Mikuni VM 26G setup on a narrow case 250
Posted: Thu Aug 03, 2017 4:19 am
Mikuni VM 26G Twin Float type carburetor setup
Refer to: http://www.mikuni.com/pdf/vmmanual.pdf
As used on a Ducati narrow case 250 engine #88103 believed to be circa 1964 production with no AAU (centrifugal advance.)
The engine is fitted with a period correct Del'Orto metal canister air filter (Ducati Scrambler type) containing within it a NAPA Gold 2719 Crankcase Ventilation pleated paper air filter (a restricted air intake is the point I am trying to make) which by the way is an excellent fit for the canister.
The exhaust system is a lightly baffled aftermarket megaphone (very little restriction.)
The subject 250 fitted as described is a 2-kick cold engine starter; a first kick is given with the choke on, a second kick with the choke off and the engine starts.
With regards to the Mikuni, this is what worked for me. Your results may vary:
Jet Needle 5DP7
Jet Needle "E-ring" Clip position: 2nd notch from the top.
Needle Jet: "PRIMARY type" 182 (NOT "Bleed type")
Slide cutaway: 2.5
120 Main Jet
Pilot Jet 40
Air Jet 0.5
Idle adjusting screw: 7.5mm from carb body to bottom of screw adjust (total distance the spring spans)
Air Screw: Set at 1.0mm from bottoming out.
The long story is the bike was sold to me with the Mikuni fitted. At 3000 rpm the bike would stop making power. Removal of the spark plug revealed the plug was wet and fouled, the mixture was far too rich. I told the previous owner (who had assured me the bike ran) what was happening and he said rather matter-of-factly that the engine ran just the same way for him, as if it was somehow normal for a Ducati 250.
The previous owner had sourced the Mikuni from a fellow that told him something to the effect that the Mikuni was jetted so as to be virtually a plug and play fit for a Ducati 250. Clearly this was not the case.
I found a company called Sudco (http://www.sudco.com/carburetors.html) and described the fitted jets to a fellow there. He said that the jets fitted were simply what the factory puts in that model by default in Japan for shipment everywhere in the world. No consideration for engine displacement, make or model, just some jets and such so the carb would be delivered with some guts in it. He also said he had a list of jets and needles that Sudco recommended as a good starting point for a Ducati 250.
I purchased the 'Ducati 250' kit and and there was some improvement, but it was far from an acceptable solution.
Working from the throttle closed position I incrementally tuned each fuel circuit from the pilot to the air jet. At least twice I found I had not ordered enough range in whatever jet I was working on (choosing several jets above and/or below the 'Ducati 250 kit' I was first sold), and I would place a subsequent order to Sudco after which I would have to wait for the new jets to arrive so I could proceed further.
Honestly though, I have a SSI29 on a Mach 1 250 and it's temperamental and difficult. It's difficult to start (the short throw of the Mach 1's curved kick-starter adds to the problem) and I've never been able to get the engine to idle with any kind of air filter, even a highly breathable foam sock filter. It's either an open velocity stack or the engine simply refuses to run. Maybe the valve sizes have something to do with it too, I can't say. The big SSI just seems a bit more crude in terms of the amount of circuits between closed throttle and wide-open throttle which presents very limited tuning options.
It's easy to see that the added complexity of the Mikuni makes for a more manageable and even-tempered throttle response, from starting and idle to full throttle. The only bad aspect of the Mikuni is that it's a Japanese carburetor on an Italian bike. Sacrilege! Forgive me Dr. T for I have sinned.
In my defense I am able to ride this bike anywhere and any time, so much so that this over-50 year-old motorcycle is my daily driver. The prominent vintage Ducati scrambler air cleaner hanging off the right-hand side boasts the name "Del'Orto' on it, and to date no one has ever knelt down and peered at the pot metal lump between the air cleaner and the engine so much as to ask "Isn't that a Mikuni?"
Jim
Refer to: http://www.mikuni.com/pdf/vmmanual.pdf
As used on a Ducati narrow case 250 engine #88103 believed to be circa 1964 production with no AAU (centrifugal advance.)
The engine is fitted with a period correct Del'Orto metal canister air filter (Ducati Scrambler type) containing within it a NAPA Gold 2719 Crankcase Ventilation pleated paper air filter (a restricted air intake is the point I am trying to make) which by the way is an excellent fit for the canister.
The exhaust system is a lightly baffled aftermarket megaphone (very little restriction.)
The subject 250 fitted as described is a 2-kick cold engine starter; a first kick is given with the choke on, a second kick with the choke off and the engine starts.
With regards to the Mikuni, this is what worked for me. Your results may vary:
Jet Needle 5DP7
Jet Needle "E-ring" Clip position: 2nd notch from the top.
Needle Jet: "PRIMARY type" 182 (NOT "Bleed type")
Slide cutaway: 2.5
120 Main Jet
Pilot Jet 40
Air Jet 0.5
Idle adjusting screw: 7.5mm from carb body to bottom of screw adjust (total distance the spring spans)
Air Screw: Set at 1.0mm from bottoming out.
The long story is the bike was sold to me with the Mikuni fitted. At 3000 rpm the bike would stop making power. Removal of the spark plug revealed the plug was wet and fouled, the mixture was far too rich. I told the previous owner (who had assured me the bike ran) what was happening and he said rather matter-of-factly that the engine ran just the same way for him, as if it was somehow normal for a Ducati 250.
The previous owner had sourced the Mikuni from a fellow that told him something to the effect that the Mikuni was jetted so as to be virtually a plug and play fit for a Ducati 250. Clearly this was not the case.
I found a company called Sudco (http://www.sudco.com/carburetors.html) and described the fitted jets to a fellow there. He said that the jets fitted were simply what the factory puts in that model by default in Japan for shipment everywhere in the world. No consideration for engine displacement, make or model, just some jets and such so the carb would be delivered with some guts in it. He also said he had a list of jets and needles that Sudco recommended as a good starting point for a Ducati 250.
I purchased the 'Ducati 250' kit and and there was some improvement, but it was far from an acceptable solution.
Working from the throttle closed position I incrementally tuned each fuel circuit from the pilot to the air jet. At least twice I found I had not ordered enough range in whatever jet I was working on (choosing several jets above and/or below the 'Ducati 250 kit' I was first sold), and I would place a subsequent order to Sudco after which I would have to wait for the new jets to arrive so I could proceed further.
Honestly though, I have a SSI29 on a Mach 1 250 and it's temperamental and difficult. It's difficult to start (the short throw of the Mach 1's curved kick-starter adds to the problem) and I've never been able to get the engine to idle with any kind of air filter, even a highly breathable foam sock filter. It's either an open velocity stack or the engine simply refuses to run. Maybe the valve sizes have something to do with it too, I can't say. The big SSI just seems a bit more crude in terms of the amount of circuits between closed throttle and wide-open throttle which presents very limited tuning options.
It's easy to see that the added complexity of the Mikuni makes for a more manageable and even-tempered throttle response, from starting and idle to full throttle. The only bad aspect of the Mikuni is that it's a Japanese carburetor on an Italian bike. Sacrilege! Forgive me Dr. T for I have sinned.
In my defense I am able to ride this bike anywhere and any time, so much so that this over-50 year-old motorcycle is my daily driver. The prominent vintage Ducati scrambler air cleaner hanging off the right-hand side boasts the name "Del'Orto' on it, and to date no one has ever knelt down and peered at the pot metal lump between the air cleaner and the engine so much as to ask "Isn't that a Mikuni?"
Jim