3c Compression test

katanarama

katanarama

Full member
morning all

I am looking at a 1974 3c and asked the dealer to do a compression check. Stone cold with throttle open it came back as:

Left    7.8 Bar / 113 psi
Centre  7.8 Bar / 113psi
Right  8.0 Bar / 116 psi

Compression tool used was rated in Bar, I did the conversion and added the psi. How do these readings look? Within spec?

cheers

Julian
 
Bit low, but who knows how accurate their gauge is?  As long as they are much the same, that's all that really matters.
 
little low for a 3CL engine but if you are looking at a bike to buy and it is at a dealers you should be requesting a Leak Down test to be done which will then give you a lot better test of the engine components
 
Bike I am looking at has 26,XXX on the clock. Its a little concerning that it about the same cold reading as one with 100,000KM.


Dellortoman said:
That's about the same cold readings as my brother's 3C, which has done 100,000 km but still runs well. So probably what you'd expect in a well-used engine.
Click to expand...
 
katanarama said:
Bike I am looking at has 26,XXX on the clock. Its a little concerning that it about the same cold reading as one with 100,000KM.
Click to expand...

Not really.

After initial running-in, the engine performance stays quite consistant for a long time.  In my experience, Laverdas start using oil around the 70000-80000km mark.  Not due to excessive bore wear, but to worn top ring grooves.  The excessive ring movement lets the ring effectively act as an oil pump.  This will not really show up in a simple compression test, doubt that a leak-down test would lead to that conclusion either.  Not unusual to see the original hone pattern after 100000km, bore wear is not an issue.

piet
 
Valve clearance and timing makes a big difference to compression. If engine tune isn't spot on the compression could be all over the place. When I dialled in the cam timing on my bike, without touching anything else, the compression increased by 30psi. My cam timing was a long way out though. 4C cams on A11 sprockets  ::). A stock 3C with factory fitted cams and correct sprockets can still be a few degrees off spec. Tight valve clearances will reduce compression too.

I wouldn't be put off by the those compression readings at all. They're in the ballpark and quite even across all cylinders which is more important than the actual number. If the bike has been sitting in a dealer's showroom for a while, there won't be a lot of oil around the upper cylinder area to help seal the rings. If you ask the dealer to run the engine for a bit and re-check the compression when it's warm, the numbers should come up significantly.

Cam
 
Julian testing compression will only give you a little information into the state of the engine there may be thousands of dollars of unidentifible issues that lurk within the engine.
Just buy it.
 
When I got my RGS I knew it would need some work at some stage (it was clear the head was knackered) but the bike ran fine, so I rode it for three years without any problems and it's only now away being fixed.  You can never be sure what's inside a 30 odd year old engine, so if it runs OK buy it and have some fun!
 
Well, around one of the buckets there was some damage to the head suggesting something had let go big time- dunno quite what, but the main thing was that Keith had to use some very thin shims on it to ge the clearances right.  Also it does smoke on the overrun (that gradually got worse)- rings as Piet says and of course valve guides.

Bloody sick of Laverda cylinder heads- the one on my bike is a late one according to Keith, and it's still shit.  Mind you, the bike is from South Africa so has probably been used very hard over long distances.

Hopefully the new head and a rebore will have the same effect as on my last bike- compression up from 130psi to 198!  Which just goes to show that my compression guage is only good for comparisons.  198 my arse. :D

 
It's not shit, Reggie, just worn out!

The later heads addressed all the little niggles the earlier ones were cursed with.  You won't find any cracks in the cam cover sealing surface, the slots for the cam blocks won't be as butchered and the skulls seem to be quite a bit thicker.  These still tend to crack, but after far more abuse.

Once Keith is finished with your head, you will be able to enjoy performance you have not yet experienced with your bike as the correct positioning of the valves in the chambers and decent valve seat widths and angles will really tranform it(back to what it once was), and the head will then certainly out-last you, and most of us as well... :D

The head is probably the single, least understood Laverda component for a backyard bodger, hence the damage we so often find in them.  Most damage stems from mechanical inadequacies of the persons performing the "serviceing", some more through abuse and slack maintenance, a little is "natural" wear and tear.

piet
 
OK, I was being harsh, it's a miracle that it went as well as it did over the last three years.  Wonder what the actual milage of my bike is- it reads about 55,000km.  I don't even know which is original- the frame or the engne. :o
 
I've read in more than one place that under 90 psi or so the cylinder will not fire, but again all cylinders being close is more important. That being said,I looked at a bevel ducati for a friend once that had almost no resistance at the kickstarte. Valves were in spec, and compression was about 90 psi per jug. Bike appeared to have been ridden hard and I was somewhat concerned about the condition of the bottom end given this, but my friend bought the bike and rode it for several years before finally rebuilding it. That was a somewhat dainty bevel Duc, not a very robust/over engineered 3cl.
 
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