10-12-2012, 12:42 PM
Joined: Jul 2010
Location: Canberra Sunny OZ
Mikuni OBI WAN'S
I’m looking for some help/input on tuning some Mikuni VM36 carbs I’ve fitted to my 82’ R100
Some background-my bike is stock apart from Dual plugged heads 9.5:1 comp pistons and epco sports mufflers.
I decided after a lot of research and procrastination to go with the Mikuni’s..they are relatively well priced, airhead simple and a common conversion. I went with the VM36’s because like the 3 little bears story I thought the 34’s might be a bit small(plenty of low/midrange not enough top end) 38’s might be a bit big (less low/midrange but better top end) and the 36’s might be just right?
I searched the web and the only supplier I found that made a kit for VM36’s was Allens Performance in the UK after trying several times to communicate ( exactly what does the kit include etc?) with them via email I gave up and searched locally in Australia for a Mikuni specialist ( http://www.mikunioz.com/index.html ) who could supply me a set of carbs with basic jetting to suit the R100 the rest of the bits I got from Rockypoint Cycles(cables) and Allens Performance ( adaptor manifold) some S/H carb to airbox elbows to suit 32mm Bings, a silicon rubber reducer (63-50mm) for a turbo from a local auto parts chain here in OZ (Supercheap) and stainless hose clamps to suit (choke cable conversion kits on order).
Pictures here http://www.flickr.com/photos/29497288@N00/
Carb specs are:
Main Jet- 230 (supplied with one size jet up 240 and down 220 for tuning purposes)
Pilot Jet- 35
Needle Jet- Q5
Jet Needle- 6FJ6
Throttle Valve- 2.5
Main Air Jet- 2.0
Needle Valve- 3.3
I fitted everything up and the bike started straight away with some choke…good start.
I have read the tuning guides and set the pilot air mixture to 1 1/2 turns out and went for a ride to warm things up, first thing I noticed was a big stumble/flat spot when trying to take off, once over this it took off with good acceleration….no worries it just needs some adjustment?
I balanced cables etc. adjusted pilot air with the idle as low as I could manage…still has the same stumble/flat spot….mmmm so I lowered the needle 1 notch ( it was in the middle to start with)….bad move worse that before, raised it 1 notch…big improvement but it still has a stumble/flat spot when you blip the throttle from idle, contact mikunioz and he tells me that when adjusting the pilot air screw you should be able to get it to die when turned in and die when turned out I could only get it die when turned in ½ a turn but not die when turned out so we decide it needs a smaller pilot jet 30 instead of 35. I fitted the new pilot jets and went through the adjustment procedure…same problem, I even found a couple of size 25 pilot jets I had in some smaller Vm mikuni’s I had from some old 2 stroke dirt bikes and fitted those…same problem.
I have reread the tuning guides and none of them say, when adjusting the pilot air, it should die when adjusting in both directions?
So that’s where I’m at, at the moment, my next move is to refit the 30 pilot jets and lift the needle another notch and see what happens? I’m also wondering if the throttle valve is the correct cut away, maybe a 2 (richer) would be better or perhaps it’s the needle jet/jet needle combo?
Looking at the Carb specs it seems to me all I got is a standard VM36 carb with smaller main jet!
So Mikuni Carb Guru’s out there please take pity on a bloke from down under who at the end of the day is looking for a more responsive carb/bike and some help to get it sorted before the MotoGP at Phillip Island.
Thanks in advance OBI WAN’S