Keith,
I did remove the the vent hoses after reading some of your other post with regards to the fuel starvation concerns. The bike seems to be running okay now but am still a bit concerned if the needle and seat fail or if i forget to cut the fuel as a manual valve is installed.
The needles on these carbs have 5 positions. I think not all 1978 bikes have this but mine is an early version built in December of 77. It even has a switch to kill the headlight. Anyway, I installed mikuni 120 mains, needle clip on the lowest position (richest), pilot screws out 2 to 2.5 turns, and the air screws out 1-1.5 turns. The needle clip was 4th from the top and probably would be best at 4.5.... Will do some plug chops later this week and see if the clip needs to be reset or shimmed.
I'm sure someone has a bowl for you here. Just be sure to get the right bowl (ex: left to right, carb 1, 2...).
A float needle valve can fail like any other part and not seal perfectly. So with a manual fuel valve (I have one too) you just have to remember and make it part of your routine. I turn it on first, before the key and choke. At shut off I turn it off first. Works for me. Unless you think you may have a real leak problem, you should only have to turn off your valve when parked for the night or when getting to work. Stops for lunch or short stays shouldn't require shutting off but I do anyways out of habit.
The stock jet needles always have 5 grooves as far as I know.
Jetting: If your bike is in good tune, compression, valve clearances, advance timing, good spark, etc, then... a 120 main jet is too lean.
A stage 3 DJ kit which is designed for exactly your exhaust/intake mods supplies 138 and 142 main jets. You want the 138. It works well. No need to test. If it doesn't work then somethings wrong other than the jetting. If you want typically better results then order the DJ kit. It comes with better quality jet needles. If you can't spend the $ then you can try seperate jetting and your stock jet needles but be aware the results are sometimes noticably inferior.
If going with Mikuni mains, them buy 130's which are approx' the same size as 138 DJ's. Each brands sizing is determined by different methods but those two sizes are nearly identical.
If stock jet needles then the bottom (5th) position MAY work but may be too rich. You can test the needles and get accurate plug/performance reads by running the bike at 1/3 to 1/2 throttle position for a mile or so at full operating temperature. Chop off the throttle for best results. Position 4 from the top is generally too lean but you can always try. I may be wrong but if using the stock needles I'd probably try position 4 1/2 first and test. But you need jetting spacers typically found in the DJ kit that are approx' .022" thick to make the 1/2 position changes. Some members say Radio Shack, etc, has thin washers that will work. If you get them, place the jetting spacer directly on top the e-clip in the bottom position. If none of those three positions work well, then you need the DJ needles. Be sure all factory nylon spacers are re-assembled on the jet needles in exact factory order. Thicker spacer on top the e-clip, thinner under the clip. Like I said, the DJ needles are far better so if you can I suggest buying the kit.
As for the pilot circuit, if using seperate Mikuni jetting and stock needles, you'll probably want one step larger pilot jets, 17.5's. You can FIRST try using the stock 15's and richen the pilot fuel screws underneath. These screws ASSIST the pilot jet and are for fine tuning. They are generally 1/2 to 1 full turn out from LIGHTLY seated, straight from the factory. With that range as a starting point, you can try 1/2 additional turn on each and test the pilot circuit at minimal (under 1/5 throttle) throttle positions. You can richen them a little at a time as needed to get the performance you need. If you reach a point where they are about 3 full turns out and you still get lean symptoms then you've gone past their effective range and that means you need the larger pilot jets and return the screws to something close to the factory settings. Slow warm ups on colder mornings and deceleration popping are more likely with the stock pilot jets.
If you use the DJ kit then that kit is designed to use the stock #15 pilot jets and only richer pilot fuel screw adjustments may be needed. Screws are sensitive and it does take time to get them spot on.
As for the side air screws, they generally end up about 11/2 to 2 turns out BUT you always adjust these by using the highest rpm method, not just setting them to some pre-determined position. Set them initially at 1 1/2 turns and then use the method to find each screws sweet spot. The bike will run better if you're careful doing this job.
Remember you must re-synch each time you disturb/change the jet needle positions. A vacuum tool synch should follow the bench synch for best results and to more accurately test the new jetting.
I have determined that the pipe is probably more for overflow than bowl venting so blocking it off would be a temporary fix. This should not affect operation but could expose me to filling the engine with fuel if the needle and seat fail. Anybody have a mikuni bowl (VM)? left side preferably...
