Carb Jetting when going 4 into 1 exhaust

On the particular bike I have, stock carb jetting is 100 mains 40 pilot. I have Lossa engineering 4 into 1 pipes on my bike.

Theres a guy on ebay who sells rebuilt carbs. By all accounts, he does an amazing job. I bought carbs from him and he said he woudl rejet to accommodate my 4 into 1.

He sent the carb rack back and it looks great but the stock jet are still in there. Hmmmm. Thoughts? These are the carbs

Anyone who claims they can jet some carbs off the bike to suit your pipe is talking crap which is probably why they came back stock.
They can guesstimate but they will still need to be on the running bike to get right.
May as well start with stock settings and go from there.
If it's just a less restrictive pipe then go for a few sizes up on the mains ( say 115's ) and work down.
You will probably end up closer to 105 or 110's
Also may also need a tweak on the air screws to fine tune the pilot mix.

zed1015 wrote: Anyone who claims they can jet some carbs off the bike to suit your pipe is talking crap which is probably why they came back stock.
They can guesstimate but they will still need to be on the running bike to get right.
May as well start with stock settings and go from there.
If it's just a less restrictive pipe then go for a few sizes up on the mains ( say 115's ) and work down.
You will probably end up closer to 105 or 110's
Also may also need a tweak on the air screws to fine tune the pilot mix.

I guess youre right. But the reviews from customers made me a believer. See below:

never mind.

650Dude wrote: On the particular bike I have, stock carb jetting is 100 mains 40 pilot. I have Lossa engineering 4 into 1 pipes on my bike.

Theres a guy on ebay who sells rebuilt carbs. By all accounts, he does an amazing job. I bought carbs from him and he said he woudl rejet to accommodate my 4 into 1.

He sent the carb rack back and it looks great but the stock jet are still in there. Hmmmm. Thoughts? These are the carbs

What model bike are you talking about? KZ650 models did not come stock with #40 pilot jets. See charts below. Also, please add your bike's exact year and model to your signature line as that helps avoid confusion. Ed

Those carbs are off a Honda.
Changes to the exhaust has less of an effect on carb jetting than changing the intake.


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Nebr_Rex wrote: Those carbs are off a Honda.
Changes to the exhaust has less of an effect on carb jetting than changing the intake.


.

Very observant. Yes Honda carbs, not for my KZ.

Whenever you go 4 into 1 from 4 into 4 you MUST make jetting changes. After some research I came across a legendary post by a guy who is now revered for figuring all this out.

All i had to do is go up two sizes on my mains, from 100 to 110 and leave the pilots alone. Here is a link to the guy and his science which has proven to be foolproof.

www.xs11.com/forum/showthread.php?t=382

"Jetting Recommendations
by Denny Zander

Here is a simple set of jetting guidelines that have worked for me. For those considering jet changes, this might help select a starting point.

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

3 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 2.5 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 Colour Tune. (See "Colortuning Carburetors" in the Maintenance Section).

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

Example from my '79 XS1100 F:
Stock main jets: 137.5

Stock pilot jets: 42.5

2 sizes for 4:1 exhaust (Jardine)

1 size for single K&N

1 size for drilled air box

4 (main sizes) - 1 = 3 or 137.5 + (2.5 * 3) = 145.0
1 (pilot jet size) or 42.5 + (2.5 * 1) = 45.0"

zed1015 wrote: Anyone who claims they can jet some carbs off the bike to suit your pipe is talking crap which is probably why they came back stock.
They can guesstimate but they will still need to be on the running bike to get right.
May as well start with stock settings and go from there.
If it's just a less restrictive pipe then go for a few sizes up on the mains ( say 115's ) and work down.
You will probably end up closer to 105 or 110's
Also may also need a tweak on the air screws to fine tune the pilot mix.

You were dead on. 110's are the way to go, leaving the pilots alone and simply adjusting the air screws.

Nice set of guide lines but, .... it is incomplete without any mention of needle jets, jet needles and slides.


.

650Dude wrote:

Nebr_Rex wrote: Those carbs are off a Honda.
Changes to the exhaust has less of an effect on carb jetting than changing the intake.


.

Very observant. Yes Honda carbs, not for my KZ.

Whenever you go 4 into 1 from 4 into 4 you MUST make jetting changes. After some research I came across a legendary post by a guy who is now revered for figuring all this out.

All i had to do is go up two sizes on my mains, from 100 to 110 and leave the pilots alone. Here is a link to the guy and his science which has proven to be foolproof.

www.xs11.com/forum/showthread.php?t=382

"Jetting Recommendations
by Denny Zander

Here is a simple set of jetting guidelines that have worked for me. For those considering jet changes, this might help select a starting point.

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

3 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 2.5 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 Colour Tune. (See "Colortuning Carburetors" in the Maintenance Section).

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

Example from my '79 XS1100 F:
Stock main jets: 137.5

Stock pilot jets: 42.5

2 sizes for 4:1 exhaust (Jardine)

1 size for single K&N

1 size for drilled air box

4 (main sizes) - 1 = 3 or 137.5 + (2.5 * 3) = 145.0
1 (pilot jet size) or 42.5 + (2.5 * 1) = 45.0"

This post is just meant to deal with the pilot/main jet sizing.

650Dude wrote:

zed1015 wrote: Anyone who claims they can jet some carbs off the bike to suit your pipe is talking crap which is probably why they came back stock.
They can guesstimate but they will still need to be on the running bike to get right.
May as well start with stock settings and go from there.
If it's just a less restrictive pipe then go for a few sizes up on the mains ( say 115's ) and work down.
You will probably end up closer to 105 or 110's
Also may also need a tweak on the air screws to fine tune the pilot mix.

You were dead on. 110's are the way to go, leaving the pilots alone and simply adjusting the air screws.

That's good to hear..
Glad to see my 40 odd years of playing with bikes wasn't a total waste of time :laugh: