March 31, 200915 yr Moderator Assuming you did the test correctly with wide open throttle I would expect a compression figure or around 100psi for a two stroke in tip top condition and 60 psi as a minimum figure. try a teaspoon of oil into the plug hole and repeat the test, this will help to seal the rings and if you get a much better reading its time to take the barrel and piston off to do some checking, the most likely culprit is worn rings or possibly worn bore or maybe even a scored bore due to broken rings or partial seizure. Could even be crankcase seals are leaking.
March 31, 200915 yr All of my two stroke bikes have between 140 - 150 psi of compression on three kicks.
April 1, 200915 yr Author Assuming you did the test correctly with wide open throttle I would expect a compression figure or around 100psi for a two stroke in tip top condition and 60 psi as a minimum figure. try a teaspoon of oil into the plug hole and repeat the test, this will help to seal the rings and if you get a much better reading its time to take the barrel and piston off to do some checking, the most likely culprit is worn rings or possibly worn bore or maybe even a scored bore due to broken rings or partial seizure. Could even be crankcase seals are leaking. I just redid the compression test wih full throttle and it only improved from 40psi to 50psi; than, I poured in a capfull of oild and it went up to 100psi. So I guess its its time for the good ol' rebuild, seeing as how I'm poor this will probably take ten years. I'll try to keep updating and see if anything new happens, also, does anyone know where I can get parts for this bike? I went to my local yamaha dealership and they said they don't carry parts ten years or older so any help on part numbers and where to find them would be great! thanks again for all the help
April 1, 200915 yr Moderator For parts try advertisers of vintage yamaha parts in American classic bike magazines ebay bikebandit.com TS may advise on more You just might be lucky and only need a new set of rings and a top end gasket set.
April 1, 200915 yr Moderator I just redid the compression test wih full throttle and it only improved from 40psi to 50psi; than, I poured in a capfull of oild and it went up to 100psi. So I guess its its time for the good ol' rebuild, seeing as how I'm poor this will probably take ten years. I'll try to keep updating and see if anything new happens, also, does anyone know where I can get parts for this bike? I went to my local yamaha dealership and they said they don't carry parts ten years or older so any help on part numbers and where to find them would be great! thanks again for all the help I can't help thinking that if it ran ok started ok then just 'quit', its not the rings. It has a good spark, and there is fuel in the tank and the carb but the plug is dry, sounds to me like the charge is not getting out the cases. Split/ stuck reed maybe?, or a crank seal.
April 1, 200915 yr Moderator I can't help thinking that if it ran ok started ok then just 'quit', its not the rings. It has a good spark, and there is fuel in the tank and the carb but the plug is dry, sounds to me like the charge is not getting out the cases. Split/ stuck reed maybe?, or a crank seal. Good point Cynic but it doesnt mean the ring are good, the oil test suggest they arent so they may be part of the problem.
April 1, 200915 yr Moderator Good point Cynic but it doesnt mean the ring are good, the oil test suggest they arent so they may be part of the problem. True, it may even be like my motor ended up, nothing actually broke just a lot of key components worn to their respective limits.
April 1, 200915 yr Moderator I poured in a capfull of oild and it went up to 100psi. Might be an idea to attempt a start when you have this better compression due to the oil.
April 1, 200915 yr Author Might be an idea to attempt a start when you have this better compression due to the oil. I did try to start it with the compression boosted and still it didn't start. I'll go take a look at the reeds right now and see what condition they are in. Also, I'm gonna squirt a little bit of started fluid in the cylinder head and see once and for all if it is really getting fuel to the motor; however, I do suspect that it is. Another thing I did come across some parts for the bike pretty cheap at nwvintagecycleparts.com, has anyone heard of them? If so are they good quality parts?
April 1, 200915 yr Author Ok, just inspected the reeds and they are in almost perfect condition; they open easily none were stuck and fuel flowed through them nicely. Also the starter fluid trick was to no avail ...now I'll replace the piston rings and see where that leads me. I did turn the motor a little with the reeds off to look at the piston and it seemed in pretty good condition; it didnt have any scratches or anything showing any major malfunction but i did notice that the oil i poured in earlier was now all over the piston. I guess it leaked through the seals, maybe thats what the fuel is doing and thats why the spark plug isn't getting wet?
April 1, 200915 yr Moderator Ok, just inspected the reeds and they are in almost perfect condition; they open easily none were stuck and fuel flowed through them nicely. Also the starter fluid trick was to no avail ...now I'll replace the piston rings and see where that leads me. I did turn the motor a little with the reeds off to look at the piston and it seemed in pretty good condition; it didnt have any scratches or anything showing any major malfunction but i did notice that the oil i poured in earlier was now all over the piston. I guess it leaked through the seals, maybe thats what the fuel is doing and thats why the spark plug isn't getting wet? ok you do need new rings, we already proved that, you will find the overbore size on top of the piston toward the front I think, you may have to scrape a little carbon off. However I am surprised thet it didnt even go with the starter fluid and now, like 'Cynic' said it may be wise to give the magneto rotor and stator a dose of looking at just on the chance that something has come adrift or slipped. You may be getting a spark but is it at the right time? Also I am unsure that you have not seen a wet plug with your failed starting attempts yet, that just doent seem right in my experience.
April 1, 200915 yr Author Ok, rings have been purchased and hopefully they come in tomorrow. Now that the magneto has been brought into the picture it does sort of make sense it could be it. None of my lights have been working except the the tail light; yet, the connections were gooding and giving resistance, I guess this could have been an early warning that maybe that magneto coil or stator were going out? I figured it was the wiring causing it when the bike ran... the spark plug has me baffled also; maybe it is getting wet and I am to blind to tell. well once the rings come in and i replace them I'll get to disecting the magneto if it doesn't work.
April 1, 200915 yr Moderator well once the rings come in and i replace them I'll get to disecting the magneto if it doesn't work. In the first instance just take hold of the rotor, put the bike in gear and try to turn it to see if its loose in any way, normally you will need to buy a puller to get the rotor off but i wouldnt rush into that yet if the rotor feels nice and tight on the shaft.
April 1, 200915 yr Ok, just inspected the reeds and they are in almost perfect condition; they open easily none were stuck and fuel flowed through them nicely. Also the starter fluid trick was to no avail ...now I'll replace the piston rings and see where that leads me. I did turn the motor a little with the reeds off to look at the piston and it seemed in pretty good condition; it didnt have any scratches or anything showing any major malfunction but i did notice that the oil i poured in earlier was now all over the piston. I guess it leaked through the seals, maybe thats what the fuel is doing and thats why the spark plug isn't getting wet? Pull the head and tell me if you have a hole in the piston. Send pictures.
April 2, 200915 yr Moderator ok, pull out the old rings and snap one of them, use this to clean out the ring grooves, you will find an expander ring in the second groove, if your new ring set doesnt have one then re-use that. If you can be bothered you can check to see just how worn the old ones are by fitting them in the base of the barrel and measuring the end gap. see Here
April 6, 200915 yr Author i took the cylinder and piston off, cleaned it up and noticed on closer inspections the piston did have a bunch of little holes and the walls were scratched a little. I think it can just be rehoned; does anyone know if a 78 std piston would fit in the 79 cylinder?
May 3, 200915 yr Author UPDATE: cylinder has been bored .50 new piston and rings bought and installed new gaskets all around. The bike starts up right away now, sounds like a jaguar from hell too! but now when I roll the throttle it dies out....it idles perfect and everything just when its given gas it stall. could it be because its not giving enough gas to the motor since its been bored? I'm thinking it has something to do with the carb
_33963e.png)
_83bc35.png)
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.