Can't get motobecane to run

I am having problems with my 1979 motobecane 50v. Recently, we rebuilt the engine to put on a ported crankcase. When we put it back together, it got no spark, despite the new points and condensor...we ended up putting on a spare stator plate that Bret had that is different from mine (3 coils instead of 2). It ran for a while, but then it would die and I didn't know why. It would run for a little while then just die. So, I figured the condensor on this spare stator plate was bad, especially since the points were really dirty. Long story short, I ended up fixing my original stator plate with the new points and condensor and now it gets spark.

However, I cannot get it to start. It clearly gets spark, I have checked and re-checked the timing and point gap, made sure the points are clean, made sure everything is wired correctly, cleaned the petcock and carburetor, I know that it is getting fuel flow because the float bowl is full and the jet is not clogged, and I also know that the compression is fine (even installed a new high compression head). Oh, and I checked that the gas was on and the kill switch is off. Nothing has worked.

Seriously, I have just no idea why the bike won't start. Any ideas?

Re: Can't get motobecane to run

Starter Fluid is extremely flammable .. its gaseous ether.. very likely to pop with the weakest of sparks and probably, with no compression at all ..

Squirt some in there.. If the engine runs for a second it does have carburation trouble of some sort (perhaps in addition to whatever weak ignition or compression issues there may be).

Re: Can't get motobecane to run

Are you using the stock airbox? I had a lot of trouble getting my '78 50V to start, then later getting it to develop any power over 15 mph. I suspected electrical, compression, clogged exhaust, etc., but in every case it was ultimately fixed by making a less restrictive air box.

I'm totally guessing here, but I think the stock jet in the Gurtner is really rich, because the stock air box was very free-flowing (I'm pretty sure the filter was just a metal mesh?) So if you're using foam or something more restrictive, you need to construct a large airbox with a lot of surface area. Even with my free-flowing airbox, any choke kills the engine... try starting without choke.

Re: Can't get motobecane to run

Thanks guys. Anthony, I actually have a dellorto 15/15 sha on it, with a 57 jet. It has a thick metal mesh air filter though, so I'll take that off and see how it works. I will also try joew's starter fluid idea. I don't currently have the choke cable attached, so that's definitely not the problem!

Re: Can't get motobecane to run

no choke?

A cold engine needs a really rich mixture on the order of 2:1 or 3:1 to start.. normal running is around 12:1 or leaner.

Choke is needed to get that rich starting mix ratio.

There have been times when my bike, which is in pretty good tune, just aint gonna start without choke.

before going too far with this, at least get everything put together and in working order.. There's a chance there's really nothing wrong.

Re: Can't get motobecane to run

Yeah, you're right -- I should really put the choke cable on. Everything else is attached correctly. However, my bike has never needed choke to start before, and two weeks ago was starting up fine without choke. Might as well eliminate that as a variable, though.

Re: Can't get motobecane to run

Mike Scouty McScoutington /

when you put the 3 coil stator (12 volt system) you may have messed stuff up. what year is your bike and does it have a zenor diode (the little heat sink under the speedo). the zenor diode is required with a 6 volt (2 coil) system to maintain a constant current through your bikes electrical system. if you do not have a working zenor you will blow condensors all day long because they get a big current spike. you may still get spark if that is the problem. if you dont have a zenor on your bike i just spouted off some useless knowledge for you. i know it sounds stupid, but i did some port work and piston work on one of my bikes and it ran and then wouldnt run. turns out the smallest piece of metal shaving had worked its way backwards back into my carb and clogged it. i know you said you cleaned the carb but you may want to check it again to be on the safe side. and what the hell you may as well try a new plug because sometimes they do silly things and dont work when you wan tthem to.

Re: Can't get motobecane to run

We got it turns out that it WAS a compression issue. My decompression valve did not fit my new head properly; I'll have to grind it out a bit. Yay, a running moped!

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