Smoking Engine - Any thoughts?

[moriniboy23]

Ive recently finished rebuilding the top-end of a basket-case Morini 501 engine - the one with nikasil bores. The bores looked peachy with no wear so I fitted new rings (North Leics. Motorcycles assured me they were 501 rings suited to the nikasil bores) and had the valve-seats recut. On starting and riding it 30 or so miles, the rear cylinder smoked like a b*stard, great billowing clouds of white oil-smoke out of the rear cylinder silencer. Both at constant throttle and apparently worse, when shutting off and reopening. There is an exhaust balance-pipe but virtually no smoke was coming from the front cyl silencer, all from the rear. A compression-test showed 180psi on the front pot and even better, 190psi on the rear, problem pot. So deep breath, engine out. I gave the barrels to my local engine-remanufacturer who measured and declared them unworn but deglazed them. I also gave them a spare pair of heads and valves. The valve-guides are well within wear-limits and the seats and valve wear eliminated, so with new gaskets and 30-mile old rings in the newly professionally deglazed bores and the second set of professionally overhauled set of heads I put the engine back together and...just the same, it still smokes from the back pot. Ive checked the crankcase and rocker-cover breathers and they are clear and not pressurising the crankcase. Just before I pull the engine out AGAIN, any thoughts anyone?

[Jade750SF2]

I do not know if this is an issue with Morini engines - check to see if the piston rings are correctly placed. Some piston rings are tapered in cross-section and are usually placed with the widest part of the ring at the base of the ring groove.
Are the oil control rings multi-piece - are the rear cylinder rings the same as the front cylinder rings for type and [if multipart] assembly?

[Steve Brown]

I would agree with the comment above from Jade, the rings are most likely incorrectly fitted e.g. upside down? Most oil control rings are one way only and have tapers or steps etc to wipe the excess oil from the bore. Gaps spacing may be a part of it but would need to be very wide and all in one line. I assume that you did check the ring gaps in the bore? Sorry if that's too obvious. Also how about an out of round bore or a porous barrel/head casting? Cracks even from the rocker box into the combustion chamber? (straw clutching time that)-try the ring fitting first, the difference in the faces can be very hard to see with the naked eye if there is no 'Top' marking on the rings.

Good luck, that sounds very frustrating!

[mad muller]

not well up on the nikasel bore so cant comment on that one, was it smoking before you rebuilt the top end? check plugs for oiling up, have a look at oil control rings on my sport the later type have a spring to keep the radial pressure on the cylinder its fitted to front and back pots, have you got the pistons the right way round its easy to get wrong check the workshop manual it has a diagram showing crown of pistons , its a drag you have to drop the engine out , I can get the heads and barrels off in situ but its 350 sport so who knows, you could try a hone on the offending pot and other rings old ones, if still useable, to see if this makes any difference , for running in I use a cheap mineral oil helps stop glazing , stick with it you will sort it. muller

[moriniboy23]

Barrels have been honed and measured by an engine remanufacturer and declared fine, it's on new new rings bought after the first engine removal/stripdown. Im using a second set of rebuilt heads with valve-seats recut and valves refaced by the same engineering firm.

Yes the 500 engine has to come out of the 350 frame each time.

Certainly the middle ring is critical which way up it goes and it's correct (checked twice now). The oil-control ring is one piece and appears symmetrical so uncritical which way up it goes.

Rightly or wrongly, I always remember the quote from the UK piston manufacturer Wellworthy's own literature: "the only ring gap that's critical is when there's not enough of one" I didn't check the ring gaps as the bores are fine and the rings new, but maybe I should have. I will on this third engine out/stripdown that is looking inevitable. if the gaps ARE too big, I really dont know what I can do about it, as the rings are new!

[Steve Brown]

If the gaps are too big you only have one option-more rings, sadly and expense. Any signs of porosity?

An afterthought but check next time you're in there, have had pistons (on other engines) that cracked in the bottom of the ring grooves. So they let oil round the rings missing the oil control completely! Have a really good look at the anyway, some types have a very small chamfer on the top to scrape the oil down. Sorry if that's egg sucking lectures again.

[moriniboy23]

Nope, that's definitely not egg-sucking for me....

[morini_tom]

A couple of thoughts. Firstly I'm assuming that the machinist who deglazed your bores knew they were nikasil?

Secondly, the nikasil cylinders are graded- make sure the letter stamp on your cylinder corresponds to your pistons!

Thirdly, the 501 heads are prone to cracking between the valves.

As I wrote on the Projects/camel 501 thread a while back:

As promised I've photographed my spare 501 heads. They're a bit manky, and I've not been in long so no time to clean them or remove the valves, but I think you can get the idea.

The first picture shows a crack between the valves in the classic place. I think I can actually see this crack all the way through to the rockerbox side but is not that easy to see without more stripping and cleaning:



The second picture shows the other head which had a much bigger valve seat insert (dark material concentric to the exhaust valve). I can't remember if the inlet and exhaust seat inserts were a single piece Siamese insert or slightly overlapping inserts with a portion scalloped out- and it's obscured by the inlet valve so can't easily see, but they were certainly touching.



Both these heads were re-seated at the same time, and have done probably 20,000km since. The head with the smaller inserts has cracked again in this time, and the one with the full Siamese hasn't... Should have spent the extra money getting both done properly!

As a history on these heads, they came from my 1st 501 engine, of unknown mileage and pretty worn in most areas. I had them re-seated before I even ran the engine, as they were cracked then, and built the engine back up with a 2+2 cam. For most of the time that engine was in my bike I was developing the dart, various intake, exhaust, cam timing, ignition and carb setups, and it only really started to perform well towards the end, which probably explains the amount of deposits on the head!

Hope you track down the issue without too much more hassle,

Tom

[moriniboy23]

Hi Tom, this is very useful, thank you.

Ive encountered that twixt-valve cracking on a 125 Corsaro before now.

I spotted some stampings on the barrel yes. Should there be corresponding stamping on the piston?

I told the engineers about the Nikasil and they either knew or took advice. The chap said (I think) when I dropped them off that he'd check but that they would probably use a paste not a honing stone

[morini_tom]

yes, the pistons are also stamped.

I've just been and taken some pictures of my spare 'B' grade 501 nikasil piston and barrel (thickly deposited, took a bit of scraping to find the mark!)

[moriniboy23]

ah brilliant. id found the Z7 stamping on a spare set of barrels but nothing else

[moriniboy23]

Well the engine's out for a FOURTH time, and the barrel and piston stampings are all fine, the new-ring gaps are all fine in the bores that my engine remanufacturer measured and declared OK, and my mechanic mate has admitted he's stumped too. Yes, both heads are cracked between the valve seats (I dont know how deeply) but they are on the front pot too and that doesnt smoke, nor can I see how a tiny crack like that could cause such massive oil-burning and from cold too, as well as hot.

[Ralph]

I dont know a lot about the 501 but when you put it back if it's
possible to swap the heads then if the oil burning is still there
and swaps cylinders you have a idea it's the head or barrel.
I suspect the head and wonder if it could be cracked between
rocker box and inlet or even exhaust port.

[pedro]

I had exactly this problem with an old Brit bike some time ago, I ended up in desperation putting the old rings back in, problem solved.
I had a set of faulty rings.

Good luck,
Pete

[moriniboy23]

Well the heads are identical front to rear on all Morinis as Im sure you know, and its the second set of reconditioned heads ive tried on the bike (measured valve-guides, recut seats, ground in valves etc) and still smoking just as bad as ever, so it almost certainly isnt that...

If it wasnt an engine-out job and a gasket-set to be bought each time, id be tempted to try one thing at a time - swap heads, swap pistons, swap rings front to rear etc, swap barrels. You can get a Morini engine out of the frame in 1/2 an hour but putting it back, balancing carbs etc is a much longer process

Ive just been back to the engine-remanufacturer and his best theory without knowing Morini engines intimately is that somehow the rear head is getting much too much oil, cant return it fast enough and from his experience it will go down even the most perfect valve-guides and be burnt. He refused to reline my virtually unworn guides as a waste of money, so its not like he's touting for work!