Filter skins = richer running?

oldguy

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#1
This weekend I resorted to using filter skins on the kids bike and he experienced 2 fouled plugs. Normally we take at least 2 cleaned and oiled filters extra along to races but couldn't this weekend. His new bike doesn't have them available yet (2002 YZ85). Also since his mom was taking him to a race 250 miles away on Saturday then bringing him straight to our local track for a race Sunday we didn't have time to clean the filter. I figured I'd put on a filter skin for Sat then pull it off Sun. Sat before the 1st moto on the way to the line he fouled out his stock plug so at the line his friends father put in a new one 1 heat range hotter. He raced both motos no problem but Sunday morning on the way to practice he felt the plug was fouled and a change was made.
When I removed the skin Sun morning before practice I was glad to see it had stopped a lot of dust and sand and the filter looked good.
Before this weeekend he had run the bike hard without fouling in 95 degree weather and on both race days the temp was mid 70's to low 80's. The filter skin was the only variable in the mix (other than temp change ).
Can the extra layer of the filter skin (very lightly oiled) richen the carb enought to cause this?
 
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#2
First off: Are these very dusty races? Just curious, you mentioned you were bringing 2 extra filters. Sounds like you are following the owners manual...this may be overkill. In reality, you should either:
1. Never change the filter, but change pistons every race
2. Never change the piston, but clean the filter constantly

ok, enough humor, seriously I think 3 filters on race day is a bit much, but that's just my opinion. Seeing some dirt on the outside of the filter is ok, it's doing it's job. As long as it doesn't make it thru to the inside (which takes a while) you are ok. The only time I have seen evidence of dirt making it through to the inside of a filter was when either it was forced through with a powerwasher, or the filter wasn't oiled properly. I have seen filters that looked like a big ball of dirt on the outside, while still clean on the inside (not that I condone neglecting maintenance).

I was wondering the same thing about richening the mixure (restricting air flow) by using the skins. On my 250 I haven't noticed much if any richening, even when the skin is pretty coated with dust. Another option is to not coat the skin with oil. You will have to clean the filter more often, but it seems to make a protective layer on the outside that the dirt can "bounce off of" rather than "stick to". An oiled filter or skin really sucks in the dust and doesn't let it go. So I think this is a good compromise to leave the skin dry. If you just oiled your filter, the skin is going to suck up some of the oil unless you let the filter dry for awhile.
 
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Rich Rohrich

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#3
Dave - If you want to send me one of the filter skins I'll pop it on the flow bench and see how much of a restriction there really is.
 

oldguy

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#4
Rich as I am interested in this factor one will be in the mail tommorow.
Motopuffs we just keep at leasst 2 extra ready on hand in case. Usually 1 filter suffices for the day but in extreme dust races we do change between motos. In this case I was concerned as he was hitting 2 races in 2 days (the first track is very sandy) with no time to service the one filter he got with the bike as I flag the AM practice and motos on Sunday (he got to the track at 11:30PM and practice starts at 7AM).
Just got word tonight that notoil will be shipping him 2 extras they made from measurements we sent them- so anyone reading this Notoil has those elusive YZ85 filters now.
 

ACS

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#5
I have used filter skins on all different bikes and not had a problem. The boys KX has done 2 hour cross country races in moderate / bad dust and had no problems without filter skins. We now use them and whilst I take it off at the 1/2 way mark we havent noticed any real difference. They are a bit of a safeguard against cloging filters. At MX races unless it is really dusty (and then maybe they shouldnt be racing) I would only really reluctantly change a filter but pulling off a skin is easy and means that you dont have to "open up" the bike in the pits. I have seen two bikes in the pressure / rush of pit work race without filters. :eek: On the new 85 could it be jetting?
 

oldguy

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#6
ACS altho the jetting isn't perfected yet with the skin on he had spooge dripping out the silencer. once the skin was off (2nd day) no spooge. 2nd day was 10 degrees warmer. Ran the bike today with no skin and again no spooge and plug is coffee brown. The skin is on the way to Rich for flow testing just to help kill the cat from curiosity. will post his findings once they are in
 
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#7
was the bike already broken in, or was that one of the first few rides when you tried the filter skin?
 

ACS

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#9
I am awaiting Rich's test results with interest. I have noticed a bit more spooge with the skins so they might be a bit od a problem. I have only ever used one at a time.
 
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#10
I think when they get sandy they do restrict airflow and cause bikes to run richer. I use them trail riding but quit using them dune riding because the sand seemed to layer up on them more than on a filter and my bike started running rich. So when testing, drag one through a sand box and check it out ;)
 
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#11
I was curious on the break in...could there have been a build up of oil from the 15:1 and richer jetting? Could the engine have been less efficient at the early stages because the ring(s) weren't sealing perfectly yet? This would cause the combustion of the oil to be less efficient. Just a thought.
 

Rich Rohrich

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#13
Still waiting for the package from Dave.
 

oldguy

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#14
Rich- still not there?
I am getting a bit fed up with the post office this makes at least 5 packages lost in the last 2 weeks:think (not counting what Okie has lost also).
I will package and mail another one today
 

Rich Rohrich

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#15
Nothing as of Thursday 8/23 Dave. :(