Runs fine with mix screw bottomed out (BS34)

[Forest]

Hello all

I bought my 1981 Special four years ago, but I have only recently gotten it approved for the roads. There are several things that need to be fixed still. At least now I can test drive it when I've made changes.

That brings me to the issue; well.. maybe it's not an issue. The engine runs fine I think. It is equipped with two BS34 carbs with what I think is stock jetting: #42.5, #132.5 and a #130 pilot air jet. I've put on UNI foam filters. The ones where the ends are closed off with plastic or PU. The exhaust system consists of two separate single walled headers with megaphone mufflers. It's those aftermarket ones. Not very large and with a reflective insert. I've padded them slightly, but they are quite open.

I would expect to have to change the jetting to a richer mix, but as it is now, I have the idle mix screws turned a quarter of a turn out on both sides. If I back them out further, the idle rpm drops. If I turn them all the way in, the rpm goes up. I guess the idle mixture must be rich. I just can figure out why. I did not expect to have to install smaller than stock jets. I have a feeling that I'm missing something. I do use enrichment, when I start up, but only at the very start. If I pull the knob out later the engine stumbles and almost dies.

What else can I say about my setup..
I haven't synchronized the carbs properly. Only roughly with my eyes on the bypass holes. Ignition timing flutters a bit due to a worn ATU, but it runs reasonably well. Oh.. and I'm just now remembering that I sometimes see a drop of fuel on the edge of the bell on the right hand side carb, when I remove the filter. I had both carbs off a couple of days ago and the intake boots were a bit wet. I don't know if that's bad.

Any ideas to what I may be missing here?
Thanks..

 

[Forest]

Oh wait. I forgot about the floats. I checked the fuel level with a piece of clear tube. I haven't had much success with this method earlier. I attached the tube, opened the drain valve and opened the petcock. There was air standing in the tube, so i blew the bubble in to the bowl. I turned he petcock off and ran the engine for a short while. Then I turned it back on and measured the level. 6 and 7 mm from the edge respectively. When I measured the brass floats directly, one was spot on 27.3 and the other was a bit lower but close.

I don't know which test to believe, but I kind of expected the level to be high

 

[JJames]

A head scratcher for sure. Most of your symptoms could point towards float height although this usually makes the engine unresponsive to mix screw adjustment. Enrichment to start while causing stumbling when warm is as it should be.
How does it run otherwise? Idle? Rev?
Have you been able to take it on the road yet.

 

[Forest]

Well.. Idle is a bit shaky, but that may be connected to slightly changing ignition timing or a lack of synchronization.
It works best under load 2500 to 3500. It's pretty smooth in that area. Longest trip I've taken is about 100km, but it has left me stranded once due to faulty electronics so I don't go very far.

 

[JJames]

Just looking at your profile pic. Those single wall headers are huge. What diameter are they. Not saying that's the cause of your issues. Hopefully others will weigh in soon.

 

[Forest]

That's my Savage. She only has the one pipe, so she compensates. Two inches inside a 60mm shell.

Let me se if I can't find some Yamaha pictures...

 

[Forest]

 

[Forest]

 

[Forest]

This is the most recent picture. I was stranded with no spark on the right cylinder, so a had to do some digging. The next two is of my fuel level test (obviously). As I understand it, I'm supposed to measure between the edge of the carb body and the level in the middle of the hose. Ignoring the capillary effect.

 

[JJames]

Stock "81 bs34s should have composite floats. Do your float bowls have a drain bolt on the bottom or a screw on the side.
Just to clarify, when you say fuel level was 6 and 7mm from the edge, was it above or below the edge (gasket?) We love pics.
Edit: you posted while I was typing.

 

[JJames]

That's my Savage. She only has the one pipe, so she compensates. Two inches inside a 60mm shell.

Let me se if I can't find some Yamaha pictures...

Doh! I was distracted by the pipe. Didn't really look at the rest.

 

[RC4MAN]

Stock "81 bs34s should have composite floats. Do your float bowls have a drain bolt on the bottom or a screw on the side.
Just to clarify, when you say fuel level was 6 and 7mm from the edge, was it above or below the edge (gasket?) We love pics.
Edit: you posted while I was typing.

In his lower photo you can see the drain screw peeking under the float bowl if that's any indication.

Curious thing about these engines. Back in the mid 70's when I built my first Street Tracker utilizing an engine from a 73 TX 650, I used K&N pods with a Hooker 2 into 1 header pipe and expected to need larger jetting, went up to 135 on main.
Started it for the first time, took it out for trial run came back the plugs were white. Went up another main jet size and out for another run. Again pulled the plugs and they were white. Went up another size and while it would start it didn't run well.
Needless to say I spent some time scratching my head thinking what was the issue.
Spoke with some others and a guy suggested going back down in jet size, that the mixture was so rich it was washing the plugs clean. Ended up going back to stock jetting, bike ran fine and color came back into the plugs.

 

[Forest]

I think my former dutch '81 is similar to a U.S. '79. It has brass floats and points ignition.

I can't be sure whats original on it. I bought it as a "cafe racer". Not as pretty as an early '70s stock bike, but it was significantly cheaper. I haven't changed anything on the carbs. I just took them apart back when i had just gotten it, and then again this sunday. It came with no name foam filters, stock headers (cut with an angle grinder) and completely empty megaphones. It didn't sound very good. Not that anything could be heard over the cam chain tensioner The mix screws were turned all the way in even then.

What do you think about the fuel levels in the pictures? Is it very bad? I am loath to change it, since the measured float height is good. I measured it with the cabs on their side so as not to put too much weight on the valves.

 

[Jim]

I am loath to change it, since the measured float height is good. I measured it with the cabs on their side so as not to put too much weight on the valves.

It's generally thought (at least by me ) that the weight of the float is factored into the height adjustment. In other words, a setting of 27mm takes into account the sag from the weight of the float. Laying it on it's side might not be an accurate check. Having said that, the levels in your pics look pretty close to me.... so I don't think that's a problem.

Re: the mix screw all the way in. As a general rule of thumb, if you run out of adjustment on the mix screw, you go up or down one size on the pilots. Since yours are screwed all the way in (lean), it means you're too rich. I'd try 1 down on the pilot and see what happens.

 

[Mike G]

I had a similar issue with my 80. Turned out to be crud in the threads of one idle screw so when it was “all the way in”, it wasn’t. I found it by comparing how far both screws stuck inside the throat . Once I cleaned the crud out, they both looked like this when all the way in.

Was easy to adjust after that and ran/runs fine.

 

[JJames]

Or p.o. put them in good and tight.

 

[dave_a]

Look at your needle jet (and jet needle) for wear/ovality. These things shake. stuff wears.

 

[Forest]

Was it the ceramic part that remained white RC4MAN? I don't really have any experience with plug reading. Actually I haven't inspected my plugs since the last changes to my exhausts. I should probably do that now. They were all black and velvety back then.

About the mix screws: They were a bit hard to turn and it was difficult for me to 'feel' the bottom, so I lubed them with a drop of WD40. I've had the carbs off since then and both tips protrude into the bore, just like in your picture Mike. The jet needles look very good. They sit in the second position from the top and have that big plastic shim under neath. I must admit that I havent checked the emulsion tubes this time. If the butterfly valves are set correctly (I aimed at the first of the three bypass holes being partly visible from the front) I don't think a bad emulsion tube could cause idle issues. If the idle backstop is turned too far in, it's another story of course.

About the pilot jet size: I suppose there are smaller jets than the 42.5, but that is the stock size for this bike/carb isn't it? I live very close to the sea and have foam filters and loud pipes, so I dind't expect that to be the issue. On the other hand it does sound very logical...
I'm just remembering now that those jets were very bright and shiny; definitely not from 1981. They have the two squares stamped on them, but they don't look very Mikuni to me. Do we trust aftermarket jets? I believe it's the VM22/210 that fit here, isn't it?

 

[RC4MAN]

@Forest , Yes

 

[Forest]

I've just received my ATU assy in the post. It turns out I've ordered a used unit, so the centrifugal weights are just as worn as the old ones It's my own fault. I shouldn't have been shopping on my phone.

I really wanted to have a good ignition timing set up before doing anything else. Hmm.. Maybe a small drop of solder will do the trick. Or JB weld even. I can see that MikesXS sells these weights without the rest of the assembly. Have any of you tried them? Are they any good?

Thank you gentlemen for all your input. I hope to be able to give an update before long.