CARBURETTOR ID and BUTTERFLY VALVE SYNCHRONISATION

ANLAF

ANLAF

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I have a couple of questions about my two sets of carburettors.

I have downloaded a video clip to YouTube at

Question 1 - what model are these carbs? I think they are BS38.

Question 2 - the video shows the butterfly valves on both sets of carbs do not reach their open extent at the same time in their pairs, and air gets past one of each pair when in the closed position. Is this correctly adjusted? If not, I wonder if you would point me in the right direction. I have a Haynes manual, but it lacks detail on this adjustment.

The bikes - the black carbs are from the bike I am running, which is said to be an 81, but I am not sure about that - the engine number is 3L1 002207, and is more likely to be an XS-SF (Eur) 78-79 according to the engine number list. The other carbs are from my project engine is said to be from a 74-75, but I am not sure about that, either - the engine number FO-254157, which the engine number list says might be an XS-2F or an XS-SF 77-78 or 78-79. I would really helpful to know the year of the bikes and these carbs.

Anlaf
 
Yes, both sets are BS38s. Neither is original to your bike, it would have come with BS34s. The black set you're running are from a '76 or '77 bike (same carb set used both years). Your other set is from '78 or '79 (again, same carb set used both years). Here's some of the "tells" - only '76 and newer carb sets were bolted together as a "bank". Only the '76-'77 set was bolted together and had those overflow drain nipples on the bowls. Only the '78-'79 BS38s had float bowls with no nipples. On your black '76-'77 set, on the intake bell outer rim, you'll notice 2 round holes at about the 4 and 8 o'clock positions. These are the air jets. On your '78-'79 set, you have those plus 2 more slightly larger ones at the 3 and 9 o'clock positions. Those extra holes are the vents for the float bowl and were only present on the '78-'79 BS38s. All the other 38s had external float bowl vents - those brass tubes coming out the inner side of each carb body about half way up.

Your carb sets are out of sync. That's why one plate is open more than the other. You simply adjust that screw on the linkage between the carbs to equalize the plates. I find the easiest way to do this (incidentally, this is called a "bench sync") is to back out the idle speed adjustment screw that stops the cable lever on the left carb all the way so it isn't touching. This will close one or the other plate fully. Now hold the carb set up to a good light source, airfilter side facing the light. Look in the back side at the plates. You should see little, if any, light around the closed plate, more shining past the edges of the more open one. Adjust the sync screw to close it up and equalize the plates. Keep turning the screw until the other plate begins to open slightly. Best sync is about midway between this point and the point where the 1st plate just closed. Work the screw back and forth to find that center point.
 
...and 5Twins

Now the I have synchronised the carbs (bench sync) I can retrace my steps through all the minor adjustments to the floats and jet needle settings to thr and get the carbs working properly again. In the meantime, I wonder if you could help:

QUESTION - what float settings are right for the 76/77 BS38s? (currently on 25mm) and on the 78/79 BS38s too?

QUESTION - what jet needle setting should both sets be on? (currently ended up on 2nd). There have been no changes to jets as far as I am aware.

Anlaf
 
Looks like you've done well on the "bench sync". I should add that once done, turn the cable arm stop screw (idle speed adjuster) on the left carb back in until it contacts the arm, then another turn or so to slightly open both plates and set an initial idle speed. You will need to fine tune that once you start the bike but this will get you "in the ball park".

Here's a little chart I put together that contains all the stock settings and jetting. The "-3" (or "-4" for some sets) after the needle numbers indicates their stock clip position .....

CarbSpecsReducedSize.jpg


If you plan mods to the bike like different pipes and pod filters, the larger mains needed for those will probably require setting the needles a step leaner to the #2 slot.

You are lucky in that you have the 2 best BS38 carb sets. As time went on, the carb sets got better and more refined. Bolting them together as a "bank" and adding the linkage with a single throttle cable was a big improvement. They are easier to sync and hold that sync better than the older 2 cable models.

Although your 2 sets look very similar externally, the '78-'79 set had some major changes internally. This was prompted by E.P.A. mandates and the carbs had to be made to work more efficiently and meter fuel more precisely. The slide was redesigned and the needle was spring loaded. If you have the opportunity to try both sets, I think you'll find (as I did) that the later set operates more smoothly through the low and midrange RPMs. I attribute that to the spring loaded needle. If you grasp the needle as it hangs out of the bottom of the slide on the older set, you'll notice you can move it up and down about 1/2mm. Now, a needle clip position changes the height by 1mm so that means this needle is free to jiggle around and change it's height slightly during operation. That doesn't make for very precise fuel metering. I can see why they had to add the spring.

Don't get me wrong, the '76-'77 version is a wonderful carb set. It has great midrange power because of the large Z-8 needle jets employed. It's just a little rougher in the idle and midrange areas than the newer set. Nothing you can't live with though, and like I said, the midrange "umph" is very good, a bit better than the later set.
 
I have pods and straight-through exhaust pipes (no silencer/mufflers), so it looks as if I should be ordering larger main jets while I have the chance. Your detailed chart shows 122.5s should be in there now. What other sizes are available to fit into the 76/77 BS38s, and what would you recommend?

Anlaf
 
Sizes come in 2.5 number increments. You'll probably need something up in the mid 130s somewhere. I would get 2 consecutive sizes to start, either 132.5s and 135s or 135s and 137.5s. Try them both and if neither is right, it will at least show you which direction you need to move in (most likely bigger). You will also want to get one size larger pilots (27.5). When you increase the mains as much as you're going to, that will make the upper midrange too rich. You compensate for that by lowering (leaning) the needles a step. That will also lean the upper part of the idle or pilot circuit and the larger pilot will be needed.

Now, you may read some jetting suggestions around the web that call for mains in the mid 140s. That will work on some of the other 650 carb sets but your '76-'77 set is the exception. The large Z-8 needle jets, the largest ever put in any 650 carbs, won't allow you to go that big without incurring midrange break-up that you won't be able to tune out with needle adjusting.
 
That is very helpful, 5Twins. I see that MikesXS has them. I'll put in an order for main jets and pilots. My 78/79 BS38s will be on the project bobber, also straight-through pipes and pods. What sizes should I order for that set?

Anlaf
 
Most of us prefer to use genuine Mikuni jets. The ones from Mike's are knock-offs and cost about the same so we don't bother with them. To get you started and save a bit of money, you can use the jets from your '78-'79 carb set in the '76-'77 set. The 27.5 pilot and 135 main should be very close to what you need. For the '78-'79 set, that is one of those "other" 650 carb sets I mentioned. You will want mains in the mid to low 140s for this set. Once again, one up on the pilots (to a #30) and needles lowered (leaned) one step. Then, it's just a matter of fine tuning once the bikes are up and running. Try one size larger or smaller main jet until you find the right one. It should be within a few sizes of the ones I've told you to start with. If you get lucky, it may be that 1st one you try. But, you won't know for sure unless you try another. That's one of the 1st rules of jetting - when you think you're right, try the size above and below to make sure.
 
Thanks again, 5Twins, I will do exactly as you suggest and use the jets from the 78/79 set. I am looking forward to getting into learning more about carbs - for years it has all been a mystery to me.

Anlaf
 
5twins said:
Most of us prefer to use genuine Mikuni jets. The ones from Mike's are knock-offs and cost about the same so we don't bother with them. To get you started and save a bit of money, you can use the jets from your '78-'79 carb set in the '76-'77 set. The 27.5 pilot and 135 main should be very close to what you need. For the '78-'79 set, that is one of those "other" 650 carb sets I mentioned. You will want mains in the mid to low 140s for this set. Once again, one up on the pilots (to a #30) and needles lowered (leaned) one step. Then, it's just a matter of fine tuning once the bikes are up and running. Try one size larger or smaller main jet until you find the right one. It should be within a few sizes of the ones I've told you to start with. If you get lucky, it may be that 1st one you try. But, you won't know for sure unless you try another. That's one of the 1st rules of jetting - when you think you're right, try the size above and below to make sure.
Click to expand...
hi .I know this is an old post.i have a yamaha xs400.just bought the bike as a project. having trouble jetting the carbs.the bike has short exhausts with mufflers ,2 pod filters,42.5 pilot jet,140 main jet, need positioning 1.this bike runs not to bad.however the right exhaust is warmer than the other.popping from the left exhaust now and again.the carb stubs are also linked with a fuel line ?? only runs on prime vacum pipe to tank disconnected. is there anything I could do to improve the bikes performance. I don't understand why the carbs are linked.bike also runs on higher octane fuel.any advice appreciated.
 
Honda600rr said:
hi .I know this is an old post.i have a yamaha xs400.just bought the bike as a project. having trouble jetting the carbs.the bike has short exhausts with mufflers ,2 pod filters,42.5 pilot jet,140 main jet, need positioning 1.this bike runs not to bad.however the right exhaust is warmer than the other.popping from the left exhaust now and again.the carb stubs are also linked with a fuel line ?? only runs on prime vacum pipe to tank disconnected. is there anything I could do to improve the bikes performance. I don't understand why the carbs are linked.bike also runs on higher octane fuel.any advice appreciated.does this bike have a rev limiter.?
Click to expand...
 

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