Talking Trash

Yup its time to trash the stock rotted out pipes, going to install a mac 4 to1 system my bikes a 82 xj 750 seca, stock and mint except the exhaust so no other mods, my book says it has 110 main jets, so I was checking into jets and found that the dynojet kits stage one starts with a big 124 main jet, seems like way to much depending on how hitachi rates there jets by flow or orifice size, thinking more like 117.5,plus a 15 thou washer under the needles, what have any of you used for just a muffler mod, but the way I found NOS mufflers for a mere 750.00$$$$$$$$$, NOT

 

Stock jets on a 750 Seca are #120 fuel.........

 

I think my bike had 112's in it as stock, and the Stage 1 jet kit had 118's and came with adjustable needles.

For jets that large you need aftermarket exhaust and a high flow air filter.

 

My 1981 Seca 750 has 118 Mains. A 4 into 1 , and "pods". The engine feels "flat" at 80 pulling up a hill, sounds lean? but it also gets about 34 MPG. Suggestions and trash talk welcome.

(it's also not running because I fried something while welding, not the ICU)

 

Took a bowl off today and it has 120 mains so I guess the 124 will be the ticket I double checked the factory book and there giving specs for the 650 and 750 but they arent very clear about sperating the two,

 

All XJ650 models (all years, except 650 Turbo) used #110 main fuel jets.

All XJ750 models (all years) used #120 main fuel jets.

From the factory.

 

Ok - stupid question: How the heck do you know what number to use?

I haven't pulled my carbs apart because I'm using the bike for daily transportation so I don't know.

 

TIME TO RIDE....#118's on a modified 750 engine is going in the wrong direction, your bike is probably running very lean (plug insulators will be bright white) and that is a Very Bad Thing to do to an engine, especially and air-cooled one.........

Here is some sample infor regarding jetting:

Here is the info regarding jetting that I found:

1 jet size for custom 4 into 2 exhaust

2 jet sizes for 4 into 1 exhaust

1 jet size for K&N filter (single inside airbox)

1 jet size for drilling out the bottom of the airbox

2 jet sizes for both single K&N and drilled airbox

2 jet sizes for individual filters

2 jet sizes for no muffler (open header)

1 pilot jet size for every 3 main jet size increase

Add up all the jet size increases and subtract one. (Remember
they go in steps of 2.0 for each jet size)

Under a mismatch condition, like individual filters and stock
exhaust or 4 into 1 header with stock filter and air box, subtract
an additional 1 jet size.

Check plug color often, sync carbs after each jet change,
make sure the floats are set correctly, and seriously consider
purchasing a Color Tune.

Make sure your carbs are in perfect working order before
making jet changes.

Additional info: Decrease main jet size one step per
2000' above sea level. Decrease pilot jet every 6000'
above sea level.

--------------------------------------------------------------------------------
EXAMPLE:

Stock = 120 Main and 40 Pilot with Y-13 Needle

4 into 1 with Supertrapp = +2 Sizes Main Jet
K&N Pods = +2 sizes Main Jet
Subtract 1 = +3 Sizes Main Jet or a 126
Subtract One More Size for Altitude of 2500' Average

I figure a 124 Main and 42 Pilot should do the trick after adjustment.

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Just be aware that the issue that most affects jet sizing is the restrictiveness of the INTAKE air flow, which is via the stock airbox/filter combination. Changing the air flow on the INTAKE side of the engine requires much more of an increase in fuel delivery requirements (jet size) than it does versus merely changing the EXHAUST flow by itself; doing both changes together may or may not have "additive" effects, depending on just how restrictive the aftermarket exhaust system is----and especially, at what RPM ranges does that restriction occur at. Not all aftermarket exhaust systems are THAT much less restrictive than the stock factory 4-to-2 system.

The 650 K&N filter probably passes more air than the stock paper filter, although the 650 K&N filter is a TINY little filter for some odd reason, so the reduced (like, about 50% reduced) surface area of the filter of the K&N filter versus the stock paper filter may negate some of the "freer flowing" aspects of the K&N filter----which is why many people say that you don't have to re-jet the carbs due to ONLY a change to a K&N filter element in the 650/750 models. Changing to POD filters, on the other hand, basically means you're going to have to change jetting, no ifs, ands, or buts about it.........

The only way to know for sure is to read the plug insulators under various conditions....idle, crusing, heavy throttle, cold outside, warm outside, etc. A real pain in the rump, which is why most people don't do it, or do it improperly, and end up with all sorts of performance-related issues when going from stock to "some other" configurations. And the worst condition of all to end up with is a lean fuel mixture condition, especially at a typical crusing or bike usage range, which can quickly eat up your engine, perhaps not all the way to catastrophic failure conditions, but enough to cause increased piston/cylinder/valve wear, and thus decreased engine performance over a remarkably short time span.

 

I found this while digging:

http://www.dragstuff.com/techarticles/h ... plugs.html

Gives some info on reading plugs.