Mikuni Carburetor Information Links

[J6G1Z]

Mikuni VM Carburetor Parts, by Part Number & Application
http://victorylibrary.com/tech/mikuni-3.htm

Mikuni VM Carburetor Diagrams & Charts
http://victorylibrary.com/tech/mikuni-2.htm

Mikuni VM Jetting Examples for British Motorcycles
http://victorylibrary.com/brit/Mik-jet.htm

Jetting Specs For Yamaha TT500
http://www.mikunioz.com/jetting_specs_and_applictions.htm

VM Round Slide Mikuni Carburettor Standard Specifications
http://www.mikunioz.com/jetting_specs.htm

Carb Jetting For Amal, Mikuni, Dell'orto, Weber, etc.
http://www.postdiluvian.org/~mason/moto/jetting.html
http://www.braigasen.com/Mikuni_jetting_chart_four_stroke.htm

Mikuni TM Flat-Slide Carb Info
http://www.mikunioz.com/tm_flatslide_jet_specs.htm

[J6G1Z]

MIKUNI FLAT SIDE RACING CARBURETOR
36mm Mikuni Model TM36-68
 
EXCELLENT PERFORMANCE UPGRADE OR REPLACEMENT FOR THE YAMAHA TT XT AND SR500 MOTORCYCLES

Mikuni’s TM Series flat side carburetors provide significant performance improvements over older round slide carburetors.

Air flows faster, smoother through the TM Series venturi and jet blocks due to the flat slide design which help create a smooth bore effect.

The high velocity air flow means a stronger vacuum at the needle jet for more precise metering and will produce better throttle responses due to the advanced operation of the TM series carb the jetting information may seem "off" if compared to the old style VM series or stock carburation figures the following information is the jetting specs for a 500cc Yamaha style motor.

Bore size:  36mm

Main Jet Size:  130

Pilot Jet Size:  12.5

Needle Jet / Series: 784-13002-P-8/568

Jet Needle:  9DZH6-50

Throttle Valve:  N/A

Air Jet:  1.0

Above information from an eBay Ad

[J6G1Z]

The following are some jetting specs provided by Mark at Thumper Stuff.

http://www.thumperstuff.com/FT500.html
360-805-0272 
sales@thumperstuff.com
17461 147th St SE
Suite 11A
Monroe, WA 98272


These specs are from a 38mm Mikuni round-slide carb removed from an FT500.
3.0 Slide
240 Main
32.5 Pilot
6F8 Needle
P5 Needle Jet
0.5 Air Jet

These specs are from a 38mm Mikuni flat-slide carb removed from an SR500.
4.0 Slide
190 Main
55 Pilot (He felt that this spec may be incorrect & that a 25-30 would probably be more appropriate)
389P2 Needle Jet
6FJ40 Needle

Please remember Thumper Stuff when it comes time to buy parts for your Ascot. He has a lot more available than what is shown on his web-site.

[J6G1Z]

Another source of carbs, parts & information is SUDCO. http://www.sudco.com

They will not give out any jetting specs, but they will assist you in selecting the correct components to purchase from them.

[J6G1Z]

Jetting specs for the Mikuni VM34 carb on an FT500

Pilot Jet - #25. You'll get a nice reliable idle that sometimes will yield 900 RPM. A #20 is too lean and the idle is not reliable. I have a #30 but haven't tried it - no need.

Needle Jet - P8. Possibly a P7 if you are running a stock exhaust but I think you will want the P8 no matter what exhaust you are running.

Jet needle - 6DH4 (already installed in the new carb)

Main Jet - #220. I'm running a free flowing homemade air filter and stock airbox which I'm sure passes as much air as a K&N. I have effectively no muffler (steel wool packed "glass pack"). You may want to play here a-bit. A #250 is way too rich, a #240 is too. I have a #230 and #210 which I haven't tried and doubt I will.

You'll want to hit the auto parts store for a piece of radiator hose to make the union between the Mikuni to the Keihin intake manifold. Buy a Dayco part number B71226, S-452. There are other numbers on the hose also - 452, 14-2861-6, S60940. You'll need to hacksaw off about 5/8" to 1/2" off the hose end to act as a filler between the Mikuni outlet and the rubber manifold. Measure the depth from the outer edge of the manifold to the raised "stop" in the throat of the manifold. A stainless hose clamp works fine to attach the carb.

VERY important!!! Make sure you have a 90 degree exit out of your carb for the throttle cable. A straight pull fitting out of the carb and you've got a nightmare in terms of cable clearance under the fuel tank.

You'll find that the fuel line connection is a-bit awkward. The fuel inlet fitting on the Mikuni is straight up. I'm going to install an in-line 90 degree elbow into the carb. Until you do you'll just have sort of a goofy looking fuel line arrangement especially if you install an in-line fuel filter (recommended).

No more yank yank yank on the twist grip to activate the accelerator pump to prime the carb on starting. No more grinding on that poor, over taxed starter. Just flip on the petcock, drop the enrichening lever, hit the starter and in two revs of the crank she'll fire - every time.

You'll notice an immediate and significant improvement in mid range torque. Install a Dennis Kirk, $15.00, 17 tooth counter shaft sprocket and you won't be toeing for sixth gear all the time and your engine will make happy noises as you go down the road. With the Kehin carb and a 15 tooth counter shaft sprocket I could pull 82 MPH in fourth and 78 MPH in fifth. With my new setup I can pull 92 - 93 MPH in fifth.

I bought my carb and jets from SunLPartsOnline.com. I gave them some very complex jet orders and they didn't muck the orders up once. They have very nice prices and you can't stump them on a jet size.

If you go the Motion Pro throttle cable route they will want you to send them your stock twist grip so they can make a custom cable. Make sure they know you are installing a Mikuni and that you want a 90 degree exit out of the carb. Lube your new cable BEFORE you install it, it does not come lubricated. I assume you've lubed cables before. If not let me know and I'll share a cool tip with you.

Courtesy of Bill from the Yahoo Ascot Owners Group.

[J6G1Z]

Carb specs:
36 mm VM Mikuni
#25 pilot jet
#2.5 slide
#210 main jet
#P5 needle jet
#6DH3 needle--dropped down as far as it will go.

Specs provided by OldGuzziGuy on the Yahoo Ascot Owners Group. These specs seem to impact fuel economy.

[J6G1Z]

....I can wholeheartedly recommend a Mikuni 38mm roundslide carb. I had one of these on my XR500 I vintage raced, and besides giving a little bit of boost in power, it is a very easy carb to live with. Starts well, idles smooth, takes sudden/quick throttle openings well (you do have to sorta roll the throttle on smoothly & quickly), & is overall easy to tune. I have some jetting specs, if anyone is interested......

Carb: Mikuni 38mm VM Round Slide

Slide Cutaway: 2.5

Needle: 6DP1, clip in the middle notch

Pilot Jet: #35

Needle Jet: Q-0

Main Jet: #220

Air Jet: #2.0


Hope this helps some!!!

Added in Edit: This was the first VM Roundslide carb I've ever had to change that "Air Jet" mentioned on the last line of the jetting specs. It's the little brass one that takes a smallish regular/slotted blade screwdriver to remove from the "belled-out", or air intake, end of the carb. Sits at approximately 6 'oclock or so in that part of the carb, and mostly controls idle/low throttle openings, if I recall......

Found on the Thumper Forum

[J6G1Z]

Copied from the Yahoo FT Ascot owners Group.

Just wanted to say THANKS to everyone on the list that had a suggestion for my mileage issues after changing from the stock 35 mm Kehin carb to a 36 mm Mikuni carb on my 83 FT500.

I have to report that after some more research and picking a few more brains that I have my mileage back to what I consider an acceptable range.  Had the bike out this afternoon and when I filled up after the ride my mileage was back up to 52 mpg!  Not quite the 60 mpg I had before, but I'm able to live with the 52 mpg and the extra performance and tractability from the Mikuni.

Final set up is a:
#25 pilot jet,
a P4 needle jet,
a 6DH3 needle set in the middle notch (could drop it a notch, but doubt if I will),
and a #200 main jet. 

The bike starts and idles well, runs strong, accelerates well, no surging at steady speeds, plug is a nice light tan color, and it hits the 6800 rpm rev limiter on the Ricks CDI box that I installed.

In hindsight I think that the mileage issues arose mostly from my use of the stock FT500 air box.  The carb was probably jetted for use with a clamp on pod filter, and was just jetted way to rich for use with the stock air box.

I'm a happy camper again!!!!!!!!!!!!!!!!!!!!!!!!!

Neil

[scottly]

I checked out a 36mm Mikuni that came off my buddy's '82 XR500. Before fitting the new carb, he had already installed a higher compression piston, a White Bros exhaust with a Supertrap, and a White Bros cam, and was a bit disappointed with the power increase. After fitting the new carb, the bike came alive!!! All the new parts seemed to be working in harmony. I believe the jetting is the same as supplied by White Bros.

#35 pilot
#220 main
6DH3 needle; clip in highest groove
1.5 slide 

[J6G1Z]

...and was a bit disappointed with the power increase. After fitting the new carb, the bike came alive!!! All the new parts seemed to be working in harmony....

Excellent post! When modifying an engine for more power, the whole package needs to be addressed either in one shot, or in steps as budget allows. All the performance parts should "complement" each other & work together as a system to flow more fuel & air through the engine.

So the loss of the accelerator pump when swapping to a Mikuni carb does not seem to be much of a loss at all I take it?


Thanks
J.

[scottly]

The '82 XR came with a 34mm Keihin round-slide; I don't believe it had an accelerator pump. When I say "came alive", I mean it turned into a monster! We always wondered how much came from the cam, and how much came from the carb. I've got both the cam and the carb, and I'm debating on which to try first. My gut feeling is that the carb with the stock cam will perform better than the cam with the stock carb. Which test would you guys like to see first: a cam swap or a carb swap?

[J6G1Z]

Well, I'm interested in doing a needle bearing performance camshaft upgrade myself someday.

Swapping on the carb, with the stock cam, will be the easiest to do & likely what most owners would probably do first.

Thanks
J.

[AndreLindholm]

It seems my stock carb is about to give in, I had a local Carb-shop look at it and they said that the vacuum parts look real sad.(which hopefully explains why its so hard to start) So I have started looking in to upgrading to a Mikuni round slide(Mikuni VM 36-4). I run my FT stock at the moment. Do you guys think that 34mm or 36mm is the best choice? Will probably install a pod filter with the new carb. And is it a bad thing to go for a carb without accelerator pump?

[J6G1Z]

There are a few Mikuni carbed bikes over on the Yahoo Owners Group.

I recall hearing that a 34mm Mikuni was good for a stock bike. If you have plans for a camshaft swap or other modifications, a 36-38mm would be better.

Whatever carb you end up using. You will need a 90 degree throttle cable outlet at the top of the carb to clear the frame.


Good luck
J.

[AndreLindholm]

Do I need buy a carb with the 90 degree elbow or is possible to buy the elbow separate?
I also found a used 37mm flatside mikuni for sale in my town, Anyone have any experience with them?
Edit: I have been searching for info on the 37mm mikuni, but I can't find any. The seller probably got it wrong in the ad.

[J6G1Z]

The elbow can be purchased separately.

Do you have a set of calipers that are indicated in inches & millimeters? Just measure the inside diameter of the carbs outlet towards the engine. Usually Mikuni carbs size is indicated in even numbers, instead of odd. Like 32mm, 34mm, 36mm, etc. Your seller probably has a 38mm Flat Slide. That seems to be about the most common size of Flat Slide out there. I have one myself that I'd like to use on my hybrid Ascot, but I think it's too big for a stock bike. On a street machine it's usually better to error on the small side vs. having a carb that is too large. A carb that is too big will give you lazy throttle response, or a bog. A smaller carb will have a higher air velocity through it & give you better throttle response. The Flat Slide carbs require a different jetting than the same size round slide carb.

Another carb option, is to find a carb off one of the modern 4-stroke single 450cc off-road bikes. A lot of the Honda XR owners are going this route. The modern carbs on these bikes have accelerator pumps & are very expensive carbs if bought new. They pop up on eBay fairly regular for a better price. You would be a pioneer & more or less be on your own trying to figure out this carb swap, as I've only heard of one guy using one on an Ascot so far. His engine is highly modified for Bonneville & has had a spigot mount welded to the head.

Be careful buying used. Sometimes it costs almost as much to swap out the internals of a used carb, as buying a new carb.

Have you checked out SUDCO yet? http://www.sudco.com/Carburetor01.html   They have a lot of information.

Good luck & please let us know what you end up doing & how it works for you.
J.

[J6G1Z]

Here is a picture of how one Ascot owner notched the frame to clear the straight throttle cable.

I would prefer to have a custom throttle cable made that has the 90 degree fitting on the end, so you don't have to notch the frame.

J.

[AndreLindholm]

I ended up not buying it, so it's probably for the best. But thanks for the advise! Got some help tinkering with my stock carb today it seems to work better now so will try to keep the original and see if will work for awhile longer. 

[J6G1Z]

The original carb is a great carb when it is freshened up & jetted correctly.

Here is a master rebuild kit: http://randakks.com/collections/honda-ft500-ascot-single/products/ft500-ascot-randakk-master-carb-overhaul-kit

Then change the low speed & main jet to a #58 & a #145. This seems to work for most FT Ascots.

J.

[Lefty]

Good stuff in here, I just read through it and I'm thinking about replacing my stock carb with either a 36 or 38mm Mikuni unit- My top end also is starting to leak a little oil and I'm going to re-seal it, and was thinking while I'm in there why not upgrade the cam also. This will happen a little way down the road because of money issues, but I'm definitely going to do it- the mods I've done so far(jets, air filter mod, & full exhaust) really didn't increase the power like I expected they would, so I am taking things a bit farther.
 
 
 

[J6G1Z]

I think one of those needle bearing camshafts would be real trick! Increase the compression a bit & a new carb. Should wake that engine right up. Those are my plans someday.


J.

[patrino]

Just finished the 34mm VM Mikuni conversion detailed below.  Details soon!

Jetting specs for the Mikuni VM34 carb on an FT500

Pilot Jet - #25. You'll get a nice reliable idle that sometimes will yield 900 RPM. A #20 is too lean and the idle is not reliable. I have a #30 but haven't tried it - no need.

Needle Jet - P8. Possibly a P7 if you are running a stock exhaust but I think you will want the P8 no matter what exhaust you are running.

Jet needle - 6DH4 (already installed in the new carb)

Main Jet - #220. I'm running a free flowing homemade air filter and stock airbox which I'm sure passes as much air as a K&N. I have effectively no muffler (steel wool packed "glass pack"). You may want to play here a-bit. A #250 is way too rich, a #240 is too. I have a #230 and #210 which I haven't tried and doubt I will.

You'll want to hit the auto parts store for a piece of radiator hose to make the union between the Mikuni to the Keihin intake manifold. Buy a Dayco part number B71226, S-452. There are other numbers on the hose also - 452, 14-2861-6, S60940. You'll need to hacksaw off about 5/8" to 1/2" off the hose end to act as a filler between the Mikuni outlet and the rubber manifold. Measure the depth from the outer edge of the manifold to the raised "stop" in the throat of the manifold. A stainless hose clamp works fine to attach the carb.

VERY important!!! Make sure you have a 90 degree exit out of your carb for the throttle cable. A straight pull fitting out of the carb and you've got a nightmare in terms of cable clearance under the fuel tank.

You'll find that the fuel line connection is a-bit awkward. The fuel inlet fitting on the Mikuni is straight up. I'm going to install an in-line 90 degree elbow into the carb. Until you do you'll just have sort of a goofy looking fuel line arrangement especially if you install an in-line fuel filter (recommended).

No more yank yank yank on the twist grip to activate the accelerator pump to prime the carb on starting. No more grinding on that poor, over taxed starter. Just flip on the petcock, drop the enrichening lever, hit the starter and in two revs of the crank she'll fire - every time.

You'll notice an immediate and significant improvement in mid range torque. Install a Dennis Kirk, $15.00, 17 tooth counter shaft sprocket and you won't be toeing for sixth gear all the time and your engine will make happy noises as you go down the road. With the Kehin carb and a 15 tooth counter shaft sprocket I could pull 82 MPH in fourth and 78 MPH in fifth. With my new setup I can pull 92 - 93 MPH in fifth.

I bought my carb and jets from SunLPartsOnline.com. I gave them some very complex jet orders and they didn't muck the orders up once. They have very nice prices and you can't stump them on a jet size.

If you go the Motion Pro throttle cable route they will want you to send them your stock twist grip so they can make a custom cable. Make sure they know you are installing a Mikuni and that you want a 90 degree exit out of the carb. Lube your new cable BEFORE you install it, it does not come lubricated. I assume you've lubed cables before. If not let me know and I'll share a cool tip with you.

Courtesy of Bill from the Yahoo Ascot Owners Group.

[patrino]

Fitment was pretty straightforward.  The tip on the Dayco hose was spot-on.  More details on that soon.

Bike info:
* Kerker full exhaust
* stock engine internals
* K&N clamp-on air filter, freshly serviced, roughly 5" long, 3-1/4" OD, 2-5/8" inlet opening
* 34mm Mikuni VM carb (this is the new bit)

Bike fires up, runs, and is rideable ... but doesn't run particularly well at a few throttle positions.  I'll be tackling these issues over the next few days (hopefully).  Notes so far:
* Running a #25 pilot jet.  The bike is running VERY rich on the idle circuit, to the point where it's puffing nonstop clouds of unburnt fuel out the pipe at idle, with no choke.  It did not do this at all with the stock carb; this behavior's new.  (The smoke is so bad I'm going to start by yanking the carb and making sure something isn't wonky with the choke.  The choke plunger moves cleanly, and when I turn on the choke, the bike dies instantly, so the choke is doing SOMEthing when turned on.)  Idle speed increases when I back out the air screw, suggesting the bike wants a smaller pilot.  I'm going to pick up a #22.5 and #20 to test.
* Big flat spot ~1/3-throttle.  Next step is to better pin down where it's at.  I marked the twistgrip with an index mark, and the switchgear with taped closed, 1/4-throttle, 1/2-throttle, 3/4-throttle, and full-throttle markings, to help pin down the location.  Talking with a friend at a local shop who's built multiple FTs, he said the VMs take the most fettling in the needle jet / jet needle area, which squares with what I'm experiencing.
* Bike revs hard when it's on the main (currently running a #230), once it climbs out of the flat spot.
* #2.0 slide
* #159-series P-8 needle jet
* 6DH4 needle

[patrino]

p.s. Anyone else running a 34mm VM Mikuni?  I used the brass specs in the original post as a baseline, but some additional data points are welcome!

[patrino]

Switched to a #22.5 pilot.  Still too rich; I've got a #20 and a #17.5 on order.