bog at 1/2 to wide open throttle

[mjc79]

just picked up a 74 yamaha yz250A the other day for 100 bucks. the guy said he had issues with it keeping an idle. turns out it not only has an idle problem, but has no power past 1/2 throttle. he gave me the original carb with the bike and the newer carb thats on it now. what im trying to find out is which is the right sized jets to use for it? the old carb had a 70 pilot and a 370 main jet. the newer carb has a 57.? pilot in it and a 320 main jet. i tried swapping them around to see if it would come out of it but it wouldnt. if any one has any ideas it would greatly be appreciated.

thanks

 

[mideastrider]

Don't know if this will help but from a Yamaha parts catalog looks like it came std. with a #45 pilot and a #350 main.

 

[mjc79]

Don't know if this will help but from a Yamaha parts catalog looks like it came std. with a #45 pilot and a #350 main.

thanks for that info. i went ahead and got those jets and tried them out. still bogging after half to wot. i tried mixing the gas to 50:1 and it still never changed anything. right now im running 40:1 which is what was in it to begin with. im kinda stumped here lol. it has newer boysen power reeds in it, i have no clue if thats the problem or not. might change back to the metal ones that came with it.

 

[RM_guy]

A bog in that range indicates it's lean at the main or the needle. Try dropping the clip one notch on the needle first (to raise the needle) to see if it makes a difference. If it does then increase the main until it feels good then go back and play with the needle.

It is very much an iterative process but I find if you start with the main it goes a bit smother. The size of the main will affect the needle so I like to get the main settled in first.

 

[mjc79]

A bog in that range indicates it's lean at the main or the needle. Try dropping the clip one notch on the needle first (to raise the needle) to see if it makes a difference. If it does then increase the main until it feels good then go back and play with the needle.

It is very much an iterative process but I find if you start with the main it goes a bit smother. The size of the main will affect the needle so I like to get the main settled in first.

cool deal i'll try that tomorrow to see if it helps. just for kicks i messed with it on lunch today and on the first kick it popped through the exhaust. so i proceeded to try and kick it again, it popped every kick for 3 times then popped like a shotgun going off lol. there was also some flames coming out the exhaust as well. dunno what was the deal there, maybe still too much fuel?.

 

[2strokes4life]

You may want to check the top end. I bought a 1991 RM 250 from a guy who said he rebuilt the top end. It was really hard to start cold, but would start first kick when warm. It wouldn't idle for anything. It also backfired like crazy. Sounded like a gun once in a while. I adjusted the carb and rejetted. I thought the jetting was off because I put a new FMF Fatty pipe, Twin Air Filter, and Boyesen reeds. I tried everything with the carb. I checked the compression and it was low (150 PSI). I pulled the cylinder off and the piston was cracked clear across the top and around both sides of the skirt. I learned the hard way that you can mess with the carb, but with top end damage you'll never get it to right. I can't believe my bike ran at all. If you bought it used it's probably worth a look anyway. I hope this helps and I hope you don't find a wrecked piston and cylinder like I did. At least I found a solution to my problem even though it was expensive.

 

[RM_guy]

That's a good point. You should always make sure everything is up to snuff before messing with the jetting.

 

[gurt]

My MX125 was giving me similar trouble. It would be fine to about 1/2 throttle and then bog and cough. As it turned out, it was too rich on the main and the needle and flooding the bike out. I leaned out the needle and the main, pulled the pipe off (and dumped the gas out of it), and kicked it a million times with the gas off and the plug out to clear it out and now it runs great.

 

[mjc79]

just to let you all know i think i found the prob. ended up taking the motor out and took it apart. the ring had sharp edges on it like it had been filed so i know it wasnt getting good compression. and also half of the piston was black and the rest was clean. hopefully a rebuild kit and rebore and rering and new piston will cure it.