CT90: Need advice from engine experts

[skiri251]

I opened up the top end of my Honda Trail CT90 which had a bit low compression.

The rings are not stuck. Haven't checked wears yet. (I don't have micrometer or bore gauge.)
Even to my untrained eyes, exhaust valve and valve seat don't look good.

What should I do? I don't have seat cutters, either. Will de-carbonizing and the lapping fix this?

 

[joedrum]

yes ..just do it thuraly and with all kinds of touch ..no wiggleing around and such unless thats the way you want the finish to look like...im sure you will do it right

 

[dan filipi]

I would lap the valves and check cylinder for any scoring.
If the cylinder looks good put it back together.

 

[AApple]

Looks like that valve and seat are pretty carboned up. A good cleaning, followed by careful hand lapping, should get it to seat fairly well. The guide may be worn, too, which could allow that problem to start with. Once you get the valve and seat cleaned, wiggle the valve in the guide and see how much slop there is....shouldn't be much, if any. If the guide is worn, it can be knurled(machining operation), or possibly a new guide installed....don't know if there is such a part even available for such a large engine.... :smilie_happy:

 

[skiri251]

Thank you all!

I will follow the advice and lap the valve.

Oh, what is the best way to de-carbonize?
I dipped the valves in SeaForm overnight but that didn't soften the deposits. Is it just a matter of scraping them with not so hard material like brass/copper/plastic?

 

[dan filipi]

I use a wire wheel.

Check also the valves aren't bent, that can cause leaking and low compression.

 

[skiri251]

....don't know if there is such a part even available for such a large engine.... :smilie_happy:

This guy https://dratv.com/honcttrail90.html has quite extensive parts.
I wanted to install his $60 big bore (yes, 54mm OD, VERY BIG :hihihi: ) piston kit but it's out of stock. :head bang:

I found another one but it costs $235 :head bang:

 

[skiri251]

I use a wire wheel.

Check also the valves aren't bent, that can cause leaking and low compression.

Of what material? I have regular(?) ones but they look like stainless steel. Isn't it too hard?

 

[joedrum]

get a couple chor boy pads and just clean them up ..its easy ,,,they cut it good ....just dont press hard with them ...more like brushing paint on ....anyway thats what i do but like i said before just use a lot of touch

 

[skiri251]

get a couple chor boy pads and just clean them up ..its easy ,,,they cut it good ....just dont press hard with them ...more like brushing paint on ....anyway thats what i do but like i said before just use a lot of touch

Wasn't sure what is "chor boy pads" but google found it for me. :hihihi:
Kitchen item, eh?

Thanks. I will go and get them.

 

[slabghost]

Valves are quite hard. No worries that a wire wheel will damage them. just use it for the carboned places only. you shouldn't chance scratching the stems.

 

[dan filipi]

Valves are quite hard. No worries that a wire wheel will damage them. just use it for the carboned places only. you shouldn't chance scratching the stems.

Ditto.

 

[sledge]

One thing I would check is the Valve Springs , it really looks like that Valve hasn't been closing good.... could be the Valve stem is bent a little .....or , a Broke or Weak Valve Spring. . . . be worth checking anyway.

 

[skiri251]

One thing I would check is the Valve Springs , it really looks like that Valve hasn't been closing good.... could be the Valve stem is bent a little .....or , a Broke or Weak Valve Spring. . . . be worth checking anyway.

Good point.
I measured the free length which was on spec but will buy the heavy duty springs anyway. It costs only $7.99 for the whole set.

 

[slabghost]

One thing I would check is the Valve Springs , it really looks like that Valve hasn't been closing good.... could be the Valve stem is bent a little .....or , a Broke or Weak Valve Spring. . . . be worth checking anyway.

Good point.
I measured the free length which was on spec but will buy the heavy duty springs anyway. It costs only $7.99 for the whole set.

Perfect! :yahoo:

 

[skiri251]

Cleaned seats and valves.

Seats look okay but the valve? It looks like corrosion not carbon.

Also the guide feels too loose. Can't tell exactly because I don't have a bore gauge but definately looser than intake side. That means new valve guide? How do I remove the old one? The manual talks about the special tool but of course I don't have it. Can I use vice grip? :hihihi:

 

[mike in idaho]

The valve guide tool is a steel drift with a step turned on the end so one end fits into the guide hole. The other diameter should be slightly less than the od of the valve guide so the tool can follow the guide out of the head. Clean as much carbon out off the lower end of the guide as possible so it won't gouge the hole on the way out. Put the head in an oven and heat it to 250-300 degrees, then drive the old guide out. Put the new guide in the freezer overnite, then quickly insert it into the head while it's still hot.

 

[scdmarx]

Once installed you'll need to ream the guide to fit the valve.

 

[skiri251]

Thanks.
Obviously I don't have a set of reamers either so I figured I should just lap the valve and see how it goes.
The valve looks okay now. (hopefully)

 

[dan filipi]

After lapping is the exhaust valve contacting the seat good?

In post 1 it looked like it wasn't seating at all.