Low compression - new rings needed?

Hi - on a warmed up V50 Monza engine I am getting only 75PSI on the LH and 85PSI on the RH cylinders. I notice I now have fine oil leaks on both sides where the top half of the cylinder head sits. Should I just tighten the bolts and reset the tappets and ignore or take the top and barrels off and check the pistons?

The bike is 35 years old but has only done about 25k.

Cheers, Ray

I have a V50 3 that has the same engine as the Monza. I found a set of rings cured my compression issues. Pistons are unobtainable now anyway… I haven’t found any and I’ve been looking for some time now.

Thanks Steve - from where did you source the rings?

I have found the rings on gutsibits - but did you manage to change the rings while the pistons were still atached to the bike or were you able to access them easily enough just by taking off the barrels? From the workshop manual it does look possible.



I also got my rings from Gutsibits and I fitted them by just removing the barrels.
New ‘O’ rings and gaskets etc. I did mine on the deck but if you have access to a ramp you can see what your doing!
The oil ring is delicate (guess how I know)

The oil ring is delicate (guess how I know)

Thanks Steve - and I am cringing thinking of that moment when a new ring just snaps!

I have kind of wimped out and will do this in stages - first I will regrind the valves and see if that improves things.

Luckily I do have a ramp now so that will help my getting older knees!

I also have to redo the carb floats - when I refurbished the carbs I set them as per the manual and when all put back together and the fuel tank turned on guess what… petrol flooding out through the K&N filters! Doh!

Unfortunately the rings come in sets; ended up with 3 sets.
I put the carb jets back to standard and all went well. Not sure if you need to change anything if you’re using K&Ns.
The jets, floats, and valves are still available from Eurocarb.
Good luck, and I know what you mean about the knees!

Leaving rings aside for a mo, the OP says he has oil coming out presumably at the head gaskets? If I interpret it right? If so, implies heads are essentially loose so are blowing past the gaskets hence the low compression?

Yes, it implies they’re loose or not flat.
If the gaskets have failed the compression will be wrong. There is the rocker oil feed up the centre but as you say Mike the gasket must be faulty for it to leak.

With the heads off the rings and barrels all looked good - I reground the valves and put it all back together and… compression is the same!

What I thought was oil coming past the head gaskets previously I now think was petrol! I had a problem with the carb floats not shutting tight and also noticed that petrol was leaking out the back of the K&Ns.

I am about to start a new ‘woe is me’ thread about how I have cocked this bike right up!

Noted …

Shouldn’t need rings at that mileage, I got to about 55k miles on my V50 before I had to do them. The bike was still running great but was pressurising the crankcase causing a lot more general oil leakage.

I should check my compression gauge in another bike in case it is misreporting! Certainly the inside of the barrels were smooth and clean so I’d be surprised if they are worn to the point I am getting half the compression I should be getting.

I believe it’s best to check with throttles wide open for best results.

The bores are Nigusil they will not wear. Mine still looked like new when I reringed mine. Main point with the compression test on yours was that the two sides were close. And yes, you must make this test with the throttles wide open.

Perhaps not much use here but I have recently tested both my Loop and T3. Blue was about 150 both sides and Jug nearer to 175 both sides. They are both 850s.

Interesting - I turned the throttle for sure but maybe not to the max - next time I will make sure the bike is really warmed up after a ride. Last time I also was having a concurrent problem where the carb floats were not shutting off and petrol was leaking out of the K&Ns - so excess petrol could also have been flooding the barrels and removing the oil from the rings - now that the carbs are fixed and the electronic ignition working as it should I will get her MOTd on Monday (29th July) and then on my return from that 20 mile round trip will check compression again - thanks all

I am delighted to report that after a run to get an MOT and back, checked compression with the throttle wide open and it was exactly 150psi on each side!

Great result :sunglasses: