aaronxs400
XS400 Enthusiast
Building a SOHC motor, lighter crank, gsxr1100 pistons (500cc final displacement), and megacycle cam, some headwork. Im thinking 36mm carbs, still need bike to be streetable. Any opinions?
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VM carb size for built xs400?
- Thread starter aaronxs400
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I was considering the same thing for the standard engine. I'd say that in order to be usable on the street, 32s would probably be more like it. Bear in mind the 650 runs best with 34s unless it has a 750 kit, and36s are getting towards the too big end. I had a set of 36s on my 650, and ended going back to the '77 BS38 CVs. I couldn't get the off idle stumble out without making both the idle and mid range rich. Ok for two strokes, but hard on four strokes to run that rich.
The CVs can be larger as they only give what the engine demands. Which is why they are almost universal on road bikes. FI can provide the same thing. Slide-needle carbs cannot, unless they have an accelerator pump and a power valve(vacuum regulated jet, common on cars). So unless you like off idle stumble, be careful with too much size. Add a big cam, which with overlap makes off idle even worse, you could have a jetting dilemma of epic proportions.
You might want to look for a set of the late model Mikuni BS series CVs from the XS500. They should be almost spot on. they are close to the XS650, but set up for a smaller engine. They very well may be the ideal carb for the XS 400 series, more, but not too much. Avoid the early Keihin version, the all alloy slide liked to bind and hang up. The ones that look like the XS650 alloy cap carbs. The 77 carbs for the 650 may also be pretty close to plug-n-play. The spacing will no doubt be wrong, but can be easily adapted.
The CVs can be larger as they only give what the engine demands. Which is why they are almost universal on road bikes. FI can provide the same thing. Slide-needle carbs cannot, unless they have an accelerator pump and a power valve(vacuum regulated jet, common on cars). So unless you like off idle stumble, be careful with too much size. Add a big cam, which with overlap makes off idle even worse, you could have a jetting dilemma of epic proportions.
You might want to look for a set of the late model Mikuni BS series CVs from the XS500. They should be almost spot on. they are close to the XS650, but set up for a smaller engine. They very well may be the ideal carb for the XS 400 series, more, but not too much. Avoid the early Keihin version, the all alloy slide liked to bind and hang up. The ones that look like the XS650 alloy cap carbs. The 77 carbs for the 650 may also be pretty close to plug-n-play. The spacing will no doubt be wrong, but can be easily adapted.
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That is why I like the 76-77 bs34 xs360 carbs they are very close to the 76-77 bs38's used on the xs650 just in a 34mm
Very wise idea. While I'm not a super fan of living with the CV carbs in an ethanol rich environment, they do allow a much larger carb to be fitted than straight slide-needle versions. And you can live with them and not rejet for every temp change like large slide/needles would require.
Staying with the Cvs does take some of the adventure out of the equation, however.
No do these guys use bs38 type jets or bs34 ? Cause I know bs 34 use bs38 main jets as air jets and their pj are different also. But I've also heard that even tho one uses large round type and thee other small they both can be used because they have the same size holeThat is why I like the 76-77 bs34 xs360 carbs they are very close to the 76-77 bs38's used on the xs650 just in a 34mm
Don't worry about ethanol jbm industries makes new diaphrams that are tolerant to ethanol fuels
They use the bs38 jets and also go in the carb bowl. Their main jet is what the 80-82 bs34 uses for the air jet. The set on the left is the 76-77 xs360 bs34/38 (pilot and main) jets the one on the right is 77-82 xs400 pilot and main.
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Nice clean bowls.
I'm not referring to the diaphragms as the source of grief with ethanol, but the carb body and jets/passages. Zamac and ethanol are a bad mix. Brass isn't a lot better.
If i recall correctly, you can use the small jets anywhere, but the large ones only some applications. I think only the head diameter is different. The threads are the same, and the sizes are the same.
I'm not referring to the diaphragms as the source of grief with ethanol, but the carb body and jets/passages. Zamac and ethanol are a bad mix. Brass isn't a lot better.
If i recall correctly, you can use the small jets anywhere, but the large ones only some applications. I think only the head diameter is different. The threads are the same, and the sizes are the same.
Yes same as the 38's, floats too.
The needle and needle jets on the left is for the 77-82 sohc xs400 (the needle with the single clip is the 80-82) the other is the 76-77 xs360 notice the extra hole in the jet and how they get progressively larger as the needle or throttle opens more giving more fuel than what the 400 carbs do
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Do you think that needle and needle jet would provide enough for the 400 engine. With pods and eventualy a 2 into 1 ? Cuase my stock carbs I kept trying to fix midrande with bigger main and raised the needle but still had dead spot. I eventually went with a canadian needle from mikes and it helped quite a bit I was able to drop back down almost to stock main. What I'm trying to get at is if I get a set of these early bs34 will the bs38 needles and needle jets from mikes work if I have the same mid range problem or maybe the clips postions will be enough 
? Cuase my stock carbs I kept trying to fix midrande with bigger main and raised the needle but still had dead spot. I eventually went with a canadian needle from mikes and it helped quite a bit I was able to drop back down almost to stock main. What I'm trying to get at is if I get a set of these early bs34 will the bs38 needles and needle jets from mikes work if I have the same mid range problem or maybe the clips postions will be enough 
Sure is!
It's from a company called winning performance. They where around back in the 1970's. This set came off a 76 xs360 I got off ebay a few years ago for $25 shipped I have only seen one other winning performance exhaust and they where on a 1976 honda cb750. So I don't think they where around a long time or made a lot of them.
I have only seen one other winning performance exhaust and they where on a 1976 honda cb750. So I don't think they where around a long time or made a lot of them.
