yz125 yamaha breaks up when it hits power band

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Johnathan · #1 Unread post by **Johnathan** » Mon Nov 27, 2006 3:00 pm

was working fine and ran its best at high speeds then one day at high speed it started breaking up please help. I changed the plugs had a new top end done on it but it still does not run at top speed it just breaks up

BuzZz · #2 Unread post by **BuzZz** » Mon Nov 27, 2006 8:55 pm

Assuming the top-end was done correctly, I would be looking at the carb. The main jet could be partialy blocked/gummed. Or the airboot could be cracked or not sealed properly. I'd clean the carb first.

You didn't mention a year of the bike :naughtynaughty:

, but if it has a powervalve, that might be malfunctioning.... but I would still be opening the carb first.

Is it still easy to start or has it become obstinant?

Johnathan · #3 Unread post by **Johnathan** » Tue Nov 28, 2006 9:31 am

Sorry its a 1995 it has only one spark plug it starts up no problem but again when it use to go into power band the engine breaks up no power if you keep trying to get on it the plug fouls out. I cant tell you how many plugs I have put into it. I tryed cleaning the carb ...one repair shop told me that I need to rejet it.... the other repair shop that has it now did the top end and stated he dont know what the problem is being he has taken it apart a few times now and its still the same...we have spent over $800. dollars so far and the guy at the repair shop really does not want to give it back with out getting it fixed....nice man...wish we could help him/us the weather here in NY has been just great for riding but the shop has had the bike for 3 weeks now...HELP...thanks for all your help

WVUChrome · Tue Nov 28, 2006 11:31 am

Just a thought, it may be running slightly rich and is getting too much fuel and flooding out when you crack the throttle open. That would probably explain the fouled out plugs. Also, you can probably clean the plugs and reuse them until you can track down the problem. No need to keep putting brand new plugs in a bike that you know is going to foul them.

Later,
WVUChrome...

BuzZz · #5 Unread post by **BuzZz** » Tue Nov 28, 2006 5:38 pm

It still sounds alot like jetting. Your mechanic should be able to tell and re-jet though.... try mentioning it to him.

If the powervalve (your 95 has one. No YZ ever came with more than one plug.) isn't opening or not opening fully, it could also be causeing this. I can't remember off the top of my head how Yamaha activated the powervalve in 95.... but I think it's mechanical, not electronic. So maybe the PV actuation system is dirty/sticky and messing with things. If it is electronic, then some troubleshooting with a meter is called for.

I would hope that whoever re-built the top-end cleaned the actual PV slide in the cylinder, and connected the actuation system correctly....... but it has happened.....

Johnathan · #6 Unread post by **Johnathan** » Wed Nov 29, 2006 1:01 pm

I stopped by the repair shop covered everything you mentioned and he said he did it all. He is still persistant in fixing the bike. He said he is going to try to put in a new carb and see if it works out...I'll keep you posted on the out come. Thank you!

BuzZz · #7 Unread post by **BuzZz** » Wed Nov 29, 2006 2:16 pm

One more thing just popped into the vaccum between my ears...

You don't have a split seem or crack in the exhaust pipe, by any chance? Especially in the fat bulge area..... also check for a large dent in the same area. Both can screw with how a 2-stroke runs.

But your guy has probably already checked for those..... can't hurt though.

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yz125 yamaha breaks up when it hits power band

yz125 yamaha breaks up when it hits power band

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Thank you for your help