chronic plug fouling

ericlachance · Jul 7, 2003

I bought a 96 yz250 a while ago, it runs great when warmed up, but I allways have trouble getting it started when cold. I haven't really had time to ride it much(work too much), but I try to start it every saturday afternoon (just to piss the neighbors off ;) ) and 'clean' it out. My problem is this, Every time I want to start it, I have to change the plug due to plug fouling. So my guess is that the pilot jet is on the rich side. I have a 46 pilot jet in it(keihin carb). I'm sort of new to jetting issues so any help would be appreciated. I wanted to know how to tell what pilot to use. I know (from reading threads from Rich Rohrich) that you can tell wich main jet to use by reading the main jet ring on the plug, but he also said that you can't tell the condition of the pilot by reading the plug. So how do you tell wich pilot to go to?

Thanks for the help

Eric

keen · Jul 7, 2003

I'd say , right off the bat, your problem may be elsewhere. A cold engine requires more fuel. If your eng. fires well warm the piliot may be ok. Text book check says if the air screw is more than 3 turns out step down on the piliot. Are you starting w/ the choke on? if so try w/ it off. MAke sure the petcock isn't on and the floats arn't running over causing the cold start problems when it sits.

kawakx125 · Jul 7, 2003

you can tell if a pilot is too lean by riding it slowly and then opening the throttle quickly. If the bike bogs and almost dies, then the pilot is too lean. If your air screw is less than 1 turn out, go richer on the pilot. If it is more than 2.5 turns out, go to a leaner pilot.

Glitch · Jul 7, 2003

eric, I used to have the same problem as you. My bike would run great with a new plug, no problems at all. Then I'd try to start it the next day and it would wet foul the plug when trying to warm it up, have to put a new plug it, or clean off old one. Then I tried a b8es instead of the usual b9es, now my bike will run just as good as before, and it wont wet foul when i try and start it again the next day. I believe the reason for this is because the 8 is a hotter running plug, so during riding, the plug is hot enough with the 8 to burn off the carbon that would build up on it. With the 9, the plug would foul when I try and start it because when I previously rode, the plug didnt get hot enough to burn off carbon that built up on it. Even though people will say that running a hotter plug is just a way to disguise jetting issues, I have tried many different jets to no avail. Stock, my bike has a 162 main jet, right now I have a 145 in it, which is a very big difference but with any bigger jets and the bike dies on me when giving full throttle, it blubbers, then dies and fouls the plug. I got a one size bigger needle, which is leaner, and have circlip in top notch, leanest. I should probably get an even bigger needle because it runs a tad rich at ~1/2 throttle but its fine for now. And for pilot I went down to a 32 from a 35, but had to go back up to 35 or else the bike would lean bog. I would like to get a leaner slide, or cutaway part of the slide to alow more air but thats too risky for me. So as of now, my bike really cant go any leaner, but its still fouls plugs with a 9, so I run an 8.

ericlachance · Jul 9, 2003

Keen, I allways start it with the choke on, I allways close the petcock right after I stop the bike, even if it's only for a short period of time. I'll have to check how many turns out the air screw is, I'm not sure.

Kawakx125, When I whack the throttle open from about 1/8 th throttle opening, the bike has no bog, It just lifts the front wheel off the ground. Like I said before, I'll have to go check how many turns out my air screw is.

Glitch, Putting a hotter plug sounds like a good Idea, I just picked up a copy of Eric Gorr's motocross and offroad performance hand book. In it, he talks about plugs cold fouling when the temperature is below 800 fahrenheit. That might explain why it runs good once warmed up, but fouls when the engine is cold.

Also, In Eric's book, (on page 21 in the faq section for those who have it) Eric talks about the Keihin Carbs with the idle circuit on the choke knob. Thats what I have. (keihin PWM) his recommendation is that you don't set the idle with the choke knob. Do you think I should turn the ckoke knob completly in so that it does not idle? when I got it, it didn't idle, but I adjusted it so that it does. Maby thats my problem, maby it's not supposed to idle. What do you guys think?

Thanks

SS

atc3434` · Jul 9, 2003

Your not running too long with the choke on are you? I need mine for exactly as long as it takes for the bike to start, then I have to take it off pretty quick, or she'll start loading up.

ericlachance · Jul 10, 2003

No, I can't even start it most of the time without changing plugs. Not fun :( I've had it for a little over 1 and a half months, Rode it twice(no time :( ) and started it up a couple times just to hear her run :) , but I think I must have about 10 fouled plugs in my tool box... I might try going to a hotter plug. Anyways, I don't ride it very hard, and use it mostly for trails, so I don't have it pinned too often. A hotter plug might be a wise choice. What do you think?

thanks

Eric

490Dave · Jul 10, 2003

Couple of thoughts Eric, starting the bike only occasoinally may be part of the problem too, while setting still, a large amount of unburned oil can settle in the bottom of the crank, that coupled with using the choke and maybe some stale gas maybe part of the problem. Jetting may be the root of the problem though, and it wouldn't hurt to check compression too.

Dave...

handleme46 · Jul 10, 2003

Ive had many similar problems, i have a stack of wasted plugs and have spent more time running into town for new plugs than actually riding my bike..:(:(
Runs great when warm, fouls at low speeds, ive tried leaner jetting but havent gotten the results i wanted. My bike is currently in peices, but when i get it back together with the newly recoated top end i fully intend to run an 8 heat range plug. They say it just disguises jetting problems,but, if it will run good with the 8 heat range plug then screw jetting prbs...right? lol just my thoughts....

Another thing, ive been told to buy the cheap plugs and buy them often, but ive tried the more expensive br9eg plugs instead of the br9es's, and they resist fouling much, much, better.

ericlachance · Jul 13, 2003

I was just thinking about this yesterday, the last time I filled it up(yes, over 2 weeks ago) I had to use some 94 octane gas with a 'teflon' additive(thats what it said on the pump) cause thats all they had except for 87 octane (I use pump gas). Maybe this additive, coupled with the fact my gas is a little old and my way of riding(I probably don't get it hot enough) might be the cause of this fouling. You guys talked about the bike bogging in low RPM's if the pilot was not jetted right, but that does not seem to be an issue. It pulls hard and strong from 0 - 1/2 throttle opening. I'm just waiting for my next pay to get myself a compression tester, I saw a nice kit on the sears website that seems to be pretty nice.

http://www.sears.com/sr/product/summary/productsummary.jsp?BV_SessionID=@@@@1043398572.1058136839@@@@&BV_EngineID=cchfadcildkimjfcehgcemgdffmdflf.0&vertical=SEARS&bidsite=&pid=00947089000

I think i'll change my gas, and if that doesn't help, i'll try a one step hotter plug.

ericlachance · Jul 13, 2003

Oh yeah, by the way keen, I checked my petcock yesterday to see if it was running over like you said, but it's not. I'll have to take the carb apart and check the floats. But if the floats were stuck, wouldn't the bike either flood(when slowing down) or run out of gas(when going WOT) from too much or little gas entering the carb? (Like I said, I 'm new to this. I had a 1980 IT125 before this yz250, and for some unknown reason, I could of soaked the plugs in oil before starting it and it would of only taken 1 kick to start it. Not kidding)

chronic plug fouling

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