Compression and Valve Clearance - Help.

750mors

750mors

Been Around the Block
Hi Guys and Girls,

Bear with me, the questions are below, but I want to share the background to help those reading picture the scenario.

I'm in the final stages of my CB750 DOHC build and have come up against something that I can't get my head around. So I thought I'd throw it out there and see if anyone can share their thoughts on the matter.

Firstly, my engine is freshly rebuilt. Bottom end by me, and the head work externally. I haven't had this engine running since and have just started the task of carrying out all the pre-checks over the past week.

Being completely honest, the first hurdle was completely my fault, I had the cams incorrectly timed. So having rectified that, my compression gauge turned up in the mail and I threw that on and realized I also have a dead No. 3 cylinder with zero compression. The other 3 are good with readings of 105 - 130psi.

I then checked the valve clearances and found that the Inlet on No. 3 has been set up to 0.6mm (.023") instead of 0.06mm (0.0023"). The exhaust on 3 and the rest are a little tight but consistent.

Could this be the cause of the zero compression?
My first thought was that naturally it must be if it can't get air in it has nothing to compress, however, looking through the plug hole I can see that the Inlet valves are still opening (albeit .54mm (0.021") less).
Would this much excess clearance be enough to not lift the valve off the seat?

Needless to say I need to remedy the clearances, which will answer my question. But while I'm waiting on a valve shim tool to arrive from the U.S. I wanted to hear from the greater DTT community and see what you all had to say.

Thanks for reading.

Matt :D
 
Too much clearance in the valve will not affect your compression - it's the other way round (too small).

Something else is wrong.


If you've had the cams in wrong and have been cranking the engine you've probably bet a valve or two.
 
The specs I had read on a new bore were >100psi, could you share your source and spec if you know otherwise rather than just telling me I'm wrong please? I'm here to learn.

When I say the cams were 180 out, I mean that instead of timing them to the crank with the cam lobes pointing towards the plug, I had them pointing towards the outside. Which is half of a rotation 360 divided by 2 is 180, so I assure you it can be done.

I did spin this engine over by hand dozens of times without resistance, so I'd be surprised if I've bent anything but it won't be ruled out.

Thanks for your answers.

Matt
 
750mors said:
When I say the cams were 180 out, I mean that instead of timing them to the crank with the cam lobes pointing towards the plug, I had them pointing towards the outside. Which is half of a rotation 360 divided by 2 is 180, so I assure you it can be done.
Click to expand...
I assure you that it cannot be done.

The numbers you have provided are a clear indication that you have issues, and if you are willing to learn, I'd start by researching how to time a cam. You absolutely, positively cannot install a camshaft 180 degrees out. No exception.

Now, if you are reffering to having the cams timed to each other improperly, that is a possibility. In which case, you had all at the same time, which is bad. You need to inspect every valve.
 
Nope. proper psi for a "new" engine should be, i do believe, around 170.
Others will correct me if i am wrong.
The lower your compression the less power your engine can make.
 
DohcBikes said:
I assure you that it cannot be done.
Click to expand...

He is right here
If the engine is set up like 2* 180 twins
You can instal it out by 180* but it wont be wrong. It just means it will be on the exhaust stroke instead of compression.
 
A camshaft cannot be installed 180 degrees out. Again, you can mistime them relative to each other (which you did not do according to your description), but there is no way to install a single cam exactly 180 degrees out. If you dont understand that it's not my problem.

Crank rotates twice for every 1 camshaft rotation. Suck Squeeze Bang Blow. I've built a lot of engines dude, not just 1. See ya.
 
d6bc0d4e6a424e5edac5d6e10c51d0bf.jpg


For the sake of my sanity, because you're both making me feel like I'm going crazy.
Here's a pic of what I had done.

Dohcbikes I'll take a breath and hope we can engage in a civil conversation, not a debate.

My cams were incorrectly installed in the positions shown when timing the engine. I'm sorry if I've used the wrong terminology here but from where I'm sitting these cams needed to be rotated 180 degrees to be in the correct position.

Matt
 
farmer92 said:
Nope. proper psi for a "new" engine should be, i do believe, around 170.
Others will correct me if i am wrong.
The lower your compression the less power your engine can make.
Click to expand...

Thanks Farmer, I'll have to verify my readings and the new gauge also.
I'm thinking I'll go through and re-set all the valve clearances on the tight valves too as they'll only get tighter.
 
750mors said:
For the sake of my sanity, because you're both making me feel like I'm going crazy.
Here's a pic of what I had done.

Dohcbikes I'll take a breath and hope we can engage in a civil conversation, not a debate.

My cams were incorrectly installed in the positions shown when timing the engine. I'm sorry if I've used the wrong terminology here but from where I'm sitting these cams needed to be rotated 180 degrees to be in the correct position.

Matt
Click to expand...

Nope.

If you wanted to rotate both cams 180 degrees, all you'd do is rotate the crank 360 degrees. Done.

For the last time. It is IMPOSSIBLE to install cams 180 degrees out.
 
750mors said:
Thanks Farmer, I'll have to verify my readings and the new gauge also.
I'm thinking I'll go through and re-set all the valve clearances on the tight valves too as they'll only get tighter.
Click to expand...


So. There is a possibility you tweaked a valve but maybe not. It's certainly worth a double check. It's the last thing you want to be surprised by after getting the other work done on them. I'm not saying pull all the valve and mic em'. OCD types sleep better when they check stuff like that though.


Either way, re-checking all the clearances can't hurt. I tend to err on the looser side of the valve clearance chart specs.
 
DohcBikes said:
If you wanted to rotate both cams 180 degrees, all you'd do is rotate the crank 360 degrees. Done.
Click to expand...

Thank you! The clear explanation to back up your statements helped. Now I understand.
 
canyoncarver said:
So. There is a possibility you tweaked a valve but maybe not. It's certainly worth a double check. It's the last thing you want to be surprised by after getting the other work done on them. I'm not saying pull all the valve and mic em'. OCD types sleep better when they check stuff like that though.


Either way, re-checking all the clearances can't hurt. I tend to err on the looser side of the valve clearance chart specs.
Click to expand...

Thanks mate. I'll definitely do the clearances as a starting point and go from there. :D
 
before you reset clearances :
you can positivily determine if a valve is bent/held open
the extra clearance could very well indicate bent valves which will be held slightly open creating the extra clearance and zero compression
with the valves closed on the suspect cylinder try blowing in the comp tester hose just with your own lungs
a held open,bent intake valve and you will have no resistance and hear the air flowing towards and through the carb for that cylinder
its not advised to use compressed air unless you lock the crankshaft from possibly violently rotating wrong direction

to use hose for the above test,you will have to remove the shcrader valve core from the tester hose plug fitting, easy peasy
 
xb33bsa said:
before you reset clearances :
you can positivily determine if a valve is bent/held open
the extra clearance could very well indicate bent valves which will be held slightly open creating the extra clearance and zero compression
with the valves closed on the suspect cylinder try blowing in the comp tester hose just with your own lungs
a held open,bent intake valve and you will have no resistance and hear the air flowing towards and through the carb for that cylinder
its not advised to use compressed air unless you lock the crankshaft from possibly violently rotating wrong direction

to use hose for the above test,you will have to remove the shcrader valve core from the tester hose plug fitting, easy peasy
Click to expand...

Awesome xb! Thanks so much, I was pondering how I could determine the valve to seat integrity without removing the head.

Legend!


Sent from my iPad using Tapatalk
 
750mors said:
Awesome xb! Thanks so much, I was pondering how I could determine the valve to seat integrity without removing the head.

Legend!


Sent from my iPad using Tapatalk
Click to expand...


If you take the cams out then you can use compressed air without having to hold the crank at TDC.
 
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