Topic: Which Jet size would you guys recommend?

[noelocol]

Hey guys,

What jet sizes would you guy use on a 75 cb750f, running 4-4 drag pipes on the exit and K&N Pod filters on the carbs?

Cheers
N

[kghost]

I would use the ones that turn my plugs a nice tan color 

Seriously tho...

Try 130's to start and I have heard of people going 140's,

[physician]

a quick search will tell you exaclty, i think it was 130. every set up is different, dont know about clip position, and slow jet

[noelocol]

Thanks guys...
Do you think 132 mains & 46 pilots would be too much?

Cheers
N

[TwoTired]

You know its not just the jets you need to size.  You also have to adjust the slide needles for 1/4 to 3/4 throttle mixture settings.

Cheers,

[noelocol]

Thanks for the advice guys!!.. Greatly appreciated!!

N

[kghost]

Personally I have never seen anyone that had to go to larger pilots....

Stock 40's should work

[mrbreeze]

Ditto on the 40's for slow jets. Main jets will be determined by altitude and temperature also.....where are ya?

[Terry in Australia]

Don't go crazy on your main jets mate, I know that the pods and pipe will lean it out a bit, but 130-140 sounds enormous, I'm running a stock engine with stock airbox and filter and a race pipe with tuned length headers and with 120's it was way too rich, so I went down to 110's, but it was still way too rich.

In desperation I took the airbox off and it ran really well, so as I couldn't leave the airbox off forever, I dropped the mains down to 105's, and now it's just about right, maybe a tad rich, (will start fine most days with no choke and not much warm up) but I'll re-fit the pods that I had on it soon, and it'll be interesting to see if I do need to richen the mixture further, or whether it really needs it.

Try riding your bike and do a plug chop to see if you really need to do anything, as it'll run like shiite if you go plonking big jets in it with no initial testing. Cheers, Terry.

[Jerry Rxman Griffin aka MuthaF'er]

I'm curious whether or not you pod guys with monster jets are getting an equivalent increase in power or is it running the same but looking much cooler?  Let's say your stock jetting was running a nice tan color plug and you got the same nice tan color plug with the pods and monster jets.

[noelocol]

I'm curious whether or not you pod guys with monster jets are getting an equivalent increase in power or is it running the same but looking much cooler?  Let's say your stock jetting was running a nice tan color plug and you got the same nice tan color plug with the pods and monster jets.

Very good question... I"ll let you know as soon as I slap these carbs in!!

Cheers
N

[TomC]

Hi Carb. Experts
     The jet size numbers such as 105 main jet. The 105 is a measurement of the diameter of the hole in the jet?
     If this is correct then going from 105 main jet to a 130 main jet would increase jet area by 153%.
     Or are the main jet numbers a measurement of the area of the hole in the main jet?
          TomC

[TwoTired]

For Keihin jets the number refers to the diameter of the orifice hole. 
#100 would be 1.00 mm ,  #130 would be 1.30mm, #40 would be 0.40 mm.

Be carefull about your area size calculation, as the actual flow is a function of the pressure differential across the orifice boundary.

Cheers,

[750 4ever]

If you run 105's with K&N and 4-4 drag pipes you'll burn your pipes and hurt your engine- --more air in-( K&N's)--and with 4-4  less back-pressure more air out ( stock mufflers create the back pressure) ---the 105's don't give enough gas to help with cooling and preformance-----keep the same ratio air to fuel  --thats the hard part ---I 've got the same combo and I run 130's.but like they say ''all bikes are different and can be effected by --altitude,octane,temp,rider,wheelies--yada yada --GOOD LUCK---

[noelocol]

If you run 105's with K&N and 4-4 drag pipes you'll burn your pipes and hurt your engine- --more air in-( K&N's)--and with 4-4  less back-pressure more air out ( stock mufflers create the back pressure) ---the 105's don't give enough gas to help with cooling and preformance-----keep the same ratio air to fuel  --thats the hard part ---I 've got the same combo and I run 130's.but like they say ''all bikes are different and can be effected by --altitude,octane,temp,rider,wheelies--yada yada --GOOD LUCK---

Ahh. you have the same set up.. Thanks so much for the advice...

The newy rebuilt carbs that I got, (that havent put on yet), were jetted to have 132 mains & 46 pilots. Im wondering if thats too much. Or Should I slap it on then tune it?
What could be the worst that would happen if I just slap those on, sync the carbs and run them?

Thanks
N

[noelocol]

Ditto on the 40's for slow jets. Main jets will be determined by altitude and temperature also.....where are ya?

Oh sorry. I missed your question...
Im located in Toronto Ontario Canada. Usually the weather here during riding season is more on the humid side rather than dry...

Thanks
N

[mrbreeze]

Whats the altitude there? I'm running 138's at sea level with no probs. Higher elevations would need smaller mains...like maybe 125- 130 depending on your elevation. I used to live in Wyoming at 5000 ft. but had stock airbox and exhaust(without baffles).Stock mains (105's) were fine but when I got to California and went Kerker 4to1 (again...no baffle) and K&N pods,it was time to jet up. It took awhile and not only different jet sizes but needle position also.

[TomC]

Hi TwoTired
     

For Keihin jets the number refers to the diameter of the orifice hole. 
#100 would be 1.00 mm ,  #130 would be 1.30mm, #40 would be 0.40 mm.

Be carefull about your area size calculation, as the actual flow is a function of the pressure differential across the orifice boundary.

Cheers,

     OK a 1.05 diameter has an area of 0.866 square mm and a 1.30 has an area of 1.327 square mm
     So 1.30 jet has 1.53 times the area of a 1.05
     If I put a 1.30 in place of a 1.05 and ran the engine under the same load and rpm I would expect to see less that 1.53 times the amount of fuel going through the jet? How much less? This makes the assumption that the engine would run properly with both jets. Not likely!
          TomC

[TwoTired]

I do not believe these are simple equations.  I know just enough about fluid dynamics to understand that flow models are highly complex.

There are boundary layer effects that become more prominent as the orifice size is made smaller.  There may also be non-laminar flow issues in relation to the fluid velocity.

The carb throat pressures are what "pull" the fuel through the main orifice.  The main orifice is the fuel restrictor for 3/4 to WOT throttle settings.  The carb runner and air filtration device between the fuel entry point into the carb throat and where it is fed from the atmospheric source have a direct relation on the throat pressure, as well as nodal effects relating to the air velocity.

If you are going to try and calculate or predict orifice diameter changes and it's effect on engine mixtures, I think I will sit back a watch as I just don't know this stuff well enough to teach.  My head spins enough these days without such mental gymnastics.  Anyway, I certainly wouldn't try it without a computer program of some sort.

Perhaps others on the forum have access to such tools?

Best of luck!

[Jerry Rxman Griffin aka MuthaF'er]

Hi TwoTired
     

For Keihin jets the number refers to the diameter of the orifice hole. 
#100 would be 1.00 mm ,  #130 would be 1.30mm, #40 would be 0.40 mm.

Be carefull about your area size calculation, as the actual flow is a function of the pressure differential across the orifice boundary.

Cheers,

     OK a 1.05 diameter has an area of 0.866 square mm and a 1.30 has an area of 1.327 square mm
     So 1.30 jet has 1.53 times the area of a 1.05
     If I put a 1.30 in place of a 1.05 and ran the engine under the same load and rpm I would expect to see less that 1.53 times the amount of fuel going through the jet? How much less? This makes the assumption that the engine would run properly with both jets. Not likely!
          TomC

Interesting concepts but more irrelevant than relevant. Bump 'em up 1 size at a time and read the plugs or take it to a dyno . This isn't a computer controlled bike that you can hook up your lap top to and "tune it in" with a Power Commander. Wouldn't that be nice.   

[Terry in Australia]

Interesting concepts but more irrelevant than relevant. Bump 'em up 1 size at a time and read the plugs or take it to a dyno . This isn't a computer controlled bike that you can hook up your lap top to and "tune it in" with a Power Commander. Wouldn't that be nice.   

I agree entirely Griffy, being the dumbass that I am, I always look for the simplest solution, so I just ratted thru three sets of carbs and found 3 sets of different sized jets (which in itself poses at least one more question, but I digress) and tried them out until I was reasonably happy.

I'm pretty much restricted to a quick blast around the block and it feels fine, but I've no doubt that over longer distances more adjustment may be required, so when I take my bike to the DMV on Friday to make riding it "legal", I'll have more opportunity to further fine-tune the mixture. Cheers, Terry.

[TwoTired]

I certainly agree that for a one-off carb/jet mod, a run and try, mod and try, is probably your most expedient.  It's even faster if you have Dyno access.

The computer model, calculation approach is the one to take if you are making thousands of examples.  Even then, what you really hope to achieve is a proprietary model or methodology that allows rapid mock up or determination of production quantities of parts.  This will give your company a time-to-market developmental advantage over your competitors.  I think this is the only way to pay back the development costs.

Or, perhaps a service to charge bike customizers?  Good luck with payback on that one, though.

Cheers,

[TomC]

Hi Guys
     My point here is that going from say 1.05 to 1.30 is a big jump. If the difference in flow is between 1.377 to 1.683, thats 10% each way, it is still a big difference.
     Clearly if you change things and your engine now runs best with 1.30 jet that what you run.
     Flow might not be exactly proportional to the area of the passage in the jet. But how big is the real world difference?
          TomC