Carbs Mikuni BS38 jets.

mike-dubai

XS650 Member
Messages
13
Reaction score
5
Points
3
Location
dubai
Hello,
It would be great if someone could help me to choose new jets before ordering my parts from US. (I live in Dubai UAE!)
I have a 650 XS special 1978 with Mikuni BS38.
I added two XS performance Pod Air Filters (http://www.mikesxs.net/parts/yamaha-xs650-57mm-tapered-oval-pod-air-filter-2-1-4)
And two bigger than stock Stainless Steel XS Performance Headpipe Set - 1.75" (45mm.) OD x 1.65" (42mm).
(http://www.mikesxs.net/parts/yamaha-xs650-stainless-xs-perf-headpipe-set)
With two Lossa reverse cone mufflers (http://www.lossaengineering.com/col...-brand-reverse-cone-muffler?variant=856209431)

Main jet: 135
Pilot: 27.5
Don't know if someone has some near conditions in the US (i don't think so!), it could help me to change the jets... The bike is not working well at all like that.

Average temp 33°C or 91.4°F (in between 25° and 40° degrees),
Average humidity around 40%.
Alt: sea level...

What will you try?

Thanks,
MIchel.
 
Last edited:
You may well find that increasing your pilot jets to 30.0 and adjusting the air/fuel screw will be sufficient . I have foam filters on my 79 BS38's and increasing to 30.0 worked well for mine. Like you I also have non standard exhausts which will almost certainly require larger main jets and adjusting the needle position but this will need to be arrived at by testing .

My advice would be to buy some pairs of 6mm main jets say 140 145 150 and start with the richest jet 150 and work your way down . You need to tune each throttle section seperately .
effects3.gif
 
Last edited:
Some of the components you've chosen may make perfect tuning impossible to achieve. Your pleated style pods don't work well on CV type carbs. Foam pods are a better choice. Your 1 3/4" headpipes are bigger than ideal for this engine. You may need to add the inserts at the head .....

http://www.mikesxs.net/parts/yamaha-xs650-headpipe-torque-inserts-pk-2

In the sizes we usually use, Mikuni mains and pilots come in 2.5 number increments. I would suggest getting 30 and 32.5 pilots, 140, 142.5, and 145 mains. The 32.5 pilots may be difficult to find as they are a rather new size offering from Mikuni, but they may be just what you need.
 
6mm main jets don't have to be Mikunki jets . They can be any generic 6mm jet with a 8mm overall length.... Delortto ....Polini... lots of different makes will fit and work just as well ;)
I've not seen any 32.5 pilots I'll keep an eye out.
edit
Found these which I believe are the correct type for a 1978/79 BS38 I know that there is also a pilot jet with no air holes in the body but can't find your post 5T
mikuni pilot jets.jpg
 
Last edited:
Mike if you can't get any over there PM me your address and I'll post you a couple of pairs
 
I use only genuine Mikuni jets. You can be assured of their quality and flow rate. But it's your bike, do what you want.
 
well I never trust anybody so I have sets of jet gauges and a venier guage to check all the jets I use on bikes , scooters and cars.;)
I was just making the point to the PO that like lots of other things on the XS650 you don't need to buy Yamaha labeled parts Generic oil seals and bearings and other components are equally as good if not better. I think this silly OEM nonsense can be taken too far sometimes
 
I didn't say buy the jets from Yamaha. They are Mikuni jets of course, but at nearly twice the price because they come in a Yamaha bag. There are many other places to get genuine Mikuni jets at their normal price, sometimes even less. Personally, I buy the 4 packs for 4 cyl. bikes. They're cheaper that way and I do lots of carb tuning so I need the supply .....

I2WDGp4.jpg
 
Last edited:
yeah i hear what you are saying 5T but I suppose I see things from a different perspective .

Here in the UK prices of XS650 bikes are double even triple the price of the USA . The same goes for parts . The USA had a huge number of XS650 and parts so prices are still low enough to be able to afford OEM parts .
I feel really sorry for all the xs650 owners down under where parts are in very short supply and extremely expensive to import .

Your advice is good especially for those new to repairingthese old bikes, you should always be able to trust genuine Mikuni parts.
 
Some of the components you've chosen may make perfect tuning impossible to achieve. Your pleated style pods don't work well on CV type carbs. Foam pods are a better choice. Your 1 3/4" headpipes are bigger than ideal for this engine. You may need to add the inserts at the head .....

http://www.mikesxs.net/parts/yamaha-xs650-headpipe-torque-inserts-pk-2

In the sizes we usually use, Mikuni mains and pilots come in 2.5 number increments. I would suggest getting 30 and 32.5 pilots, 140, 142.5, and 145 mains. The 32.5 pilots may be difficult to find as they are a rather new size offering from Mikuni, but they may be just what you need.


Hello 5twins, thanks for the info. You're not a funny guy!
I have already the inserts but would like to try without. I don't understand why they buy bigger pipes just to reduce them with the insert....
I rode already with the pods (and stock exhaust), the bike works fine but not with the new pipes....
I will buy two or three differents jets and one or two main pilots and try to tune...
Thank you again, Michel
 
You may well find that increasing your pilot jets to 30.0 and adjusting the air/fuel screw will be sufficient . I have foam filters on my 79 BS38's and increasing to 30.0 worked well for mine. Like you I also have non standard exhausts which will almost certainly require larger main jets and adjusting the needle position but this will need to be arrived at by testing .

My advice would be to buy some pairs of 6mm main jets say 140 145 150 and start with the richest jet 150 and work your way down . You need to tune each throttle section seperately .View attachment 88682

Thanks for the info Peanut, i should find what i need...
 
Bigger diameter pipes flow better down away from the head but these motors like a smaller diameter coming out of the head for the first few inches. Hence the inserts. You "fool" the motor into thinking it has smaller diameter pipes. So, you get good flow out of the head and also down the line away from the head. Larger diameter pipes are supposed to move the power band higher up in the RPM range and also cause some low RPM power loss. A back to back comparison with and without the inserts would be good. That will probably show you this.
 
CV carbs are very forgiving and can mask over minor jetting glitches. For this reason, when jetting them, you have to work them hard to reveal any potential jetting issues. You need to test the various RPM ranges and the transition areas between circuits using large amounts of throttle, even full throttle sometimes. If you don't test like this, you may not find the glitches.

When sorting the mains, an important area you test through is the midrange to main transition range. This is from about 4K to 5.5K. As you increase your main jet size, eventually you will start to get break-up through this range under heavy throttle applications. This indicates the bleed-over from the larger mains into the upper midrange is making it too rich. You fix this by leaning the needles a step. You can then continue to increase the mains but you usually can't go much bigger before the break-up returns. This is pretty much the limiting factor for the main jet size on these carbs. Now, leaner needles and/or needle jets could be installed and that would allow even bigger mains, but none are available for these carbs so we're kinda stuck with what we've got.

CV carbs rely on a smooth flow of air to create vacuum and lift the slide. The pleated style pod disrupts this air flow and that can mess with the slide lift. The slide can actually flutter instead of lifting smoothly. Sustained slide lift on a CV carb doesn't occur until late, like 4K RPMs. This is the same area you'll be watching if trying to sort your mains. The pleated pods can contribute to the break-up there and make it occur sooner than if you had better smoother flowing foam pods. From the break-up you're getting, you may assume your main is too large when it may not be. This was the problem the pleated pods caused on my bike. I felt I needed another size larger on my mains but the upper midrange break-up it caused wouldn't let me run it. I switched to UNI foam pods and was able to pop that larger main in no problem. All the upper midrange break-up had disappeared.
 
CV carbs are very forgiving and can mask over minor jetting glitches. For this reason, when jetting them, you have to work them hard to reveal any potential jetting issues. You need to test the various RPM ranges and the transition areas between circuits using large amounts of throttle, even full throttle sometimes. If you don't test like this, you may not find the glitches.

When sorting the mains, an important area you test through is the midrange to main transition range. This is from about 4K to 5.5K. As you increase your main jet size, eventually you will start to get break-up through this range under heavy throttle applications. This indicates the bleed-over from the larger mains into the upper midrange is making it too rich. You fix this by leaning the needles a step. You can then continue to increase the mains but you usually can't go much bigger before the break-up returns. This is pretty much the limiting factor for the main jet size on these carbs. Now, leaner needles and/or needle jets could be installed and that would allow even bigger mains, but none are available for these carbs so we're kinda stuck with what we've got.

CV carbs rely on a smooth flow of air to create vacuum and lift the slide. The pleated style pod disrupts this air flow and that can mess with the slide lift. The slide can actually flutter instead of lifting smoothly. Sustained slide lift on a CV carb doesn't occur until late, like 4K RPMs. This is the same area you'll be watching if trying to sort your mains. The pleated pods can contribute to the break-up there and make it occur sooner than if you had better smoother flowing foam pods. From the break-up you're getting, you may assume your main is too large when it may not be. This was the problem the pleated pods caused on my bike. I felt I needed another size larger on my mains but the upper midrange break-up it caused wouldn't let me run it. I switched to UNI foam pods and was able to pop that larger main in no problem. All the upper midrange break-up had disappeared.


Wahoo,
I should have read this before buying parts...
Anyway thank you very much for your explanations.
I will let you know the settings or changes...

Michel.
 
6mm main jets don't have to be Mikunki jets . They can be any generic 6mm jet with a 8mm overall length.... Delortto ....Polini... lots of different makes will fit and work just as well ;)
I've not seen any 32.5 pilots I'll keep an eye out.
edit
Found these which I believe are the correct type for a 1978/79 BS38 I know that there is also a pilot jet with no air holes in the body but can't find your post 5TView attachment 88683
Hey, I’m going through the pilot jetting process right now my self. My 78 kz750 has BS38 Carbs on it, and I’ve been getting my pilots from Jetsrus.com. If you haven’t check them out do so. Also, the OEM BS38 pilot just had 6 holes originally, not 8 (based off my research of course). Good luck!
 
I have a 77 with a 750BBK, Omar's pipes and foam pods. The 76-77 have a big Z08 needle jet which fattens the midrange. I am at 130 mains(up from 122.5 stock) and 30 pilot (up from 25). This combination is very rideable and delivers goooood performance. Your 78 carbs have a Z02 needle jet which I assume is leaner.
 
Last edited:
He has a Kawasaki KZ750 twin. Yes, it uses BS38 carbs, but they are jetted differently than our 650 ones. I have a PDF manual for the bike and according to it, the needle jets are a Z-4. The pilots are a #45 and being that large (and also the fact they only have 6 air bleed holes in the sides), I'm going to guess they are the BS30/96 type, not the VM22/210 type that the '76-'79 650 carbs use.

Mikuni-Pilot-BS30-96.jpg


Mikuni-Pilot-VM22-110.jpg


On our 650 BS38s, carbs older than '76 do use the BS30/96 type pilots, and they are sized in the 40s just like in the KZ carbs. The VM22/210 pilots used in the later BS38s are sized in the 20s. The reason there is such a big size difference is because of what the jets flow. The BS30/96 pilots flow a fuel/air mix while the VM22/210 pilots flow straight fuel (the air is added after the jet). The end result is pretty much the same pilot fuel/air mix strength, it's just achieved in different ways. The float bowl air passages differ on the two types of bowls .....

BS38 Bowl Types.jpg


..... and you must match the jet type to the bowl. That manual I have only has drawings, no actual float bowl pics, but I'm going to assume the bowls look like the 650 BS30/96 bowls above. Air is delivered to the bottom of the pilot jet so it can make and flow a fuel/air mix.

As far as re-jetting goes, I think they will follow what is done to our 650s, and that would be just one or two sizes up on the pilots. A #125 is speced as the stock main jet size so mains in the mid 130s are probably the most these carbs will take.
 
Back
Top