BS38 Tuning Issue

[79josh81]

Could be, what about spark? poor spark can make a cylinder act rich. Were floats leak tested?
lotta carb info here. keep reading. Mailman’s XS2 a full on restoration

Spark I’m not sure. I still have the breaker point set up on this bike but it’s all recently been serviced along with the advance unit and rod. Also has brand new plugs, wires, and coils. I suppose I could swap the plug wires and see if the left side starts having issues. When I got the bike stock and before I did anything to it I seem to remember noting the same issue on the right side.

The fact that I can turn my right side mix screw in all the way and still have a good idle tells me it’s a fuel problem somewhere in the carbs.

As far as the floats, I think they’re good. I didn’t do the dunk test but I know I checked them when I pulled them and they looked good and there wasn’t any fluid sloshing around inside of them.

 

[79josh81]

Assume???


The pilot circuit relies on a vacuum ...........as long as the circuit is in the fuel then it will draw the fuel into the idle and pilot circuit........don't think this will cause to much of a lean/rich running.................lean or rich circuit is more controlled through the mains and needle setting............Yes float height is critical for this........

At the end of the day it is a combination of getting all of the sums to match

That’s what I originally thought but I didn’t know if I was missing something with all this talk about float heights.

My main issue is that my right carb pilot mix screw can be turned in all the way and the right cylinder still runs fine all by itself. Obviously the pilot mix screw isn’t doing it’s job (unless fuel is bleeding through somehow from a bad o-ring or the enricher) because I should be able to completely shut of the idle’s fuel flow from turning in the pilot mix screw all the way.

 

[79josh81]

Once the wife and kid go to bed I’ll get out there and start checking some of this stuff. My wife calls my motorcycles my girlfriends. And she doesn’t like when my girlfriends take away from family time otherwise I’d be out there right now tearing shit apart.

 

[79josh81]

So yesterday I finally got at the 79’ with the BS38’s because I’ve been tied up working on an 81’ lately. I ordered some new pilot mix screws and needle jet o-rings ahead of time from mikesxs.

So my issue has been the right side carb like I stated earlier. I pulled the carbs, drained them, and started pulling them apart. Before I took them apart, I tested the slide diaphragm and choke by raising the slide and plugging the hole and both slides dropped very slowly. When I let my finger off the hole they both drop fast. I flipped the carbs over and removed the float bowls and everything looked normal. I tried to remove the float bowl gaskets to measure float height and destroyed one of the gaskets so I ordered some new ones. The floats measured out within spec. I then removed the floats and pulled the float needles and set them aside.

At this time, with the carbs upside down, I pushed in the slides just to see if they would return to the idle position with the springs upside down. The left side slide made it back to the idle position, the right side slide (the side that’s having the issue) seemed to get hung up and wouldn’t return to the idle position unless I helped it a little. Not sure if this would cause the right side to run richer but it was something to note as I disassembled the carbs further.

Next I pulled out the pilot mix screws. Everything, as far as the tips of the screws go, looked normal as far as I could tell. Both mix screw springs were present. The o-ring on the left side mix screw (the side not having any issues) was broken/torn-through so I’ll be replacing that. With the choke seeming to be working from the slide drop test and the mix screws looking good, I’m still not understanding what would cause me to be able to completely close the right side pilot mix screw and still have the bike be able to idle on only the right side cylinder by itself.

Then I pulled the carb tops and removed the slides from the carb bodies. I checked the diaphragms against a bright light and didn’t see any holes. The one thing I did notice is that the right side diaphragm seemed thinner than the left side. More floppy I would call it I guess. But with the costs of new slides and diaphragms and the fact that the diaphragms I have seem to not be leaking, I’m just going to reuse them.

Getting back to the right side slide sticking issue that I mentioned earlier, I examined the slide and the carb body to see if the slide would be hanging up on any gunk or anything and didn’t notice anything. I reinstalled the slides and just tested them by pushing them through the carb bodies by hand and they both seemed to slide through easily with neither getting hung up on anything. I will clean the slides and carb bodies with some solvent and a rag though before I reassemble to make sure they’re clean. As for the springs, they’re the same size and both seem to have around the same pushing force when I squeeze them together. Not sure how else to tell if a weak spring is causing the “sticking” issue I mentioned before. I did look at getting new springs but wasn’t able to find any for the 78-79 bs38’s.

Next I pulled the needle jets, or tried to. I know it was mentioned that the o-rings could be bad and causing my right side symptoms but they seemed to be in place pretty solid. In fact, they were completely seized in and I wasn’t able to push them out by hand, even with the little bit of liquid wrench applied. I ended up having to resort to a needle nose pliers to kind of break them free. They got marred a little bit on the rim of the jet but not bad and nothing I did distorted any of the portion of the jets where the actual holes are so they should be fine. Once I broke them free I was able to work them out. Both o-rings were intact and seemed to be fine but I’ll be replacing them with the ones I ordered from
Mikes.

So that’s as far as I got so far. Any thoughts on anything that I mentioned? I’m still not understanding my right side mix screw situation and why it doesn’t completely shut off the idle when completely leaned out (all the way in) like the left side does. And I’m still not understanding what might be causing the right side to run a little rough and more rich than the left at this point.

 

[TwoManyXS1Bs]

When the mix screw has minimal or no effect, it's discharge port may not have enuff influence, overshadowed by fuel flow out the bypass (transition) ports.

Have a look at post #90.
http://www.xs650.com/threads/on-the-road-again-at-40-the-bike-that-is.49360/page-5#post-511978

 

[79josh81]

So I read the link....are you saying that my throttle plate on my right side might be open further than the left causing fuel to be richening and drowning out the mix screw setting in the right carb because more of the right side transition hole being exposed?

 

[gggGary]

Describe your synch method?

 

[TwoManyXS1Bs]

Could be.
The important thing is learning how these contraptions work...

 

[gggGary]

Are you talking about the needle jet o-rings?

No he's at the base of the emulsion tube, AKA needle jet, center bottom of carb body. During your work did you drop, clean the the emulsion tubes? I use that name for a reason. It's kind of overlooked that much of the fuel delivered to the intake should be in the form of an emulsion, a fine frothy mixture of fuel and air. Those tubes are cross drilled with fine holes to help make the emulsion. When overhauling most to all those holes are often found plugged!
I'm riffing from (poor) memory but that's one of the points of evolution through the carb years, later carbs are better than early units at making that more easily burned mixture. A big change with the BS34s was moving the pilot circuit up into the body so it receives it's supply as an emulsion rather than straight fuel. Early morning, no research done. LOL
Find and use a small brass or copper tube to act as a driver from the top if the needle jet is reluctant to come out. Use a hair drier to help soften decades of varnish acting like glue also.

 

[gggGary]

 

[79josh81]

Describe your synch method?

I use the fluid manometer method (yardstick type but not on an actual yardstick). I don't have carb boot barbs so I have nipples that I screw into the ports on the sides of the carb bodies. I hook the tubes from the manometer to those. Then I adjust the throttle plates and throttle stop screw accordingly until the fluid is level with each other in the manometer.

 

[5twins]

With the slide assembly removed, I tap on the tops of the needle jets with a wooden dowel. I don't over-do it. If they won't move, I resort to more heat and more penetrating oil. Sometimes they need to be partially driven out then back in several times to loosen them up enough.

 

[79josh81]

Could be.
The important thing is learning how these contraptions work...

Yeah I know. It's difficult to get because I don't completely understand how all of the little holes work together yet. I have a general understanding though. I wish there was a Magic School Bus episode of the BS38 and BS34 carbs.

 

[79josh81]

No he's at the base of the emulsion tube, AKA needle jet, center bottom of carb body. During your work did you drop, clean the the emulsion tubes? I use that name for a reason. It's kind of overlooked that much of the fuel delivered to the intake should be in the form of an emulsion, a fine frothy mixture of fuel and air. Those tubes are cross drilled with fine holes to help make the emulsion. When overhauling most to all those holes are often found plugged!

Yeah I just pulled the needle jets the other day. I made sure all of the holes were cleared, cleaned them again, and put new o-rings on them. The old o-rings were definitly flattened out so I guess we'll see if the new ones help.

 

[gggGary]

I use the fluid manometer method (yardstick type but not on an actual yardstick). I don't have carb boot barbs so I have nipples that I screw into the ports on the sides of the carb bodies. I hook the tubes from the manometer to those. Then I adjust the throttle plates and throttle stop screw accordingly until the fluid is level with each other in the manometer.

While your carbs are off do a visual confirmation that the butterflies are exactly equal. If you slowly close the idle speed screw, the last tiny sliver of light looking through the intakes is a VERY accurate gauge, IMHO better than any stick, drill bit, etc. method. The thing is if you need to adjust far from this with a manometer (say more than 1/4 turn of the synch screw) that's prolly an engine, not carb issue..........

 

[79josh81]

With the slide assembly removed, I tap on the tops of the needle jets with a wooden dowel. I don't over-do it. If they won't move, I resort to more heat and more penetrating oil. Sometimes they need to be partially driven out then back in several times to loosen them up enough.

Yeah I was eventually able to get them out after using some penetrating oil and working them back and forth a few times.

 

[79josh81]

While your carbs are off do a visual confirmation that the butterflies are exactly equal. if you slowly close the idle speed screw the last tiny sliver of light looking through the intakes is a VERY accurate gauge, IMHO better than any stick, drill bit, etc. method. The thing is if you need to adjust far from this with a manometer (say more than 1/4 turn of the synch screw) that's prolly an engine, not carb issue..........

Yeah that's how I usually get them close before I put them back on the bike. And then it's a very little bit of adjustment to get them right with the manometer. Definitly less than 1/4 turn.

 

[5twins]

There are 4 tiny holes that feed the pilot jet's mixture into the main bore. All must be clear. 3 come directly from the pilot jet, the 4th goes to the mix screw then in to the main bore. The mix screw has control over the flow through this one passageway. If some or all of these tiny holes are plugged, the mix screw isn't going to have any affect .....

The pilot jet is mounted in the float bowl but it flows it's fuel/air mix into the carb body through a small hole in the bottom of the carb .....

Blow carb cleaner through that with compressed air and watch for it to exit out the 4 little holes in the main bore. The mix screw will need to be removed and then the outside of it's hole blocked with a finger. If you block the outside mix screw hole and the three holes, cleaner will only come out the inner mix screw hole. Block the outer and inner mix screw holes and cleaner will only come out the 3 holes. Block all 4 inside holes, blow cleaner through the outer mix screw hole, and it will back-flush through and come out the pilot inlet hole on the bottom of the carb.

Another issue could be if you replaced the float bowl gasket and didn't remove all the punch-outs. Sometimes the punch-outs are stamped but the "dot" of paper isn't removed. Leaving some in would block some of the passageways .....

 

[79josh81]

When I get outside I'm going to take a look at the transition holes like twomany mentioned earlier. I wish I would have checked them right when I pulled them off the bike. The sync might be all messed up now since I had the support bracket on the bottom and the top butterfly plate off already.

 

[79josh81]

Thanks 5twins. That's golden as far as helping me understand the pilot circut. I'll follow that routine when I get outside and make sure all those passage ways are clear. As far as the gaskets, I never replaced them but I recently just ordered new ones so I'll make sure the holes are all punch out when I get them.