490 fouling

[yamahonky]

my 1982 yz490 has a new top end, headwork, porting, and new crank seals, I have the pilot screw 3 turns out, the clip in the top position, and the stock main jet, running a BR7ES plug premium pump gas and Torco synthetic at 50:1, I ride in the woods, and never really rev it up very high, after hour long rides and the bike running great, the next time I try to start it cold it just coughs and won't start. With a new or cleaned plug it starts right up. The plug is dry and black after a ride, I am getting tired of cleaning plugs :bang: Also when you start it up cold, it acts like its running out of gas. it didn't do that with the pilot 2 turns out. please help me, yamahonky

 

[matt-itude]

It would be good if someone that has worked on the 490s would tell us the typical air screw setting but if your not reving it up I would assume you are less than wide open throttle most of the time. The black plug still indicates rich. the three turns out on the screw make me think a size smaller on the pilot jet and retest/ride. most bikes when jetted spot on are cold blooded but when warm run crisp, clean, and dont fowl plugs, oh yeah restart easy, and cold once you figure out there routine. sorry I rattle on sounds rich to me and by your described ridding style Im guessing pilot or needle.

 

[yamahonky]

Thanks, I think I'll try to lean the air screw some more, or maybe try a BR7EG or one of those plugs that the nose sticks out farther.

 

[Mike's 2003 YZ250]

I wouldnt go any furhter out on the air screw. Id go down one size on the pilot jet and put the air screw back in a turn and a half or so. Do a search on jetting, there is plenty of jetting threads, just to give you some ideas of what other guys have done with their bikes. Mike :cool:

 

[matt-itude]

carefull anytime something sticks further into the cylinder we dont want it to touch the piston. you said the needle was in the top position. meaning all the way lean. it may be time for a new needle as in different size or mabey your stock one is worn down. the air screw will only have so much adjustment and 3 plus turns sounds a little excesive but usable for diognostic purposes. if it works better further out change your pilot jet.

 

[yamahonky]

how do you change the pilot jet and how much difference does a one step leaner one make?

 

[matt-itude]

well if you know how to chainge the main then you can figure the pilot out. even if you dont you should pull the float bowl off the carb then tip it up side down and usually right next to the main jet you will see a flat head brass screw head looking peice (the bottom of your pilot jet) it will either set about flush or a1/4 inch or so up inside (you may need to look up in next to the main jet to find it) remove and replace. I like to blow carb cleaner through it whenever I have the pilot out just to help keep the passages clean and check to make sure it isnt plugged.
so are you telling us it gets better the leaner you make it?

 

[matt-itude]

oh yeah with pilot jets one size is a pretty good step you almost never have to change more than one size from stock.

 

[yamahonky]

my bike runs rough in the low speed with the screw 3 turns out, but it never quits or won't start when its warm, just when you try to start it the next day cold. With the screw 2 1/2 turns out, it runs smooth but it dies halfway through a ride, the main and needle jet are spot on because it runs smooth and has tons of power once you open the throttle, should I set the screw where it runs smooth and use a hotter plug? BR7ES the insulater is sooty and black. Should I mix more oil in the gas to lean the mixture? It takes about 6 kicks when cold, about 4 when hot.

 

[matt-itude]

when you say it dies half way through a ride do you mean you come to stop and it dies? do you mean your riding along and it dies? pleases elaborate on when it dies. It still sounds rich to me. and if you are thinking the main jet is correct you wouldnt want to mix more oil and lean it out. i assume you arent mixing you premix incorrectly (too much oil) I mean by this that the whole premix thing isnt new to you.

 

[JTurn36178]

I had a thought. Does your bike have points or a cdi?

If points...There is a possibility of (a hot engine) changing the timing, or high resistance causing fire to be intermittent.

If points.. File slightly and see if it cures it.

Good Luck,
JIm

 

[zoommx]

The 490's all had CDIs. On a little KTM my kid had, it would foul plugs leaving the plug black and dry. It had low compression, replaced the rings, and problem disappeared. Could have been the kid started riding harder, but more likely, in my mind, the top end.