Motorcycle Repair: changing my exhaust, little honda, wonded

Question
I apologize up front for being long wonded here... I want to swap out the exhaust on my 1980 Honda CB750K. It has an aftermarket 4-2 with megaphone mufflers. The bike runs and sounds great, but the megaphones get in the way of the centerstand, so it drags around corners and going over the curb in and out of my driveway. I know that changin the exhaust can alter the tuning of the bike. How do I know what else to change? Is there some way to measure the flow of my current exhaust and compare it to the new one? If the new one is more- or less-restrictive, what will that do to my performance if I don't change the jetting and air intake? How do I know what jets I need if I have to change that? I'm really hoping that it's not just trial and error. Thanks for the help!

Answer
Stu, being that the exhaust was already changed from OEM, perhaps the jetting was already changed, as well! You may have to do ... nothing.

Each exhaust system has different dynamics, however. A 4:1 will function some differently than a 4:2, however Honda's CV carbs are somewhat forgiving as they automatically adjust to a certain extent for flow changes in the system.

I remember a Brit magazine doing a hop up on a similar bike and even with no air filters the jetting change was just a size or two richer.

I think that with the more restrictive 1980 carbs, Honda eliminated the primary main jets, which gave more options to the jetting picture. I think you are back to just the idle jets and mains again. I have a little Honda tuneup guide and it isn't clear if the 80 750 still has both main setups. The book shows #68 and #102 pri/sec mains. If that is the case, you will probably want to go to 70/105 combo, assuming that you are going to some kind of reasonably quiet system with a little backpressure.

Unless you have access to a dyno, it will be trial and error, based on how the engine reacts to the exhaust change. You can use the choke to help guide you along, though. If you encounter a "flat spot" roll on a little choke and see if the mixture change helps or hurts the performance at that point. That will tell you if you went too far one way or the other.

In general, you can count on going up about one size on the mains from OEM jetting to compensate for a different exhaust system and a bit more if the air filter system has been changed, as well.

Bill Silver