1976 Running Rich

[5twins]

On these BS type carbs, they are true mixture screws. They adjust the amount of fuel/air mix going into the carb. That means turning them out makes things richer. The fuel/air mix from the pilot jet is delivered into the carb's main bore through 4 tiny holes. Three are clustered together at the top of the bore by where the butterfly plate closes. The fourth is off to the side and comes from the mix screw .....

The flow out of the mix screw hole can be adjusted, the flow out of the three holes cannot. The only way to change the mixture strength coming out of them is to change the pilot jet size. The mix screw gives you control over roughly 25% of the flow and is why it is for fine adjustment only.

 

[Wilsonsk]

Thanks for the great explanation. I got my needles moved down 1 slot, E-clip is now in the second groove from the top instead of the middle. I’ll get out and ride it some in the next couple hours. I also ordered 132.5 jets, If I didn’t do it curiousity would kill me not knowing, I have the 135s that came with my kits as well as the 27.5 pilots.

 

[5twins]

Well, that's one of the first rules of jetting - when you think you're good, try the size above and below to confirm. You'll never know otherwise.

On the other 650 carb sets, one to three up on the mains is the norm for mods like yours, but your '76-'77 set is different because of the very small original mains (#122.5). Your carb set usually requires more, like 4 to 5 sizes up. But, the very large Z-8 needle jets it came with will limit the size to the low to mid 130's. Many of the other sets can run mains into the low 140's because they came with smaller, leaner needle jets, but you'll never get that big on your set. Like I mentioned earlier, 132.5 will be about it.

 

[5twins]

I'll also mention that the factory recommended mix screw setting for your carb set is 1.5 turns out. But this is just a recommended starting point. From there, you need to fine tune the screws for best running, for fastest and smoothest idle. You should find that within a half turn either way from that factory recommended setting. If you have to go more, that indicates the pilot jet needs changing.

 

[Wilsonsk]

Thanks again for all of the great info. I've avoided this process when it has come up in the past, I did a CM400 a few years back, left the restrictive exhaust collector thing on the bottom and left the air box so I didn't have to try to re-jet, the Keihins on that thing are pretty weird - anti-backfire cut-outs, accelerator pump, 2 main jets - I opted not to learn on that one. With this one stock exhaust isn't an option, the exhaust I found looks and sounds great, just need to get it dialed in, again, thanks for the forum here and all of the help, I never would have considered leaning the needle, it seems counter intuitive when you are generally try to make everything richer. The bulk of my experience is usually just cleaning and putting back together and mostly snowmobile Tillotsons (yuck) and Mikunis.

 

[5twins]

What you have to know and realize is that the three circuits in you carb overlap the one next to them a little, and changing one will "bleed over" and effect part of that adjoining circuit ......

Larger mains also richen the upper part of the midrange, eventually making it too rich and causing stumbling under heavy throttle applications. Changing the needle effects both the lower portion of the main and the upper portion of the idle circuits. You need to do it to fix the stumbling caused by larger mains but it's a double edged sword. It also leans the upper part of the idle circuit and can cause a flat spot there, requiring the larger pilot as well.

It can also make the midrange rather flat and unresponsive. For that reason, I like to lean the needle as little as possible, just enough to cure the upper midrange stumbles. Sometimes that can be done with just a half step adjustment. To achieve that, you shim the needle with a half MM thick washer. Mikuni needle slots are 1mm apart so adding a half MM washer effectively gives you that half step setting.

 

[Wilsonsk]

Thanks for the tip, I ordered a pack of washers. Just curious, are the needle jets not removeable/interchangeable on BS38s? The Keihins in the Honda are, my carb kits for those came with new ones.

 

[Wilsonsk]

I got out and rode it, it is barely rideable, starts right up, spits and sputters, left cylinder seems to fire intermittently, really can’t get it up past 4,000. Could it be that lean on the pilot to cause it to run that bad? Or did I cause some other problem while I was in there? Suppose i need to change back my needles and see if it goes back, or I put in 27.5 PJs in see if that helps?

 

[5twins]

Changing the needle setting shouldn't make it run that bad. Maybe you screwed up on the reassembly. Maybe one of the diaphragms isn't seated correctly in it's groove. I would install the 27.5 pilots, I'm pretty sure you'll need them if you leave the needles leaned a step.

 

[Wilsonsk]

Took the slides out again, found 2 suspects, when I pushed up on the needle on the left slide, it just went up about half way, then hit the plastic stopper, which was wedged cockeyed about halfway down, not sure if I did that on assembly this morning or disassembly tonight, can’t be sure, but i double checked the clip being 2nd to the top on both and that the stopper was all of the way down on both. The othe suspect was the left side battery power for the coil, I was just generally looking at the wires that get disturbed when mounting and dismounting the tank and left one just came unplugged, the female side was stretched out, I gave it a squeeze and it fits much tighter now. I didn’t change the pilots yet, one variable at a time. I put it all back together, BIG difference from the bottom up to 5,000, like the front wheel pulled up a couple inches on its own, at 5,000 grand it’s a bit sputtery, didn’t have the road to see what happens above that, but holy crap, really woke up the bottom end. Could the pilot cause trouble up there? I’m at 130 mains, clip in the second groove from the top on the needle, stock pilot. Still idles great.

 

[5twins]

No, the pilot jet shouldn't have much of an effect at 5K. Yes, it would still idle OK with the needle leaned. What that effects is the transition from idle to midrange, say going through the 2 to 3K range. It may create a flat spot there.

 

[Wilsonsk]

Any thoughts on what’s happening at 5K? Is that about where it transitions to mostly main jet? Does it need the needles dropped one more notch? Bigger mains? Feels like a pretty distinct change when it hits that spot. I need to ride it some more and get a better feel for what’s going on, really woke things up down low, I don't see spending a lot of my riding time over 5,000, pretty frenetic up there, but I'd like it to at least run decent.

 

[5twins]

Personally, I don't like to run the needles at their extreme ends (last slot) but you could try it I guess. Yes, 5K is the upper part of the midrange to main transition area. Usually, when the main gets too big, you will get break-up in this area (4 to 5K) but only under heavy or full throttle applications. It will usually run through the range OK using light throttle. That's why for testing, you have to use lots of or full throttle. You won't normally ride like this but for testing, you need to do it. These CV carbs are rather forgiving and will mask over small jetting "glitches" unless pushed hard.

This break-up in the upper midrange is usually the limiting factor on how big a main you can run. You "fix" it by leaning the needle a step but if you then go up another size on the mains, that usually brings the break-up right back. So, you have to step the main jet size back down.

 

[Wilsonsk]

Ok, I'm going to put a new set of plugs in it and ride it a little before I make any other changes. I got my .5mm washers today, so I suppose top slot with a washer could be an option, or go back to 127.5 main jet. I haven't tried the 127.5 with the needle leaned 1 groove yet, I really need to have that data point.

I was on it hard last night when it was stumbling at 5,000, I let it idle for 5 minutes or so, putted for about a half mile, then got on it hard. The difference in throttle response up to 5K was very noticeable, not worth it if it doesn't run smooth though. I haven't tried easing past it yet. I'll get a better feel for what it's doing, get it good and warmed up and see what I have.

Thanks for taking the time to coach me through this, I know you have done it hundreds of times and I did sift through old threads but they are never 100% relevant to someone else's situation. I love this bike though, the size, the power range, the weight, just feels very right, I've had a dozen bikes or so over the years, this one is my favorite already.

 

[5twins]

OK, for shimming the needle in your style of slide, the needle sits down hard on the bottom of the slide, so to shim it, you will need to place the shim under the e-clip. That will raise (richen) the needle setting the equivalent of 1/2 step. Placing the shim washer on top of the e-clip won't change the needle height, won't do anything. So yes, to get a setting 1/2 step leaner than where you are now (the #2 slot), move the e-clip to the #1 (top) slot and place a shim washer under it. That will give you a setting equal to 1.5 (if there was such a thing).

I happen to be running a couple sets of carbs like this right now. They are '78-'79 carbs that I've fitted XS400 5Z1 needles to. This needle is richer at the beginning and end of it's travel but leaner through the rest. This leaner needle allowed me to go up another size on the mains but did require another half step leaner (from #2 to #1.5) to do so.

You asked in an earlier post about changing the needle jet. There's really not many others available except what came in other 650 carbs so folks don't usually mess with them. The needle jets in these 650 BS38s are all "Z" sizes (an 8 in your set, a 6 or 2 in the other sets). This is quite a large series. Many other bikes use similar CV carbs with the same style of needle jet, but from what I've seen, they are all much leaner (X or Y series), probably too lean. Because of the large Z-8 needle jet in your carb set (biggest of any 650 carb set), you get very strong midrange performance, about the best of any 650 carb set. Change to a leaner needle jet and you would no doubt lose some of that. So, I'd keep working on sorting the mains. Yes, try the 132.5's to rule them out but you may need smaller. If you're still running the stock airbox and the mufflers are your only change then yes, maybe a smaller main is called for. The 132.5 mains I was running were with those nice, free flowing Commando mufflers and UNI pod filters.

 

[Wilsonsk]

Thanks for the great info, I understand the washer half-step process. I'll stick with playing with mains for now. I pulled the plugs just now and put in new. When I was running everything stock in the carbs - 122.5 MJ/middle groove/25 PJ - plugs looked good, tan/brown. I swapped to the 127.5 MJ (no other changes) and probably rode 30-40 miles, much of that in the 55 mph range. I swapped to 130s and dropped the needles down one grove and it ran horrible (pretty sure unrelated to those changes) and I rode it literally a couple miles last night. The plugs were black, likely due to the rich stock needle jet position? At 3,000 rpms running around back roads I was probably never out of the needle valve, now that I think about it, ALL of my riding is, and most likely will be there. I'm just going to ride it as is for a bit. Current set-up:

130 MJ
needle leaned 1 step (clip in 2nd groove to the top)
25 PJ (I haven't changed this, the bottom end has still been pretty flawless, maybe if I sort out some of the other things I'll look closer at those. Pulling the carb bowls off would be good anyway, this thing sat for 26 years with gas in it, came surprisingly clean, but in the past I usually find stray hunks of junk even after a thorough cleaning, you get stuff flowing again and junk loosens up).

 

[5twins]

Be aware that if your carbs were really gunked up and sat that way for some time, the gas varnish can eat away at the jets, making them bigger and richer. Maybe that's part of your issue here. Your pilots may no longer be the #25 size, they could be bigger now, and that's why you haven't needed to change them when you leaned the needles. Also, the needle jets may have gotten eaten away and could be larger and richer as well. Since the metering holes are so small, it's pretty hard to tell just by looking at them, and difficult to measure in any way. Sometimes you can tell by comparing them to a new jet.

I recently replaced some needle jets in my '78-'79 carbs. Over the last few years, the plugs have gradually gotten darker and darker. Nothing was changed in the way the bike was set up, or jet sizes, so I figured it might be the needle jets wearing. I found some N.O.S. originals on eBay and comparing them to the old ones, you could see the hole in the old one was bigger. The old jet is on the left, new one on the right .....

The new jets lightened the plugs up considerably.

 

[Wilsonsk]

All of the brass in the carb is pretty black. I went out and rode it some more, the sputtery stuff lasts up to at least 6,000, I didn’t really fight with it past that, everything is awesome til about ~5,000. Tomorrow I’m going back to the 127.5 mains and try that with the needles where they are. Can you by new z8s? I see niche cycle has needles for mine, worth replacing them or just move forward with the knowledge that they may be big?

I think I’ll start working back to the 127.5s again next, I may pull the bowls and look at things, make sure some debris didn’t find its way in there.

 

[5twins]

You can't get any of the original needles or needle jets anymore but they are making repops. I usually like to use genuine Mikuni parts and jets but in this case, you don't have much choice. Needles don't often need replacing but you can look yours over to see if there are any obvious worn areas. I've replaced needle jets more often than needles.

It does sound like your mains are too big. What are your mods exactly again? Just those mufflers with the drain strainers in them, and the stock airbox. Who knows how well those mufflers are flowing. They may not even be flowing as much as a stock set, lol. So, you could need very little in the way of a main jet size increase.

 

[Wilsonsk]

I think I agree on the mods I have not really being much different than stock. The mufflers have a perforated disc about midway in the muffler, I added the strainers, then cut 3 3/4” holes in the sides of them, not much restriction, but not nothing either. My air boxes are stock including foam on the outside of the element, all in very good shape. It occurred to me in the middle of the night that I could leave the 130s in and run to Dennis Kirk and get a set of pods and see how it runs with those. I bailed on that idea this morning and put the 127.5s back in, it’ll be tomorrow before I can ride it. You never know, that half step to 125 might be the ticket. I think the pods may be a decent idea, I’d like to figure this configuration out first, the experience working through the process should make that a little easier. I’m not interested in wringing every last hp out of it, ultimately want it to run good, but if there is some easy hp to be had, and I’m already up to my elbows in jets, now’s as good a time as any.