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it has taken sometime to find the likely reason for smoke out the left exhaust which can come and go...bad on start up, bad after a good run, worse when left on side stand or banked left bend.........
I have taken the carbs/airbox off ready to strip the head.
In the left carb by lifting the piston and poking my finger in I wipe out clean fresh engine oil after the bike has been standing, it is in the intake.
I have little experience with a bonnie engine, I am sure somebody will know how oil is finding its way there.......head gasket/studs/bolts??
It would be great to know and any tips dealing with it.
What is the view on copper or composite gasket?
Thanks

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What year?

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Bear in mind that these engines did not originally come with valve stem seals. It is reasonable to assume that oil may be sucked into the intake port when the engine is running under high vacuum or that oil would run down the valve stem/guide when the engine is off and has been sitting for a while. An engine with a lot of run time on it since the last valve job was completed would be even more prone to trouble than a new, tight engine would be.

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1979 matching numbers thanks.

Last edited by RGM; 10/30/20 8:57 pm.
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79 crank breather vented into the air box- on the timing side
Is it possible that a bit too much oil has been sent to the airbox and breathed in?

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Copper. They have worked since the dawn of time. Why change?


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Oil could be leaking down between the guide and the head. These holes can become scored with successive drifting in and out of guides.

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Originally Posted by AngloBike
79 crank breather vented into the air box- on the timing side
Is it possible that a bit too much oil has been sent to the airbox and breathed in?
Definitely the easiest to check. The tube from the case to air box is on the left side.
Only a problem with significant blowby, examine the air filter.


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Another idea which comes to mind is wet sumping. Left on the side stand, without the DS crank seal, the primary can fill with oil.
Leave it on the stand for a couple of days and drain the primary to see how much comes out.


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ok, I will check these things thanks. The air filter and air box are dry, I do not think it is excess crank pressure as it tends to be left exhaust that is bad and it is not a constant problem. It seems to get oil from the top end in the bore and intake when standing, it then clears after a smoky start until the engine warms up after a few miles, then it smokes on and off...lots of it though. So I am guessing warm thin oil finds the way into the bore, perhaps even drawn in.
I have previously checked wet sumping and only ever drain a little out the engine after a run but will check the primary.
I heard somewhere it is not unknown for engine oil to track down the studs/bolts from the top end and then into the intake?

Last edited by RGM; 10/31/20 11:27 am.
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Has anyone "ported" the intake? Possibly they broke through to the stud hole or they broke through and sleeved it but did not seal the tube.

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The claim is 79 did have valve guide seals . My 79 does....


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I will find that when it is dismantled but I am not aware of any mods to her or porting

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Be sure to do the big washer mod as outlined here and here

To determine if the offending oil is coming down from the top end (valves, guides, studs etc) simply disconnect and plug the rocker feed for say 10 ~ 15 minutes running. If the smoke stops, at least you'll have narrowed down the area to look at.

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Originally Posted by Hillbilly bike
The claim is 79 did have valve guide seals . My 79 does....

My ‘79 doesn’t have them and I know it was original. No seals are listed in the 1979 parts catalog either.

The 1982 parts catalog does list a part number for seals so they were added sometime between when the two catalogs were written. Maybe halfway through the 1979 model year?

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The guys that build my heads used seals on intake valve stems for awhile thinking that it might slow oil consumption. Didn't see much if any improvement in that department but guide wear increased. So no more seals.
Have to guess that factory seals was done for some sort of emission control?

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well. the inner head studs/bolts and possibly washers seems like it could be a good possibility. The symptoms of oil could be right.
As for valve stem seals the chaps at my local Triumph specialist told me long ago the Bonnie 750 can have totally clapped valve guides, sound like a bag of old nails but still not smoke or burn much oil as a result.
Sadly I cant take it for a run at the moment to block off the oil feed which is a good idea to prove oil is coming from the top end.
Think I will just pull the head and see...watching for lose inner bolts as I go too.

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...hi; in one of my 79 engines, I have similar oil badly smoking; previously this last reassembly, did not smoked.
I have those thicker washers installed. Those studs are tight and with the 18 pounds. I checked them the four times that I re torqued the head.

After removing again, visually you do not see anything different; valves and springs were put in the same place.
When the welder refilled the head; he put old valves in the guides to prevent material over the seats...

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I started dismantling this evening, the left carb intake, inlet rubber and intake port had oil laying in the bottom! right intake dry. The bike had been standing before work for a good few weeks.
I have taken pictures, but do not know how to add them to the post!

Last edited by RGM; 11/11/20 8:42 pm.
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Further update, a little guidance would be great seeing as its my first Brit bike.
Head off, the head gasket is very very oily and the pistons/bores are wet with oil especially on the left which was smokier, Both inlet tracts have oil in them which could have drained in from the top of the pistons with the slope rearwards on mainstand (back wheel sits on ground)
So it looks like the head gasket could have been better.
Next, I am unsure if it has been ported...both intakes have a sleeve showing on the inner head bolts protruding into the intake port. The sleeves look dry...I do not think they let oil in.
However, the valve guide on number one inlet is stuffed...it wobbles and I could see it has turned in use by the carbon marks/shiny bits.
It would seem the valve guide fitting to head is loose and worn.
Finally one of the rear inner head bolts was missing its washer...so never mind about bigger washers, it had nothing!
In a nutshell the B---er up brigade have been at it before me.
I am unsure if the pistons are standard shape...they are oversize though.Looks like plus 40 before I clean them and read the top better.
I am assuming I will need an machine shop to ream oversize valve guides where required?

Last edited by RGM; 11/12/20 1:41 pm.
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The mind boggles at the time and effort folks went to to bugger these things up.

Of course I've never done anything bad to them 😉

Note that guides can be bought in oversize sizes.

Here is 4 thou oversize for example.

https://www.triumph-spares.co.uk/t120-inlet-valve-guide-bronze-004-oversize-70-3827-004


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If the guide hole is worn out of round or tapered, then it will require accurate remedial treatment before a suitably oversize guide can be fitted.

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Hi RGM, The sleeve you see in intake port is normal. It's a hollow tube they put in the mold when they cast head. The head bolt goes through it.

Loose guide in head explains a lot. As Koan stated oversized guides are readily available. The machine shop can/will custom turn/fit the oversized guide. You need to find machine shop skilled in Triumph head repair to have it reamed & appropriate oversize guide installed. Be very careful of the shop. If they ream guide bore crooked they have to grind the dickens out of the seat to get it Concentric. This will ruin seat. This is routine work for a skilled shop with the proper equipment such as a Serdi head machine or the like. These heads are gold & hard to come by. Don't let anybody mess it up. Advice on best shop will have to come from UK members that have good personal experience with such shop. It will not be cheap.

Regarding pistons, rings, if needed you can remove cylinders & look at pistons & rings. If all ok, simply blow off rings with compressed air or wipe best you can. DO NOT REMOVE RINGS!! If ok, you can simply put cylinder back on. Rings will find their old places in a few miles. I've done this hundreds of times to good results, like replacing base gasket or resealing tappet blocks. A pair of real ring compressors & piston boards to hold pistons makes this a very simple task. Lots of ways to compress the rings, but the real tool is a gem.

If you remove rings all bets are off. Often then the rings will slightly distort & it will burn oil & smoke.

If bores look good, you may well want to just leave cylinder alone. Get head repaired & installed. Then reevaluate the smoking. All you'd be out is gaskets & time.

It is very sad to see what some owners do to these bikes. It is said these bikes are simple to work on. Compared to a modern twin cam 32 valve V-8 with cam adjusters they might be. But they demand precision work & every thing done just so. Then they really do work good & can be quite trouble free. Very reliable.
Don


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