I'm refining the carb jetting on my Wbat 94 for road use. I want to be able to ride moderate distances instead of trailering the bike. I have increased the main jet to 170 and get a slight "burble" (four-stroking) at WOT-- stock main is 160.
The issue, I think, is the midrange, 1/2 to 3/4 throttle at highway speeds 45-60 mph in 5th gear. I don't feel any burble and a plug cut looks a tad lean (photo below). And at 1175*F the EGT is a couple hundred deg hotter than "typical".
Tomorrow I'll test more, and see what raising the needle a notch or two does. Needle is a 4E1 in 3rd notch and needle jet is an N-8.
I need to decide at which point I need to go to a richer needle or to a larger n.jet instead of raising the needle a couple of notches.
Suggestions?
Last edited by Bill2001 on Sun May 07, 2017 2:00 am, edited 2 times in total.
The plug looks fine to me. Doing plug readings is a bit of an art form. Once the plug has run in the engine as long as yours has, it isn't easy to interpret just where the coloration has come from. You are probably looking at the porcelain and thinking it is lean. I am looking at the outer ring and thinking it is just fine.
As for mid range tuning, you say it runs clean. I don't know what would be wrong with that scenario. If you want to try one notch rich on the needle, try it and see, but if it is running cleanly now, I doubt you will need to change to a different needle or jet. Once you deviate from stock jets, you are literally on your own.
Bill, I too would prefer to see a bit more "chocolate color" on the insulator - especially for road use with extended fairly steady-state operation at fairly high engine speed.
Ed
It was the Jet Needle. It had a 4F15 needle in #3 which I initially raised 2 notches to #5. Too rich!! Burbled badly between 1/4 and 3/4 throttle. Ended up using that "F" needle in notch #4. The EGT on a highway-speed run went from 1175*F to below 1000*F, so it is good to go. Plug is a nice chocolate brn.
I have a 4E1 needle that I will check the mix with. I doubt that one needle step is the same as one notch, so I can fine-tune it.
There is still a slight burble at 4/4 throttle, I went from a stock #160 Main to a #170 Main.
I've got a Mikuni Tuning Manual on the way. It should have a lot of details and specs.
I'd like to order some other jet needles to test. Stock is an 4E1 and the other needle I have is a 4F15. I understand the "4E-" and "4F-". I presume that the last number is a revision#, "1" being the basic straight taper and "15" being much later modifications of the basic taper to give fine mixture corrections. I presume that the closer that last number is to One, the less modified it is.
Right?
The needle and needle jet number progressions for Mikuni have nothing to do with Hodaka engines or the carbs used on those engines. Search the web and find a needle and jet chart for Mikuni carbs. The numbers group items by general size, "4" needles being shorter than "5" needles for example. The letters and numbers on the needles that follow the main number relate to, but are not mathematically linked to, the number of tapers, the length of the tapers, and the angle that results. The only way I have found to identify needles and needle jets for the purpose you describe (experimental jetting) is to read the chart.
