a bunch of the holes in the carb, no compressed air though. Now the bike runs, but seems to top out at 20 to 22 according to the speedo, and the acceleration doesnt seem as good as on some of my previous successful rides. Now, there was one part of the carb that seemed like it may have been still a little clogged, and I think it is what you are calling the main jet. It is a brass piece that seems to be held in with a screw going into its side. I loosened the screw, but poking at the jet with a nail did not seem to work at getting it out, and I didnt want to damage it. I suppose I can find the diagram to the carb somewhere on this site? When running without the exhaust, no gas was spitting out the exhaust port as you said should happen, so this is probably a few starvation problem?

One other question I am curious about is what kind of performance my carb should be getting. On the carb was printed "AR2-10-737" and elsewhere there were some numbers followed by a "D". Is this one of the carbs used on slower models? On the front fork, the max speed is listed as 30mph. So Im thinking I have a 10mm carb, but is that proper for a 30mph model? Can I use any Gurtner carb successfully, with no machine work?

Thanks again

Re: Motobecane 50V wont start, performs poorly

🇮🇹💦 Of the Loin /

I have the AR2.12-737 carb, which is for a 30mph model. It's different than what you said so you may have a 20mph carb. Although I'm not positive on that.

Re: Motobecane 50V wont start, performs poorly

Yes it does seem I have a smaller carb than I should have. Well, the bike seems to start now reliably. I even got it up to 25mph. Unfortunately my decompression cable detached from the end piece and I have no idea how to fix that, but Im sure thatd be cheap anyway. Well, Im off to look for a new carb.

Re: Motobecane 50V wont start, performs poorly

Midlife Crisis Alan /

How many miles are on the bike? Engine gaskets or seals could be an issue.

I'm going through the same motions on a motobecane that I bought in non-running condition as well - I've had it with the gurnter carb, though, and I'm switching to a Dellorto. I'll let you know if it solves my problems (although mine seems to be a fuel starvation issue involving the inlet on the carb itself).

Re: Motobecane 50V wont start, performs poorly

The bike has about 3500 miles on it. Im going to get a compression tester and test to see exactly what my compression is, but I was told it was good when I bought the bike. The wierd thing is, when I first got the bike running, it seemed to be faster than it is now, at least as far as acceleration. Ive since cleaned the carb again, and cleaned the exhaaust to some degree. I suppose I may even try to clean the carb a 3rd time, but my local motorsports place has a DellOrto 14-12 for $75 so I may just buy that. But definitely let me know how it works for you. What size is the DellOrto you plan on getting?

Chris P

Re: Motobecane 50V wont start, performs poorly

DON'T GET A 14.12, GET A 15.15.

That's not a great price for a 14.12! You can get a 15.15 from your favorite online moped shop for cheaper than that! Seriously the 15.15 will give you better performance. You will need to get the bendy intake though.

Re: Motobecane 50V wont start, performs poorly

Is that the intake I saw for a Puch for like $40?

Re: Motobecane 50V wont start, performs poorly

Also I need to ask, how well would the 15.15 work with my stock exhaust?

Re: Motobecane 50V wont start, performs poorly

just fine

Re: Motobecane 50V wont start, performs poorly

after reading this post i went out and started to switch from the stock carb to a spare sha 14-12 i have on my 50v

i had to use a dremel to bore out the plastic piece that connects to the intake

ill let you know how it runs when im done

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