38mm Mikunis?

Spondonrider

Spondonrider

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Location
Colorado
Looking for input on the use of 38mm Mikuni RS flat slides. This is for the modified 120 triple in my Spondon. Currently it runs 41mm FCRs which are a bit too large and which I am planning to swap out for something smaller. I have a smaller set of FCRs, I think 39mm. I expect these would work but due to the design of the frame and fuel tank the FCRs, if mounted as a single bank of three, rub the underside of the fuel tank. As it is now the right most carburetor is separate and mounted with a slight inward angle. This requires a second throttle cable and results in a very heavy throttle action. Today while cleaning the FCRs I grabbed a broken RS body I have and see that it should mount up just fine with no tank interference issues. So now I'm thinking I should get a set of Mikunis and sell off these 41s and the 39mm FCR's. My question then is which size to look for, 36mm or 38mm? 
Going by how the 41mm FCRs work, I expect the 38mm Mikunis to be fine. I have much more experience with the FCRs though so am looking for the thoughts from those who have perhaps traveled this path before.

Motor specs are: 1116cc (I think), J&E high compression pistons, ported head, lightened crank, bigger valves, 3-1 taper tubed exhaust. Motor makes around 100hp at the rear wheel.

Thanks for your input,

Ed
 
Dellortoman said:
Maybe you need a set of Red's angled inlet stubs. They tilt the carbs back by about 10 or 15? (not sure of the exact angle) which gives more room under the tank.
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Its not top clearance, its on the side. The FCRs are wider in the slide body than the Mikunis and the difference is enough..just. However on the Mikuni the accelerator pump well does hit the lower edge of the tank. On a bank of Mikunis though the accelerator pump is mounted to the middle carb so I can cut the carb body and float bowl to gain plenty of clearance.
 
Thought about it Grant. The thing is that its a custom made tank and would be expensive to modify, its beyond my skill set. Taking into account the amount of modification needed, the time it would take, and the cost, with the possibility of it being pretty well screwed up, its easier overall for me to swap out the carbs. I do have the Mikuni RS' on both my Executive and Montjuic so while I've more time and experience tuning FCRs I'm OK with the Mikunis.
 
Modifying the tank may work out cheaper than a new set of carbs, linkages, cables etc.
Plus you would not have he hassle of getting them dialled in.
If it was me I would get quotes for each option then make my mind up.
 
Hi Ed,

you can move the accelerator pump to the center carb on the FCR without problem. You only need to swap the float bowls and move the actuation arm to the center carb, preferably on the 3-in-a-row configuration with a single actuation rod. Have that setup on mine, works great. Also have moved the single cable actuation between carb 2 and 3 (center and right), which gives also more clearance on the right side.

On the SFC and also the Jota, I had to gently (!!) hammer the underside just some millimeters to make some room for the top right edge of the right side carb. so - depending what your tank is made of, you could try the same (if its alloy f.e.) or cut out a piece and close the hole with some more space (in case its fiber).

If you need any parts for doing that, I have some bodies laying around as well as some other parts, float bowls with pump and without I think etc.

I also have the single actuation for the choke made to fit our triples, laser-cut out of stainless steel.

cheers

Lothar
 
Laverdalothar said:
Hi Ed,

you can move the accelerator pump to the center carb on the FCR without problem. You only need to swap the float bowls and move the actuation arm to the center carb...

On the SFC and also the Jota, I had to gently (!!) hammer the underside just some millimeters to make some room for the top right edge of the right side carb. so - depending what your tank is made of, you could try the same...

I also have the single actuation for the choke made to fit our triples, laser-cut out of stainless steel.

cheers

Lothar
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Hi Lothar,

Yes, I already have the pump in the center and I have hammered the underside of the tank as much as I dare. Still not enough room. The FCR body is just too wide. I've put a stripped body in the location to check clearance and it hits on the top outside edge of the carb body. Nothing on the carb to remove or relocate to gain clearance. Thanks for the offer on the choke plate but mine are downdrafts, no choke/enrichment circuits. This would be another benefit of the Mikunis. While its OK not having a choke on the FCR's, it would be nice and I can easily set that up with the Mikunis.
 
Hi Ed, being in Colorado you could get your local surfboard maker to make a mold of the tank! :D
Seriously though, being a custom made tank, it's just insurance if you ever need a replacement.
Hate to think how much it would cost scratch building a custom tank compared to popping out one from n existing mold.
What is the tank made from?
I'd hate to think how many hours I put into scratch building the Motodd tank, would equate to a several thousands of dollars:o if someone was paying for it. :o :o
Really had no choice as the original ally tank was continually cracking and I doubt I could have brought a replacement.
Happy to have molds of everything now, piece of piss next time, well sort off!
 
Grant said:
Modifying the tank may work out cheaper than a new set of carbs, linkages, cables etc.
Plus you would not have he hassle of getting them dialled in.
If it was me I would get quotes for each option then make my mind up.
Click to expand...

I guess its a matter of what I'm comfortable with. Finding someone I trust to modify and reweld this tank would be tough but I'm quite comfortable with setting up carbs. I hate having anyone work on my bikes, especially this one. As for cost, I'll have 3 41mm and 4 39mm FCRs for sale. Once cleaned up and on Ebay they should cover the cost of the Mikunis.
 
motoddrob said:
Hi Ed, being in Colorado you could get your local surfboard maker to make a mold of the tank! :D
Seriously though, being a custom made tank, it's just insurance if you ever need a replacement.
Hate to think how much it would cost scratch building a custom tank compared to popping out one from n existing mold.
What is the tank made from?
I'd hate to think how many hours I put into scratch building the Motodd tank, would equate to a several thousands of dollars:o if someone was paying for it. :o :o
Really had no choice as the original ally tank was continually cracking and I doubt I could have brought a replacement.
Happy to have molds of everything now, piece of piss next time, well sort off!
Click to expand...

Hi Rob, We think alike, I've often thought I'd like to have a carbon fiber copy of this tank made up, with the required modifications underneath, to use instead of the original. The current tank is aluminum and was made by a gentleman named Jack Hagemann. He was an old Southern California hot rodder who became a metal worker, the kind of guy who would take a sheet of aluminum and pound out a new front end for your Bugatti. He did a lot of custom cars, race cars, classic stuff, but as far as I know this is his only motorcycle tank. I think he was a friend of Lance Weil's and was thus talked into doing this one. Jack passed away back in 2010 I believe.
 
An excellent reason to get a carbon copy made, put the original safely on a shelf, and use the copy on the bike
 
have a set of 38's in stock already converted
not certain though if they are mine or Jotaman's we have done that many carb swaps over the years

 
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