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MKIII smokey rebuilt engine

Submitted by felix_hall on Sun, 28/02/2016
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Commando

Hello again

This is a follow up to my first plea for help (on the batter/ charging/ starting) on my new to me (sort of) mkIII.

It came from my uncle, who put a lot of time in over a period of years but ended up frustrated because he could never get it to run. Sad. He used to live on a 750.....

In rebuilding it he (I believe, although am not 100%) had cylinders rebored, new pistons, rings, valve springs, guides, seals. The head sealed. The carbs ultrasonically cleaned. And enough people looking at it that I guess nothing obvious was missed.

It then sat. For at least a couple of years.

I am a complete beginner here. I'm really hoping someone can tell me whether I'm doing the right thing, and what I should be thinking about.

I now have it running, but there's quite a lot of oil getting into the left cylinder. I have no idea whether from the top or the bottom. Don't know how I could possible tell....

Symptoms are:

- Lots of blue smoke at start up. Which then seems to clear (this may just be because I'm moving. But then once the engine's hot return with a vengeance. Enough to have been pulled over....

- Oil spitting out of where the exhaust joins the cyclinder (and from there all the way don the side of the bike). My theory here is that when the plug gets fouled the cylinder goes a couple of cycles without firing, causing oil to mix with fuel and spit out - partly because it doesn't seem a constant flow

- The plug fouls. The one on the the other side gets pretty black too. But every so often the idle drops 50% (normally its idling very fast at 2k) and stalls. Swapping the plug out sorts it. I have learned to carry them and a spanner. And also have burned fingers....

At the moment what I'm basically doing is working through it - trying to get enough miles on the bike and engine to run it in/ shake it down so its easier to work out what's going on.

Is that the right thing to do? Assuming I can trust it (see the charging thread) I can do so, as long as I'm not creating further problems. But it seems extreme..

An aside....no choke. ever. Is that normal?

Another aside....there's oil all over the place but I reckon not just from here. I'm embracing that at the moment as part of the joy of 70s bike ownership....

Let me know any thoughts. They'd be much appreciated.

Felix

Submitted by david_comeau on Sun, 28/02/2016

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Since it is a MKIII, it should have a RH4 head. And since about 2000 most people know that almost 50% of RH4 heads have cracked heads at the intake guides (one or both sides). When the head was being worked on, was it cracked checked before proceeding with the guide replacement? Standard procedure here in my shop..

otherwise a compression test and especially a leak down test can help get data to help diagnose the problem.

Submitted by peter_shand on Sun, 28/02/2016

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Oil in the cylinder is either getting past the rings, getting past the inlet valve seal/guide or getting into the intake to the cylinder affected.

You say it has been rebored, you need to check how good it was done (right size, right finish) and is the bore straight and parallel, those three legged spring hones are useless for honing a cylinder they will neither keep a straight bore nor keep a round bore. Have the rings been fitted properly/ not broken

Has the inlet valve stem seal been damaged/missing? you say the valve guides have been changed, how exactly was that done? If the head was heated up and the guide hammered/pressed out (as it says in the manual) it will have scored the bore in the head, old guides should be machined out with a counterbore almost to the head bore size just leaving a thin shell of the old guide to press out. doing it this way removes most of the interference fit of the old guide to the head thus you will not score the head. A scored valve guide bore in the head will leave an ideal channel for the oil to get into the cylinder. You might have a loose valve guide if the work been done was shoddy.

Check that the oil tank has not been overfilled and the engine drawing oil in from the oil tank breather

You might have a porous/cracked head, not unknown on a mk3

you might have a damaged head gasket that is drawing oil in from the push rod tunnel

Is the cylinder head flat?

All of the above will require checking/correcting

Is it still fitted with amals? if so the no choke senario is normal The air valve slides need to be raised for normal operation.

Years ago when i had amals on my previous mk3 i thought they were the nearest thing to uncontrolled fuel injection. tickle, start, ride... never used the choke. An SU carb was a revelation, full choke to start, exhaust going blue in a couple of minutes and an achievable 80 plus M.P.G.

Years later and my current MK3, again fitted with an SU carb i got 86 to the gallon on a VMCC run following a friend of mine on a 1925 two speed Scott Flying Squirrel. Admittedly our average speed wasn't high (though the scott would cruise at 50-55 mph) we did 186 miles that day and the scott didn't miss a beat. and it went up Sutton bank near Thirsk at a fair old clip!

Regards

Peter

Submitted by robert_tuck on Mon, 29/02/2016

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I agree with all the above but would heavily suspect the oil is mostly leaking between the guides and head the main suspect being an exhaust one. Its also likely that the oiled up plugs are adding to the problem. Plugs now made don't cope well with excess carbon or oil as the insulator finish has been changed to suit modern lean,clean burn engines and absorbes the muck and does not clean easily, I have tried lots of things but not the dishwasher (I'm not that stupid!).

Submitted by martin_freeman on Mon, 29/02/2016

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Dear Felix,

you are experiencing full range of joys of ownership.

I have been here 3 years ago, here is something for you to try before dismantling the engine to check valve guides etc.

Before trying to start it get that 2 litre ice cream tub and drain oil from bottom of engine; you may be surprised how much there is. Now put it back in the tank if like me you are from Yorkshire or top up tank with fresh. Now hopefully you will have less smoke on startup. If this is the issue either service oil pump end float or ride it every week.

there have also been issues with available replacement pistons and rings allowing oil past mine took a good 1000 miles to settle down.

good luck

martin

Submitted by felix_hall on Tue, 01/03/2016

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Thank you, everyone.

Martin - I am indeed, but it's what I signed up for. The way I see it is this is a good time of year to be dealing with problems......

I don't think the issue is the oil slipping down into the sump. I ride it every week. So it would have to be doing so pretty quickly. And the smoke stays with me even after riding for an hour or more .

My first plan is, as Martin says, is to keep riding it in the (possibly remote) hope that once the pistons and rings (which are new) settle down that sorts it out.

Assuming it doesn't I'll then be in a better place anyway to start opening it up........

Thanks again. If you see a cloud of blue smoke in NW London that's probably me.

F

Submitted by robert_tuck on Tue, 01/03/2016

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To try to identify the cause it has been said that if you block off the oil pipe to the head for a while and the smoke stops you know the problem is in the head. The rockers dont need much oil so a 5-6 mile run would not hurt and may help direct your efforts.The smoke would take a while to stop till you burnt off the oil in the silencers.

 


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