Topic: Jetting for a Big Bore Kit

[colab]

I have to re-jet my carbs in my 72 cb750. I have a wiseco 836 bigbore kit... a longer travel hotcam and I've ported and polished the heads. Can anyone give me advice on jetting. What size for the main jet and idle jet would you recommend? Recommended products/where to get them? Will i have to reposition the needles?

Any advice would be greatly appreciated.

J

[crazypj]

How much percentage wise is the engine bigger than stock (clue, 736 vs 836)
 thats the starting point (about 12% bigger) 100 main would give 112, plus a bit for safety so 115~117.5.
 You probably don't have stock exhaust or air box and may have head ported to increase flow (or else why bother with big bore kit?)
 you may want to go a bit bigger and come down to where you need to be and start in the 120~125 range.
Although there are thousands of 836's out there, all engines respond differently for various reasons
 there are formulae to get fuel flow in cc/min but I don't know how to translate them to Kei-Hin carbs which use hole size instead of flow.
PJ

[754]

Start off like he said above with jets maybe 2 steps larger.. but that is the START POINT!

 Then start on plug readings and jet accordingly, do not try to choose your jets by internet..

[pampadori]

my 810cc uses 130 mains and 40 idles. I'm going to dyno soon with a wideband.  i don't trust plug reading as much as the wideband.  i think its something to consider doing, take a few different size mains with you to the dyno and see what gets you around 13:1 afr.

[754]

I am betting you will show more HP with 125 mains..let us know..

[eldar]

There is a huge range of jets being used. Some here have gone to 140s even. Pretty large but sometimes, that what it takes.  Just remember that idle mix is every bit as important as the mains. You also need to tune for the midrange. Many people try to get the ratio between 12 - 13.1 Not spot on but just a tad rich. Of course you should also try different plugs. Many 836 engines run the D9 plug instead of the D8 as they generate more heat. Also from what others here have said, you will probably need higher grade gas, not just regular AND you may need to retard your timing a little.

[Jerry Rxman Griffin aka MuthaF'er]

Some things you didn't mention: your elevation ; your current jetting ; your exhaust system ; your air cleaner system

While pampadori is running 130 mains in his 810, I have 115 mains (stock is 105) in my 810 with a stock airbox and K & N filter and a Kerker with street baffle. I do not have head work but I am running a Yoshimura Daytona cam with strong springs and a stock airbox with K & N filter. It is slightly rich, HOWEVER I am at 7000 feet. This set up allows me to run 10K rpm with power so the slight richness is good. You might not be running 10K so you might not need to jet it for 10K either.

I've kinda used a 1 size increase for the displacement increase, 1 for the cam and springs with higher rpms, 1 for the exhaust and K & N filter MINUS 1 for the elevation. If I had head work I'd probably try 1 more increase. If you're at lower elevation I'd go along with 754 and try 125 mains but if you're running pods you may have to go bigger after the 125s. I'd work out the mains first then needles, well let's just say I hate needles! Why not start in the middle clip position, see what throttle response you have then do plug readings and adjust accordingly.

[pampadori]

I am betting you will show more HP with 125 mains..let us know..

well i couldn't wait for the dyno.  seat of the pants says 130 felt much better up top. 

the 125 jets run very very good though.  just lacking a little of the umph the 130 jets had. 

[Gittfiddler]

Where are you guys getting your jets?

[Jerry Rxman Griffin aka MuthaF'er]

The Honda dealer.

[crazypj]

Are you running stock pilots and needles with the 130 mains?
 If your in USA most dealer also have K&L catalogue, they list various size jets (pilots and main jets)
Hey people, fill in a general location on your profile, this site caters for all SOHC people, not just USA
PJ

[pampadori]

yes, stock needles and pilots.  needles i think are in middle setting. (from memory).

[SohcCBs]

I just dropped the jet "needle" down one position the the second from the top...as seen in my avitar.

[low-side]

     Like everyone has been saying, it varies widely.  My Dad has a '76 750F (K style carbs) with an 810 kit, pod filters, 4 into 1 exhaust and an Andrews "A" grind cam.  He runs his pilot screws at 3/4 turn, needle clip 1 from the bottom (1 richer than stock) with a 135 main jet.  Runs like a bat out of hell. 
     My bike has an 836 kit, pod filters, Kerker 4 into 1 exhaust with a 4" Supertrapp spark arrestor, and a Webcam 63a cam.  My pilot screws are set an 1 1/4 turns out, needle clip 1 from the bottom, and a 130 main jet.  It will probably end up with a 125 main jet because right now it will run straight through the bottom 3 gears, but it won't accept any more than 5/8 throttle after 5000rpm in the top two gears.  The leaner I make the main the more throttle it will accept on the top end. 
     Anyway, good luck with it.   - Troy

[SohcCBs]

low-side has a very good point...and I failed to mention that I had gone with pods bigger jets, lower clip on the needles.  It all depends on how you want to ride.  Mine had been set up to run very well at 120+, and I ran it that way on a weekly basis.

After a little while (getting older), I decided that was going to get too costly.  So I detuned the bike for the bottom end.  I lost the 6000rpm explosive character of the bike in favor of passin' a "good" Harley off the line b4 shifting into second.  That's really lots more fun than runnin' 120+ anyway.

I'm also running #38 pilot jets...
....so you can set the bike up many different ways.

[mrbreeze]

I just dropped the jet "needle" down one position the the second from the top...as seen in my avitar.

Thats where I ended up also....stock 750,open Kerker 4to1,K&N pods,Dyna 2000,89 ft. elevation,138.5 mains. Last year I moved up into the foothills(1750 ft.) and it ran OK but last month, I went over the mountains and decided to take the mains down to 130. It ran noticibly better here at home and got me over the mountains with no probs (topped out at 9600 ft. that day..Sonora pass).