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1978 honda cx500
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Discussion Starter · #261 ·
Ok so i finally got time to check the bike out, i looked at the exhaust valves, and as i rotated the motor by hand, i couldnt see any gap at all on the exhaust side.
 

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Ok so i finally got time to check the bike out, i looked at the exhaust valves, and as i rotated the motor by hand, i couldnt see any gap at all on the exhaust side.
Were you using a feeler gauge, or were you just trying to check for free play by wiggling the rocker?. Was this true for the exhaust valves on both the left and right cylinders?

Have you read the factory service manual on how to set the valve clearance?
 

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Discussion Starter · #263 ·
I used a feeler gauge,
Were you using a feeler gauge, or were you just trying to check for free play by wiggling the rocker?. Was this true for the exhaust valves on both the left and right cylinders?

Have you read the factory service manual on how to set the valve clearance?
I used a feeler gauge to check them. Not sure if this helps but when i crank the bike, the side with no compression spits out a lot more air out of the exhaust then the good side. So air is definitely escaping.
 

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Discussion Starter · #264 ·
I used a feeler gauge,

I used a feeler gauge to check them. Not sure if this helps but when i crank the bike, the side with no compression spits out a lot more air out of the exhaust then the good side. So air is definitely escaping.

Would this mean the valves are stuck open? It just seems like air is going straight through
 

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Set the motor to TDC on the cylinder in question..

Observe the position of the exhaust valve.

Turn engine one full turn to TDC on the same cylinder {remember to rotate clockwise from the front}. Again observe the exhaust valves position.

You may need to rotate the motor a number of times to figure this out.

You are looking for the position where the valve is less depressed.

When you have found this position make the adjustment.
 

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Would this mean the valves are stuck open? It just seems like air is going straight through
If a valve were stuck open it would become bent by contact with the piston.

... Not sure if this helps but when i crank the bike, the side with no compression spits out a lot more air out of the exhaust then the good side....
Since this is a long thread I don't remember if your bike has stock exhaust including the H-box. If so, remember that the exhaust gases from the two cylinders get mixed within the H-box.
 

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Discussion Starter · #267 ·
If a valve were stuck open it would become bent by contact with the piston.



Since this is a long thread I don't remember if your bike has stock exhaust including the H-box. If so, remember that the exhaust gases from the two cylinders get mixed within the H-box.
I took the exhaust off since om removing the motor to install the raesan unit. So im feeling the exhaust port directly when i crank the bike
 

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A valve only has to be off of its seat to release all compression, not necessarily all the way open at maximum lift where it can be struck by the piston..

I would check this out as if it is anything else your motor has serious problems.

If the head has been retorqued this also closes up the valve clearances.

Question - have you adjusted the valve clearances yourself? It is easy to get this aspect wrong if you're inexperienced with the job. TDC comes around twice in a 4 stroke cycle.

Suck squash bang blow..

Clearance is set at the top of squash {compression} not blow {exhaust}, when the exhaust valve is open.
 

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Note that the piston rises to Top Dead Centre twice per cycle and the valves are both closed during the compression stroke but the exhaust valve is open during the exhaust stroke.
Line Font Technology Machine Engineering


The valve clearance basically measures how far the cam (or in the case of our pushrod type engines the cam, pushrod and rocker) must push the valve before it starts to open. If the clearance is set too tight the valve won't close tightly enough to seal against its seat. If there is no clearance the valve may be held slightly open all the time so pressure can't build during the compression stroke.

Ok so i finally got time to check the bike out, i looked at the exhaust valves, and as i rotated the motor by hand, i couldnt see any gap at all on the exhaust side.
You need to establish whether the engine is at TDC exhaust or TDC compression to check the clearance. Turn the engine with a wrench until the TR mark is lined up with the bumps at the sides of the timing port and wiggle the rockers to see if the valves are closed (no movement = the valve is being pushed open). Of one or both of the valves is open you are probably at TDC exhaust, not TDC compression so turn the bolt on the front end of the crankshaft a full revolution until TR is lined up again and see if the valves are closed (rockers can be wiggled).

If there is no gap when the engine is at TDC on either stroke loosen the nut on the adjuster and turn the adjuster until there is a gap. Measure the gap with your feeler gauges (at this point it doesn't matter what the gap is as long as you know), rotate the crankshaft a full turn and measure the gap again.
  • If the gap is now greater than it was you are at TDC compression and you can adjust the gap as specified in the FSM.
  • If the gap is now smaller you are at TDC exhaust so rotate the crankshaft another full turn until it is back where the gap was greater and adjust the gap as specified in the FSM.

NOTE: As Phreak mentioned, when turning the engine manually always turn the wrench clockwise, as if you were tightening a bolt. This is because the hex that you put the wrench on is actually the head of a bolt screwed into the end of the crankshaft and you could loosen the bolt if you turn it the other way.
 

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Discussion Starter · #270 ·
Note that the piston rises to Top Dead Centre twice per cycle and the valves are both closed during the compression stroke but the exhaust valve is open during the exhaust stroke.
View attachment 210114

The valve clearance basically measures how far the cam (or in the case of our pushrod type engines the cam, pushrod and rocker) must push the valve before it starts to open. If the clearance is set too tight the valve won't close tightly enough to seal against its seat. If there is no clearance the valve may be held slightly open all the time so pressure can't build during the compression stroke.


You need to establish whether the engine is at TDC exhaust or TDC compression to check the clearance. Turn the engine with a wrench until the TR mark is lined up with the bumps at the sides of the timing port and wiggle the rockers to see if the valves are closed (no movement = the valve is being pushed open). Of one or both of the valves is open you are probably at TDC exhaust, not TDC compression so turn the bolt on the front end of the crankshaft a full revolution until TR is lined up again and see if the valves are closed (rockers can be wiggled).

If there is no gap when the engine is at TDC on either stroke loosen the nut on the adjuster and turn the adjuster until there is a gap. Measure the gap with your feeler gauges (at this point it doesn't matter what the gap is as long as you know), rotate the crankshaft a full turn and measure the gap again.
  • If the gap is now greater than it was you are at TDC compression and you can adjust the gap as specified in the FSM.
  • If the gap is now smaller you are at TDC exhaust so rotate the crankshaft another full turn until it is back where the gap was greater and adjust the gap as specified in the FSM.

NOTE: As Phreak mentioned, when turning the engine manually always turn the wrench clockwise, as if you were tightening a bolt. This is because the hex that you put the wrench on is actually the head of a bolt screwed into the end of the crankshaft and you could loosen the bolt if you turn it the other way.
Thank you for the visual, it was very helpful. So I did what you said and for sure, the gap on the exhaust side was huge, the rocker on that end had a lot of wiggle room. So i adjusted it, and even though im a novice im very confident it was adjusted correctly. So i started to crank the bike to test for compression, and its still 0. Ill double check everything but thats my update so far.
 

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Discussion Starter · #272 ·
OK, if you have adjusted the gap correctly and at the correct stroke it is time to consider the possibility of a bent or sticking valve.
If thats the next step, should i take apart the top end of the cylinder? Im sure i could find a video somewhere on it
 

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Did you go around twice?

THE GAP ON THE EXHAUST WAS HUGE.

Do you feel competent to remove a head.?

Aside from head removal, repair and refitting you'll have to do retorques.

And adjust valve clearances.

Don't get ahead of yourself. It may cost you.
 

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Discussion Starter · #274 ·
Did you go around twice?

THE GAP ON THE EXHAUST WAS HUGE.

Do you feel competent to remove a head.?

Aside from head removal, repair and refitting you'll have to do retorques.

And adjust valve clearances.

Don't get ahead of yourself. It may cost you.
I went around a few times actually to make sure i was seeing the right thing. And the exhaust gas was definitely HUGE when i readjusted them. Like it was probably around a 1/8th inch gap. And i feel confident, in terms of repairing things im always very percent confident since i worked as a mechanic in highschool and did a lot of engine work on cars. Since im new to bikes my only issue lately has been diagnosis of the problems im having. Once i know where to look im comfortable taking things apart.

But im triple checking everything now and it all seems fine to me.
 

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Fair enough. I just don't want you pulling stuff apart if you don't need to.

Do you have any way you can pressurise the cylinder to see where your compression is going?

The large gap makes me think it has been gapped on the exhaust stroke as a large gap on the compression stroke results.

Go one engine rotation back to TDC. What gap do you have here?
 

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Discussion Starter · #276 ·
Fair enough. I just don't want you pulling stuff apart if you don't need to.

Do you have any way you can pressurise the cylinder to see where your compression is going?

The large gap makes me think it has been gapped on the exhaust stroke as a large gap on the compression stroke results.

Go one engine rotation back to TDC. What gap do you have here?
I do have an air compressor i can probably hook up to it.
And i went one rotation to TDC and thete is no gap at all.
 

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Go back to the large gap TDC and gap to spec there.

Then rotate through and check on both TDCs again.

It should go from the gap to a depressed exhaust valve @ 360 degrees and back to the gap you set
 

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Discussion Starter · #278 ·
Go back to the large gap TDC and gap to spec there.

Then rotate through and check on both TDCs again.

It should go from the gap to a depressed exhaust valve @ 360 degrees and back to the gap you set
Ok, the gaps are good when i do that, everything seems correct. When i rotate the engine again , right before i get the the TR mark again, the exhaust valves pop back up from veing depressed. Are they supposed to be fully depressed at that mark?

Also i hooked up my air compressor to it, and i blew air through when i had it at TDC for the compression stroke, and the air was coming through the exhaust port
 

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... right before i get the the TR mark again, the exhaust valves pop back up from veing depressed. Are they supposed to be fully depressed at that mark?
Depend if you are on the exhaust or compression stroke

Also i hooked up my air compressor to it, and i blew air through when i had it at TDC for the compression stroke, and the air was coming through the exhaust port
If you had the engine at TDC on the compression stroke and the valve gaps are properly set then no air should be coming from the exhaust port. Compare the positiion, height, of the exhaust valves on the left and right cylinders with the appropriate cylinder on TDC compression. That might tell you if you have bent valves which are not closing properly.

The other possibility is that you have burnt exhaust valves.

If you decide to remove a cylinder head make sure that you buy a good head gasket, such as the ones sold by Murray's Carbs. Use copper gasket spray on the new gasket before installing and follow the recommended procedure for torquing the head bolts. You will be doing that a few times.

If you find bent valves or burnt valves it might make sense to source another cylinder head.
 

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Discussion Starter · #280 ·
Depend if you are on the exhaust or compression stroke



If you had the engine at TDC on the compression stroke and the valve gaps are properly set then no air should be coming from the exhaust port. Compare the positiion, height, of the exhaust valves on the left and right cylinders with the appropriate cylinder on TDC compression. That might tell you if you have bent valves which are not closing properly.

The other possibility is that you have burnt exhaust valves.

If you decide to remove a cylinder head make sure that you buy a good head gasket, such as the ones sold by Murray's Carbs. Use copper gasket spray on the new gasket before installing and follow the recommended procedure for torquing the head bolts. You will be doing that a few times.

If you find bent valves or burnt valves it might make sense to source another cylinder head.

Hmm ill test that out to see. I saw an old post on this forum where someone had a similar issue as me, almost exactly the same, and his valve heads were chipped badly. So maybe thats the issue. Is there a way for me to remove the cylinder head without removing the engine? Just as a preliminary check?
 
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