First Generation Ascots > Tech Section
Mikuni Carburetor Information Links
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!
--- Quote from: J6G1Z on October 14, 2013, 07:19:02 pm ---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.
--- End quote ---
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.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version