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YDS3 Big Bear Scrambler only firing on one?

1267 Views 4 Replies 2 Participants Last post by  yds3guy

I just un-froze a YDS3 250 twin. Slightly rusted on cylinders, honed them and put in new rings. Cleaned out carbs. New plugs, cleaned up the points.

I go and start it and it will run strong on the left cylinder and sometimes the right cylinder kicks in. Usually the right kicks in when I initially start it. If I put my fingers ofter the right carb it will suck some more fuel in and sputter a few times and then nothing. I have taken the carb apart a few times, fuel is in the bowl, all the jets are clean.

If I take the right plug out and rest it on the head it sparks nicely. Switched plugs, same running behavior.

I'm leaning toward a bad condensor.... Will get new points and condensor and go from there.

How would I be able to tell if it was a lower seal crapping out? There are no leaks from the crankcase.

Any other things to look for?
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Check your points, and point gap settings. When mine was firing on only one, it was the points. Also, check your head bolt torque. You got a lot luckier than I did... I had to do a full .75mm bore on both cylinders to get them useable.

The carbs can be very finicky on these bikes as well. Not only do you have to sync them when on the throttle, but you also need to make sure that they're even at idle, or one will load up quick. They need to be darn near perfectly synched, or this engine will foul in a huge hurry. My dad used to keep several spare sets of plugs in his toolbag under the seat...
This bike is not going to be ridden across country so I did not opt to bore it. Lightly honing the cylinder actually came out nice and there were no deep pits or gouges.... Rings were stuck. Got one set of the old rings loose and the other piston I was not as lucky... So, got two new sets of rings. I did not spec out the clearance, getting good compression so I will leave it be.

I hear you on the sync of the carbs.... Cable slack must be just right, then set the idle with the top screws... Then air screws... I have fixed a few of these old bikes, most times the previous owner has put the throttle linkage and slides in wrong... As with this bike...

On to new points and condensers.

Thought I would follow up with the solution. Decided to switch the carburetors to see if it was a carb issue. When I switched them the right cylinder ran and the left did not... So, I knew it was a carb issue. Took the faulty carb apart for the 10th time. All jets were clean on the carb. Looked at the bowl and noticed that there was a hole that was clogged. The whole led to a chamber that a brass pipe from the carb was in. Cleaned out the little hole and put the carb back on. Bingo! Both cylinders fired.


Glad to hear you got it figured out. Where abouts are you from? If you're close to PA maybe we could get together sometime.
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