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Tech Section / Re: Mikuni Carburetor Information Links
« Last post by patrino on February 14, 2020, 10:56:39 am »
A couple more changes and we're close ...

Best test-ride yet with the P4 needle jet and a #27.5 idle jet.  Still some flat spots at certain throttle positions but much much better than before.
Tech Section / Re: Mikuni Carburetor Information Links
« Last post by murdo on February 13, 2020, 01:32:36 am »
Good work. Keep plugging away at it and it will be worth the effort.
Tech Section / Re: Mikuni Carburetor Information Links
« Last post by patrino on February 09, 2020, 08:32:44 pm »
Well ... slow going but making progress!

I kept dropping idle jet sizes, all the way down to a #10 (the smallest available for a 34mm Mikuni VM!) but the bike kept chuffing clouds of fuel smoke out the pipe when idling.  Less smoke with the #10 than with the larger idle jets (e.g. the #30, the #25, etc.) but still quite the fog machine.  I triple-checked the float level (good), visually reinspected the float needle and seat (good), confirmed the floats floated (they did - no holes), and that the float needle actually shut the fuel off when necessary (it did).


I was ready to throw in the towel, but figured even though the bike with the throttle slide closed is 'technically' on the idle circuit only, leaning out the needle jet / jet-needle circuit might teach me something.  So I swapped the P8 needle jet for a leaner O6 needle jet I had in my jet box.  And ... voila! 

After I installed the O6, I fired the bike up with the #10 idle jet.  There wasn't a whiff of fuel smoke out the pipe.  It idled for crap so I reinstalled a #22.5 idle jet (it was handy).  I got a decent idle (with not a whisper of fuel smoke), and a test-ride showed that the #22.5 is still too lean, but that we're close! 

I've had the carb off ~12 times throughout this whole process (I only make one change at a time) and am happy to report that I'm optimistic again!

I have a P2 and P4 needle jet on order (the P8 is clearly way too rich; the P2 and P4 are roughly halfway between the P8 and the O6).  Next step will be a #27.5 idle jet with the O6.  Maybe the O6 needle jet is the ticket, but if not I'll have the P2 and P4 on-hand.

I reckon the lesson is that while 'technically' the bike's only on the idle circuit with the throttle slide closed, if the needle jet is WAY too large the signal from the engine's intake can be strong enough to pull fuel through the needle jet (enough fuel to mess up tuning of the idle circuit).  I suspect that this might especially be true when you've got a big (500cc) cylinder pulling through a relatively small (34mm) venturi.

I'll keep folks updated, but again, optimistic again!

Tech Section / Re: Mikuni Carburetor Information Links
« Last post by patrino on January 29, 2020, 09:49:59 pm »
My #17.5 and #20 pilots showed up today; will report back soon!
The Thumper Years: Modified To Full Customs / Re: Dutch FT project
« Last post by kansloos161 on January 28, 2020, 09:14:52 am »
Bike is still not running.
Still problems with the starter.
I think the starter control unit died. Anyone any idea how to bypass it or test it to see if it is broken?
Tech Section / Re: Mikuni Carburetor Information Links
« Last post by J6G1Z on January 27, 2020, 08:58:05 am »
Thank you for adding to the information.

Tech Section / Re: Mikuni Carburetor Information Links
« Last post by patrino on January 26, 2020, 10:54:33 pm »
Switched to a #22.5 pilot.  Still too rich; I've got a #20 and a #17.5 on order.
Tech Section / Re: Mikuni Carburetor Information Links
« Last post by patrino on January 16, 2020, 07:27:05 pm »
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!
Tech Section / Re: Mikuni Carburetor Information Links
« Last post by patrino on January 16, 2020, 07:25:55 pm »
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
Tech Section / Re: Mikuni Carburetor Information Links
« Last post by patrino on January 14, 2020, 11:01:21 pm »
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 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.
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