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Ironhead Sportster Motorcycle Talk (1957-1985) For all those that wanna talk about Ironhead Sportster Motorcycles

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  #1  
Old 14th August 2008
lowplaces lowplaces is offline
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Question 79 Sportster Crankcase Pressure.

I have a 79 sportster, it recently started leaking oil at the base of the pushrod tubes, and also where the tach cable going into the crank case.

I suspect that the problem is overpressure in the crankcase and not gasket seals so much.

It also seems to be losing power and the throttle becomes non responsive once it warms up really good, maybe after 15-20 minutes.

Any ideas of where to start looking to idintify this probelm? I would much rather figure it out myself instead of taking it to a shop. Turning the wrenches is half the fun.
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Old 14th August 2008
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First thing is to measure the cranking pressure with a compression gauge. It should measure about 160 PSI and both cylinders should be the same or close. If it measures low then you are losing compression and getting blowby which will presurize the case like you said.
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Old 14th August 2008
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I think readings in the 120-130 psi range are fairly normal for most stock Ironheads. I read this recently on the Hot Bike site and thought it was interesting:

An engine with aluminum heads can usually support a compression ratio of roughly 1.0 to 1.5 points higher than an equivalent cast-iron head engine without incurring detonation. This is because the aluminum acts as a heat sink and pulls heat from the combustion chamber. From a power standpoint, this means that all things being equal, including the compression ratio, an iron head engine will make more power than an aluminum head engine because more heat will remain in the combustion chamber, thus producing higher cylinder pressures. To make up for lost heat and power, an aluminum head engine must run a higher compression ratio than an equivalent iron head engine.
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Old 14th August 2008
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Is there any oil making its way back up into the air breather and soaking the filter?

There is a one way flapper valve in the cam chest cover behind where the rubber vent hose connects. It limits the positive pressure as it is quite weak but is there to stop the reverse flow of air back into the cam chest and bottom end.

The base of the push rod tubes will leak if not seated snuggly or if the O rings are torn or pinched. Try taking a flat slot type screw driver and hammer to tap around on the bases making them fit solidly against these O rings. Make sure the oil is not coming from the return tubes as they are barely long enough and do leak commonly when over tightened or not sitting fully into their bases.

The tachometer needs a thin copper washer under the hex nut to seal it against the aluminium cam cover. If it leaks at the knurlled collar and cable junction it may need an O ring on both sides of the cables flared end piece before overly tightening the knurlled collar down. I find this one the toughest one to always keep dry.

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P.S Welcome to the Forum there are getting to be quite a few members now with the 1979 year of Ironhead
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Old 14th August 2008
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The air filter is clean, but the crank vent tube is not connected there anymore it vents down to the ground now, there is some oil that vents with it, but nothing significant. A drop here and there.

I will try to buy the pressure gauge to test compression this week. This bike sat mostly for a year or 2, it seemed ok the first few times out but then it started blowing oil around the lifter tube bases, then some around the tach cable connection, I reseated all and that didnt seem to help much. I also notice some slight bubbling around the base of one of the lifter tubes which is what leads me to believe it is a crank pressure issue.

Could a ring be sticking after it sat, or a valve. is there something I can add to the cylinders or gas to loosen them up if so.

Another thing I was thinking is, This engine has about 2k miles or so on a rebuild. Could this be the lifter getting out of adjustment? could this cause the issue. I think I will try to follow the Sticky about adjusting lifters tonight and see what happens.

Last edited by lowplaces; 14th August 2008 at 19:42.. Reason: Add a thought
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Old 14th August 2008
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Look in the oil tank while the engine's running and make sure oil is returning to the tank (you should see oil movement other than vibration-induced). Also, check for any kinked or mashed oil lines. With the loss of power you noted, excess oil could be building up in the bottom end because of a restriction or oil pump defect. I'd think oil would be blowing out of the breather if that was the case, though. If the compression's bad, then blow-by could be causing your problem, too.
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Old 15th August 2008
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Ok checked the oil tank, Oil is moving as I would expect. Adjusted the lifters tonight, they were slightly off but not bad. replaced air filter and plugs.

Started and warmed it up to do a compression check, ran about the same as before, good until it warmed up then started running like crap.

ran compression check on both cylinders. front was about 90lbs back was about 35. ACK!

Put a couple of oz of oil in the cylinders, front went to 110lbs, back stayed at 35.

So is this telling me that there are valve issues?
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Old 15th August 2008
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Honestly you might have a few things going on in that rear cylinder. What did the sparkplugs look like? When you add oil to the cylinder the compression should go up some, so it looks like you need to remove the cylinder heads to see what's going on in there. You could have a hole in the piston and that could be your pressure problem. What kind of seals are you using on the pushrods, you mentioned that the bike sat for two years. If you have cork seals replace them with the blue silicone seals.
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Old 15th August 2008
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Quote:
Originally Posted by lowplaces View Post
The air filter is clean, but the crank vent tube is not connected there anymore it vents down to the ground now, there is some oil that vents with it,....
This probably is your problem.
That tube is supposed to connect to the intake manifold or carb somewhere, which pulls a vacuum on the crankcase on a 79 model.
There is no internal non-return valve on the 79 to stop air getting sucked in the breather. Just a plastic baffle, according to IronMick's extensive research on the matter.
With the breather vented to atmosphere through that loose tube, you engine now sucks in 1000cc of air on every half revolution as the pistons go up the cylinders. Then the other half of the revolution, your engine is pumping 1000cc of air out that tube. Or trying to.
Doing that anywhere up to 6000 times a minute is what causes excess crankcase pressure in many cases, and can rob power also. (Although if power loss is too bad, you might have worn rings as well. )

Earlier model Sportsters had a timed breather pre-77, and vented straight out. 77-78 modles had a vent tube with a non-return valve like a PCV valve on a car. Search this site for FOOFOO VALVE for more background on this.

You need to either hook that breather pipe back up to the intake manifold, or instal a foofoo valve on it to stop the bottom end of your motor being a big one-litre air pump. You can put an earlier HD foofoo valve on, or get a Krankvent aftermarket one. Again, search this site for foofoo valve and all will be revealed.
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Old 15th August 2008
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Did you do the compression test with the throttle wide open? Makes a big difference. I know on my 79, with the throttle wide open I have 175# on front and 165# on rear on the compression check. Also on my 79 I have an S&S carburetor and they recommended the vent go to ground like it was when I got it, they recommended taking it to the air filter housing on 80 and up. I took mine to the air filter housing anyway because of the occassional drop of oil in the garage from the vent, but it idled irratic at times and ran worse. I redid mine and it vents out the bottom of the bike now. Does sound like something major happening in that rear cylinder though. Could be a bad valve not seating 100%. I would pull the head if it was mine just to take a peek at it.
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