My son's YZ 85 is fouling spark plugs all the time. He is a beginner rider on a closed course MX track. I have reset the pilot air screw to 1 1/2 turns out, moved the needle to the first grove and went down one size on the main jet The bike has fouled BR9EG NGK more than once. Fuel / oil mix is 94 octane Ammoco pump gas 40:1 with Yamalube. I know that he is not running the motor hard enough to keep the plug clean. I am going to run a BR8ES next time out. Any other suggestions ? How about race fuel or octane booster?
YZ 85 fouls spark plugs
- Thread starter KE100
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NO HAND
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- Jun 21, 2000
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Octane booster is not going to help you unless the fuel at the pump doesn't meet the minimum octane requirements for you engine to run safe. Race fuel may help with plug fouling but only once jetted right for that specific fuel. In the end, if you keep the stock jetting with race fuel, you will foul plugs just as often. Your best bet is to find at what places in terms of throttle opening the bike runs rich. Remember your pilot air screw and low speed jet will affect only from closed to 1/4 throttle, needle clip 1/4 to 3/4, and main jet 3/4 to wide open. If he is not runninh the motor hard enough to keep the bike on the pipe for a long time, then it is safe to try the BR8ES.
When we first got my Son a YZ85, we had problems fouling plugs also. It only fouled plugs when we were warming the bike up for races in the cooler weather. I went down one size on the pilot jet and this seemed to take care of the problem. Since your Son is new to MX and the fact the pilot controls the fuel from idle to 1/3rd throttle you might want to do the same. As for race gas we started running C-12 race fuel along with Yamahalube R-2 and this really helped the bikes performance also. We also installed a procircuit pipe and silencer at the same time and ended up going back to the stock jetting. This bike in stock form has very little bottom end and needs to be ridden in the mid range and top end. I would think as your Son gets more experience keeping the bike on the pipe you will see most of your problems go away. Good luck!
HiG4s
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- Mar 7, 2001
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My son's YZ85 was fouling plugs too. I did exactly what you did, (one size leaner on main and dropped the needle, per our Yamaha shop mechanic's reccomendation) except I run 32:1 with Castrol TTS. Plus I switched to an 8 plug and it has not fouled a plug since.
I also tried one step down on the pilot but the bike heasitated too much off idle so I went back to stock pilot Also as he is getting use to the bike he revs it more so I have gone back to a 9 plug. It still spooges a little but doesn't seem to foul plugs anymore.
I also tried one step down on the pilot but the bike heasitated too much off idle so I went back to stock pilot Also as he is getting use to the bike he revs it more so I have gone back to a 9 plug. It still spooges a little but doesn't seem to foul plugs anymore.
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- #5
We bought the bike in September when the tempeture was in the low 70's and riding it at a larger outdoor track (longer straightaways) with the Stock carb adjustments, 32:1 Yamalube & Premium pump gas. The orgimal plug (BR10EG) lasted a couple of rides and only fouled when he dumped it over. The plug was black and wet with lots of spooge coming out of the pipe. The weather got a lot colder and started riding at a shorter indoor track. I then changed to a BR9EG and the fouling problem got worse. The colder weather should make it run leaner, but you couldn't see it on the plug. The indoor track we go to doesn't have real good ventilation so I need to warm the bike up outside then shut it off and wait until his class is up. The moto's are kind of short, about 15 miniutes and the track has only one straightaway about 250 feet long coupled with his skill level I know that the combustion chamber tempeture is not is high as it should be. That last couple of rides I ran the stock pilot air with the screw out 1 1/2 turns, stock needle in the first grove and one size smaller in the main jet and a BR9EG. I pulled the plug afer every ride, still black but not wet and a very small amout of spooge. It's fouled 3 BR9's in this setup. I changed the plug to a "8" and warmed it up at the house doing netural run ups and checked the plug......and it's starting to turn black. I guess we need get back to the track before I make any other adjustments
When I had my 99 YZ80, I hardly fouled plugs (don't ask why, it's beyond me, I should've been changing them every 2 minutes with the way I rode it).
The one thing I found helped me, confidence wise & ability to rev the bike harder, was changing the gearing. I preferred 13/50 but generally ran with 12/48 due to difficulty in obtaining the 50 tooth rear sprocket.
I'm a trail rider (slow at that), but maybe that'll help your son. No idea what the jetting was, but ran race gas/motul 800 40:1.
Michelle
The one thing I found helped me, confidence wise & ability to rev the bike harder, was changing the gearing. I preferred 13/50 but generally ran with 12/48 due to difficulty in obtaining the 50 tooth rear sprocket.
I'm a trail rider (slow at that), but maybe that'll help your son. No idea what the jetting was, but ran race gas/motul 800 40:1.
Michelle
- Thread starter
- #8
I am considering going to sythetic oil which I understand it burns cleaner, but should I run 32:1 or 40:1 ? I am also thinking about going one tooth smaller on the drive sprocket which will increase the engine RPM for the same track speed he rides at.:think
KE100
the jetting is definately your problem, if your son doesn't keep the bike "in the band" you'll need to lean the pilot.
also 40:1 isn't enough oil for a small bore. It probably is right now, but once your boy starts really screaming the bike it will need more oil. the smaller the bore, the harder it is run and the more revs it produces, the more oil is needed. Most of the pro's use more like 20:1. This also effectively leans the bike out because there is less gas per volume.
the jetting is definately your problem, if your son doesn't keep the bike "in the band" you'll need to lean the pilot.
also 40:1 isn't enough oil for a small bore. It probably is right now, but once your boy starts really screaming the bike it will need more oil. the smaller the bore, the harder it is run and the more revs it produces, the more oil is needed. Most of the pro's use more like 20:1. This also effectively leans the bike out because there is less gas per volume.
HiG4s
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I don't know, my son doesn't really keep the bike on the band and he doesn't foul plugs but they often look dark and the bike does spooge. I tried to put the next smaller pilot in and the throttle response from throttle off to hard on developed a bad hesitation and I had to go back to stock.
And about that jetting change for ratios. Do the math. From 32:1 is about 1/4 jet size leaner than 50:1.
And about that jetting change for ratios. Do the math. From 32:1 is about 1/4 jet size leaner than 50:1.
And about that jetting change for ratios. Do the math. From 32:1 is about 1/4 jet size leaner than 50:1.
how do you calculate that?
to make that statement you would have to know the volume per unit of time of fuel that the current jet size allows to pass, and also the difference of flow of the next jet size. you would then need to derive a ratio of fule to jet size increment, and the calculate the different fuel ratios for the corresponding jet sizes. that's an pretty complex calculation. I agree that the ratio is leaner, which is what he needs, but to convert it to a jet size equivalent is stretching it .
how do you calculate that?
to make that statement you would have to know the volume per unit of time of fuel that the current jet size allows to pass, and also the difference of flow of the next jet size. you would then need to derive a ratio of fule to jet size increment, and the calculate the different fuel ratios for the corresponding jet sizes. that's an pretty complex calculation. I agree that the ratio is leaner, which is what he needs, but to convert it to a jet size equivalent is stretching it .
- Thread starter
- #14
synthetic oil at 32:1, BR8EG and 13 tooth drive gear is going to be my next setup. The pilot air screw is now at 1 turn out......thinking ahead if I went to the next leaner needle which grove should I start in? If I install the next leaner pilot jet how many turns out should I start with? What are the first symptoms a laen air/fuel mixture in terms of engine performance? Surging or a flat spot? I really haven't been able determine anything about the engine from reving it up in netural
HiG4s
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Time has nothing to do with it, you are making it much harder than it is.
All you have to do is compare the % increase in actual size for one jet size to the % of increase in the amount of gas in 50:1 from 32:1.
In 51 gals of 50:1 there is 50 gals of gas. In 51 gals of 32:1 there is 49.5 gals of gas. That is a 1% increase in the amount of gas that will be flowing through any given size jet at 50:1 over 32:1.
I can't find the info right now, but I believe that the average difference from one main jet size to the next is about a 5% increase.
you were trying to convert % fuel to jet size. it absolutely does have to do with time. hole (pipe) size flows at an exponential rate. if you double the pipe size the flow vate at a given velocity increaces ie. a 2" pipe flows 4X the amount of fluid as a 1" pipe. if there is a 5% increase in the size of the oriface of a jet, that will result in much more than a 5% increase in fuel delivery. you have to have a conversion in any equation to convert from one unit ( fuel%) to another unit( jet size). what I was asking is what your conversion factor is and how do you arive at it. i just wanted to know the math behind your assertion. if you are right then i just wanted to know the correct math behind it, but as it stands i don't see a credible relation b/t the two. I know there is, but i am curious about the #'s.
HiG4s
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Sorry, I didn't mean a 5% increase in size, I meant a 5% increase in flow. That is why at smaller jet sizes the next jet size is in smaller increments than at larger jet sizes. I believe they try to make each next size jet approximetly a 5% increase in flow. Actually if you check the thread with the link to needle charts currently going on. It shows that at full throttle, (which is full open main jet) there is 9% increase in flow to the next size jet (from the base line) and 3% decrease to the next smaller jet. I would assume the base line was not an actual reading as it is a straight line. So it would probably be more like 6% each way.
here's the link
http://home.att.net/~wbarrow/jd/jd_index.html
here's the link
http://home.att.net/~wbarrow/jd/jd_index.html
good link, i saw it a few days ago.
i wasn't really debating your statement that more oil is leaner. I mentioned that earlier in the thread, i was just curious on how you arrived at that specific ratio. I agree that more oil is essentially an incremental "jet" change which is why i suggested it in the first place. also an 80 needs more lube due to the high rpm's fast riders run them at. I see now that your #' were derived from JD's info. I just thought you "guessed" or pulled them out of the air.
still love ya though.
i wasn't really debating your statement that more oil is leaner. I mentioned that earlier in the thread, i was just curious on how you arrived at that specific ratio. I agree that more oil is essentially an incremental "jet" change which is why i suggested it in the first place. also an 80 needs more lube due to the high rpm's fast riders run them at. I see now that your #' were derived from JD's info. I just thought you "guessed" or pulled them out of the air.
still love ya though.
- Thread starter
- #22
Wow, what a differnce the synthtic oil made! The motor was easier to start and required a very short amount of choke time. I could tell a differnce just warming up the motor. After one ride the plug was brown one side of the tip on the opposite of the side electrode, black under the side electrode and the end of the side electrode was black. The track was packed and we were able to run 1 moto.:mad: We're in the ball park now......I'm now undecided about the drive gear
