Full throttle after carb service - Now seems OK. For safety would like to understand reason, can anyone advise please?

[lowside]

I'll try to keep this short.

Keihin PWK 28.  Completed rebuild on carb, cleaned.  Fitted after market idle and mixture adjusters.  Also fitted inline fuel filter.  Fitted new grips and levers.  Fuel was present throughout starting procedure, no leaks as far as I can tell, manifold boots all seated.  Play in cables correct at all angles of steering.

Initial start with choke bike went to full throttle, did not respond to lanyard.  Blipped throttle and bike then returned to high idle.  Adjusted idle screw, bike idled normally but slow to return to idle after blip.  Believe that will be mixture related. I think the after-market adjusters are slightly different specs to OEM, as I recorded the number of turns when removing then OEM parts and installed in reverse.  Will re-adjust tomorrow.

What I would like to know is, if I've not touched the throttle sliders or needle, why would the bike suddenly launch into max throttle?  I had noted that the clack from the carb upon releasing the throttle was not as loud as I'd expect prior to the first start.  The click heard from the carb at full throttle (bike off) was normal.  Could it be lubication of the sliders?  They sat for 2 days wrapped up in towel.

I believe for some reason, the sliders became stuck, but as I've not touched them aside from removal and installation (they are complete), for safety reasons I'd like to try and understand why this may have happened?  The sliders now make the expected sounds in the carb, whereas on initial start they did not, but nothing has changed that I'm aware of.

As it stands, the bike runs and idles in and out of gear, but I think the mixture needs adjusting slightly.  Will test again tomorrow.

I'd appreciate any suggestions of what to check given the above, with emphasis on potential issues with the sliders.

Other than this, everything went pretty well for my first serious go at spannering. lol.

Thanks for reading, hard to explain when you're new at this

[lineaway]

No matter, just working on the carb can cause the cable to be displaced. Changing out both screws will make the whole thing a mess at the first attempt at starting. Better to leave the idle adjust screw all the way loose and then adjust. Not all aftermarket adjusters are an upgrade. Most are poor quality. Your are lucky you did not cold sieze the motor.

 That was not a carb rebuild, just a little disassembly. You should be able to tell if the air filter was seated right when you remove it and the grease is on the filter and the box evenly. It would be quite costly if you suck dirt through a 2 stroke. Sounds like you are figuring it all out. Better to keep asking questions than not. Good luck.

[lowside]

4 hours ago, lineaway said:

No matter, just working on the carb can cause the cable to be displaced. Changing out both screws will make the whole thing a mess at the first attempt at starting. Better to leave the idle adjust screw all the way loose and then adjust. Not all aftermarket adjusters are an upgrade. Most are poor quality. Your are lucky you did not cold sieze the motor.

 That was not a carb rebuild, just a little disassembly. You should be able to tell if the air filter was seated right when you remove it and the grease is on the filter and the box evenly. It would be quite costly if you suck dirt through a 2 stroke. Sounds like you are figuring it all out. Better to keep asking questions than not. Good luck.

Thanks lineaway.  Very lucky about the motor yep.  It completely blindsided me but the throttle blip sorted it so it only revved out for about 5 seconds.  I'm almost sure it was a stuck slide after disturbing the cable with carb reinstall and throttle grip removal.  Thought maybe I'd twisted it, but I think you'd feel tension in such a case.  I know what to listen out for next time.

The screws were visually identical to the OEM parts held side by side, but I hear what you're saying as these adjustments are so slight.  It was about 1 turn off on idle and fuel is probably 1/8th of a turn out.  The parts are CSP and looked to be well machined, but next time out I'll use calipers and take a more cautious approach as you suggest.

The air filter check will work for coverage and seal, but it's a slightly different shape top and bottom and I think I remember reading that some filters have varying foam densities across their surface so wanted to make sure it was the correct orientation, as well as sealed all around.  Hoping someone who has one can confirm on that other thread.

Thanks again

[stpauls]

"Did not respond to lanyard".  Do you mean that you have a magnetic lanyard cut-out/kill switch, but when you pulled it off the engine kept running?  Or, do you mean that it did not respond to the throttle?  Important to be clear, because they would have two completely different causes and therefore solutions.

I suspect you meant throttle, in which case my best guess is that the throttle cable was not seated correctly into the top of the carburettor, or that the cable was not seated correctly into the throttle.  A third guess is that the throttle cable was not seated correctly into the carburettor slide.

Engines have been known to race at full throttle even when the kill switch has been off, but this will have completely different causes and solutions.

[Glayne]

6 hours ago, stpauls said:

Engines have been known to race at full throttle even when the kill switch has been off, but this will have completely different causes and solutions.

Pretty sure he meant this^^^ but I am also confused. Did it return to high idle with the lanyard un-attached?

[lowside]

6 hours ago, stpauls said:

"Did not respond to lanyard".  Do you mean that you have a magnetic lanyard cut-out/kill switch, but when you pulled it off the engine kept running?  Or, do you mean that it did not respond to the throttle?  Important to be clear, because they would have two completely different causes and therefore solutions.

I suspect you meant throttle, in which case my best guess is that the throttle cable was not seated correctly into the top of the carburettor, or that the cable was not seated correctly into the throttle.  A third guess is that the throttle cable was not seated correctly into the carburettor slide.

Engines have been known to race at full throttle even when the kill switch has been off, but this will have completely different causes and solutions.

Thank you, I see what you mean.  Kill switch did not respond when bike was at max revs.  Before resorting to taking the cap of the spark plug, I blipped the throttle.  RPM instantly lowered to a high idle.  Once at the high idle, it did respond to kill switch.  I have since tested the kill switch repeatedly and it is functioning apparently correctly.

After that, everything worked normally once I'd adjusted the idle screw about 1 turn out.

I checked the throttle cable, its routing and seating to the carb prior to first starting.  It all looked and felt fine.  Correct play in throttle, lock nuts in place and tight (I had not adjusted it) and it was seated correctly on the carb with the gasket nice and tight fitting all around.  The only thing I noticed before I started the bike, was the lack of the usual loud "clack" of the slides in the carb when letting off the throttle.  It was a more muted sound.  Now, following the second start which I was able to control, the "clack" of the slides returning is at what I'd call normal volume.  Thats why I'm convinced something in the throttle slides became slightly stuck and has now unstuck itself.  I wondered if they had dried out and after the first start, were lubricated again?

I am led to believe that at maximum revs, a kill switch may not always work because the engine is able to self ignite, similar to how a diesel engine works?

Thanks for responding, all of this is very helpful when trying to learn.

[lowside]

4 minutes ago, Glayne said:

Pretty sure he meant this^^^ but I am also confused. Did it return to high idle with the lanyard un-attached?

Hi Glayne.  No.  Once I realised the kill switch hadn't worked, I left it attached rather than drop it on the floor.  I cut the fuel.  My next step was removing cap off spark plug ASAP.  Before I got off the bike to do that, I blipped the throttle and the RPM dropped instantly to the high idle (which I know the cause of).  It all happened so quick.

[lowside]

To close this off, I've just got finished sorting out the mixture and idle.  It's running brilliantly now and the hanging idle has disappeared.  It rev's smoothly and freely so I'm confident its OK.  A mm or so too much free play on the throttle, so will sort that tomorrow.

I've noticed the clutch is dragging a bit, always was hard to get into neutral. I think I'll do an oil change, take it for a ride and see what happens.  Can't spend all the time in the garage.

Thank you to everyone who took interest and tried to help out

Edited June 3, 2023 by lowside

[Glayne]

42 minutes ago, lowside said:

To close this off, I've just got finished sorting out the mixture and idle.  It's running brilliantly now and the hanging idle has disappeared.  It rev's smoothly and freely so I'm confident its OK.  A mm or so too much free play on the throttle, so will sort that tomorrow.

I've noticed the clutch is dragging a bit, always was hard to get into neutral. I think I'll do an oil change, take it for a ride and see what happens.  Can't spend all the time in the garage.

Thank you to everyone who took interest and tried to help out

I don't believe that a clutch should drag but I do believe that they are designed to be hard to accidentally find (false) neutral. My *trail* bike can find neutral a couple times a ride, nobody wants that in *trials*sections.

I don't understand why my foot can't find neutral on the *trials* bike most the time but by hand can?

[lowside]

MI6 couldn't find neutral on my bike if the engine is running 😆.  I suspect it's probably related to the clutch drag, as if the clutch is engaged, neutral will stay hidden.  You're right in that you definitely don't want to find neutral accidentally, but the drag is enough to move the bike on its own, unless I sit on it fully, so it's quite strong I'd say.  I don't think its by design.

I am expecting the oil to be grim, based on everything else I've found so far.

[lowside]

Oil was the colour of dark caramel.  Drained out about 650cc and replaced with 75W.  No improvement, clutch still drags in 1st gear, enough to pull the bike along if I stand up.

I'll make a new thread about clutch drag to try and document it.  Is that accepted on this forum?  I thought it might make the content more useful for people in the future, thats why I do that rather than bundle it all into a single long thread.

[feetupfun]

11 hours ago, lowside said:

Oil was the colour of dark caramel.  Drained out about 650cc and replaced with 75W.  No improvement, clutch still drags in 1st gear, enough to pull the bike along if I stand up.

I'll make a new thread about clutch drag to try and document it.  Is that accepted on this forum?  I thought it might make the content more useful for people in the future, thats why I do that rather than bundle it all into a single long thread.

Yes. I suggest you put your new thread in the forum for the brand of bike you are working on.

If it's a Beta there's a few novels worth of reading about clutch drag there already.

[lowside]

39 minutes ago, feetupfun said:

Yes. I suggest you put your new thread in the forum for the brand of bike you are working on.

If it's a Beta there's a few novels worth of reading about clutch drag there already.

Will do, thank you.  Mine's a Sherco, although I have been reading those threads about the clutch drag on Beta's too.  I think its a different type of clutch on mine, diaphragm type.