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Carb mods

10 Nov 2019 10:43 10 Nov 2019 11:54 by FrankC.
FrankC
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Carb mods #1
I got this spare carb body sitting around minding its own business. With some time waiting on a 1W5 pipe for the 400 and a friend with newly acquired harbor freight welder, I have a little time. The old carb is just too polished, any way with the port work done a 35mm will only help. It took about an hour to oval bore the carb to 35mm. I used a couple of rough sanding rolls on the dremmel and a half round medium file. The sanding rolls cut fairly quick at the work and I didn't have to worry about carbide bit chatter. The file ensured that both the front of the carburetor and the bell mouth side of the carburetor lined up perfectly and I noticed that it also prevented me from belling the carburetor from front to back and keeping it very flat. Without both in combination it would have gone wrong. So I worked only the top half of the carburetor so that at the halfway point it starts to crescent into the oval shape. I will try to take some pictures and post them up in another post if I can right now I just don't know how with my phone. As far as knowing how much to cut I didn't. What I mean is I just measured the carburetor beforehand and then as I went checking periodically till I got to the one millimeter mark. Eyeballing also works. It wasn't as hard as I thought it would be. I feel there is no way that you can enlarge the carburetor other than oval boring because you have those are passages in the Bell mouth of the carburetor then you have to deal with the throttle slide cutaway being effectively enlarged. so with the oval boring I believe that all I have to do it deal with is changing the main chat a little bit. I don't recommend the oval boring as yet as I have not tested the results but like I said I will try to get pictures later I will take some with my camera. Edit- also I just let the sanding roll do the work with no pressure from me and kept it moving. As far as the file kept that moving in that I did not lift and ran it forward and backward against the work. I know when filing you need to run it in one direction but with the soft material of the carb the teeth on the.file did not round over and this gave faster results and more control.
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10 Nov 2019 17:23
RacerClam
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Carb mods #2
35 is better than 34 but still too small to make good power, What kinda port work has been done ? Reeds ? Try this for great working carb mods .

Rich

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10 Nov 2019 18:27 11 Nov 2019 19:25 by FrankC.
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Carb mods #3
I I like what you did to those carbs those are so nice. I effectively have raise my all my ports by 1.3 mm, widened 1mm, except the boost by 2mm. Widen the exhaust port I believe three sixteenths of an inch to 65% of the bore. I cut the auxiliaries to come in flatterer, but did not hook them.the boost part I cut the bridge out of the cylinder liner and filled it in for a steeper angle directed at the intake side of the spark plug. Did a little case stuffing, and some in the intake, and a little in the tunnels Added some boyesen ports and dropped the intake by 3mm. Also matched the 2 intake holes in the piston to the intake port and added a hole in the piston around 5/16 inches below the bottom ring land angled up to the crown. I have the head waiting a squish test to see if I can get that to around 50 thou. I am believing that pipe can be made to work, but really it will need mods. The 76 IT 400 pipe looks almost bolt on but the 77, 78 routes more in a stock fashion albeit I believe exits on the opposite side. As far as the Reed valve I just have the stock 4 pedal stainless valve but I rounded the ribs out for better flow. And the intake boot I matched to the carb throat and rounded the transition to those ears that extend into the valve. I really don't like the shape of that boot, something to do with an offset carb I guess. I am glad to hear from you.
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11 Nov 2019 10:16
RacerClam
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Carb mods #4
THanks for the compliment on carbs. Maybe we should have a discussion . feel free to give me a call 702 858-9177

Rich

racerclam
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16 Nov 2019 03:38
PMK
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Carb mods #5
Back in the late 70s, I modified a Mikuni on my RM125. Essentially the same way, done by hand. Kept lower section stock and increased upper section to maximum at slide bottom edge.

Best carb bore setup I have ridden is the SmartCarb on my OSSA MAR. Not round, not oval, but kind of triangle. Very small radius at the bottom but opens huge at the top.
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16 Nov 2019 18:39 16 Nov 2019 19:14 by FrankC.
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Carb mods #6
That's excellent. Does not have to be oval or round. I saw pictures where the top half is hogged out larger than the bottom half on a round slide carb. Got fuel? Edit- just ordered a pipe for a 78 it250, cost 89 bucks with shipping off eBay and got the carb together a few days ago, except it has a broken breather nozzle. Anyone know where to get an elbow for a carb? Thanks.

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17 Nov 2019 09:24
Pedalcrazy
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Carb mods #7
Wow you guys should work for NASA...unless you already do?

1978 DT400E
1976 DT400C
1973 RT3
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17 Nov 2019 11:53
FrankC
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Carb mods #8
I know. I can get a job moving their furniture around.:whistle:

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17 Nov 2019 19:33
RacerClam
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Carb mods #9
Try Speed and Sport in PA 866-455-6831 the got losts of nos parts

Rich

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