Has anyone got any idea of when it is and what to do about too much pressure/backpressure? Just put on a 2 into 1 (unknown brand but using a original fishtail muffler), I only put it on to try out for the best sound and, despite me not liking fishtails all that much, I've found my sound) but, I'm now fazed with another problem, it appears that I got too much pressure/back pressure going down the pipe ! It manifests itself when I'm starting up, sometimes the starter even appears to be slipping and no, I don't have a kick starter at all, pretty stock standard -98 FLSTF (guess I'm lucky about that, well to familiar with reverse kicks on my old bikes). Anyway, to get back to pressure, how much can be classed as bad ? I seem to have the carbie jetting sorted out, but, should I now retard the ignition as well ? Any help will be gratefully appreciated.

 

It is doubtful that your starting issue is related to exhaust backpressure. Exhaust backpressure requires the engine to be running to develop enough flow to fill the pipe before creating the level of pressure needed to affect engine performance @ idle.
To much back pressure could prevent your exhaust valve from closing properly which will show up as sluggish performance.

 

As Chris said the pipes aren't effecting your starting nor do you have to retard your timing. You will have to rejet your carb for the new exhaust flow.
Unfortunaly the Fish Tail pipes do cerate a lot of back pressure and with this back pressure your going to get reversion into the cylinder which takes the power away that you had with free flowing exhaust. The only way to correct the reversion is change the pipes.
We are looking for an exhaust flow of 280 to 300 feet per second....higher than 300 fps will over scavenge the cumbustion chamber which means the pipes pull out the charge in the cylinder and on to the road behind you. If you can't keep the charge in thecylinder you can't burn it.
Lower than 280 fps will under scavenge and this will let exhaust residue lurk in the pipe (like a cork) which will slow the exhaust down even more and then get sucked back in the cylinder afterblow down leaves it's low pressure area in the cylinder and some of the pipe. (something like the old patato in the tail pipe trick). If you fill the cylinder up backwards (reversion) then there is no room for a fresh charge to come from the intake.....hence your power is dropped down also.
Pipes either make a build or it kills the build, there are thousands of pipes out there forHarleys but only a hand full really work well.
So bottom line is this...what's more important, sound or performance. Once you determin which oneyou want then go with it but this is one of those cases where you can't have both......

 

the starting issue could be ignition timing, but if its stock ignition, this is not likely. a better guess would be your starter clutch or ring and pinion gears starting to shear. Either way, the primary cover has to come off, and if the gears look good, change out the starter clutch and you should be good to go.

 

How can one check for exhaust flow? Or is that something only a manufacturer does during the design stages?

 

There are formulas to figure out the speed, but it doesn't stop there. The baffle plays a big part....is it restrictive or open. I guess 6 years of dyno tuning you just know which ones work and which ones don't.

The formula for MEAN FLOW VELOCITY is (average piston speed / 60) x (bore [squared] / exhaust pipe inside diameter [squared])=FPS
Now to find AVERAGE PISTON SPEED to plug into the formula AVP is (stroke x rpm) / 6= average piston speed