Author Topic: tuning specs  (Read 725 times)

0 Members and 1 Guest are viewing this topic.

Offline Lewis

  • C-Grade
  • **
  • Posts: 144
    • View Profile
tuning specs
« on: May 16, 2018, 10:23:30 AM »
Would anyone have tuning specs for a 34mm Mikuni fitted to an Ossa Phantom engine? Engine is used in a track bike not on the dirt. I'm having trouble tuning it  in the lower revs- it blubbers and is reluctant to pull cleanly till the revs increase, then it clears itself and pulls well.

Offline OverTheHill

  • Legend
  • *****
  • Posts: 767
    • View Profile
Re: tuning specs
« Reply #1 on: May 16, 2018, 02:23:01 PM »
Not sure on carb specs except drop the needle a notch at a time to see if it help [clip up needle down], make sure the pilot's not drowning it from over richness every time the throttle is closed off from speed then when you crack it on again. Cutaway number 3.5 is leaner than 2.5 etc. But--my initial thoughts are--is it over ported in which case lengthening the header might get some of it back down low & help the transition into the hard power up top. Not being up on Ossa's [did have a Mk8 Bully in "75/"76] so is the Ossa reed valve inlet?, if so i had a case with my bucket racer which ok was only suzuki 100 mudbug based motor so nothing exited but i'd done some crazy over porting & couldn't cure this damn blubber right where the power came on hard. Was rich symptoms but leaning it way off didn't cure it & just about stuffed it experimenting. Along came a mate of my kids & listened or rode it [years ago] & said REED FLUTTER--realy i said. Double them up--i had a yamaha reed housing welded into the back of the cylinder. So ok i doubled the yamy steel reeds & fixed it 100% on the spot. That guy went onto work for a race team in the UK. I'm still working on my old crap lol. Kids eh--what would they know!!. Sorry if none of that computes with your problem--just ignore me woffling. Cheers.
Edit fixing missing letters--keyboard--.
« Last Edit: May 16, 2018, 02:27:02 PM by OverTheHill »

Offline Slakewell

  • Legend
  • *****
  • Posts: 3474
  • Slakewell Motordrome
    • View Profile
Re: tuning specs
« Reply #2 on: May 16, 2018, 05:37:18 PM »
Lewis
This a very open ended question a bit like why is wife cranky? Many variables.
1) If it's an old carby it could be just F#ucked
2) Is the pilot circut clean?
3) Are the reed vlaves in GC?
4) Is the ingnition timming spot on?
5) Why did my wife just get upset?   To many unknowns ?
Current bikes. KTM MC 250 77 Husky CR 360 77, Husky 82 420 Auto Bitsa XR 200 project.
Kawasaki A4 125 78 for sale

Offline John Orchard

  • Legend
  • *****
  • Posts: 2649
  • ^^^ Jan 1984
    • View Profile
Re: tuning specs
« Reply #3 on: May 16, 2018, 09:26:49 PM »
Would an Ossa Phantom have reeds?  Lew do you know anyone with a rolling-road we can do some plug-chops on? Or a quiet road? Firstly workout which circuit is running rich; pilot jet or needle, then lean it out.
Johnny O - Tahition_Red factory rider.

Offline sleepy

  • Legend
  • *****
  • Posts: 563
    • View Profile
Re: tuning specs
« Reply #4 on: May 17, 2018, 09:43:13 AM »
Would anyone have tuning specs for a 34mm Mikuni fitted to an Ossa Phantom engine? Engine is used in a track bike not on the dirt. I'm having trouble tuning it  in the lower revs- it blubbers and is reluctant to pull cleanly till the revs increase, then it clears itself and pulls well.

Has it run clean with any other carb? Sounds like a normal piston port 2 stroke, runs like crap till it gets on the pipe.
Recently helped a guy build a 76 CR125 Honda engine for a track bike and he took it to a mate with a dyno and payed him to spend 4 hours trying to get it run clean between 6 and 7000 rpm only to be told that the engine builder(me) had got everything wrong in the porting and compression ratio and the bike would be unrideable.
The little 125 only very gets ridden between 10k and 13K so in your case what are you trying to achieve? If you are talking full throttle then you are on the main jet regardless of RPM. I would guess the main should be around the 340 as a starting point.

Offline OverTheHill

  • Legend
  • *****
  • Posts: 767
    • View Profile
Re: tuning specs
« Reply #5 on: May 17, 2018, 04:56:29 PM »
Consider my essay up a bit just incase something's relevant but if it's genuinely "rich blubbery" [& not over porting big hole in the start of the powerband] as you come on the pipe so need to give it more throttle than should need to get past it so to speak. If rich though you'll just have to jet it down whichever part of the carb it is in relation to where it happens. No use holding wide open from an idle to get past it in revs but ride it increasing slowly until--right there--then decide where 'right there 'is'-- like 1/3 throttle & maybe cutaway to rich or needle clip too low. Or if its an old carb, quite likely the needle jet [emulsion tube] where it meters at the top is worn oval--that plus wear on the needle but definitely the metering bit. If bad enough you can hold the tube up to the light & see the elongation at the metering point. Back a few haircuts when i had hair--i had a YZ125K so "83 or a bit later probably "85 so had lots of use, bought second hand & was a bugger sitting on the start line & had to constantly rev the shit out of it to be clean when the flag dropped [anyone remember flags lol]. So got a new needle & jet [the tube] std size as came out & just transformed it. I know you have 5 clip grooves & all that but if there's significant wear then leaning off the clip will help but not fix it & if say you need to go from richest to leanest clip to get it to improve then that'll bugger it up somewhere else. Could do it to prove a point & if mostly sorts it then go & replace the bits with new items. Actual if ya talking 34mm it's probably what my old K had on std back then so might be same carb. Sorry to harp on, but that's me done. Thanks.--except i'll probably have to edit mistakes--old keyboard.

Offline Lewis

  • C-Grade
  • **
  • Posts: 144
    • View Profile
Re: tuning specs
« Reply #6 on: May 18, 2018, 08:17:03 AM »
Thanks for the input guys. Engine has just had a rebuild so new everything-timing is spot on- no air leaks- porting is standard Phantom and the pipe is a standard Phantom type. It runs cleanly with a 38mm Mikuni.With the 34mm fitted and using a 35 pilot it keeps running with the air screw all the way in. I've gone up to a 40 and now it stalls with the air screw in. 1 1/2 turns out it idles however I've not had a ride on it as yet as I'm reluctant to take it out on the road for a test run. I'll need to wait till the Broadford ride day and keep fiddling with it. I THOUGHT it may have been too rich low in the rev range initially due to the blubbering and reluctance to rev till around half throttle, that's about where it clears itself and takes off.

Offline sleepy

  • Legend
  • *****
  • Posts: 563
    • View Profile
Re: tuning specs
« Reply #7 on: May 18, 2018, 09:43:45 AM »
Try a leaner slide as it sounds like it is getting on to the main circuit to early. Leaner means higher number. Could also try dropping the needle a clip or 2.
Or just put the 38 back on it as it is a track bike not that needs top end.