1971 yamaha 125 2 stroke at1-c problems

smstavinoha88

i was told to come to this site because yall are supposed to be really helpful with diagnosing bike problems so i really hope yall can help.. I recently restored an old enduro bike and it ran great.. after riding it for close to a month it started to lose power. when the bike was running good it would top out around 60 with the rpm's reaching 6500. everytime i rode it the top speed would get lower and the engine would start to bog down at lower rpms until finally it just stopped running. i put a carburetor kit in it and new rings and pistons during the rebuild. the kit had a few pieces that didnt fit so they werent installed. i believe they were the needles that weren't the right size so i left the old ones in because they seemed fine. do you think my problem is fuel related or possibly electrical? any help would be greatly appreciated, i'm ready to give up!
thanks, steven

Rizingson · Posted: Thu Dec 30, 2010 12:28 pm Post subject:

Welcome to the VJMOG forum. There are a few things that could be causing your power loss. Does the bike have normal power prior to being fully warmed up? If so I'd suspect you could have a partial seizure condition due to new piston/rings clearances to tight. Try a richer mixture or increase injection oil. You could also have a fuel delivery problem/lean condition at WOT causing excess heat. Recheck float level also (maybe too low). Clean air filters and exhaust baffle system is also necessary for normal performance. Check timing per the manual and for high speed power ensure mechanical advance is operating properly. Good luck!

smstavinoha88 · Posted: Sat Jan 01, 2011 8:35 pm Post subject:

yeah see the problem is that it actually stopped running now.. thats whats crazy. i messed with it yesterday and with a full tank of gas after the first kick start it it will try and run for MAYBE 2 seconds at an extremely low idle and then just quit and wont even come close to starting after that first attempt...

Rizingson · Posted: Sun Jan 02, 2011 7:17 am Post subject:

This could change everything, but I believe it still is a fuel delivery problem. Just hard to tell for sure if it is starving or flooding. Were any parts from you carb kit used other than gaskets? Check the petcock flow rate by disconnecting the tube at the carb and see if it is a steady full stream. Reconnect and start it, after it dies, check the plug. I'd guess it is wet with fuel (I'm assuming it was clean and properly gapped to begin with). So go back to the float or float valve and see if height is right or valve is damaged or an incorrect new part. I'm thinking the plug is dry after setting awhile and just gets flooded as soon as you turn on the fuel and start kicking it over. Is the enrichment/chock valve seating properly?

smstavinoha88 · Posted: Sun Jan 02, 2011 8:44 pm Post subject:

well i've taken the carb apart a few times but i'm not positive on what all the parts names are that are in it.. what is the enrichment/chock valve? and what would you say the proper setting would be on the float?
thanks alot,
steven

Rizingson · Posted: Mon Jan 03, 2011 7:18 am Post subject:

Sorry I meant enrichment/choke, not chock. Here's a link to your carb parts
http://www.bikebandit.com/houseofmotorcycles/1971-yamaha-motorcycle-at1c/o/m6922#sch208204
Different manufactures use different names for that valve. Yamaha in this case calls it a starter plunger. In reality it enriches the fuel mixture at kickover and running speed, by opening a larger port/fuel path that's easier to draw fuel from with vacuum. A choke restricts the airflow through the carb will increasing vacuum to draw more fuel through the normal jets (depending on throttle position). Just remember when you use the enrichment valve on a two stroke you MUST leave the throttle in the closed position for it to be effective.
As for float level, I think it's about 15mm but I'll see if it's in my manual later, or someone here should have that info.
If you don't have it, now would be a good time to find a manual for your bike!