New member 2001 1200 sporster rear bank exhaust sucking in

[Jakeschultz]

Hey y'all!
So I'm trying to figure out if this is normal or not.
I have a 2001 sportster 1200 that I've put into a 1975 ironhead frame. The bike runs great, but I noticed the rear exhaust had a different sound then the front. So I whipped out my dollar and did the old trick. On every 3rd or 4th revolution the exhaust on the rear bank would suck the dollar back in. So I swapped the wires on the coil with no change. I can't find a solid answer about this anywhere, and that's why I've come here.
Thanks y'all.

[koko3052]

If it does it when warmed up, I would then unclip the pushrod covers, jack up so the rear wheel is off the ground, put tranny in high gear so that you can turn the engine over slowly with the rear wheel, take spark plugs out & turn the engine over to see if the exhaust pushrod gets really loose.
Maybe for shits & giggles do a compression test first.

[Jakeschultz]

Tomorrow I'll start with a compression test, and go from there.

[Jakeschultz]

Compression test results.  Front bank 120 (after about 5 revolutions) rear bank 120 (after about 6/7 revolutions). In my opinion those are great results. Think I should still inspect the exhaust push rod?

[Jakeschultz]

Any suggestions fellas?

[rigidthumper]

Lose the drag pipes- what you're feeling is simple reversion.

[koko3052]

Was the throttle wide open when you did compression tests?

[Jakeschultz]

They are drag pipes, and I haven't put back pressure washers on them yet. Am I harming the bike by using drag pipes? Also what's the purpose of having the throttle wide open for a compression test? That doesn't make sense to me as it wouldn't effect the results. No spark, or fuel is getting to the cylinders.
Again I really appreciate y'all chiming in!

[Ohio HD]

Yes the throttle needs to be wide open, it's restricting the air flow when shut.

As Rigidthumper said, reversion is occurring, no back pressure, no buffer in the exhaust.

[Jakeschultz]

Hu that's a new concept to me as far as compression tests go. But I work on cars. And the idea for a compression test is to actually see how much air pressure is being compressed in a cylinder. So adding air doesn't make sense to me, but again I'm a car guy. I'm super open to learning. So I'll try it with the throttle open. But can I ask what is the purpose of adding air to a compression test on this harley motor. I'm just trying to learn guys. Sorry if everyone is rolling there eyes ha

[Ohio HD]

Cars you hold the throttle wide open too. Any four cycle motor, that's the procedure. 

[Jakeschultz]

No offense but what am I missing...? I've been an old car guy for years!!! Seriously restoration and motor builds. Never once have I held the throttle wide open for a compression test. So I'm trying to understand the reasoning behind it. Again you're trying to test the compression level of a sealed cylinder, if a valve isn't fully closed or piston ring isn't seated ect ect it allows more air into that cylinder. Thus creating less compression. Again I'm just trying to understand why you'd want to let air flow into the cylinder during a compression test. Not trying to be a dick y'all. Just trying to learn.

[Jakeschultz]

Also I'm not doubting anything. I'm totally gonna do it with the throttle open ha.

[Ohio HD]

No offense but what am I missing...? I've been an old car guy for years!!! Seriously restoration and motor builds. Never once have I held the throttle wide open for a compression test. So I'm trying to understand the reasoning behind it. Again you're trying to test the compression level of a sealed cylinder, if a valve isn't fully closed or piston ring isn't seated ect ect it allows more air into that cylinder. Thus creating less compression. Again I'm just trying to understand why you'd want to let air flow into the cylinder during a compression test. Not trying to be a dick y'all. Just trying to learn.

If you can't get a full charge of air in the cylinder, then what are you compressing?  A less than optimal charge of air in the cylinder. Been doing it that way since I started working on cars, in automotive school, 1975. 

[Jakeschultz]

Well Daggum! I just learned a lesson! Thank you so much very noteworthy. I'll do another compression test tomorrow, and post results. Thanks y'all!

[rigidthumper]

Don't think of it as adding air, think of it as removing restrictions. The manifold (plenum) on a Harley isn't big enough to feed one cylinder, so if the throttle is closed, there's a restriction.

[FXDBI]

No offense but what am I missing...? I've been an old car guy for years!!! Seriously restoration and motor builds. Never once have I held the throttle wide open for a compression test. So I'm trying to understand the reasoning behind it. Again you're trying to test the compression level of a sealed cylinder, if a valve isn't fully closed or piston ring isn't seated ect ect it allows more air into that cylinder. Thus creating less compression. Again I'm just trying to understand why you'd want to let air flow into the cylinder during a compression test. Not trying to be a dick y'all. Just trying to learn.

If you can't get a full charge of air in the cylinder, then what are you compressing?  A less than optimal charge of air in the cylinder. Been doing it that way since I started working on cars, in automotive school, 1975.

Same has I was taught in the 70's in trade school.  Also was taught to do it dry then try with a squirt of oil in the cylinder and see if it changes. A increase with oil will show ring seal problems. All tests were always done with throttle wide open air cleaner removed.  Bob

[Jakeschultz]

Followed the procedure test results...
Front bank 165psi
Rear bank 160psi

[Hossamania]

Seems good to go. A little better than the 120 you originally got.
You could try putting a 1/4" bolt through each pipe about 2" from the end, it breaks up the reversion, and has been proven to help running issues and add up to 2 horsepower.
Or, get some better pipes. Don't get me wrong, I love drag pipes, but they are not perfect. You're not hurting the motor running drag pipes as long as your jetting is proper. Run it and have fun!

[Ohio HD]

Build yourself a couple of lollipops. Tack weld the washer to the eye bolts. Or bolt them together like this. You want about a 1-1/4" O.D. washer. Set them at about 45° and see how it runs. Probably pick up low end power you never knew you had.


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[Jakeschultz]

Yeah I had originally decided to make some of them when I started the build. It was just low on the list, but thanks to y'all it has moved to the top ha.

[TexNorth]

This is a good example of reversion due to drag pipes and the effect on a dyno run

https://www.youtube.com/watch?v=KjKUKhHQLHg

[Jakeschultz]

Wow great stuff! I guess losing the drag pipes is probably the best option. I'm gonna make some washers in the meantime. Any suggestions for pipes?
Thanks y'all!

[chaos901]

Interesting video, I knew (or believed) that some resistance was necessary in the exhaust just never knew why. 

[chopper]

Very well done video!