Need help with valve clearances

[mr blackstock]

G'day,

Last week I decided to synch the carbs, some recent injector cleaner useage improved the idle but she seemed in need of a balance.  I did a compression test before I started and discovered 2 pots were low, so off comes the rocker cover and I checked all valve clearances, most of the inlet ones were tight, .07 and .10.  The exhaust side were a little loose.  i ordered more shims and set about installing them.

Job done, now all inlets are between .12 and .15.  I decided to do another compression test to be sure, bugger me the two pots that were originally low, around 100psi are now 80psi and 10psi.  I checked the clearances again, still good gaps.

So, I re-install the original shims and the gaps are tight again, but now the compressions are back to their previous levels of 100psi and 110psi?!

The previous owner informed me that the bike has a mild cam, a mechanic recently did some head work and confirmed this. 
My question for the wiser ones out there is this:

Would a mild cam require different valve clearances, ie: tighter or looser over a stock cam?  My initial thoughts are no, but my understanding of cam timing is slight.

I hope someone can help,
Cheers, Gareth

[yamaha fj rider]

This might only make a slight change in compression. Did something go wrong during the second compression test? Retest was my first thought? Were the shims marked correctly and seated properly?

Kurt

[mr blackstock]

I retested all compressions quite a few times, when I put a shim in I usually rotate the camshaft a few times to be sure all is good. 

cheers, Gareth

[FJ_Hooligan]

Sound like a valve is being held slightly open.

Perhaps you mis-calculated the shim size? 

Or, maybe you mis-read the original shim size?

Or, did you go the right way with the new shim size?

[aussiefj]

As long as there is any clearance so the valves were fully closed I would have thought the compression would be the same.

John

[yamaha fj rider]

If the only thing that was changed, was the valve shims the problem has to be there. Shims are not always what they are marked. Measure the new and old shims, thickness and diameter. Hope this helps.

Kurt

[Pat Conlon]

As long as there is any clearance so the valves were fully closed I would have thought the compression would be the same.

John

John, you would think so, but remember that the valve clearances are checked on a stone cold engine and the compression test should be done on a warm engine.
What little (tight) valve clearance you may have on a cold engine is gobbled up by expansion on a warm engine.

Tight valves when cold, are leaky valves when warm. When you have a tight valve it is common to see a increase in compression when the shims are adjusted.

That's why in the tune up procedure you always start with valve adjustment.
As your compression numbers change it is prudent to re syncronize your carbs. Airflow changes.  Pat

[fj1289]

The previous owner informed me that the bike has a mild cam, a mechanic recently did some head work and confirmed this. 
My question for the wiser ones out there is this:

Would a mild cam require different valve clearances, ie: tighter or looser over a stock cam?  My initial thoughts are no, but my understanding of cam timing is slight.

I hope someone can help,
Cheers, Gareth

Gareth - most aftermarket cams specify the same clearances as stock.   You can play with the clearances a little if you want to tweak the power characteristics - but I'd stick with stock numbers for a street bike.   

Also, I'd put the valves to stock clearances, synch the carbs, put in new plugs and enjoy the kookaloo!   I wouldn't put too much worry into the compression readings unless it starts to definitely feel weak.  How do the slides react on CV carbs during a compression check?   I'd think they'd be all the way down.  Maybe pulling the carbs completely would give a more accurate and repeatable result??  I'll let the CV carb experts chime in and set me straight...

Chris

[mr blackstock]

G'day,

Thanks for all the good advice, I have been busy hand sanding a heap of shims to get the optimum sizes.  I think pat Conlon hit it on the head, I did the first comp' test when the bike was warm, the second one I did when the bike was stone cold, and I am guessing that due to worn piston rings on 1 and 2 may explain why the reading was so far down.  And some shims were way off their number?!  Surely one grinds off the number so as not to confuse the bloke following you?!

I have since reassemble, fired her up, she sounds good, equally hot across all pipes.  Now I can balance her.  I figure it is one of those things like Fj289, don't fret too much, start her up and ride her like you stole her! (sort of)

Thanks all, Gareth

Below is a pic of all the readings as they are now, I am happy with them I reckon.  Missing shim numbers mean i did not lift it out to change.

[oldktmdude]

G'day,

Thanks for all the good advice, I have been busy hand sanding a heap of shims to get the optimum sizes.  I think pat Conlon hit it on the head, I did the first comp' test when the bike was warm, the second one I did when the bike was stone cold, and I am guessing that due to worn piston rings on 1 and 2 may explain why the reading was so far down.  And some shims were way off their number?!  Surely one grinds off the number so as not to confuse the bloke following you?!

I have since reassemble, fired her up, she sounds good, equally hot across all pipes.  Now I can balance her.  I figure it is one of those things like Fj289, don't fret too much, start her up and ride her like you stole her! (sort of)

Thanks all, Gareth

Below is a pic of all the readings as they are now, I am happy with them I reckon.  Missing shim numbers mean i did not lift it out to change.

Gareth, print a couple of these out to transfer your shim info onto. It's a good way to keep a record for your next adjustment.   Pete.
http://www.fjowners.com/index.php?topic=1800.0