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Sportster Motorcycle Air intake, Carburetor, EFI, Fuel, and Exhaust Problems, advice and/or how tos for Sporster and Buell motorcycle carburators, Electronic Fuel Injection (EFI), Air Intake, Fuel and Exhaust.

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  #1  
Old 27th December 2015
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Default Crankcase vent question.

I plan on modifying the crankcase venting on my '13 Iron while I'm off next week. I am keeping the stock ham can and just want to get those nasty oily watery gasses out of my intake. The bottom rear corner of my air filter has accumulated a fair amount of oily residue over the last 9K miles.

I am concerned about the small pinhole of the stock vent bolts. If I vent to the atmosphere, I won't have the benefit of engine vacuum pulling the gasses out and don't want added crankcase pressure causing leaks and robbing power.

So my question is, should I drill out the holes in the breather bolts? And, if so, how much?

Stock bolts are on the right of this picture that I stole from the internet.
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Old 27th December 2015
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Vent from the cam chest is best.
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I have all the parts and pieces laying around from other projects, to put together a system from the air box. I don't have any money to buy anything else. I could wait a week until, but I wanted something to do while I had a week off.
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Well drill them out to 3/16ths and be happy. It will work just fine as long as the poppet valves are healthy.
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Old 28th December 2015
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The problem in my experience is lack of adequate crankcase venting and what you want to do is replace the breather bolts with solid bolts as this cuts off the oil from getting into the air cleaner. This has been a problem for way more than 20 years. As suggested above you want to vent from the cam gear case. The more you run at higher rpm the more oil you blow into the aircleaner. The oil in the rocker box will eventually run back down when it gets deep enough. Engine must be able to huff in and out as both pistons rise and fall together. The 45 degree cylinder V is deceptive in this way as there is only a single crank pin. So as the pistons rise the engine must inhale 54 cubic inches of air and as they fall it must exhale 54 inches of hot air + any blowby- that is an 883 - the 1200 is 74 inches both ways at 5000 rpm so that is a lot of huffing and puffing. These need a LOT of breathing capacity. A lot!
If pressure builds too high you can blow out gaskets and push oil out the seams.
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Old 28th December 2015
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First of all, let's clear up some misconceptions. Stage 1, stock displacement engines do not need cam case vents. The biggest problem occurs when the stock umbrella valves blow out and quite working as one way valves. The purpose of one way valves is to allow the air in the crank case to be pushed out as the pistons go down, then the valve closes to prevent air from being sucked back in as the pistons go back up. If no air can be sucked back in, the crankcase cannot pressurize. My umbrella valves blew out at about 15,000 miles and Harley considers them a normal replacement item.

There are several brands and styles of external one way valves and different kits available. I bought after market banjo bolts with holes way larger than stock and connected them to a Krank one way valve on my 2006 1200 Roadster. After 35,000 miles of aggressive street riding, I have no problems with crank case ventilation, leaking seals, excessive blow by or what ever.

One advantage of venting to atmosphere is cleaner combustion, and possible need for rejetting or retuning EFI.

Drilling out your banjo bolt won't do anything because, as far as I know, they can only connect to the air cleaner and vent crank case vapors back through the carb. That means if you want to get the oil vapor out of the air cleaner, you will have to purchase new banjo bolts, tubing, Krank vent, and re-tune carb/EFI as needed.

Another consideration, if you leave the head bolts open, without valves, too much air will be sucked into crank case causing the oil to thermally breakdown sooner. That happened to me when a weld on a cross over pipe cracked and caused oil to drop to 30 w in 2,000 miles.

If you go with big displacements, higher rpms, worn out piston rings, you have another set of problems.
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Last edited by XLXR; 28th December 2015 at 04:50..
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Old 28th December 2015
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Think it through. What is pumping volume? What is an air spring? Do aluminum cylinders swell and grow when they heat up? HD engines rely on the cyl growth partly to maintain the seal. All air cooled engines have significant blow by and how could it be other wise?
Look at the diameter of your carb which only fills one cylinder at a time 40mm- so the engine needs 40 mm to fill one cyl to maybe 100% volumetric efficiency.
The same thing is happening under the pistons as to air displacement except that the air change is double for both cylinders at once and to this you add the blow by whatever it is.

You will pick up *free hp* by reducing the pumping losses associated with XL engines through less crankcase pressure under the pistons as they move on the downstroke- you are losing torque right there.
***
Always choose the cheapest approach like a cheezy little tiny filter and a small diameter hose like XLXR recommends. How much air can that thing exchange?
***
4000 rpm and 74 cu inches =512,000 cu inches of air per minute through a 1/4 inch hose how long?
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Quote:
Originally Posted by 69xlch5213 View Post
Think it through. What is pumping volume? What is an air spring? Do aluminum cylinders swell and grow when they heat up? HD engines rely on the cyl growth partly to maintain the seal. All air cooled engines have significant blow by and how could it be other wise?
Look at the diameter of your carb which only fills one cylinder at a time 40mm- so the engine needs 40 mm to fill one cyl to maybe 100% volumetric efficiency.
The same thing is happening under the pistons as to air displacement except that the air change is double for both cylinders at once and to this you add the blow by whatever it is.

You will pick up *free hp* by reducing the pumping losses associated with XL engines through less crankcase pressure under the pistons as they move on the downstroke- you are losing torque right there.
***
Always choose the cheapest approach like a cheezy little tiny filter and a small diameter hose like XLXR recommends. How much air can that thing exchange?
***
4000 rpm and 74 cu inches =512,000 cu inches of air per minute through a 1/4 inch hose how long?
You are missing the main point that a external vent with one way valve does not allow air exchange to take place as the pistons move up and down. The problems occur when you take the one way valve out of the system.

Quote:
All air cooled engines have significant blow by and how could it be other wise?
Mine doesn't, at least no more than any air or water cooled engine with 35,000 miles on it.

Last edited by XLXR; 28th December 2015 at 05:08..
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After reading a big long thread on the cam chest vent it certainly sounds like that is the way to go. Unfortunately, I don't have the money for gaskets, fittings, drill bit and tap or a krankvent at the moment.

I may just decide to wait until I can gather up all the pieces needed to do the cam chest vent.

To be clear, I am not having any issues with excessive blow by or any other issues at the moment. My only intent is to get the gasses out of my intake tract. I simply don't want my engine ingesting that stuff. My concern is causing other issues by causing an increase in crankcase pressure.
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I have looked at the tiny little poppet valves in the rocker box and I am telling you there are vent slots in the right side of the crankcase that allows the crankcase to pressurize the cam case and this forces pressure up the pushrod tubes into the head. This is PUMPING VOLUME so as the pistons fall on the power stroke pressure rises in the crankcase the cam case and the rocker boxes. The only way air gets in and out of the engine is through those two tiny breather bolts as the rest is sealed. The oil mist condenses in the rocker boxes which are VERY crudely cast and can get into the air cleaner as it runs around in the rocker box.

This is a VERY common complaint. You are refusing to think this through. That little rubber umbrella valve MIGHT be able to seal a soda bottle if you did not shake it too much.

I have seen this on my own brand new 883s. Now sit down and THINK- where does all that air and blow by go? Most engines will have one cyl rising and one falling in pairs so the pumping is swapped back and forth between the cylinders and venting requirements are usually minimal. This is NOT the case with HD XL and BT 45 degree engines.

Pull off your heads and prove it to yourself.

I build and run these engines- try to learn from racers and high performance people - why are certain things done? Why do we use catch cans and lots of venting?
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