Rebuild Kits for 83 750 twin???

Where is the best place to get a couple of carb rebuild kits for an 83 750 twin?

Wildh2oskier wrote:

Where is the best place to get a couple of carb rebuild kits for an 83 750 twin?

I found some kits for my '79 750 Twin made by Keyster or Keystar as I recall. Don't know if they fit yours.


www.loudfastugly.com/carbkits/carbkits.htm

76-80 have BS38 carbs, 82-83 have BS34.

But 76-79 BS38 kits won't work in 80 BS38 carbs, I tried it and couldn't get it to run right, took the kits out of the 80 carbs and put the parts in 79 carbs and they worked perfectly. I don't know what the difference is between the 76-79 carbs and the 80, but there must be something.

Mid-range tuning of the 76-79 BS38 is MUCH richer than the 80 BS38... not sure why they changed their concept of tuning in 80... The main jets and pilot jets look to be about the same as I recall. Not sure what parts were included in the Keyster kits but if it included jet needles, I doubt the 76-79 jet needle would work well in the much larger 80 needle jet... What components are in the Keyster 76-79 kit that effect tuning?

wiredgeorge wrote:

Mid-range tuning of the 76-79 BS38 is MUCH richer than the 80 BS38... not sure why they changed their concept of tuning in 80... The main jets and pilot jets look to be about the same as I recall. Not sure what parts were included in the Keyster kits but if it included jet needles, I doubt the 76-79 jet needle would work well in the much larger 80 needle jet... What components are in the Keyster 76-79 kit that effect tuning?

main jets, pilot jets, and needles. The main jets were correct stock (125), the pilot jets were richer than stock (45) for the 1979 model which came with 40's, and the needles were definitely richer because they were about 0.1" shorter than stock. They did have five slots, but if they are used in the center slot, they will be MUCH richer than the stock setup in my 1979 BS38. I just kept the stock needles.... and ended up having to rich to the lowest slot with the MAC pipes. I'd wager the shorter aftermarket needles in center slot would be about right? Anyway, the needles look like they will work but have to be adjusted to a different slot.

Not sure I agree in principal about the needle jets. The taper has to be thinner as the NEEDLE JET size went from X-4 to Y-3 in 80. I am not sure you could make it work decently in the much smaller needle jet. The needle jet is the thing that the jet needle seats in and the hole size meters gas in conjunction with the taper (and length) of the jet needle. The 40 to 45 pilot jet jump would means that the air jets associated with the pilot jets must be a lot bigger as I don't think that the slides have cut outs. On VM carbs with slide cut outs (the half moon shape at the bottom), the larger the cut out the bigger the pilot jet is needed. The air speed slows down dramatically with the larger cut outs but there is more volume necessitating larger pilots. In general, smaller openings with quicker air speed require smaller pilot jets and the latter (small cut outs) are easier to tune in the idle circuit.

wiredgeorge wrote:

Not sure I agree in principal about the needle jets. The taper has to be thinner as the NEEDLE JET size went from X-4 to Y-3 in 80. I am not sure you could make it work decently in the much smaller needle jet. The needle jet is the thing that the jet needle seats in and the hole size meters gas in conjunction with the taper (and length) of the jet needle. The 40 to 45 pilot jet jump would means that the air jets associated with the pilot jets must be a lot bigger as I don't think that the slides have cut outs. On VM carbs with slide cut outs (the half moon shape at the bottom), the larger the cut out the bigger the pilot jet is needed. The air speed slows down dramatically with the larger cut outs but there is more volume necessitating larger pilots. In general, smaller openings with quicker air speed require smaller pilot jets and the latter (small cut outs) are easier to tune in the idle circuit.

By visual, the needles in the Keyster kit are shorter and taper faster if laid side by side with the stock ones and the five clip slots are aligned. They definitely will run a lot richer than the stock needles I have, not sure if they are designed for another year or that's the only needle they could find to put in the kit. They definitely don't match my needles.

The 83 as well as the 82 twins both use Mikuni BS34, stock jetting for the BS34 both years are: 115 main jet and 45 pilot jet, jet needle is 5C50 for the US model and has very little taper and 1 clip. If you are gonna keep the stock airbox, these should be correct. In order to get my 82 to work w/the K&N pods I replaced my stock jet needles/needle jets w/those from a Canadian XS650 Mikuni BS34. I could not find a kit so ended up purchasing my replacement jets individually.
BTW before I replaced the jn/nj's I tried just about every combination of jets from 115-150 mains and 45-55 pilots. The 145/50 combination was about the best, until I changed out the jn/nj's but still had a bad bog between the transition from pilots to mains.