MC 5 125 Carburation ?

[Doug Bridges]

I am starting to sort out my new project bike (1976 125 MC5). It came with 3 carburaters a 36 Bing with no slide and in peices,a 36 Lectron that looks good and a VM30 Mikuni that looks like it is brand new. The guy I got it from was unsure if any of them were used on this bike, but he seemed to think that the lectron had been on it. He got the bike from his brother who he said also had a 250 MC5 and owned a KTM dealership at one time. I tend to think that the 36 carbs were not used on this 125 as they would be a bit too big. I am tempted to try the 30 mikuni. It is probably a bit to small, but I will be useing this bike for trail rideing and enduros so I don't need a real pipey bike. Perhaps there is someone with a 32 Lectron that would want to trade for my 36 lectron, They sound like they are good carbs , but I have never used one. Has any one used a 30 mikuni on a Ktm 125 ? If so I could use some help with a starting point for jetting. Does anyone want to trade a 32 Lectron for a 36? Any help is apreciated.
Thanks

Doug Bridges
76 MC5 125

[Larry Perkins]

Doug,

Many of the MC5 125 bikes indeed ran a 36mm Bing.  Some of the first ones had 30mm & 32mm Bings but had lean running and there were motor troubles because of it.  The Lectron carbs are great and very glitch free once it has the right metering rod.  I think Mike Lenz had a post recently on Bing jetting for his MC5 125.  Might search that or perhaps Mike will see this and chime in.  

On another note, the MC5 125 is not the best of trail bikes.  It and the 175 are a bit pipey no matter how you jet them.  sorry for that bit of info.  They just have a lot of port for a small bore bike.

Larry P

[Doug Bridges]

Thanks for that info Larry. I must say that I am surprised that a 125 can take a 36mm carb, it must have some big ports. I sifted thru some more old postings and found a reference to a guy named Larry Brown. It said that he had ran a 36 Lectron on his 125 and it ran very strong. Since there is a 36 Lectron that apears to be in good condition included with this bike I think it would be safe to assume that there is a good posibility that it has been used and is possibley set up correctly for the bike. I think I will give it a try and see what happens. The Bing is too far gone to mess with when I have a good Lectron sitting here and it does have an extra metering rod with it. When it comes to rideing trails with it I will have to hold on and hope for the best .
Now I need to figure out what type of throttle cable to use as the only one I have is broken. Will a standerd cable used with a Bing work on a Lectron? I also need to find some tech/tuneing info for the Lectron.
Thanks again

Doug Bridges
76 MC5 125

[Larry Perkins]

Doug,

Gary Ellis, who is a member here can probably fix you up with a cable or at least point you in the right direction.

Larry P

[tofriedel]

Doug,

For what it is worth, my 77 GS 250 has an Electron and I believe it is a standard cable. AL B probably has a Terry cable that will work.

Tony

[brian kirby]

Not Penton/KTM but sort of related, my '82 Husky XC125 came with a 38mm Mikuni and it was WAY over carbed. I replaced the 38 with the smallest in that body which was a 36mm and it ran better but still had too much carb. A 34mm would have been perfect but the body is smaller I would have had to rig something up with a different intake manifold.

Brian

'73 Berkshire

[G Ellis]

Doug give me a call or email me your phone number, I think I can help you. 217 260 0237  Later Gary

[Mike Lenz]

Doug,  I am running a 36mm Bing on my 125 MC-5.  But for the woods I would run a 32mm Bing or 34 mik.  My 125 is ported for MX.  However, I am having a problem that is new to me, with the jetting I believe.  Maybe some of you reading this post can help me out with some suggestions.  Right off low idle I have a dead spot.  At first I thought it was too rich so I went down on the pilot after messing with the air screw didnt help.  But after riding it more and going all directions with the pilot and air screw I now think it may actually be to lean right there.  I have to push start it and get the engine spinning faster before it will lite.  While pushing it I have pop with no throttle, but when I give it a little throttle it dies, until I get the engine turning over fast enough that it gets past my dead spot, then it lites off.  It runs very well mid and top range. I went so far as trying a different carb, thinking something may have been lodged in the internals of the carb.  No luck, same thing!  Help!?

[brian kirby]

The 125 might not generate enough vacuum to draw fuel through the pilot circuit. It doesnt matter what jetting you put in the carb if the engine can not generate the minimum vacuum to draw fuel. I have never seen this on a motorcycle, but I've seen it a lot with poorly set up watercraft. One SeaDoo even had this condition intentionally. SeaDoo wanted their new 951 engines to be fuel efficient at the typical "cruise" speed so they engineered in a window of throttle area that would cause an extremely low vacuum signal. The combination of the depth of the carb, the distance of the throttle valve from the intake and the length of the intake tract caused it and no amount of jetting would fix it because there was not enough vacuum from the engine for jetting changes to matter. I dont know if that is your problem, but it sounds just like the symptoms that SeaDoo gave so its a possibility.

Brian

'73 Berkshire

[Mike Lenz]

Thanks Brian!  However I have ignition at low low idle area, its when I give it some gas it seems to starve, right off a very low idle??

[brian kirby]

Right, the condition I was talking about with the SeaDoo is a resonance thing like an expansion chamber only its happening on the intake side. The pulse waves could cause this at different RPMs depending on the intake tract. Its possible with a smaller carb you would change the point this resonance would occur where you would not feel it. Of course, this could have nothing at all to do with what is wrong with your bike, I was throwing out some ideas.

Brian

'73 Berkshire

[Mike Lenz]

Hey thanks I will try a smaller carb and see if it goes away.
Mike