jetting

[mikeyboy1]

the 750 I'm redoing has aftermarket exhaust on it,I think.there are 4 pipes ,black, into one.looks like a header.and a short chrome muffler.I'm wondering if I need to change the jets.I've never had it running but it should be ready in a few days.any ideas?
thanx  Mike

[LukeM]

Depending on the model, Honda did change carbs and/or carb settings.  The F models (Super Sport) did specify either different jets or different carbs (can't recall, I'm away this week, and my manual isn't at hand). I would say that, if it's running well, do a plug chop (search for "plug chop" on the home page search bar), and see what they look like.  You're looking for a light brown on the porcelain insulator if the mixture setting is good.

With a 4 into 1, and what appears to be a shorty or stub muffler, the airflow will be greater, and stock carbs with a more open exhaust will tend to run lean (more air for the same amount of fuel) compared to stock.  This is why bikes with pods and/or open exhaust will richen things up a bit to keep the motor happy.  Same goes for core'd exhaust (baffles removed).

Once you have a baseline, then you can experiment with different mixture settings, jets, mufflers, flux capacitors, etc.  This way, you can go back to carb setting A, jet size B, etc.

Hope this helps.  Back to work for me.
Luke M

[mikeyboy1]

thanx Luke.still a couple days from starting it up.it may be OK,never ran it.wish me luck

[Re-run]

Depends on if you have a k or f bike. Either way, I would just put it at the stock settings and fire it up and check from there. Ultimately, I doubt you will need to change jets at all. Adjustment on the air/fuel screw is needed anyways, so I wouldn't worry about jetting. Seldom do exhaust changes really require jet changes unless you go from a full exhaust to some short drag exhaust.

[gkmaia]

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[Re-run]

You can absolutely do a plug chop of your bikes idle. You can do it was any rpm range you want, technically.
Say you put in plugs and start the bike and warm it up. You then let it idle. If it is rich or lean, your plugs will certainly show it.

A chop is also useful at WOT. A person has no other indicator for the mix at WOT than to do a plug chop.

A plug chop just doesn't need to be done at idle or midrange because if the carbs are tuned properly with the a/f screw, then a chop won't be needed.

From there, the needle position only determines when the mains become richer, not how rich.

In most cases, a plug chop is done by wringing the piss out of the bike and climbing through the gears to maintain high rpm and load on the motor. Once 5th is reached, the rider pulls the clutch and hits the kill at the same time and brakes to a stop with the engine off. Once cool enough to reach in, then the plugs are removed to inspect. That is a WOT plug chop and only tests the mains.

As for installing an o2 sensor, that linked ebay isn't too useful as it does not measure accurately. I thought about it a few years ago and the narrowband sensors don't cut it. A person needs to use a wideband to get accurate readings.
Not only that, but a bung MUST be welded into each header pipe. This is the only way to accurately test each cylinder's A/F. Plus, unless you get a heated sensor, they must be close to get hot enough to read properly.

There was a discussion on this a few years ago here and this was the end result. So, while o2 sensors are certainly feasible, you need to use wideband(this is the type that is mounted in the exhaust header of a car), 4 bungs with plugs, and of course the display. The narrow band only measures if the reading is over or under. this is the type of sensor that is used further downstream for exhaust readings.

More info on the difference between narrow and wide band.

[Lucky 1]

The most important question is: did you change the intake from stock?

If you did you will have to re jet.
If you did not change the intake then all you will need is a mixture adjustment.

[Re-run]

That's about the long and short of it. Obviously just changing a filter does not require work unless you go from a paper to k&n or uni. Those usually just require a bit of tuning since a person is still using the air box.