Topic: backpressure

[Chris Liston]

Quick question.  Does more or less backpressure make the mixture leaner or richer?

Thanks

[eldar]

I think less back pressure makes the mix leaner as it allows for more flow.

[ProTeal55]

I believe you are right. More air = lean.

And remember , Lean is Mean !!

[cb(r)]

I believe you are right. More air = lean.

And remember , Lean is Mean !!

teal again: like the bike!  sorry for the slight hijack on the thread

[eldar]

Yeah well I would like to meet the chick on yours cb(r)!

[ProTeal55]

Yeah well I would like to meet the chick on yours cb(r)!

I 2nd that....

[HondaMan]

Quick question.  Does more or less backpressure make the mixture leaner or richer?

Thanks

The lower backpressure on these engines causes leanness in the midrange. Adjust this with the needles in the carbs: raise for more richness. Lean out the idle a little at the same time.
At higher RPM the mixture is controlled more by the airbox characteristics than by the pipes.

[Dusthawk]

Don't mean to hijack here but...................................

Rita, up there in my avatar has straight pipes and I was told that if I put a bolt in the pipe it'll create enough backpressure to help the carbs balance out without trying to figure out new jets. Am I barking up the wrong tree here?

Peace and Long Rides,

Jeff

[eldar]

Probably have to be a big bolt to do anything.

[ProTeal55]

Rita, up there in my avatar has straight pipes and I was told that if I put a bolt in the pipe it'll create enough backpressure to help the carbs balance out without trying to figure out new jets.

Anything that will restrict the exhaust flow will create backpressure. How much it creates depends on how big the obstruction is. A 1/2 bolt in a little pipe is a decently large 'thing" for the exhaust to go around, and will prob. give you some added backpressure.

I know allot of "hardcore" HD bobber guys who swear on little tricks like this..

[eldar]

Yeah anything can restrict flow but the question is how big will it have to be to actually make a difference. It does help that our headers are only 1.5 inches or so.

[ProTeal55]

Yeah anything can restrict flow but the question is how big will it have to be to actually make a difference.

I would be trial and error I guess. 
Would be nice t oget some Dyno time, and try a few different things to
see what actually works, and what doesnt...

[eldar]

Instead of a bolt, you could install a plate at the end which would act as a restrictor also. The amount of restriction would be determinded by how much of the pipe was closed by the plate.

[ProTeal55]

I agree.
I have been drilling out the "holes" in my factory pipes mainly to get more sound outta them , but I
did feel alittle more top end charge, prob. due to the less backpressure.

Amazing what a drill and Unibit can do 

[Philly550K1]

in addition to the sound changing, observing the effects on the powerband is part of the fun of tinkering with the exhaust. 

one of these weekends, Listo ...  before i perfect my "SOHC EXUP valve" invention,  i still need my butt kicked to change my tire and chain....  i only have a few more burnouts left in my bald rear.

now there's a quote.....

[Dusthawk]

Rita, up there in my avatar has straight pipes and I was told that if I put a bolt in the pipe it'll create enough backpressure to help the carbs balance out without trying to figure out new jets.

Anything that will restrict the exhaust flow will create backpressure. How much it creates depends on how big the obstruction is. A 1/2 bolt in a little pipe is a decently large 'thing" for the exhaust to go around, and will prob. give you some added backpressure.

I know allot of "hardcore" HD bobber guys who swear on little tricks like this..

I have a friend I talked to today and he said the best thing to do is put a thumb screw in, you know the one that has the big flat grabber thing, and that way it can be turned to add or subtract backpressure then held by a nut on both sides of the pipe wall.

Sounded good to me, but any other ideas or experiences would be welcome before I go nuts with a drill

Peace and Long Rides,

Jeff, the Mad Driller

[Chris Liston]

in addition to the sound changing, observing the effects on the powerband is part of the fun of tinkering with the exhaust. 

one of these weekends, Listo ...  before i perfect my "SOHC EXUP valve" invention,  i still need my butt kicked to change my tire and chain....  i only have a few more burnouts left in my bald rear.

now there's a quote.....

Geterdone Philly and lets go riding

[HondaMan]

There used to be a popular device called "Snuff-R-Not", which consisted of a washer welded to a nice, spring-loaded detent knob. You'd drill a 1/8" hole all the way through the pipe's end, then install. You could turn it to "closed" position (washer hole only) for less noise, or more backpressure, and open it almost empty (except for the shaft) just by burning your fingers. Honda 350 scramblers were often stripped of their baffles and these things installed to get back the 3 HP they lost to the CB's pipes.

When the 750 started getting shorty pipes, tuning problems abounded. I used to install these so the owners could play out their tune & tone fanasties to their heart's content. We sold 'em by the boxload. Don't know if they're still around, though...