Back pressure

[Geezerman35]

Running fine with dual pipes completely open, ending mid frame.
While riding, I feel intermitting shudders. Not an engine miss. Does not seem to be fuel problems as it occures at various speeds and power usages, Low speed, going thru the gears, applying gas, letting off on gas etc. Cruising under power at a steady highway speed, just fine.
What symptoms can I look for that might indicate that I need to adjust the exhaust?
I have had a recent new clutch replacement. That seems to be operating fine, no slippage. Shudder was evident before that and was noticed after changing the exhaust pipes.
I am running a longer Paugho frame and had been hoping that a chain tensioner might solve the problem. Having recently installed a tensioner (Monster Garage), the shudder is still there.
Appreciate any helpful input,
Son in law keeps telling me I'm paranoid and to shut up and keep riding
Thanks,
Geezerman

 

[gggGary]

Just because you are paranoid doesn't mean it's it's not out to get you.
No specific It's gotta be this but
Are you running a head brace?
Are all the engine mounts tight?
Chain and sprockets in good condition?
All welds checked and good
Steering bearings good and properly tensioned.
Forks good oil proper level
Cam chain valves set correctly
Fuel level, jetting correct
Cush springs on the back of the clutch broken
Parallel twins are shakers; it might just be a sympathetic vibration of say your "oil bag" while passing through a certain RPM range in certain gears.

 

[Geezerman35]

Thanks,
I'll check over all of these points
Bike didn't used to do this.
Son in law says it is harmonics acting up, probably what you are inferring as sympathic vibration.
Thanks,
Geezer

 

[ReycleBill]

Harmonics aka, sympathetic vibration usually only happens at the same RPM or in the case of tire balance, at the same MPH. For example: My fairing rattles at 3500 RPM no matter what gear I'm riding in but doesn't rattle above or below 3500 RPM. So when riding I get through the gears and cruise above or below the rattle speed.

Sympathetic vibrations can come and go based on speed or RPM but you'll always notice a pattern to them. A new sympathetic vibration indicates a new problem.

Check all the things gggGary said. And if that doesn't do it then try baffles or lollipops in your pipes. Open pipes always add more vibration at some point in the RPM range but they can also cause reversion (reverse air flow) which can be detrimental to your exhaust valves and cause strange things to happen.

 

[Geezerman35]

Thank you, Guys.
I had "lollypop" baffles at one time (thumbscrews).
Now that I have made some changes since then, I may stick them back in and see what happens.
Geezerman

 

[gordonscott]

Here is a piece from "The Horse" it sums it up best.... "Backpressure? One of the common misconceptions I hear is “you need more backpressure” This is about the same as “you need more choke” and makes about as much sense. The engine needs to move air in, combustion products out, or “breath” if you prefer. If you want backpressure, try farting through a boatswain’s whistle. An engine’s displacement is constant, but its volumetric efficiency can be maximized. The basis behind backpressure is not completely wrong though, as at first it seems that leaving pipes off would allow the maximum amount of exhaust to escape. But, this leaves only the upward movement of the piston to push the gases out after the initial pressure release. Properly sized pipes route the combustion products such that their velocity sucks out the remaining gasses that the piston can’t push out. If the pistons push out 80 ci the pipes could help pull out the remaining 10ci that the heads hold. On the other end of the scale, a pipe that’s too restrictive doesn’t allow all the gases to escape.” So in short a great system can give you a little extra perfomance while a bad one (to restrictive) can hurt perfomance significantly.

 

[Geezerman35]

FYI Guys:
I reinstalled the thumbscrews in the ends of the pipes. I have them straight on, not crossways.
My 'imagined' shudder, has all but dissappeared. I get just a very little now at low RPM with the engine under load. Like 25/30 MPH in high gear

 

[ReycleBill]

FYI Guys:
I reinstalled the thumbscrews in the ends of the pipes. I have them straight on, not crossways.
My 'imagined' shudder, has all but dissappeared. I get just a very little now at low RPM with the engine under load. Like 25/30 MPH in high gear

That sounds about normal.

 

[ReycleBill]

Here is a piece from "The Horse" it sums it up best.... "Backpressure? One of the common misconceptions I hear is "you need more backpressure"This is about the same as "you need more choke"and makes about as much sense. The engine needs to move air in, combustion products out, or "breath"if you prefer. If you want backpressure, try farting through a boatswain's whistle. An engine's displacement is constant, but its volumetric efficiency can be maximized. The basis behind backpressure is not completely wrong though, as at first it seems that leaving pipes off would allow the maximum amount of exhaust to escape. But, this leaves only the upward movement of the piston to push the gases out after the initial pressure release. Properly sized pipes route the combustion products such that their velocity sucks out the remaining gasses that the piston can't push out. If the pistons push out 80 ci the pipes could help pull out the remaining 10ci that the heads hold. On the other end of the scale, a pipe that's too restrictive doesn't allow all the gases to escape."So in short a great system can give you a little extra perfomance while a bad one (to restrictive) can hurt perfomance significantly.

Makes perfect sense if you chink about pipes like a chimney on a fireplace. If the chimney is too short then it won't draw and the fire won't burn as well as it could. And if the chimney is too tall then all the heat gets sucked out of the top and doesn't warm the house. But it does burn a lot more wood. Put a squirrels' nest in the chimney and everything gets backed up.

Of course, chimneys work by convection and not pressure but the analogy works just the same.

 

[wacko]

With straight unbaffled pipes is backfiring on deceleration normal or a result improper jetting?

 

[ReycleBill]

With straight unbaffled pipes is backfiring on deceleration normal or a result improper jetting?

It's generally agreed that jetting will solve this issue but so will baffles. The popping indicates dangerous leaning that can burn valves and pistons.

Personally, I like to use lollipops because they are more easily tuned than changing jets. Cheaper too.

 

[wacko]

It's generally agreed that jetting will solve this issue but so will baffles. The popping indicates dangerous leaning that can burn valves and pistons.

Personally, I like to use lollipops because they are more easily tuned than changing jets. Cheaper too.

What are lollipops?

 

[wacko]

What are lollipops?

Never mind. I figured it out. Homemade washer baffles. I think I will give that a try.

Thanks

 

[gggGary]

I pulled the muffler off my 305 scrambler back in the day and put on snuff r nots. Was running em open and got pulled for for"excessive noise" cop looked the bike over and said wouldn't it be quieter if you turned those? I said YES SIR turned them and he walked away.

snuff r nots still exist some guy out in Washington state made them for years.

 

[wherearewe]

Sorry for asking but how would you jet for an adjustable setup? Or do they not make that much difference? I know they can influence sound a lot...

 

[ReycleBill]

Sorry for asking but how would you jet for an adjustable setup? Or do they now make that much difference? I know they can influence sound a lot...

Fact is, you probably can't accurately jet for an adjustable setup but you can get close. If it were me I'd jet for how I'm going to be doing the majority of my riding.

Not sure where you live but for me jetting can be a a bit of give and take. For me a single day's riding can see altitude changes from sea level to 5000 feet. There's no way to just for that so I jet for about 1000 to 1500 feet above seal level where I spend most of my time.

I would apply the same thinking to adjustable exhaust.

That said: If I were running open pipes the adjustable pipes would be the solution to my jetting issues.

 

[hoffman900]

Back pressure within itself isn't a good thing. High velocity through the exhaust is a good thing!

The higher the velocity through the pipe, the more pressure drop. When the valve cracks off the seat, more exhaust is evacuated before the piston starts pushing on it (notice where the exhaust valve opens BBDC ). The more exhaust you can get out before the piston starts coming up, the less pumping loss you have. In addition, an exhaust at a proper tune length (for a given rpm range) creates a negative pressure pulse that arrives at the cylinder during exhaust / intake valve overlap. This negative pressure causes more intake charge to enter the cylinder (think of it as a bit of a reverse supercharger). This increases volumetric efficiency (VE), and is a big part of why engines can have over 100% VE.

That being said, it is possible to 'over scavenge' the cylinder. Generally this is seen as the engine wants more gas without an increase in power (extra is getting sucked out at valve overlap). This is a good sign, but you need to make some changes. To combat this, the professionals, now with a more efficient exhaust system, use a smaller exhaust valve (since most of it is being sucked out before the piston pushes up). This means they can get away with a larger intake valve as there is now space in the chamber for it, thus making more power!