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Question on carbs!!
- Thread starter rotsen423
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Sportster-2001-951C-Stock
Well-Known Member
That carb would be a Mikuni Super BN, maybe the 40i model. There should be a model number on the carb and it should be listed in the shop manual. You'll need to swap jets accordingly to work with your combo, info as to which jets are appropriate should be in the shop manual.
As far as installing dual carbs goes it depends. 787 yes they were configured both ways and the dual setup should breath better but you'll need the hardware to go with such as intake manifold and brackets, linkage, etc.
I'd say go for it if you had a parts ski, but not sure I'd spend the dough piecing it together. If you're compelled perhaps @minnetonkaforme can assemble a kit of good used parts with everything you need?
As far as installing dual carbs goes it depends. 787 yes they were configured both ways and the dual setup should breath better but you'll need the hardware to go with such as intake manifold and brackets, linkage, etc.
I'd say go for it if you had a parts ski, but not sure I'd spend the dough piecing it together. If you're compelled perhaps @minnetonkaforme can assemble a kit of good used parts with everything you need?
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Are you sure? Everything I have seen says the 720 is rated at 85 HP with single or dual carbs.
I am pretty sure they are same the HP with dual or single carb. I think when they went to the accel pumps is when the dual carbs went away. The dual setups the main jets are like 130 and only 38MM design and the singles are 40mm w/160 mains. I would take the one carb just for easiness and less parts to buy and clean.
Sportster-2001-951C-Stock
Well-Known Member
I prefer fewer carburetors from the perspective troubleshooting is simplified, as well.
These carbs can be tricky to calibrate, not always so easy to just slap a couple on and it runs correctly from the get-go unless you're duplicating an already established factory tune out of the service manual. You literally need to duplicate everything including the flame arrestor due to it's an important part of the tune also.
IDoSeaDoo
Well-Known Member
I'm not sure that duals would breathe better or not, after all, the limiting restriction is the port window on the case, or perhaps the rotary valve itself. There is a little more restriction of having a manifold to direct the air through the single carb, but I think it's negligible. I'm not sure why the duals are better. I know the 587 had a smaller carb, and it had a triangular portion where it bolts to the flame arrestor. Now that was definitely a different carb, but you're saying that the 717 carb looks identical to the 787 but is just bored smaller? Interesting.
I also agree, the single carb is much easier to tune. Dual carbs are twice as much of a pain, and twice as expensive to work on.
I also agree, the single carb is much easier to tune. Dual carbs are twice as much of a pain, and twice as expensive to work on.
Seadoodoc
Active Member
The cases are 38mm openings. Mikuni are taper bore 40mm to 38mm. Huge discussions over years why twin 44mm or 46mm carbs makes such huge performances gains . I even rode a boat with twin 50mm. Bottom line single carb is better fuel economy, huge tunning margain of error impossibilty, but the twins will bark with way more authority on low end and mid range.
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