ST50 Shifting issue + leak

Hi,

Two questions, that I hope some of you dedicated people can answer.

I had an engine restored, and the guy did a terrible job.
He had used lots of old worn and damaged parts, for an example a cylinder with a very large crack running through it, "fixed" with some kind of sealant and a thick layer of paint. This helped explain why it was smoking insanely to begin with.

The timing chain tensioner arm was old and worn as well, and did not apply enough pressure on the chain. The gear shifter spring didn’t apply enough pressure either, making it nearly impossible to go into 3rd gear.
Tappet caps was of poor quality and made in China, not original ones as otherwise agreed. Bad functioning clutch mainly due to clutch plates with quite a bit of wear. Worn clutch guide, wrong clutch nut used, and damaged lock plate as well. Several washers were missing.
The list was long.

Had the engine restored again, and all of these errors corrected.
This time it was done properly by a guy who used to own a moped repair shop back in the 70s and 80s, and who has a very good reputation. The engine now seems to be running good. No more white smoke, and seems to be going steady when idling.

Only one thing I'm wondering about, and one issue.

The issue is that the carburetor seems to be leaking from the air screw.
It seems to happen after a good run, a few hours after and in the coming days. Then it seems to stop.
As from what I can understand the fuel level shouldn't even be all the way up here?
This screw doesn't have an o ring, and shouldn't have one. Any of you who have experienced a similar issue? I haven't notice any fuel dripping from the overflow, not seen any traces on the ground at least...

The other thing is the gear shifting.
It shifts fine from the 1st gear to the 2nd, but only if my speed is at a maximum of around 10 – 12 km per hour. If I drive with a speed above this, i simply can’t shift gear. Whenever I push the lever nothing happens, but as soon as the speed drops I’m once again able to shift gear just fine. The gear pattern here is 1st gear up, Neutral one down from 1st gear, and the rest down.

The same thing seems to happen quite frequently when I try to shift from the 2nd gear to the 3rd. If I drive 25 - 28 km per hour or above, I’m often not allowed to shift gear.
Downshifting is however working perfectly at all speeds.
If I shifts gear before hitting these speeds, the clutch also engages perfectly fine every single time.
Is this normal for a bike like this?

Kind regards,
Thomas

The fuel leak...
You're right, the fuel shouldn't even be able to go that high in the carb.
The float could just be stuck. Sticking on a poorly fitting gasket, possibly binding on the pin or perch.
The float could be leaking...have a hole in it allowing fuel to leak IN...then it don't float.
The float height could be out of adjustment...float height should always be checked, and set when you clean, service or rebuild your carb. It's pretty easy while the carb is in your hand.
The float needle valve could be bad. It's the thing that stops the flow of fuel when the float rises to the correct height.


Hopefully racerx has some thoughts on your shifting problem.
Please tell us if you have a manual clutch, or a semi-automatic clutch??

kirrbby said:

The fuel leak...
You're right, the fuel shouldn't even be able to go that high in the carb.
The float could just be stuck. Sticking on a poorly fitting gasket, possibly binding on the pin or perch.
The float could be leaking...have a hole in it allowing fuel to leak IN...then it don't float.
The float height could be out of adjustment...float height should always be checked, and set when you clean, service or rebuild your carb. It's pretty easy while the carb is in your hand.
The float needle valve could be bad. It's the thing that stops the flow of fuel when the float rises to the correct height.


Hopefully racerx has some thoughts on your shifting problem.
Please tell us if you have a manual clutch, or a semi-automatic clutch??

Click to expand...

Hi, thanks a lot for your reply.

I've had the carb apart once before, in order to change the main jet.
Have a picture from when the guy who did a bad job with the motor rebuild it for me.
I've attached it, gasket looks rather new, but of course that doesn't mean that it fits perfectly.
Should I simply just remove the carb from the bike, and inspect it closer to make sure, that the gasket isn't binding to anything?
When I've opened the carburetor, I take it that I can follow this video to check the float height, or got any other good ideas for my kind of carb?


Could you perhaps explain more about checking the float needle valve, and how to check if the float is actually taking fuel in, - should I soak it in fuel in a bucket and see if it drowns?

Yes, would be great if he perhaps could let us know his thoughts on the shifting "issues". Clutch was rebuild, new clutch plates and so on. Perhaps it's working exactly how it should be, but would like to have it confirmed though, since this is new to me.. But so is the bike
The clutch is semi automatic on this model.

Thanks a lot for your help so far.

To check for leaking floats, I submerge them in carb cleaner for a few days by tying a heavy bolt with wire to them in a jar. Shake them by my ear to see if I hear any fluid in them. I also check to see if the floats themselves are level with each other. When rebuilding a carb I always replace the float needle AND the brass seat with OEM.

I've never heard of a 3speed semi auto with a 1up and 2down shift pattern. What year is your bike and engine?
Your shifting problem might be just that the clutch needs adjusted or there is a bent shift lever in there. I'm also wondering what the condition of your shift forks are in.

The ST models, marketed outside of North America, were more road-oriented. 1d/2u a.k.a. "international" shift pattern was the norm.

As for checking floats for pinhole leaks, heat & cold can be used. Place the float on a freezer, then submerge it on warm ~120-140F water. If there's a leak, you will see the telltale bubbles.

Shake it first. If you hear liquid, there's a leak.

Your shifting issue...it may very well be technique.

The clutch is basically a centrifugal type clutch.
When the RPM s are high, it's engaged. When the RPM s drop...at idle, it disengages and allows a smooth shift.

The shift lever does 2 jobs.
It acts as a clutch lever/pedal. Just like your other bike, or your car. When you move the shift lever up or down it disengages the clutch...just like your car...then let it out easy...just like your car.
The shift lever also changes gears each time you move it up or down...disengages the clutch and changes gears simultaneously.

Riding along, in gear, at high RPMs, let off the throttle, allow the RPM to drop slightly, then change gears slow(er) just like in your car.

I've heard it's tough to find neutral with a 1d 3up...or similar, shift pattern, and semi-auto clutch. Maybe easiest when stopped and at idle.

You maybe need to adjust the clutch preload adjuster too. Were you able to make sense of that procedure?

@cjpayne and @racerx - The bike is a model that was imported to Denmark in 1973, ST-50, also known as the Dax, so yes, it has a slightly different shift pattern.

Thanks for your suggestions regarding checking for a potential leak, I'll have to do that soon. I actually noticed that there was a bit of fuel leaking from yet another place, see the attached picture.
I did however try to quickly check the float adjustment (didn't see any signs of fuel inside the float), made it cut off the fuel a bit sooner, but didn't change anything. Issue still remains; when I've been driving for a longer trip, I see this small amount of fuel leaking in the following 2-4 days, and then it stops once again after a while. I also feel that the bike has a harder time keeping the engine running when I slow down my pace, for an example in order to stop. Might also indicate that there is still getting too much fuel to the carburetor perhaps?
Anyhow, reckon I'll still have to test the float as suggested by you.

@kirrbby @cjpayne funny that you mention this kirrbby, I wasn't really aware when I finally had assembled the bike. Assembled everything from scratch, except for the engine that is. I thought I could just switch gear by pushing the lever down, boy was I surprised Both managed to make several engine brakes, and get it to do a wheelie changing to the 2. gear
cjpaye, the engine ought to be in good condition. Several things were changed during the second time where I had one take a closer look at it, also related to the gears, since it had a hard time shifting to begin with. I think all should be good now in this regard however.
If you scroll all the way down to the bottom on this site, you can see the box with all parts that were changed.. So at least hope all is good, but don't know it of course.
http://mechmania.net/2019/06/12/1-year-later/

Hoping that kirrbby is on to something with the shifting technique, since this would (perhaps) be the easiest to cure.
I've had no gear machine before, so no practice, only from a car.
I see you say that the clutch disengages when the RPM's drops, and allows for a smooth shift.
Does this mean, that it's actually normal, that I can't make it shift from 1st gear (1 up) and to Neutral (1 down from 1st gear), right after I've let go of the throttle? Second gear on this bike is 2 down from 1st gear, meaning that it has to go through Neutral first, before I can get it in second gear. It is however not possible to do as soon as I releases the throttle, unless RPM is already quite low before releasing it - but after a while it is.. As soon as it drops in RPM.. Meaning that all of this is perfectly normal, and that it ain't about that I can't switch gears at certain speeds, but more that I can't switch gears immediately after having released the throttle?

Sorry if it sounded confusing, just trying to figure out what's normal here and what's not, since I've never had a machine like this, - but I appreciate all the help you can give me.
You mention that it's not easy to get it in neutral, - maybe easiest when stopped and in idle. But I need to get it in Neutral right, - in order to go from 1st gear to 2nd? I've attached a sheet from the user manual, - how it shows that the gears should be changed as well.
BTW; I've tried adjusting the clutch, no real changes here in this regard. Makes me believe even more that it's just me, and not the bike...

Sorry for these rookie questions guys! I owe a round

Starting off in neutral...
While at idle, simply click up 1 into first and let off of the shifter while sitting still. You'll be in first gear, but not moving...because at idle speed the clutch is NOT engaged. Then, slowly roll on the throttle. The clutch will begin to engage as RPMs go up, and you will begin to move.

Then, when you begin to wind out 1st gear, let off of the throttle, when RPMs drop a little, click all the way down, PAST neutral and into 2nd gear. Let off of the shifter as you begin to roll on the throttle. Now you're in 2nd.

When you wind out 2nd gear...let off the throttle...click all the way down again into third.

Because you have a semi-automatic clutch, you can stop completely while in 3rd...or ANY gear, without stalling the engine. Simply let off of the throttle and stop.
The RPMs will drop to idle speed and the clutch will disengage, allowing the bike to sit at idle...while running IN gear.
If you try to take off again...in third gear, you'll have NO power to speak of...same as in your car. But while stopped, at idle, you can change into ANY gear, or into neutral, and start over.


Neutral is halfway BETWEEN 1st gear and 2nd gear. So when you want to go into neutral...
While the engine is at idle speed, move the shift lever HALF WAY up from 2nd gear...OR... HALF WAY down from first gear. If you move the shift lever too far, you'll go PAST neutral and into another gear.

I am well-familiar with the Dax version of the CT70. The Dax was available in both 49cc & 72cc flavors, unlike the North American-spec CT70. That 1d/2u 3-speed was also never available on the CT70. No matter, semi-automatics are called that for a simple reason...the centrifugal (automatic) clutch action is only below engagement rpm. Above that speed, your clutch operates as a full manual. What you may not realize is that the shifter linkage also disengages the clutch, when the shift lever is moved to the limit, in either direction. You really need to understand how a typical hand clutch is used as that helps to understand what you're dealing with...and...proper shift technique.

Regardless, this can be learned "from scratch" we all began there, at some time in the dim & distant past. Try holding the shifter fully depressed, then giving the engine a little bit of throttle. The engine should freewheel, if the clutch is properly adjusted. This is the same, mechanically, as squeezing the hand lever of a typical, fully manual, bike. With practice, you can teach yourslef how to coordinate throttle application and clutch action, in other words, how to modulate the clutch & gear changes. The biggest mistake most new owners make is releasing the shiftter too quickly. Not modulating the throttle is a close second.

I see your link in your signature line now. NICE! Very very nice.
http://mechmania.net/2018/06/24/restomod-100-complete/

If you rebuilt the engine to the same standard as the rest of the bike, then it should be nothing more than learning shifting technique and possibly a minor clutch adjustment to set things right.

Get used to "burping" the throttle when you shift.

Thanks a lot for all the feedback. Been way too busy with work lately but will come back within a few days!

Hi all,

Finally came around to come back on this. Thanks a lot for the kind words about the little machine. I appreciate that! Sadly the engine was the only part that I didn't dare touch myself, and that's the only thing that's been giving me quite some issues.

I followed all of your advises regarding the carburetor.
Checked for leaks, but there were none to be found. Adjusted the float level, but that didn't help. Checked how high the fuel level was, and actually realized that it wasn't too high. It was nowhere near the air screw, but still fuel was leaking, which puzzled me quite a bit. Adjusted the kick stand, and that didn't do a thing either.
I then gave up on it, and tried a brand new reproduction carburetor, and all of my issues were solved. No more leaks!
It was really quite weird issue, and can't come up with an explanation as to what might have been wrong with the carburetor.

I then started to take a look at the shift issue - thanks for the guidance all of you.
It actually appears that it isn't me who's using a wrong technique.
Normally neutral should be somwehere between 1st gear and 2nd gear, as far as I can understand, but not with this model apparently!
I found this information:
https://advrider.com/f/threads/shifting-variations-on-old-school-bikes.1106909/

It mentions the following shift pattern "1 up 2 down - positive neutral stop both up & down w/auto clutch - Honda C100, C102 & CM91".
It would appear that my bike is following the same pattern, and that one full stroke down changes from 1st gear to Neutral, and that a second full stroke will change it from Neutral to 2nd, meaning that going from 1st gear to 2nd requires a double kick.
Also found a discussion about it here as well:
https://lilhonda.com/index.php?threads/dax-shift-pattern.10076/

Meaning that the only question I have left is whether it's normal, that the rpm's need to drop to a very low level, before it's possible for me to change from 1st gear to Neutral. Nothing happens when I try to change gear if the engine is high in rpm's, but as soon as they drop to a low level, it engages perfectly.

Tried running the bike yesterday, and it's going really well. Only one issue; noticed that it's dying on me when making a standstill in 1st gear. Then the rpm's continues going lower and lower, until it dies completely after approximately one minute, unless I give it a bit of throttle. Managed to solve it by setting the rpm higher with the throttle screw, but not really the solution I wanted to use.
I then noticed that there appeared to be coming a little bit of oil from the head gasket, which might explain the behavior.
The engine head is torqued with 10 nm (7.38 ft lbs), so I'm considering to try and tighten it faster at around 12 nm (8.85 ft lbs) - hopefully that'll solve the issue? Don't hope that it's too tight.

Once again; thanks for all your help so far.

If it will idle normally, while in neutral, but not when in gear...I'd suspect that the clutch is not disengaging, at idle rpm. That may be the underlying problem. Try holding the shifter at the limit of its travel, in either direction, after a shift; the engine should freewheel, same as if you were pulling the hand lever of a conventional, fully manual, clutch. If the bike still moves, the clutch needs attention. It's possible that something was assembled incorrectly. Fortunately, for you, the clutch can be removed & reinstalled with the engine left in place.

@racerx it freesheels fully as it should, when I do as you suggests, so it seems that the clutch is working as intended. I'll be on a short vacation the next few days, but this coming Sunday I'll check if tightening the engine head any further will stop what appears to be an oil leak, as well as this little issue. I do however not have the faintest idea, if such a minor leak as this could ever be the reason as to why it's slowly dying when in gear and at a standstill. I'll give an update on the matter as soon as I've done a bit of tightening and taken it for a ride. Thanks for the help so far.

If you have a leaking head gasket, that could be part of the stalling problem...or the cause. More likely, though, it's carburetor-related.

As for you clutch & shifting issue, I think that's just a matter of time & practice. Gear changes can be, and are, executed quickly. What you need to learn is how to modulate the clutch & throttle application, in both (upshift & downshift) directions.

@racerx actually tried another Dax yesterday, it appeared to act identical with the shifting issue, meaning that it wasn't simply enough to let go off the throttle in order to switch gears from 1st to 2nd, but that I actually needed to wait a few seconds for the RPM's to drop to a significantly lower level if I had pushed it a bit hard in 1st gear, before it was possible for me to switch gears.
I therefore now assumes that it's the way it is, - that all I need is a bit of patience.
Normally I also wouldn't rev it that high in 1st gear, but I needed to see what that little bike could accomplish

Tightened the head nuts a bit more (to 12 nm / 8.85 ft lbs), and it made the leaking quite a bit better. There is however still a tiny bit leaking, not even enough to make it drip, but it's visible. Checked it after a 30 km. drive yesterday.
Don't dare to tighten it more though and exceed the recommended specs, in risk of stripping anything.

One weird thing I noticed however; two of the head nuts rotates themselves around 1 millimeter back when I have just torqued them. Like a little spring, it's being pushed right back out as soon as I remove the wrench. After that they stay put. This "issue" was also present with a torque of only 10 nm / 7.38 ft lbs.
Guess I'll have to reinspect the head studs (looked fine last I checked though), or try and swap the head nuts around.

The little Dax still dies after roughly half a minute to a minute in 1st gear at a standstill.
There's even a new carburetor on it, since I tried everything in order to fix the leak from the airscrew on the old one, but nothing ever helped - tried all the aforementioned good suggestions.
Now the problem with the leak is solved though, so that's a least something
Reckon I'll have to play around with other settings.

One good thing however; it drove really well yesterday during the trip.

It takes practice and time, to coordinate the throttle/rpm/shift lever movements. If you've never mastered shifting with a manual clutch, this is going to be more difficult...because...what needs to be done is translating & replicating manual clutch shifting. You see, when the lever is moved to either travel limit, the clutch is being disengaged the same as if you were squeezing a hand lever to work the clutch. That's why it's a "semi automatic". It uses a centrifugal assembly to engage above a certain rpm, beyond that, centrifugal assembly fully engaged, it becomes a fully manual clutch. You're unaware of that, since there's a cam plate assembly, as part of the shift linkage, inside the engine.

You must coordinate engine rpm & torque load to shift gears, same as with a manual. Any of these transmissions will fight you, if the load, throttle application and rpm are not properly coordinated. The gears are constant-mesh. When you shift, the gears move in pairs...it's their drive dogs (fitting into their respect pockets on the next gear) that engage and disengage. Those drive dogs have a "reverse taper" (bigger toward the ends) that mechanically lock them together. Without those, the tranny would jump out of gear, while riding...a symptom of worn gears.

I'm guessing that you gear changes are rough, with an audible "snap". That's a bad sign...telling you that you're not shifting correctly. Seriously, it is possible to execute smooth, seamless, gear changes with a semi-auto. It's just not intuitive, you have to think about what you're doing, then practice refining your technique, long enough until it becomes almost second-nature.

If the head nuts are backing-out as the engine runs, something isn't right. There aren't many parts involved: 3 acorn nuts, 1 hex nut, 3 flat washers, 1 sealing washer, 4 studs, head gasket. If it were mine, I'd probably replace the head gasket, nuts & washers. That's less than $10 worth of parts and not much work, either.

As for why it dies at idle, that diagnosis could be more complicated...along with the correction. If carb adjustments (pilot airbleed, a.k.a. "idle mixture" and idle speed) won't do the job then, based on the info you've provided, I'd suspect a leaky intake valve. Take a look inside the intake port; if it's blackened, the intake valve is leaking. That's a very common malady with heads of this vintage and, it can persist for a long time before manifesting as poor performance or, ultimately, a non-running engine due to sever compression loss.

For now, is the insulator where it should be...between the intake & carburetor? The manifold should only have a gasket between it & the intake port flange on the head. I've seen guys move these between the head & intake, which results in an over-cooled intake...or omit it altogether, causing fuel percolation inside the carb a.k.a. "vapor lock", especially at idle or during the heat soak period immediately following engine shutdown.

@racerx thanks for the detailed explanation, it's appreciated.

Only manual clutch I've mastered so far is the one in my car. It takes a bit of practice yes, especially since you're engaging the clutch at the same time as you changes the gear. I am beginning to be able to shift more smoothly through the gears now, so it's definitely getting better.

What puzzled me was simply just, that I wasn't able to upshift from one gear to another, if the RPM's was really high in say 1st or 2nd gear. Then I couldn't roll off the throttle for just a second and then switch. Nothing happened, wouldn't go into gear. But as soon as the RPM's dropped (if I had pushed it quite hard in 1st or 2nd gear, perhaps too high) I would be able to switch to a higher gear instantly.
As long as I respect that, don't rev it crazy high (and one really shouldn't, i know!) it upshifts every single time perfectly.
Downshifting is perfect, the clutch engages perfectly fine when I keep the lever in either the upper or lower position, it doesn't jump when I switch from Neutral to 1st etc. So clutch adjustment is spot on (and tried various other settings as well).
I simply wasn't aware that I had to wait several seconds before being able to upshift after having released the throttle, if RPM's was high, - but I guess that's what you're also saying about coordinating engine RPM's and torque load to shift gears, and that all is perfectly fine in this regard then

Sorry if the explanation was a bit confusing regarding the head nuts.
They aren't aren't backing out when the engine runs. They stay put. They are however "springy" when torqued. The two head nuts on the left side of the engine "springs" around 1 mm back out (0,04 inches approx.) as soon as I removes the load by removing the torque wrench. They stay where they are supposed to stay after this, and don't move any further. Threads on the studs looks just fine, same does the head nuts, tried switching them around, same thing happened. Don't know how it's even possible, and what makes them do this - it happens so I can see it with my bare eyes, and feel it when they reaches their correct torque and I release the pressure. Only the left side.
Anyhow, perhaps it means nothing, just never seen this before.

If it keeps leaking (even though it's just a very tiny bit) I reckon I'll follow your advice and try to change the head gasket, even though a new one was used when the engine was assembled (didn't do it myself).

Regarding the idling; might be able to keep it running in 1st gear when it's at a standstill if I adjusted the idle speed, but then I would have to adjust it quite high, much higher than the idle speed that it accepts perfectly when in Neutral.
I'll try and play a bit around with the idle mixture, and see if I can do anything here.
A leaky intake valve is worth checking, thanks for the tip. I will try and take a look at that after having fiddled with the above mentioned settings. And yes, the insulator is where it should be, so that's at least something!
Once again; thanks for all the help so far.