3C Valve Clearance & Compression

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laverlesmains

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i've had the 3C for almost a year, I figured it was time to check the valve clearance & compression.    My feeler gauge is SAE/metric with even SAE increments, so I had to interpolate the metric readings a little.

Having said that, if I've done this right my valve clearances on a stone cold engine are:

Cyl 1  Intake .21  Exhaust .21  Compression 82psi
Cyl 2  Intake .16  Exhaust .13  Compression 82psi
Cyl 3  Intake .12  Exhaust .21  Compression 88psi

This is the first time checking valves on a Laverda, but it seems a pretty simple thing to do.   Seems like Cyl 2 is awfully tight, as is the intake on Cyl 3...  I would have thought that valves would have loosened up over time.

Anything unusual here, or should I just go ahead and get the right shims & go for .20 Intake and .25 exhaust.

Thanks, Tim
 
laverlesmains said:
i've had the 3C for almost a year, I figured it was time to check the valve clearance & compression.    My feeler gauge is SAE/metric with even SAE increments, so I had to interpolate the metric readings a little.

Having said that, if I've done this right my valve clearances on a stone cold engine are:

Cyl 1  Intake .21  Exhaust .21  Compression 82psi
Cyl 2  Intake .16  Exhaust .13  Compression 82psi
Cyl 3  Intake .12  Exhaust .21  Compression 88psi

This is the first time checking valves on a Laverda, but it seems a pretty simple thing to do.   Seems like Cyl 2 is awfully tight, as is the intake on Cyl 3...  I would have thought that valves would have loosened up over time.

Anything unusual here, or should I just go ahead and get the right shims & go for .20 Intake and .25 exhaust.
Click to expand...

That is the common misunderstanding... the valves tighten up instead of loosening.  Result of valve seat recession, or burnt valves, should the carb settings have been way out in the past.  Get yourself the correct shims, if you can.  If the engine has considerable mileage on it, the shims might already be too small to allow further adjustments.  Either you grind the shims to the required sizes, ie, less than 1.85mm, the smallest factory shim, or you bite the bullet and have the head reconditioned properly with new valve seat inserts and whatever else might be necessary.

Were the compression readings attained with an open or closed throttle?  If it was wide open, you've got other problems as well... good running triple should show at least 110-120psi.  Or are you running C7 cams?

piet
 
Piet

the readings were obtained w/ the throttle closed, and the indicated mileage on the engine is around 12,000 miles.  The cams are marked "A11".


Cheers, Tim
 
Cosi said:
im not really qualified but

Inlet 0.20mm

Exhaust 0.25mm
Click to expand...

Inlet is specified as 0.20-0.25, exhaust 0.25-0.30.

I always err to the loose side, gives you some time before the valves close up again to the lower figures.  

Mostly quite fiddly attaining exact values, therefore the tolerance. ;)

Tim,

Your maladjusted valves shouldn't influence the compression values to a great extent, they're not off by enough to do that.  With only 12000 miles on the clock there should also be plenty of life left in the head...

Check compression with either a cold or warm engine, but remember to do the same every time, otherwise your findings will be quite useless for comparison.  And always at WOT!

piet
 
Sounds like a plan Piet.    I'll take her apart and check it all out.

Thanks, Tim
 
Revised compression readings (with WOT)

Cyl #1 - 140psi
Cyl #2 - 130psi
Cyl #3 - 140psi

blindingly obvious in hindsight - need WOT to have something to compress during the compression test..  D'Oh !!
 
laverlesmains said:
Revised compression readings (with WOT)

Cyl #1 - 140psi
Cyl #2 - 130psi
Cyl #3 - 140psi

blindingly obvious in hindsight - need WOT to have something to compress during the compression test..   D'Oh !!
Click to expand...

Blindingly obvious maybe, but I think you may be the first person to actually say why it should be measured at WOT.....

Heading for cover now, as threads are searched for evidence to the contrary  (bolt)
 
So blindingly obvious, nobody thought it needed mentioning, until it was and become blindingly obvious (scratch)
 
I'm just curious.  Does your 3C have the pistons that are about 8:1 compression ratio?  Do you use "regular" grade fuel?
Those look like pretty good numbers to me, but then again the engine has very few miles on it.
 
I've been informed by my bike's previous owner that the pistons he put in it have a nominal compression ratio of 10.5:1.

Compression tests were generally around the 140 psi level.

Then I dialled in the cams. After adjusting the cam timing it jumped to 180 psi.

Sounds impossible because if you multiply atmospheric pressure by 10.5 you get about 155 psi. Where does the other 25 psi come from?
The answer is that the air gets hot as you compress it (that's what makes a diesel fuel mixture ignite), which causes the pressure to rise further.

Cheers,
Cam
 
IIRC my Mirage 1200TS has a compression ratio of 8:1.  It uses regular grade fuel without a whimper.  When I bring her out of hibernation soon, I plan to check the compression as well.  I'd be happier than a very happy rider to see 140 psi.
 
Poffer said:
The early Jotas were 10:1 and later ones a bit less.
Click to expand...

Talking Mondial pistons with Wolfgang on the weekend, he told me something I'd never heard before; high compression "Jota" pistons could be either 10.5:1 "Jota Sport" or 10.1:1 plain Jota. Apparently a lot of UK Jotas got the Jota Sports ones, but, as usual, not all. They both have the same Mondial casting number (2556) but the 10.5:1 pistons have the crown 1mm higher than the plain Jota ones. I have to replace one piston (it ate something solid but the DPO put it back in the engine anyway), so I'm buying a high compression one and machining the necessary 1mm off the top.

Ken
 
Hi Ken,

That kind of makes sense. When I bought an early Jota from Richard Slater, he said "it's got the proper lumpy pistons". Just to me sure though, before you go machining, make sure the pistons were different and not the heads!

(see guys, I am getting used to this measuring thing  :-* )

Pete
 
I'm just curious.  Does your 3C have the pistons that are about 8:1 compression ratio?  Do you use "regular" grade fuel?
Those look like pretty good numbers to me, but then again the engine has very few miles on it.
Click to expand...

sorry for the delay Lawrence, was traveling this week.

Not sure about the pistons as I haven't had the head off yet.  I use 93 rated California gas..    Not sure why, I just figured the bike would need premium grade.  Also, I'll take the bike out for a spin today and then try the compression readings with the engine warm as suggested by others in this thread.

Cheers, Tim
 
so with the engine warm (which means the valve cover burns your hands trying to remove the sparkies) I get:

Cyl #1 - 149
Cyl #2 - 140
Cyl #3 - 149

 
That's more like it. :D

Almost consistant with your cold readings too.  Check the next time you service the valves to monitor any changes.  Wouldn't worry too much about the 9psi difference, probably a very slight valve clearance differance to the outer cylinders.

piet
 
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