free website stats program Wet Sumping Explained | Harley Davidson Forums

Wet Sumping Explained


BOT Machine
One common problem that scares people is wet sumping. This is basically when you have too much oil in the bottom of the crankcase and it either blows out the breather or the flywheels throws it up on to the piston skirts and the motor smokes badly or both.

There are a couple of things that will cause this. Most of the time it is because of a check valve in the oil pump that is not seated This is very common in Harley Sportsters built before 1977 and softails fall into this group occasionally.

Usually this happens after the bike sits for an extended period of non use. On the older Sportster it usually blows a big puddle of oil out of the breather tube attached to the bottom of the cam cover. Sometimes it will smoke, sometimes not, it just depends how much oil is in the bottom of the motor. Usually if the plugs don't foul, it will clear itself out just by running the motor awhile. If it happens repeatedly at shorter and shorter intervals it is probably time for a new oil pump or at least a rebuild. The problem happens because of the oil tank being higher on the bike than the motor which causes siphoning of the oil into the engine.
On later twin cam engines, a misaligned oil pump can also cause this problem. There have been some issues with a incorrectly installed "O" ring on the oil pump mating surface from the factory which allows engine oil to enter the cam chest cavity and the scavenging section of the oil pump cannot keep up with the task of removing it fast enough. When this happens, you will experience a sluggish engine and possibly engine oil blowing out the breathers. The sluggishness is caused by the flywheels in the motor running through the excess oil in the crank case area.This condition on a newer bike that is under warranty should be taken care of by the dealer under warranty.
Hey Mr. Data,wats up?
Would vibrating olso be a sign of wet sumping? S.E. air filter verey oily and in throtle body behind air cleaner. and on oil change book i have called for 4 qt,s dose that sound right on 07 street glide 96"? thanks Keith in Reno.
Not so much vibrating but lack of power. Your bike should take about 3 1/2 Qts on an oil change, then warm it up and check the level on the stick. If you put in 4 Qts of oil, you will be over full and your oil problem would be the result when it purges through the air filter. There could be around 1/2 quart left in the engine after draining it in the oil galleries.
Hey Mr. Data,

I have a bottom breather evo that I went to 88 inches on.
I built the motor and then it sat for about two years.
The oil pump is a TP engineering classic Oil pump.
I primed the pump as per instructions and fired the motor up.
It is sumping like crazy. I check the oil line routing and it was right.
I then performed a check valve clearing as per Tp Engineering website.
Fired it up and still is sumping like crazy.
Could have installed the breather gear off one tooth ?
Thanks for your help!
Check your oil pump to make sure yours is the same.On a stock H.D oil pump there is a tiny hex allen plug just to the left of the oil pump lines on the oil pump that you can blow air into. be very carefull that you don't blow air into very hard. You have to take off the oil lines and the face plate of the oil pump. put you thumb on the oil pump gear and shaft while you are doing this as you can screw this up and you would have to then go inside the engine case and re connect the shaft woodrift key and tiny clip.
Rember to drain all of the oil from the oil tank first and mark which oil lines go where they are to be.

Last edited: