Oily 450

[IanHood]

My 450 Desmo starts easily, runs nicely and blows no smoke however, after even a short ride I am left with an oily mist on the side cover that would appear to come from the carb bellmouth. Additionally, the plug also appears very oily. Anyone have any ideas before I pull the head and barrel?

Ian

[LaceyDucati]

Ian,

Mostly with Desmo's, guides and guide "O" rings are the issue (if not the piston rings). The "O" rings end up like wood and can only be removed by cutting them out in sections. Also when replaced, the "O" rings can then be "chopped"/damaged when putting the valves in, due to the sharp edges on the valve half ring groove. Same issues apply to the rocker spindle "O" rings and sharp edges in the pin bores. If you examine a used "O"ring with an eye glass you will most likely see what I mean. The solution is to remove the offending sharp edges, to avoid damage. Usually I end up wasting a handful of "O" rings in the process of eliminating this issue. Patience is a virtue here, and various home made scrappers for internal holes is the way forward. Valve edges are best radiused on a lathe with a small wet stone.

Ultimate solution - Maybe upgrade to a spring valve head, where you get the same performance without all the design faults

Regards Nigel

[IanHood]

Thanks Nigel, I think it must be a rings issue because it was the same with my springer head on the bike and I know that head is good because a nice chap called Nigel Lacey sorted that one for me

Ian

[LaceyDucati]

Hi Ian

Did I, LOL.... You may have to remind me of the details. Hopefully that's not the issue then, what sort of piston is it? Maybe give me a call in the week if you want to talk it through. Same number as before.

Regards Nigel

[IanHood]

Piston and rings were also from you Nigel and from memory, Omega. I thought I had measured everything right and got all the ring positioning correct but something must be amiss

Ian

[themoudie]

Aye Ian,

I have been following this thread, with a chuckle at the banter between you and Nigel. However, how much mist is oil, or fuel and to what degree, after 50 - 100 miles, is it dripping from the underside of the side cover? Or, is it just a sheen on the side cover?

The reason I ask is that my 450 MKIII, is running a 173 cam and a 32 mm AMAL MKII carb, with no air filter. The cylinder head inlet has been enlarged to have a smoothline between the carb and the inlet, it was 29 mm. The base of the inlet has been left alone and the roof of the port opened up and some material removed from far end of the port to try and aim the charge downwards, rather than scudding straight across the top of the piston and out of the exhaust! A dimpled finish, like a golf ball, was then applied to the whole of the inside of the inlet port, rather than the old school mirror finish. BUT, it is NOT a dyno achieved pukka gas flow job!

Prior to the above set up, running a standard 450 OME soft cam, a AMAL 30mm MKI, with no air filter and no porting of the inlet tract, I too would get an oily/fuel mist/breath on the side cover that eventually would run along the underside of the cover, but it always blew off. With the latest iteration, the oily/fuel mist still appears on the cover after a couple of hundred miles, but never enough to run and get blown off.

My 1953 AJS 18S did the same oily/fuel mist over the oil tank from the AMAL monoblock and I have always assumed that singles, are liable to do this breathing and more especially the greater the overlap on the cam.

Maybe the piston rings need bedding in? I hope that you can get it sorted to your satisfaction.

Good health, BillR

[IanHood]

Bill,

It’s just an oily smear on the side cover but it appears after even a very short run of 30 mins. It’s never heavy enough to ‘run’ or drip but if there’s enough oil in the combustion chamber to create that mist, I don’t understand why I don’t also see some blue smoke.

Ian

[themoudie]

MMmmmm!?

Aye Ian, I think a conversation, with Nigel, rather than my witterings might be productive!

Are you spinning the engine up in the lower gears, 5,000 rpm plus since the piston and rings went in? Blethermaskite would probably say "6,500 rpm more like"! I am thinking that the rings haven't had a chance to "bed in", as you describe the spark plug as being "oily" as well. There being no sings of blue smoke on the overun, would tend to indicate that there is not a lot of oil about, from wherever it is coming and certainly when I ran the 450 MKIII, without any valve seals on the tops of the guides, a blue haze wasn't long in appearing on the overun and the oil/fuel sheen on the side panel was slightly tacky from the oils greater viscosity.

If you have somewhere a bit quiet, I'd warm the engine through and then give it some throttle and see what the side cover, spark plug colour and the end of the exhaust look and feel (slippery, black carbon) like, after a good gallop.

Whichever path you choose, I wish you well with the beastie and that you are able to get some miles under the wheels.

Good health, Bill

[IanHood]

Well, I think I’ve solved the problem and I’m embarrassed to say it was my fault. Pulled the barrel off to discover the two oil rings were perfectly aligned (instead of 180 degrees to each other and therefore making a nice gap for the oil to run through. I must have been distracted when I put it together.

Still can’t figure out why I wasn’t seeing any blue smoke from the exhaust though. Just waiting on some fresh oil now before taking it on an another run.

Ian

[themoudie]

Aye Ian,

Pleased to read that you have found the culprit for the "wet" blow-back and black spark plug. I hope you can give the beastie a canter afore long.

Good health, BillR