Tillerman6

OK now I convinced myself. Overnight I realized why the engine has to come apart again. But it is not what I was thinking. I think the crank is bent or out of alignment. You can see a slight wobble on the fly wheel outer face when the left cover is off and the engine is running. It looks like maybe .005- .010" at the outer rim of wobble. There might be some excess play between the big end of the con rod and the insides of the flywheel halves too. I don't have the specs on the 250 A model for that. And that measurement might be different around the circumference of the fly wheel because if the pin is bent, there will be less clearance at one angle than at another. ( I did not measure that) So like you said, if it gets fixed now it will live on. Does anyone have any ideas about who might be able to re-build the crank? New bearings? Possibly a new rod? I think I will try and get all the bearings before I tear it apart again. If any of this stuff is "unobtainium" I won't have the bike down for too long. And I wondered why the exhaust pipe was wearing scratches on the plastic rear fender when they are mounted almost an inch apart! LOL!

I don't doubt that one bit. It's much cheaper to build them that way and not worry about vibrations or out of balance conditions. On the other hand if you are trying to concentrate on your riding accuracy and balance it does not help to have your hands go numb or having the seat shake so hard it makes your ass and feet tickle. I know there is always going to be some vibration and that is normal, but it's hard to measure and even harder to do anything about it. I guess that's why they invented twin cylinder engines in the first place, But we are not there yet with trials bikes. After this next meet I will know more and make a decision. Or someone may come up with a weight for the TY piston that is different than the 213 grams. Since I have not owned this bike for more than a few years, it could have something else wrong or different about it that the previous owner did not pass on to me.

Copemech, I don't mean to be obstinate, so please excuse my terse reply. Are you suggesting that B &J racing has some expertise in the area of balancing a 2 stroke engine? Or something like that?

While I was researching piston and crankshaft balance for a 2 stroke, I ran across this calculator. Don't know anything about it, but this guy has put some work into balancing as far as doing the math and having all the right number crunching abilities. Somebody let me know if you try it. I think it's about thirty bucks. Don't know if it's good or bad, just FYI.http://www.dragonfly75.com/moto/calculators.html

Well with the heavier piston it it the side stand would slip and the bike would roll backwards. Also, while riding it , my hands would go numb in about 30 minutes just put-putting around. No high revs allowed.My chain saw was much smoother than this thing was. Now it's about 50% less vibration- at least it's back to where it was after i put my old and much lighter piston back in.! Somebody on here must have a bonafide 438-11631-00-96 or 98 laying around that they could weigh and take pictures of? Thanks for trying anyway Lineaway.

It was making a hell of a racket like metal on metal pinging. but it does not do that now.

I know that the front forks stopped vibrating so hard that the bike was backing up on the side stand while it was parked in the driveway idleing after I went back to the much lighter old piston. Maybe the right l one is even less weight?

Lineaway, maybe you have an old stock Ty250 piston? If I knew how much yours weighs, then that will help eliminate that the piston might still be the wrong one! It should weigh 213 grams and the pin should weigh 57 grams if it is like mine????

0899F AND UNDER THAT 234M07000 . Says it fits YZ250 76-79 and IT250 78-80

For what reason?

No I have not changed the main bearings. there is no detectable looseness in any of the bearings that I can feel. Triple checked the rod and the upper and lower needle bearings. crank halves do not appear to wobble when the cylinder is off. Bike idles fine, pulls good (no wheelies) But while we're on that subject, - can you change the main bearings without splitting the cases?

Guys, So after getting my parts back from Millenium Technologies- new bore on Ebay cylinder and new piston- I noticed quite a bit of vibration. So today I tore it down again and weighed the new piston, pin, upper end bearing and compared those weights to the ones on the original parts. As I expected, the new piston was significantly heavier than the old original piston, even though the piston size was still the same. Old Piston 213 gm New piston 241gm a delta of 28 grams. Old pin 57 gm New pin 60 gm delta of 3 grams The remainder of the parts were too light to measure on the scales I borrowed, so I am forced to ignore those results. So with this much difference in weight it is no wonder that the bike is trying to shake itself to pieces! But there is no way to shave enough material off that new piston to make it match the weight of the old one! So the hard choices seem to come down to : A- try to find new rings? 434-11610-00 (standard size) B - I may have found a set of 1st oversize rings - but can these be shaved down enough to work in a standard size bore? C polish up the old rings and use them again? The scratches are only about .005" deep (I am guessing) and the piston has some scuffing, but is not too bad .Fresh bore on a different cylinder. The new piston ran in the fresh bored cylinder without a problem except for the vibration. D If you have any ideas? I am very tempted to put the old piston back in as is if I can't find any new rings for it anywhere else?

No problem Lineaway, Just trying to help.

TheoldStuff, That looks like a great mod to do, but do you remember if you had to tear the bike all apart to get the nuts welded on the inside of the frame for that footpeg mod? I have some brackets that i got with the bike that lower the pegs but don't move them back much at all. They will add about 3 pounds to the weight of the bike if I put them back on. If you do this mod, are the brake and the gearshift going to be too far to reach without taking your foot off the pegs?

Old gas? Do not use gas with Ethanol in it Premium non-ethanol gas only. Do not open the throttle at all when cold starting the bike (owner's manual) Spark must be blue-white and strong. Check compression as well. Timing - points open 21mm BTDC. on the flywheel circumference. Works for me.(suggestions from Feetupfun)

rock hopper, I saw your good comment on changing out the stock TK carb for the Kehin pe28. Can you give me some details as to how much money and rework this would entail? my TK is about on it's last legs. Thanks in advance! Tillerman6

dannyboi, I think there is a typo in that message. I never heard of a "YTZ" 250. I HAVE heard of a YZ 250, and in the years 76-79 you might find a piston that will work for you.

dannyboi, This took me about 6 weeks to finally find a new piston that would fit. In my case the bore itself probably did not change that much, but most shops will want to have the cylinder to assess the damage if there is any and see how much oversize the piston will have to be to make it fit. This could end up with a re-bore to slightly oversize or even a new sleeve, but in any case the shop you choose will have to make those decisions. But anyway I got you the Wiseco part number for the piston I am using now. 234M0700 the s/n is I-158656 if that helps. The original bore size was 2.7539" but there is a taper that they machine into the cylinder as a matter of course. I would strongly recommend that you weigh all your old parts to the gram to compare with the new ones before you start assembly again. Incidentallly, this same part number is the same one they use for the YZ250 76-79 and the IT 250 88-90 The skirt has a semicircle cut out of the carb side lower edge instead of having two holes in it like the original. Also the rings are narrower than the original. There will be detailed instructions that come with the new parts from Wiseco. Follow these to the letter. You will probably have to size the new rings into the empty cylinder when you get it back. Believe it or not, but the upper ring and the lower ring have different end gaps that they specify in the instructions. If you end up with the same part number would you do me a favor and weigh the new piston, rings, pin and circlips and let me know what you get for numbers? I may want to eventually make the old weights match the new ones just for peace of mind. My original piston was somewhat scored up from a partial seizure, but it was the original diameter. I figure that since the bike was a 74, no one could fault Yamaha for not building in lots of quality to make it last that long. I am still running the same con rod lower end bearing and con rod, flywheel and even the carb is holding up well. Also you will need a new base gasket and probably you will want to anneal the old copper head gasket to make it re-usseable. The arrow edge goes forwards. The studs for the head have long and short threaded portions. The long portion goes down when you put it back together. I had to use the full torque on all the fasteners on the cylinder to keep it from leaking (No lock tite needed, but a little anti-seize compound on the threads is a help. You may also want to lap your head sealing surface again to get rid of any scratches on the sealing surface where it touches the copper gasket. I got all my work done at Millenium Technologies mostly because that's their specialty and they do about 2000 re-bores a month on all types of engines. They are not cheap, but they have excellent customer service and you will get a written estimate before they start work. I ended up calling Wiseco to source the piston and top end bearing, wrist pin and circlips 1800 321 1364. They should have all the oversizes in stock. But before you order another piston and it turns out to be the wrong size, you might want to get your cylinder inspected and then ask them for their recommendation about what oversize you might need. It saves a lot of headaches in the long run. Good luck and let me know how it turns out! Tillerman6

Try Wiseco or Wosserman Give them your original part number for your original piston. I went thru that sourcing issue with my TY 250A and finally got one that works from Wiseco. I can get you the part number if you need it.

You are 100% right about that. I'm not used to dealing with the magnetic field and all the paths to ground. You and Feetupfun are a great help and I am putting the 12 volt source and the 12 volt incandescent light bulb setup right now. I suppose folks have thought of this stuff 50 years ago, but it's all new to me. I may end up putting a CDI on this bike if I can't get it to stop kicking my foot backwards.

Finally got the 74 250A bike back together after the new cylinder and piston, but the kickback problem is still there and probably worse than before. I noticed that the TY250B manual (Cycleserv- Australia) shows a diode between the primary coil and the high voltage coil,) page 97) but that publication does not show any part numbers for anything. If anyone has any idea of what that part number is- I would like to hear about it. The same manual says that the diode is to prevent the engine from "running backwards" Right now the engine kicks back about 19 times out of 20, so this is getting old real fast. Once the engine is warmed up a bit, it will start with less kick back events, but still does it. I do not think that the kick back is from anything hot inside the chamber because it will kick back when it is stone cold on the first kick. Today before I tried to start it again I set the timing with less advance. The points are opening now at .113" before TDC on the dial test indicator.Before that, it was set for .116", but it will kick back with either setting. The piston is brand new and the cylinder head have been cleaned of any excess carbon deposits. The plug is the B7ES specified with a .020" gap. The kick starter lever pawl will engage the starter at a point just aft of where the kick starter is straight up, but the engine only spins over one revolution for one kick before the kick starter arm hits the footpeg. Most of the time the engine will kick back before the stroke is even half way completed. I have tried indexing the kick starter arm further forward to give more spin to the crank shaft for each kick, but this is awkward because the arm wants to fold up under your foot. So I think what is happening is that the plug is firing too soon. The only way this can happen is if the magneto is generating an erroneous (and possibly opposite current pulse) thru the system. Just from the feel of the kick starter, it seems to happen when the piston is about an inch from TDC on the way up. But while the points are being adjusted, and the crank is being turned by hand, of course there are no indications of any reverse currents being generated by the magnets. I made my own current source from a couple of AA batteries and an LED diode for a load. There is a milliampmeter in series with the LED and this circuit is put in parallel across the points by hooking one lead of the ammeter to ground and the other end of the LED to the black wire coming from the high side of the primary coil. With the 3 volt supply, this circuit runs about 50 milliamps with the points closed and 46 milliamps when the points open. The change is very precise as far as the relationship of the piston travel and the points opening, so that aspect is really not in question. I did see where Feetup fun said to disconnect the HV coil from the primary coil during the time when the points are being tested - good call as this would increase the difference between the current readings due to the points being open or closed. I guess it's time to hook up my oscilloscope to the black wire and see if I can see any high voltage spikes or abnormalities that are being sent to the High voltage coil. The trick is going to be trying to figure out if there is more than one pulse per revolution reaching the HV coil, or if there are negative and positive swings in the pulses themselves. It's a 2 channel scope and maybe I can hook something to the flywheel and get a "1 per" timing signal off that. Then chan 2 will be a sample of the primary side of the HV coil. just spin the engine over with an electric drill. If chan 2 is making more pulses than chan 1 then it proves that the HV coil is getting more than 1 pulse per revolution.

Never did find the missing needle bearing. Put the new cylinder and piston in today and got it fired up. Did not hear anything unusual, so I guess it is not a problem. Wondering about break-in proceedure now. What is the preferred method?

I like the kerosine idea better than water. And I had some terrible results from WD 40 with making things rust, so that one is out as well. I will have to remove the engine from the frame in order to shake the cases well enough. I have some magnetic tape that might fit between the crank case weights and the inside of the cases. I am not trying to use the old needle bearing again. I am sorry my sad story sounded that way. I have a new needle bearing set and a new pin and piston and pin clips, new rings of course. I might get real energetic and clean out the shop floor 100% into the driveway and see if anything shows up on the floor. that would be the easiest route if it works. Just now I rotated the crank with a bunch of 2 stroke oil in the cases and I could not feel any dragging or friction. There is enough oil in the cases to coat the crank shaft weights as they dip into the oil, but they are not showing any signs of skuffing or disturbing the oil coating as the weights rotate thru the oil. So now I'm wondering what is the down side of splitting the cases? Are there any parts that would be hard to find/replace such as special gaskeIts or anything that would be risky to do? And the other question is : Assuming that all the above sloshing and dumping does NOT turn up anything - then what? I did not see anything fall into the engine, and there is still the possibility that the bike was put back together without the roller at some earlier point in time. So would you just put it back together and fire it up? I could put all the old parts back on it too. I still have the old piston, cylinder and rings etc. That way if it goes ka-pow it will only be the old piston and cylinder at risk. I would use a new upper roller cage with this scenario of course, but having a few minutes or hours on the engine without a failure would probably rule out the chance that there was still a gremlin lurking inside without exposing the new piston and bore to the risk of FOD damage.

Due to the home projects I had going (new boat shed) plus some upcoming winter weather and the extended time it took to get a cylinder re-bored and plated, new piston, etc, I have not actually started putting the engine back together. That is because of one other nagging problem that is bothering me. That is the missing needle out of the upper needle bearing cage that was part of the engine when I tore it down. I noticed that it was missing from the cage when I took the piston off the con rod. There was a rag under the piston, but with my luck it could have found it's way down into the cases under the flywheel. I have looked everywhere for it, and I even got a fiber optic light cable down under the con rod between the flywheel weights to look for it, but I can't see anything out of the ordinary. I looked on the shop floor of course and tried a magnet as well. but got nothing. So the next step is to split the cases and look for it inside. This step is beyond my expertise and tool selection so it would probably cost around 150.00 to do the inspection (I am guessing) but at least I would have the peace of mind knowing that there is no foreign object lurking in the cases waiting for the worst possible moment to come loose and ruin the new piston and bore that I have already spent 450.00 to get to this point. If anyone has any advice or expertise in this area, I would be very glad to hear it!