Partial Seizure or Fuel Vacuum?

[KayAitch]

Hi,

I'm part way through the running in process on my little KH100 after I did the top end: one run until warm on the centre stand, and 4 rides or so starting at 5-10 minutes and gradually working it up to 30 minutes or so while varying the RPM and not spanking it. So far so good, engine seems smoother and happier already and no signs of the overheating/partial seizure I was getting pre-rebuild. Engine gets nice and warm, I'd say hot to the touch, but there is no loss of power or change in performance, no noises, along the way. It seems to have done the trick.

However, today a nice day of 21C I took it for the longer ride I've done and twice I had the engine stop suddenly on me. I am 99% sure it is not a seizure and is not due to overheating. I am fairly sure it is due to my tank cap not venting properly and the fuel flow cutting. This despite being certain it was fixed after a nice long soak in some white vinegar - now when opening the cap I get nowhere near the sound of opening a Coke bottle like I was getting before. You see, once it stops I open the tank cap and and give it one kick, it roars into life and shows no negative/hot symptoms.

So, I thought I'd ask, what are the signs of a partial seizure? Am I right in thinking there warning signs that show up before hand like gradual loss of power as it gets hot, engine losing it's smoothness, generally getting baggy?

The rebuild has been otherwise flawless until now. Thanks for any help. I love this little bike :)

[anotherguy]

It is probably a lean seizure due to lack of fuel. Fix the cap before riding further. Sooner or later you'll kill it.

In reality there's no such thing as a partial seizure. If it sticks and stops running it's a seizure. There are varying degrees of damage from seizure. Pull the pipe and have a peek into the exhaust port to see the damage. Post up some photos if ya can.

[KayAitch]

Cheers anotherguy. Now after getting the tank cap vents much cleaner I'm wondering whether it's that or something further down the line... any ideas? If it is the cap, any other ideas on how to get those little vent holes clear?

Will pull the exhaust and have a look and take photos ASAP before any more riding. I hope I haven't messed this up :(.

Just to clarify the terminology, no fuel = seizure? I presume if no fuel/oil gets up into the combustion chamber above the piston, then the rings get no oil and they stick, right? I'm a complete noob.

There is one other thing I have noticed that the bike has nearly always done. That is, particularly in 1st or 2nd and early in the ride (after warm up, of course), it will often not rev completely freely. At low rpm it will be fine and then it will only rev up until a ceiling and 'stick' at a constant rpm. After a few accelerations it will stop doing it. What could this be?

Thanks for your reply.

[JAB]

Are you using the oil pump or premixing? How are the crank seals? Have you tested your venting theory by letting the fuel free-flow into a container
to see if the flow stops at some point? BTW-ccol bike.

[JAB]

Originally Posted by KayAitch

There is one other thing I have noticed that the bike has nearly always done. That is, particularly in 1st or 2nd and early in the ride (after warm up, of course), it will often not rev completely freely. At low rpm it will be fine and then it will only rev up until a ceiling and 'stick' at a constant rpm. After a few accelerations it will stop doing it. What could this be?

Thanks for your reply.

That MIGHT be what is called "loading up". 2 smokes need to clear out excess fuel/oil before running crisp. If you are trying to keep the revs low, this might explain it.

[KayAitch]

I am using the oil pump to the right setting and it is definitely working. The spark plug, new from the beginning of the running-in process looks just about perfect. If anything it looks slightly rich, but not too rich.

I haven't been to the crank seals yet. Anything a noob like me can look for? I guess I'm going through a process of elimination and I was kind of hoping it would end at this top-end rebuild :) It's an old thing so I'm fearing the worst.

A while back before I cleaned the cap I needed to empty the tank and the flow would be fine for a while and reduce to a trickle/nothing, then if i opened the cap it would start again. On the basis of what I have read this lead me to confirm the venting/flow issue.

It'll be cooler when it stresses me a tiny bit less :)

[KayAitch]

Originally Posted by JAB

That MIGHT be what is called "loading up". 2 smokes need to clear out excess fuel/oil before running crisp. If you are trying to keep the revs low, this might explain it.

Oh, I haven't been intentionally keeping the revs particularly low at all. I've been winding it up to say 70% of redline (just a guess, no taco) on my running-in rides.

This 'rpm ceiling' is occurring straight out of the driveway for the first 2 or 3 accelerations in 1st and 2nd so maybe that is just loading up from warming up? I do blip the throttle a bit. Perhaps once warm it should be given some heftier twists to clean it up?

[JAB]

Originally Posted by KayAitch

Oh, I haven't been intentionally keeping the revs particularly low at all. I've been winding it up to say 70% of redline (just a guess, no taco) on my running-in rides.

This 'rpm ceiling' is occurring straight out of the driveway for the first 2 or 3 accelerations in 1st and 2nd so maybe that is just loading up from warming up? I do blip the throttle a bit. Perhaps once warm it should be given some heftier twists to clean it up?

Sounds like you have the venting issue on the run. As far as the seals are concerned, due to age the seals may be hard and leaking air. Is this a rotary valve engine with the carb inside the engine case? If so, that may be another place for an air leak. The crankcase sealing can be leak-down tested, but I have no way to tell you the right way to do that. To change the seals (if they are even leaking) you have to split the cases. The loading up issue should pretty much clean out once up to operating temp. It may come back a bit if the engine is "lugged" at low RPM but should clean out again with a twist of the wrist. Gotta love the little old 2 smokes.

[KayAitch]

I believe this to be the case. Pre top-end rebuild I would ride along and release the cap every 5 minutes or so because I was aware of this venting issue (I have pulled the hose off the tap previously after it has cut out to find no fuel coming out of the tap. Also, emptying the tank I would get a little run out of the tap but then it would stop. Releasing the fuel cap would start the flow again).

During the running in I wasn't bothering to open the fuel cap because I thought the vents were clear.

What does a leaky crankcase do that would cause this? Again, apologies. I'm learning as we go :).

Yes it is a rotary valve with the carb inside the engine case. I have heard of leaks between the carb and the mount before. Again, would this cause this cutting out condition?

The loading up does clean up after a couple of accelerations, thanks.

I really hope I don't need to split the case.

Thanks again.

Originally Posted by JAB

Sounds like you have the venting issue on the run. As far as the seals are concerned, due to age the seals may be hard and leaking air. Is this a rotary valve engine with the carb inside the engine case? If so, that may be another place for an air leak. The crankcase sealing can be leak-down tested, but I have no way to tell you the right way to do that. To change the seals (if they are even leaking) you have to split the cases. The loading up issue should pretty much clean out once up to operating temp. It may come back a bit if the engine is "lugged" at low RPM but should clean out again with a twist of the wrist. Gotta love the little old 2 smokes.

[JAB]

Originally Posted by KayAitch

I believe this to be the case. Pre top-end rebuild I would ride along and release the cap every 5 minutes or so because I was aware of this venting issue (I have pulled the hose off the tap previously after it has cut out to find no fuel coming out of the tap. Also, emptying the tank I would get a little run out of the tap but then it would stop. Releasing the fuel cap would start the flow again).

During the running in I wasn't bothering to open the fuel cap because I thought the vents were clear.

OK, so you need to fix or replace the cap.

What does a leaky crankcase do that would cause this? Again, apologies. I'm learning as we go :).

An air leak on a 2 stroke will make a lean condition that could cause a seizure. If the seal on the clutch side leaks you will suck oil out of your transmission.

Yes it is a rotary valve with the carb inside the engine case. I have heard of leaks between the carb and the mount before. Again, would this cause this cutting out condition?

Maybe not so much at the carb and mount, but check that, but the rotary valve may have seals that leak. I don't know if that would allow air or oil or what to leak past.

The loading up does clean up after a couple of accelerations, thanks.

When in doubt, gas it.

One more thing to check. Pull the baffle out of the muffler and make sure the openings are clear. If it is clogged, may cause it to run crappy in the higher RPM's.

I really hope I don't need to split the case.

Me too.

Thanks again.

.

[Sierra Thumper]

Originally Posted by KayAitch

Hi,

I'm part way through the running in process on my little KH100 after I did the top end: one run until warm on the centre stand, and 4 rides or so starting at 5-10 minutes and gradually working it up to 30 minutes or so while varying the RPM and not spanking it. So far so good, engine seems smoother and happier already and no signs of the overheating/partial seizure I was getting pre-rebuild. Engine gets nice and warm, I'd say hot to the touch, but there is no loss of power or change in performance, no noises, along the way. It seems to have done the trick.

However, today a nice day of 21C I took it for the longer ride I've done and twice I had the engine stop suddenly on me. I am 99% sure it is not a seizure and is not due to overheating. I am fairly sure it is due to my tank cap not venting properly and the fuel flow cutting. This despite being certain it was fixed after a nice long soak in some white vinegar - now when opening the cap I get nowhere near the sound of opening a Coke bottle like I was getting before. You see, once it stops I open the tank cap and and give it one kick, it roars into life and shows no negative/hot symptoms.

So, I thought I'd ask, what are the signs of a partial seizure? Am I right in thinking there warning signs that show up before hand like gradual loss of power as it gets hot, engine losing it's smoothness, generally getting baggy?

The rebuild has been otherwise flawless until now. Thanks for any help. I love this little bike :)

I bought a kawi 250 air-cooled dirt bike from a shop when I was a kid that had a fresh rebuild. It would run for 15-20 minutes if I remember right before gradually losing power then tightening up. After it cooled for a couple minutes it would fire up and run great for another 20 minutes or so till it happened again. I was breaking it in properly, proper pre-mix ratio......when I took it back to the shop they said they had the piston to cylinder clearances too tight. They offered to fix it or give me a refund (shops were actually cool back then ) so I applied the money towards another bike. That kawi had multiple min-seizures and didn't affect the running abit, not all seizures are solid permanent lock-ups. But they all do damage
You did pretty much describe how it feels...and the motor will lock up temporarily....you'll know if its seizing or not.