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Old 10-12-2012, 12:58 PM   #1
ejute OP
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Location: Canberra Sunny OZ
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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:
VM36-4 -
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 ˝ 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

Mark
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Old 10-12-2012, 02:14 PM   #2
983Bob
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Hi Obi. I have vm 36 s on my 76 900 which is stock except for the carbs. The main jet works out to 210,[I wouldn't go higher than 220] 2.5 slide, Q-0 needle jet stock needle in the middle position and 22.5 idle jets with the mixture screws at about 1.25 out. This motor is very similar to yours and starts and idles as well as the Bings with gas mileage being about 2-3 % less. My Rs 77 which is done to the hilt with Tm38s much head work and Staintune 40mm mufflers uses 220 mains 6fj40 needles and 17.5 idle jets. It seems that unless you have a full race motor with lots of cam and a racing exhaust, you don't want to go too rich. I took the 36s off my racer and for the street, dropped several sizes on mains needle jets and idle jets to the specs above. It sounds like you are fairly close with what you have, so if the problem can't be solved with jetting, check to make sure that your air tube to carb adaptors aren't causing a shrouding problem somewhere. You can check this by going for a short ride without the tubes or air filter hooked up and see if that improves things. Also bear in mind that slide carbs won't respond to large throttle openings at low revs as well as the bings. When I nail it, I like at least 4-4500 on the clock.
Hope this helps
Bob
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Old 10-14-2012, 01:59 AM   #3
ejute OP
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Quote:
Originally Posted by 983Bob View Post
Hi Obi. I have vm 36 s on my 76 900 which is stock except for the carbs. The main jet works out to 210,[I wouldn't go higher than 220] 2.5 slide, Q-0 needle jet stock needle in the middle position and 22.5 idle jets with the mixture screws at about 1.25 out. This motor is very similar to yours and starts and idles as well as the Bings with gas mileage being about 2-3 % less. My Rs 77 which is done to the hilt with Tm38s much head work and Staintune 40mm mufflers uses 220 mains 6fj40 needles and 17.5 idle jets. It seems that unless you have a full race motor with lots of cam and a racing exhaust, you don't want to go too rich. I took the 36s off my racer and for the street, dropped several sizes on mains needle jets and idle jets to the specs above. It sounds like you are fairly close with what you have, so if the problem can't be solved with jetting, check to make sure that your air tube to carb adaptors aren't causing a shrouding problem somewhere. You can check this by going for a short ride without the tubes or air filter hooked up and see if that improves things. Also bear in mind that slide carbs won't respond to large throttle openings at low revs as well as the bings. When I nail it, I like at least 4-4500 on the clock.
Hope this helps
Bob
Bob thanks for the reply,
I think your R90 would have smaller valves than my 82 R100 which has the “large valve heads” so I’m to believe, the spigots on my heads are approx.52mm. This may? account for some differences between the jetting.
I did run the carbs without any air filtration fitted and it made no perceptible difference.
Cheers
Mark
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Old 10-13-2012, 12:07 AM   #4
Kai Ju
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VM38s on R100

I'm running VM38s on my '77 R100/7 which is stock down to the stock clamshell airbox and elbows.
Your Q5 sounds like it's way too rich.
Here is what I'm running:

Description Spec
Main Jet 210
Pilot Jet 25
Needle Jet 166 P-8
Jet Needle 6DH3
Slide Cut Away 2
Main Air Jet 0.5
Needle Valve Ass'y 3.3

The engine pulls really strong throughout the revrange, It's a little cold blooded and I'm thinking going up a bit on the pilots to take care of that.
Mixed surface streets and freeway I get 40 mpg which is up from the 35 mpg I got with the Bings.
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Old 10-14-2012, 12:34 AM   #5
ejute OP
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Quote:
Originally Posted by Kai Ju View Post
I'm running VM38s on my '77 R100/7 which is stock down to the stock clamshell airbox and elbows.
Your Q5 sounds like it's way too rich.
Here is what I'm running:

Description Spec
Main Jet 210
Pilot Jet 25
Needle Jet 166 P-8
Jet Needle 6DH3
Slide Cut Away 2
Main Air Jet 0.5
Needle Valve Ass'y 3.3

The engine pulls really strong throughout the revrange, It's a little cold blooded and I'm thinking going up a bit on the pilots to take care of that.
Mixed surface streets and freeway I get 40 mpg which is up from the 35 mpg I got with the Bings.
WELL...running too rich? that’s what I thought...I just got back from a 430km or 267M run at highway speed approx. 120km/h or 70 -75 mph. I calculated my fuel consumption to be about 47M to the us mpg running on Aussie 95 octane fuel.
I think I was getting around 36-37 us mpg on my Bings that are due for some TLC.
Plug examination (I should have put in new plugs before left..but didn't??) They look light coloured I think, definitely not dark or sooty.
I did notice when cruising at the above speed that when rolling on and off the throttle to maintain speed, it felt a bit hesitant kinda missing but when I opened the throttle at this speed it would pull strongly.
When I do a cold start, it is hard to warm up, it dies and stumbles when pulling away or accelerating after turning a corner etc. When warmed up it runs… not too bad.
As I mentioned above I have refitted the 30 pilot jet and have the air mixture screw at 1 ˝ turns out and the needle set at the second notch up from the bottom (bottom being the pointy end)230 main jet

SOOO….Wot do ya reckon?
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Old 10-13-2012, 12:29 AM   #6
Rob Farmer
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Since the Mikunis were just a copy of Amals the Amal tuning guide may put you in the right direction - Amal tuning guide here http://www.amalcarb.co.uk/downloads.aspx

Ejute. If you need anything from Allens Performance just let me know I'm not far from them and frequently pass by.
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Old 10-13-2012, 07:12 AM   #7
Airhead Wrangler
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Quote:
Originally Posted by Rob Farmer View Post
Since the Mikunis were just a copy of Amals the Amal tuning guide may put you in the right direction -
...or maybe the Mikuni tuning manual for that matter:

http://www.mikuni.com/pdf/vmmanual.pdf
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