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Old 06-28-2010, 08:28 AM   #1 (permalink)
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Research oil blow by

I have read the stories about oil blow-by running out of the air cleaner and all over the bike since way back in the evo days when these things started with head breathing into the air cleaner. I have come to a few conclusions. It seems to me that I have not seen a stock engine with this problem. It also seems to me that I do see this problem on every free breathing modified engine with a head breather including S&S. Earlier on I had experimented with my evo and found that I could change my exhaust richen up the beast and change just the air filter element with a K&N and not have any problem. But, changing the back plate to a freer breathing unit always resulted in oil blow by. I am not sure what this means exactly but I am hoping others more in the know will chime in and or others who have done similar experiments. Is it a matter of the freer breather just not creating enough of a vacuum to pull the oil mist into the intake to be burned? Has anybody with a modified Twin Cam engine with this problem tried changing back to the stock air box but keeping the higher flow air filter itself? If so did that cure the problem and also just how much power did they actually loose? My stock airbox on my new Twin Cam 96 looks like it is capable of flowing a good amount of air. I'm going to just change the filter element only and leave the stock air box. I am also going to install my new Vance and Hines Pro pipe and have my bike tuned using a TTS Master Tune. I'll see what that does. Also I just changed my oil for the first time and put in damn near the entire 4 quarts called for. The bike is currently stock and it does not blow out one drop. Lets do some research or talk here about research already done.
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Old 06-28-2010, 08:33 AM   #2 (permalink)
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I think you are correct in your research. I have the SE air intake on my Ultra and have some blow by. The service manager took me out to his bike and the service writers bike, and they both have the same situation with the SE intake. I run my oil about 1/2 quart below full, and the situation is minimized, but not eliminated.
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Old 06-28-2010, 09:55 AM   #3 (permalink)
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It is not so much that these free breathers cause blowby, but they do allow you to see it. A slightly modified motor like yours (and mine) wont generate any more. But the stock aircleaner backplate collects it up and funnels it into the intake to be burned. The flat plate like on our aircleaners lets it blow out and run down the front of your right saddlebag. Such is the price of horsepower.
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Old 06-28-2010, 11:05 AM   #4 (permalink)
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I wonder what would happen if you put a curved fitting in the airbox that would point the breather fitting right into the intake so it would have a better chance of sucking the oil into the engine to be burned. I wonder if it would interfere with the airflow much, and would the oil that normally drips out the filter or blows back on the bike cause the carb slide to get gummed up, don't know if it would have any effect on a FI bike.
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Old 06-28-2010, 11:13 AM   #5 (permalink)
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Fartnokker is reading this now saying WTF is this?
I've owned both head-breathers and bottom-breathers (my current FXR is a bottom-breather), and had instances of blow-by with both of 'em.

I've tried routing it back into the intake, which isn't too bad until the bike sits a bit & leakdown fills the crankcase up & causes a gusher. I've tried routing to a small K&N filter, back when I was running a Forcewinder air cleaner on an Evo Sportster, which still dripped a bit on the motor.

Finally, now, I have my bottom-breathing Evo 1340 running with a hose that vents out beneath the frame and just ahead of & to the right of the back tire, along with my carb's float bowl overflow hose. I know it isn't the most environmentally sound solution, but I got tired of cleaning oil off my bike. Now, when it has been sitting for a week or more, I'll roll it out of the garage & put a catch container under the vent hose before cranking it up, to catch the inevitable gusher.

I've heard of people who swear by the Krank Vent to keep blow-by from happening, but have no experience with it myself.

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Old 06-28-2010, 04:23 PM   #6 (permalink)
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my dealer is trying everything in his power to discourage me from just replacing the pipes and air filter "element" only and not the backing plate to. If I'm satisfied with the power and performance of the 96 now and I change just these items and then use a TTS Master Tune to richen her up, I know I'll be happy. I don't care for the line of Bull that she will run rough and not able to be tuned without changing the airbox back plate as well. What a crock.
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Old 06-28-2010, 04:39 PM   #7 (permalink)
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Fartnokker, next time the bike sits and you have a leak-down situation, try removing the oil dipstick before you fire it up. I have a 1990 Evo and this was suggested to me a while back. It made a world of difference.
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Old 06-28-2010, 04:40 PM   #8 (permalink)
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Back in the 90s, the first thing we always did to an Evo was pull the restrictor plate, plug the crankcase breather fitting on the air cleaner housing, re-route the hose toward the bottom of the bike and install a crankcase breather filter to the end of the line. The Evo was always starved for air from the factory.
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Old 06-28-2010, 05:23 PM   #9 (permalink)
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Well, the old sportster ('91) always left a trail
'till the air cleaner housing bit the dust and had to get
replaced with a little round one. Took the aftermarket
breather and dumped it on the chain where it finally does
some good... Chain seems to last longer now...
(maybe that'd keep your belt from squeekin'...)
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Old 06-28-2010, 05:46 PM   #10 (permalink)
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a trick... next time you break into the heads, under the breather baffles there are small holes that gum up causing this problem- drill those holes to 1/8 and it will go away.
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Old 09-25-2012, 12:48 PM   #11 (permalink)
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In approximately 250 miles I lost a quart down the side of my engine. Even if the breather did redirect the oil into the intake, I'd still be a quart low. Will a larger oil pump be more efficient and redirect the oil back into the crankcase? As I understand, the scavenger lug is attache to the oil pump.
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Old 09-25-2012, 01:42 PM   #12 (permalink)
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I've had four Twin Cams to date and none have had any blow-by issues or used any oil. Even the over-bored high compression hot rod I'm riding now. Maybe I'm just lucky but I wouldn't have any doubts about installing another SE or Big Sucker to a Twin Cam.
My Evo's were another story though, and just like everyone else I always routed the breather to below the frame (so they could mark their parking place).
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Old 09-25-2012, 06:42 PM   #13 (permalink)
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Nitram_b4 is reading this now saying WTF is this?
I have a bike punched out to 97" big sucker intake, cams and free flowing pipes, carbed and the new SE cam plate and pump.
I never get a drop of oil out of the breather, it is slightly oily when you open the air cleaner up, but never a drip.
Back when it was stock, it puked oil all the time....go figure
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