CT90: Need advice from engine experts

[skiri251]

I thought the fact that there is shiny band around the valve indicated the good contact. I guess I need a dye (?) or something to check the contact. Where can I get it?

 

[dan filipi]

I thought the fact that there is shiny band around the valve indicated the good contact. I guess I need a dye (?) or something to check the contact. Where can I get it?

yes it should be good then.

 

[scdmarx]

You can use a magic marker. Put about 5 or 6 lines evenly spaced around the shiney part of the valve. Then seat the valve in the hole and rotate it as though you were lapping it. Then when you pull it out you can see where it made contact. Ideally, your contact line will be in the center and continuous all the way around. There is a spec for how wide the contact line should be, but I got a feeling you're gonna settle for good'nuff.

 

[slabghost]

Agreed. To all above posts. However I'd still wire wheel that junk off the backside of the valve. That will make it flow a bit better.

 

[skiri251]

You can use a magic marker. Put about 5 or 6 lines evenly spaced around the shiney part of the valve. Then seat the valve in the hole and rotate it as though you were lapping it. Then when you pull it out you can see where it made contact. Ideally, your contact line will be in the center and continuous all the way around. There is a spec for how wide the contact line should be, but I got a feeling you're gonna settle for good'nuff.

Thanks. Yes, for now, good enough is good. If I can get a big bore kit, I will ask the shop to do it.

 

[skiri251]

Agreed. To all above posts. However I'd still wire wheel that junk off the backside of the valve. That will make it flow a bit better.

I tried Dremel wire wheel.

Next size up for me is 2" one from HF.

Not sure it is carbon. Looks like corroded/melted metal to me but is that possible?

 

[scdmarx]

Not sure it is carbon. Looks like corroded/melted metal to me but is that possible?

I'd say it's carbon. Unless it's been sitting at the bottom of the ocean for a hundred years. Carbon can be stubborn sometimes. If you don't have access to a wire wheel on a bench grinder, you can try soaking it in a cup of Berrymans carb cleaner overnight to help loosen it up.

 

[AApple]

The seat area looks ok, but the carbon needs to be removed, for sure. If ya don't, there's a chance some will break off while running, and get down in there, either into the cylinder, or back between the valve/seat, and then you have the same problem ya had to start with. If you can't find a good solvent, or carb cleaner to soak it in, try using Easy-Off oven cleaner....that'll definitely loosen it up after soaking for a couple of hours. Ya gotta be careful tho, and make sure you have that stuff contained, and wear gloves/eye protection. :ahem:

 

[dan filipi]

Chuck that wire wheel in a drill, lock the switch on, stand on the drill with one foot with the wheel over the edge of something to keep it in the air, there's your bench grinder.
Have at that carbon.

 

[skiri251]

Chuck that wire wheel in a drill, lock the switch on, stand on the drill with one foot with the wheel over the edge of something to keep it in the air, there's your bench grinder.
Have at that carbon.

Tried that and now the valve is shiney!

As for the seating, tried magic marker also but not sure. The ink on the valve disappears but hard to see it is transferred to the guide. Will go out and see if nearby auto parts store got contact blue. Or I guess I can try the method shown in the factory shop manual. Fill the chamber with oil and apply 28 PSI air pressure from in/ex ports and see if bubbles appear.

 

[slabghost]

Or you could install the valves and pour in a very light oil in the combustion chamber and let it sit an hour. If it doesn't leak it's most likely good.

 

[skiri251]

Or you could install the valves and pour in a very light oil in the combustion chamber and let it sit an hour. If it doesn't leak it's most likely good.

Both valves failed air bubble test miserably. Then I put ATF in the chamber and it is still there after 4 hours. Is ATF too thick for this test?

Air bubble test failure may be due to weak valve springs? I can push and open the valves by hand.

In any case, when the new springs and other parts arrive this Friday, I will put them back together and measure the compression. Shouldn't be worse than before.

 

[slabghost]

I'd say that is bad springs.

 

[MagnAndy]

Looks to me like the valve may have been set too tight to allow it to close.

 

[AApple]

I wouldn't expect it to hold air very well(bubble test) with just the cleaning, and hand lapping. That's not really an indication that the valves won't seal while running, tho. If you wire-wheeled them after lapping, you might wanna try re-lapping, and see if that improves the bubble test results. Without actually grinding the valves/seats properly, that's prolly about as good as it's gonna get. New springs will certainly help. I think you'd be good to go with what you have, with the new springs. just my .002 k:

 

[skiri251]

I re-lapped it after the wire wheeling. This time with coarse lapping compound (I was kinda desperate) followed by fine one. A bit wider band appearred as a result.

I am not sure what the air bubble test proves. 28 PSI air pressure. I would say the valve area (dime size) is around 0.7 square inch. That's around 20 lbs pushing the valve. Like I said, I can push and open the valve with my fingers fairly easily.

I am not going to race my CT90 and it was running okay on the desert lake bed. I just want to be a little more competitive against XR100s I ride together. :hihihi:

 

[AApple]

I just want to be a little more competitive against XR100s I ride together. :hihihi:

:smilie_happy: :mrgreen: :smilie_happy:
When you get that small of a diameter spring, it doesn't take much to push the valve open. Heck, in some cases, you can push the valve open on a small block Chevy by hand. I think you'll be ok with what I have seen, but...I ain't no small engine guy. Others here prolly have way more experience, and can/will give more help. :builder:

 

[skiri251]

Hopefully I will get a bit higher compression.

XR100s. They are really nice little bikes. I can buy used one (10 year old or so) for $700+. But I don't think I can get rid of CT90 easily (unless I give it away) so decided to spend some $s on it. (New $30 Chinese carb, torquey cams, etc.)

I wish I could buy that 104cc big bore piston kit..

 

[scdmarx]

With the valve spring installed, fill the runner behind the valve with a light solvent. You could use water if that's all you got. Then count to 10 (10 seconds) If you can count ten seconds before it starts to leak passed the face of the valve, that's the acceptable minimum as far as sealing. The longer it will hold without leaking, the better. But any less than 10 seconds, really ain't worth putting back together.

 

[skiri251]

With the valve spring installed, fill the runner behind the valve with a light solvent. You could use water if that's all you got. Then count to 10 (10 seconds) If you can count ten seconds before it starts to leak passed the face of the valve, that's the acceptable minimum as far as sealing. The longer it will hold without leaking, the better. But any less than 10 seconds, really ain't worth putting back together.

I tried lacquer thinner.

Exhaust: no visible leakage after 60 sec. Then I saw some wetness so touched the valve by finger and smelled thinner.

Intake: no leakage after 60 sec. I was touching and smelling every a few seconds and no smell at all.

Tried Exhaust again: same as intake above but no smell this time after 60 sec.

So I think they are as tight as ________ (insert your favourite expression here). :hihihi: