The continuing saga of the 66 Yamaha YDS 3

Back when I first got the bike, and it still had the off road sprocket, I would take the wife and with friends would ride about 25 miles to the national forest and spend time just crusing the logging roads. This time I was going shopping and was at freeway speed for about 10 minutes. I was passing a car in order to take my exit and was at WOT, but only about 6k rpms, when it siezed up.
I had been tinkering with the mixture screw for several weeks attempting to stop a high speed miss and had just changed the plugs from the originals. The carbs had been cleaned but otherwise had all the original parts with no modifications. I hadn't noticed the missing exhaust gasket until I went to reinstall the pipes. Thought I might had dropped it but after cleaning the garage I couldn't find it. (Not that the garage was in need of cleaning, right)
The oil pump has been on and off several times due to a leaking flange gasket but there was smoke from both exhausts and temperatures were about equal. There was a noticable difference in the sound of the pipes as the cylinder that failed had a sharper snap to it. Timing was reset and the snap was reduced but could still be heard.
It's all back together now awaiting fluids and a dry day. Will let you know how it goes.
Thanks to all for the information.

 

Was just cleaning my tank for repaint and found the vent tube blocked. What do you think of the following:

Bike is a '66 Yamaha YDS3 that had been in the garage for about 40 years with an occassional ride around the block every 10 years or so. Decided to bring it out when the price of fuel went sky high. Cleaned it up, including the carbs and it fired right up. Have been trying to tune since and have been bothered by a high speed "burbble" at WOT after a minute or so on secondary roads. Finally thought it was running good.

Went for a ride to the store. About 5 miles at highway speed (60+ mph) and was at WOT but only about 6k rpm while passing a slower car to make my exit. Suddenly siezed up. Burned a hole in one piston and melted aluminum into the rings.

With the blocked tank vent is it possible my "burbble" was partial fuel starvation and under freeway conditions for several minutes that the float bowls went dry throwing it into a sudden extremely lean condition resulting in the meltdown?

 

yes. The side that "holed" first was the side with slightly more advanced ignition timing or slightly leaner carb. It would have had to have been in a PERFECT state of tune to blow up both sides at once.

Your scenario is very plausible, especially if the plugged vent was such that it allowed SOME gas to keep flowing so that the bike didn't feel like it was running out.

 

Well I finally put some fluids into the bike and tried to fire it up. Took a couple of kicks (too old for an electric starter) but it started. Seemed to run ok but developed sudden noise. Think it's piston slap in the rebuilt cylinder. Checked the compression and it has 155 in the old jug and 130 in the rebuilt. Hope it's just that the rings haven't seated yet. Took it around the block and it seems to run well. When I got back I discovered a major oil leak from the oil pump cover. Opened it up and dried it out. Appears leak is behind clutch cover where the oiler lines pass through the case. I seem to remember a gasket between the cover and the crankcase. I think this gasket is damaged and I am getting crankcase oil through the gasket and into the oil pump cover. Will have to tear that part back down and replace the gasket. Otherwise it runs and just needs carb tweeking. I'm not closing the thread until I have a few more miles on it. The last time I closed the thread it siezed the next day.
Thanks to all for the advise.

 

Yeah geez whatever you do don't close the thread. Hang in there Bob, a whole bunch of us are pulling for you and the old Yammie. Keep us posted.

 

Pulled the side cover and found the gasket damaged where I thought. Cut some gasket paper and replaced the area. I took it out again this evening. (50 degrees, almost summer) Rode a little farther and think I have the leak stopped. Ran well to start and then seemed to lug down, hard to keep running at the stop sign. Noticed the right side carb overflowing so beat feet for home and put it away. Time to pull the carbs again.

Quick question. Will a plug chop work on a two stroke?

 

Quick answer. YES.

 

whats a plug chop?
and..... two strokes are a female dog to work on, ill tell ya

 

810 posts and you ask what a plug chop is? :lol:

 

Schooter, I never thought I'd see the day but since I got my Maxim I now prefer plug chops over pork chops. Still kind of partial to lamb chops though.................. :lol: