Big Bore Kit (BBK) tuning thread

willis

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As I get close to finishing up my BBK install I have been searching many threads on carb tuning. I thought it might be nice to pull info from those running BBK's in their bikes on how they have them set up and have them and list all in one place. If you are running a BBK please share with us what your setup is (carb set, jetting, airfilters, exhaust, ingnition, etc...). If we get enough participants we could possibly make this a sticky to help anyone else going this route in the future.

My '77D setup is as follows:
BBK from Yamahaxs650.com - I had the issue with the mount holes that needed reamed out, which has been resolved.
78/79 BS38 carbs - Need to figure our my base jetting and needle position to get started.
Stock airboxes & filters
Custom high pipes 1-5/8" diameter. No mufflers, but Snuff-R-Nots installed at the end. Also running the exhaust port optimizers
Tytronic ignition w/ stock advance assembly.

*Also feel free to add any other info to share with those looking into this mod.... likes, dislikes, things to watch for...
 
My 72 XS2
BBK
Stock BS38 carbs with stock jetting
Stock air boxes and filters
Stock headers with Commando mufflers from Yamahaxs650.com
Boyer ignition set slightly retarded.
 
So your stock jetting runs well with the BBK? I was under the assumption you would need to go up on jet sizes.
 
It makes sense that stock jetting will work. After all a carb is just an atomizer and siphons fuel based on the volume of air drawn through it. Doesn't much matter if it's 325cc or 375cc, it will atomize fuel based on the air sucked through.
 
This is a tough nut to Crack. Reason, I have a 77D with the big needle jet so my numbers may be off. I am at 2 up on the pilot to #30 and I am 3 up on the main at #130. Hopefully the generalization of going up 1 or 2 on pilot and main get you a good starting point. When I bought my bike the bike it was up 6 on the main and stock pilot. It ran but idle was rough, it was raspy at part throttle and snap and crackle decel.
 
So far it runs great. Plenty of color on the plugs.
I think the reasons I didn't have to go up on the jetting is I'm running stock air boxes and filters. So the air doesn't pull in any easier than before. And in 72 the EPA wasn't as strict as later years.
 
It makes sense that stock jetting will work. After all a carb is just an atomizer and siphons fuel based on the volume of air drawn through it. Doesn't much matter if it's 325cc or 375cc, it will atomize fuel based on the air sucked through.
It makes sense that stock jetting will work. After all a carb is just an atomizer and siphons fuel based on the volume of air drawn through it. Doesn't much matter if it's 325cc or 375cc, it will atomize fuel based on the air sucked through.
Yes it makes sense until you factor in many of these bikes had epa compliant setting that were a bit lean for best operation. Add that many fuels are 5% weaker than 40 years ago. (#135 - #137.5, 1 step up, represents just a few percent increase) Finally some suggest that 750 BBK make an extra 10hp and that all suggest that jetting changes are appropriate.
 
If you look at the jetting specs for the TX750 twin Yamaha made in '73 and '74, you'll see they are virtually the same as the 650s from that time. So, you'll be re-jetting mainly for your other mods, not the change in displacement. That being said, let's take a look at your '78-'79 carb set. Unlike GLJ's early set, yours were E.P.A. mandated. I think this mainly affects the idle/pilot circuit and the lower midrange, but I feel some minor re-jetting will be needed.

With the '78-'79 carb set, Yamaha implemented a big (huge actually) main jet increase. They went from the #122.5 used in the '76-'77 carb set to a #135. That's a 5 size increase. To make it work, they had to lean the midrange and they did that by reducing the previous Z-8 needle jet to a Z-2. Now, even with this smaller needle jet, the upper midrange was still right on the verge of being too rich. For this reason, any increase in main jet size on the '78-'79 carb set, even just one step up, will make the upper midrange too rich and cause stumbling under heavy throttle. So, with any increase in main jet size, you will need to lean the needles a step as well. But, when you do that, you can create a flat spot just off idle, so you usually need to increase the pilots a size to fix that.

So in your case, you could start off with stock jetting and take a few short runs, but then I'd start experimenting. I'd go one up on the pilots, one up on the mains, and lean the needles a step to start. After a couple test runs, I'd go up another on the mains. The '78-'79 carb set also sometimes needs 2 up on the pilots. But you'll just have to run the bike and see. Every one of these seems to respond a little differently to mods, and what jetting is needed for them.
 
Thanks 5T, I will be ordering some new jets from Niche in the near future, and will get an assortment of several sizes to keep on hand. The carb set I have has 140M and 42P jets installed currently. Those seem a bit big, even for the mods I will have. The carbs were purchased from an unknown parts lot, but are very clean and look ready to go.
 
That pilot jet seems very wrong. That carb set had a 27.5 stock of the VM22/210 type. What's in there may be the BS30/96 type. You'd better check that.

650 Pilot Jets.jpg
 
Also verify the float bowls are the right ones for that carb set, the VM22/210 type .....

BS38 Bowl Types.jpg
 
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