LANCER
Serious Thumper Alliance Member
Offline
Savage Beast Performance Parts
Posts: 10801
Oklahoma
Gender: 
|
spacepirates wrote on 02/01/12 at 06:59:26:backed the air screw out two and a half turns, seems to idle better now (definitely higher rpm and smoother) but pops on decel and surges a bit at low throttle.
beginning to think an overly rich air screw was covering up a lean pilot. choke use on the BS40 usually was a full minute, now it is down to 15 seconds or so after which the bike bogs down.
midrange still seems ok, no surging or hesitation.
top end/WOT still good. doesn't bog down or hesitate, no real noticeable speed increase when backing off from WOT.
Before turning the air screw out i had the bike randomly die on me a few times while sitting idle at a stop light. no warning, no bogging down, just idling away and then out like a light. one time was because i was out of gas. sure wasn't used to it dying so fast, the old BS40 sputtered for a good 10-15 seconds before it finally kicked the bucket. this roundslide just chugs along happy as can be until it up and decides it won't have no more and quits. kind of nerve wracking, but once i go through a few tanks and get the miles-per-tank sorted out it won't be so bad.
i'm hoping that upping the pilot jet will help, because I really don't want to adjust the jet needle or needle jet, as that involves taking out the slide, removing the throttle cable and spring, and then putting all those things back together. kind of a pain in the rear.
I've only pulled the plug once, after six miles or so of city riding (so sitting idle, then up to 1/2 throttle, then idle again) and it was a light brown, which is encouraging compared to white or solid black.
When you say low throttle, how much have you turned it ?
Keep in mind that when trying to determine what size pilot jet is needed, you are working with idle to 1/4 throttle.
With the engine off, mark the idle position with a pencil/marker/whatever on the side of the switch housing and the edge of the throttle grip. Then turn the throttle all the way to full throttle and mark that spot on the grip using the mark on the housing as a reference point. Then mark half way between the two marks for half throttle, and do the same for 1/4 and 3/4 throttle.
Now you know with some degree of certainty when you are at or below 1/4 throttle and can use this to see how the pilot system is working.
Also keep in mind that on the Mikuni VM carb, turning the pilot screw OUT is "Leaning the mix" & turning it in is "richening the mix".
If the low range performance improved when turning out 2.5 turns, then you are almost to the point of going down one size on the pilot jet...not up in size.
As always, whether on the VM carb or the stock carb, 3 turns out on the pilot screw is the max safe point. Once at that point it is time to change the pilot jet and start over adjusting the pilot screw.
On the VM carb, if the rpm continues to increase as you turn the screw out and you get to or near 3 turns then its time for a smaller pilot jet. You are leaning out the fuel/air mix in the low throttle range and if the engine likes it then give it more of what it wants...less fuel/unit of air.
It will run more efficiently; more power; smoother running; better mpg.
If it's happy, you will be happy. 
Review the symptoms for RICH & LEAN conditions in the 3 general throttle ranges and always recheck for exhaust air leaks which can really spoil your efforts to tune the carb...especially where backfiring is concerned. And check the plug after every jet change & and adjustment you make. If checking the low range then never exceed 1/4 throttle when doing a test run. For a midrange check stay within 1/4 & 3/4.....high range, stay above 3/4 but preferably at wide open until you hit redline and then cut it off, stop and when possible pull the plug and check it.
It can be a real pain and time consuming but its either that or pay for a dyno that can put it under load for a full range test.
Have fun dude !
The info on symptoms, tuning & more should be in the tech section.
If you have difficulty finding them you can go to www.mikuni.com and go to the bike section where they have excellent info and "how to".
|
Pages: 
