Topic: JET SIZING TIPS

[HedNut]

I can't take the credit for these tips...but they sound very handy and on the mark.
Enjoy.....

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

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 2.5 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

With this jet configuration I get 32-38 mpg on the open road, Smooth idle, very strong exceleration from off idle to 80+ mph, and steady pull past 120 mph. My "F" has 65K mile on it, of which I have put 33K with this jet configuration.

I have applied this to the '78-'79 carb set with great success. These guidelines should work for the '80-'81 carbs also. The only part I have not tested is pilot jet changing on the second generation carbs.

Happy Jetting

[MOD27]

So if i have 105's in mains and i put k&N pods on and i have a 4into2 with no baffles I would need 125's?

[hapakev]

I think 115's, according to the math.  Thats assuming that the "open headers/no baffle" cancels out the 4to2 muffs.

[gsnickets]

I think the stock jets on my 72 500 are 100 correct me if im wrong just what i have found and the idle jets are 40 also correct me if im wrong

i have 4 individual velocity stacks and cut off exhaust they go 2-1 on each side stop right after foot pegs

i can only find jets in like 120 130 140 and 150 from my math i need 110 or smaller and a 42.5

[RFogelsong]

So something's not working out here, if you have a stock airbox and a open header that would be no change at all?

by my understanding:
+2 for open header
-1 from instructions
-1 for mismatch
=0 jet size change??

[lucky]

I can't take the credit for these tips...but they sound very handy and on the mark.
Enjoy.....

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

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 2.5 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

With this jet configuration I get 32-38 mpg on the open road, Smooth idle, very strong exceleration from off idle to 80+ mph, and steady pull past 120 mph. My "F" has 65K mile on it, of which I have put 33K with this jet configuration.

I have applied this to the '78-'79 carb set with great success. These guidelines should work for the '80-'81 carbs also. The only part I have not tested is pilot jet changing on the second generation carbs.

Happy Jetting

Are you talking about mainjets,idlejets or both?

[maduncle]

Righto - I am scouring the web looking for jet size advice.

I have a 1977 CB750F2 wit ha stock engine, running a 4 into 2 into 1 set of pipes finished off with a Lossa Engineering muffler.

I am also running one of Tintop's antipod filters.

The carbs have 115 mains in - but after today's ride and some sound advice I reckon I need to upsize the jets.

Based on the theory posted here - if the stock jets were 105:
 - the 4 into 2 into 1 pipes add 2 jet sizes
 - the antipod probably counts as individual pod filters and add 2 jet sizes
 - and then I subtract one jet size

I should go up three sizes from the 105's which I think makes it 112 (105, 108, 110, 112, 115, 118, 120)

- but the 115's aren't doing the job.

[lucky]

Righto - I am scouring the web looking for jet size advice.

I have a 1977 CB750F2 wit ha stock engine, running a 4 into 2 into 1 set of pipes finished off with a Lossa Engineering muffler.

I am also running one of Tintop's antipod filters.

The carbs have 115 mains in - but after today's ride and some sound advice I reckon I need to upsize the jets.

Based on the theory posted here - if the stock jets were 105:
 - the 4 into 2 into 1 pipes add 2 jet sizes
 - the antipod probably counts as individual pod filters and add 2 jet sizes
 - and then I subtract one jet size

I should go up three sizes from the 105's which I think makes it 112 (105, 108, 110, 112, 115, 118, 120)

- but the 115's aren't doing the job.

I have the same set up and you need to make sure the accelerator pump and nozzles are all working on those carbs. (stock carbs right?)
If any of them are not working it will never run right.
When the engine is OFF, Gas in the float bowls, and the air filters are OFF, turn the throttle and make sure you can see fuel squirting out of all FOUR brass nozzles in the venturi.

As far as the main jets, usually when they say one step up it does not mean 112 up to a 115. Use #120's or 125's
Usually they are not sold in every increment.
If it coughs or spits when you try to advance the throttle it is LEAN.
Remember 0-1/4 throttle air mixture screw and pilot jet.
0- 3/4 throttle slide needle position. You may need to put a shim under the slide needle. Start with one brass shim=.020. Make sure you file or sand the shim(washer) because it can have burrs on it from being punched out during manufacturing.
Wide open throttle mainjet.
IF you hold it wide open in high gear on the road and it keeps accelerating main is ok.
If you hold it wide open on the road in high gear and it hits a flat spot and will not go faster it needs a bigger mainjet.

Also check your plugs  for condition after mainjet run.

[lucky]

So something's not working out here, if you have a stock airbox and a open header that would be no change at all?

by my understanding:
+2 for open header
-1 from instructions
-1 for mismatch
=0 jet size change??

Yes that is right.
Stock air filters and aftermarket exhaust may only need a mixture screw adjustment to keep it from back firing if it is too lean.

[Retro Rocket]

So something's not working out here, if you have a stock airbox and a open header that would be no change at all?

by my understanding:
+2 for open header
-1 from instructions
-1 for mismatch
=0 jet size change??

Yes that is right.
Stock air filters and aftermarket exhaust may only need a mixture screw adjustment to keep it from back firing if it is too lean.

{from Cliff} I am also running one of Tintop's antipod filters.

The filter Cliff is using is flows more  than the stock box or pods. Cliff, i have seen plenty of guys run 120's with pipes and filter changes, so i would probably buy the 118's and the 120's{never hurts to have a selection of jets}, but also make sure everything else on the carbs  work properly...

[maduncle]

THanks guys - the bike is going over to Terry's this week - since he has the skills, the tools, the parts and the tenacity to get this fixed. I will be the apprentice and watch and learn (if I can).

I reckon we start with 120's and see what that does.

[lucky]

Some CB750's in other parts of the world came stock with 150 mainjets.
Many times you can use a wide choice of main jets. Too small of a mainjet is the real problem.

Most of the time you are not riding with Wide open throttle.
Getting the mid range and throttle response is very important.

If you give it a quick blip of the throttle and it stumbles or quits, you have more work to do.

[lucky]

THanks guys - the bike is going over to Terry's this week - since he has the skills, the tools, the parts and the tenacity to get this fixed. I will be the apprentice and watch and learn (if I can).

I reckon we start with 120's and see what that does.

Would you please report back and let us know the final outcome as far as you complete set up?
I am sure it will help others.

[maduncle]

THanks guys - the bike is going over to Terry's this week - since he has the skills, the tools, the parts and the tenacity to get this fixed. I will be the apprentice and watch and learn (if I can).

I reckon we start with 120's and see what that does.

Would you please report back and let us know the final outcome as far as you complete set up?
I am sure it will help others.

Not a whole lot to report Lucky -  apart from the fact that the 130 size main jets in the stock carbs with a Tintop filter fitted and 4 into 2 in to 1 headers and a Lossa muffler worked out okay and the bike was running well before I sold it last week.

[Cross-over]

Guys, I've got a question about the main jets. My CB750F1 '76 café is finisht and have driven it yesterday. The mixture is to lean and today I wanne place some new main jets, only I really don't now wat size I've to buy.

Some specs:
- Pod filters
- Custom made 4in4 with megatons
- Original size main and idle/pilot jets

The shop I've called does have some jets in the sizes 120, 130, 140 and 150. What size is likely the best?

[caferacer750]

ok so ive shimmed the needle that help out get but at wide open i get a flat at 6000rpm and my main jets are at 135.5 size with a 4 into 2 exhaust and a bed box intake and im a little lost are the 135.5 bigger and a 120 now i can see the hole in the 135.5 is bigger than the 120  in just not sure   

[crazypj]

That's a really rough and ready guide and it doesn't work with CV carbs.
 You know cylinder capacity, you know max rpm, you know you need about 13:1 air fuel ratio for max power, you work out the minimum fuel flow for the known numbers.
When you have cc/min flow, you find jet calculator to switch Mikuni to Kei-Hin (as Kei-Hin use hole size not flow rate)
 It's still a guess as the intake and exhaust efficiency is going to affect airflow.
Most engines only achieve 100% at a very specific rpm, usually 75~80% is nearer the real airflow but when everything comes together it could be 110~130% of calculated flow
Instead of jet numbers, a percentage increase may be safer and more accurate (or less inaccurate?  )
'Pods' and lightly modified exhaust or long pipes need around 10~12% increase, open pipes and too big a carb can need 25~30%, short open pipes, around 20~25%.
Too short a pipe with pods, 30~50%, plus you will gain a lot of noise without any increase in power (until you hit 16K  )

[lucky]

I can't take the credit for these tips...but they sound very handy and on the mark.
Enjoy.....

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

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 2.5 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

With this jet configuration I get 32-38 mpg on the open road, Smooth idle, very strong exceleration from off idle to 80+ mph, and steady pull past 120 mph. My "F" has 65K mile on it, of which I have put 33K with this jet configuration.

I have applied this to the '78-'79 carb set with great success. These guidelines should work for the '80-'81 carbs also. The only part I have not tested is pilot jet changing on the second generation carbs.

Happy Jetting

And what years are "2nd generation carbs"

Are you talking about 1977-78?

[lucky]

Righto - I am scouring the web looking for jet size advice.

I have a 1977 CB750F2 wit ha stock engine, running a 4 into 2 into 1 set of pipes finished off with a Lossa Engineering muffler.

I am also running one of Tintop's antipod filters.

The carbs have 115 mains in - but after today's ride and some sound advice I reckon I need to upsize the jets.

Based on the theory posted here - if the stock jets were 105:
 - the 4 into 2 into 1 pipes add 2 jet sizes
 - the antipod probably counts as individual pod filters and add 2 jet sizes
 - and then I subtract one jet size

I should go up three sizes from the 105's which I think makes it 112 (105, 108, 110, 112, 115, 118, 120)

- but the 115's aren't doing the job.

Thats why the OP said he did not talk about "2nd generation carbs. The 1977-78's
On those i would use 120 mains and #45 idle jets. I know that works.

[lucky]

So something's not working out here, if you have a stock airbox and a open header that would be no change at all?

by my understanding:
+2 for open header
-1 from instructions
-1 for mismatch
=0 jet size change??

Should just need a mixture screw adjustment. (Lucky)

[lucky]

So something's not working out here, if you have a stock airbox and a open header that would be no change at all?

by my understanding:
+2 for open header
-1 from instructions
-1 for mismatch
=0 jet size change??

Yes that is right.
Stock air filters and aftermarket exhaust may only need a mixture screw adjustment to keep it from back firing if it is too lean.

{from Cliff} I am also running one of Tintop's antipod filters.

The filter Cliff is using is flows more  than the stock box or pods. Cliff, i have seen plenty of guys run 120's with pipes and filter changes, so i would probably buy the 118's and the 120's{never hurts to have a selection of jets}, but also make sure everything else on the carbs  work properly...

I agree with that.

[lucky]

That's a really rough and ready guide and it doesn't work with CV carbs.
 You know cylinder capacity, you know max rpm, you know you need about 13:1 air fuel ratio for max power, you work out the minimum fuel flow for the known numbers.
When you have cc/min flow, you find jet calculator to switch Mikuni to Kei-Hin (as Kei-Hin use hole size not flow rate)
 It's still a guess as the intake and exhaust efficiency is going to affect airflow.
Most engines only achieve 100% at a very specific rpm, usually 75~80% is nearer the real airflow but when everything comes together it could be 110~130% of calculated flow
Instead of jet numbers, a percentage increase may be safer and more accurate (or less inaccurate?  )
'Pods' and lightly modified exhaust or long pipes need around 10~12% increase, open pipes and too big a carb can need 25~30%, short open pipes, around 20~25%.
Too short a pipe with pods, 30~50%, plus you will gain a lot of noise without any increase in power (until you hit 16K  )

You are mostly right but it is 14:1 not 13:1.
But you still have to make changes with actual jet sizes.
I wish everyone would tell the whole story...NOT just mainjet size.

IDLE jet ,MAIN jet and needle position. Mixture screw too if possible.

[crazypj]

13:1 for max power, 14.7 is stochiometric ratio if it was possible to get 100% efficiency at all rpm/throttle openings
Engine will often be running at around 16~17:1 at cruising high gear, medium rpm small throttle opening
Idle/pilot jet rarely needs much if any increase as the air filters are not restrictive at low rpm

[caferacer750]

Ok I've found what's wrong the 135 main jet is to big and the power valve was frozen
Got it freed up and put in the one 20 jet and it's better but not 100 percent And my bike is a 78 cb750 k

[Dimitri13]

2 jet sizes for 4 into 1 exhaust

2 jet sizes for no muffler (open header)

Do you add these two together if you've got 4-1 headers with no muffler?