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A65 Oil Leak at the Carb?
#803339 03/29/20 10:27 pm
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This bike is so maddening. It refuses to idle well despite a fresh rebuild and new Concentrics. The left carb seems to be the issue. I get a lumpy idle and grey smoke whenever I twist the throttle. I have an oil leak up top. Since it has close to 100 miles since the rebuild, I popped of the tank and valve cover to A) solve the leak, B) re-torque the head, C) reset the valve clearances. In the process I checked the plugs (both black, very slightly wet) and the compression (180 both sides). Did all that and let it set 24 hours to cure the sealant). I thought my oil leak was from around the valve cover, but its not. It isn't from the oil feed line either. As near as I tell, it is leaking oil around the left carburettor and, I'm pretty sure it's leaking into the intake channel as well, causing the smoke.

Is that even a thing? Do the inside studs run far enough into the head to be an oil source? The ends of the outside studs are through to the exterior of the head.

I am not kidding, there is NO oil coming from the valve cover or the oil feed. It truly seems to originate at the carb flange. It is dirtier than the circulating oil from the tank and does NOT smell of fuel.

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'67 BSA Lightning--A collection of Helicoils held together by matching numbers
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Re: A65 Oil Leak at the Carb?
Redshirt #803344 03/29/20 10:59 pm
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Indeed that sounds strange. I don’t think a65 carb studs bisect any oil ways. Did you replace the intake valve guides? That could be a source.

Re: A65 Oil Leak at the Carb?
Redshirt #803692 04/01/20 9:27 pm
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So I completely cleaned up the head and carbs of all oil with brake cleaner, then added UV leak dye to the oil and ran it. The oil is seeping up from around the spark plug hole on the left cylinder. Still not making sense.

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'67 BSA Lightning--A collection of Helicoils held together by matching numbers
Re: A65 Oil Leak at the Carb?
Redshirt #803717 04/02/20 6:48 am
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Is the uv dye showing around that spark plug hole?


Now let’s all have a beer beerchug

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (undergoing restoration)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

Re: A65 Oil Leak at the Carb?
Redshirt #803730 04/02/20 2:23 pm
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For many years I was a plumber, I learned early on that where you see the water ain't necessarily where it is coming from. Working on the second law of plumbing (1st cold always on the right, 2nd [***] flows down hill, 3rd Friday is payday) I would be looking ABOVE the spark plug hole, it is probably just collecting there in sufficient amounts to be detected.

Re: A65 Oil Leak at the Carb?
Dave Martin #803797 04/02/20 11:11 pm
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Originally Posted by Allan G
Is the uv dye showing around that spark plug hole?

Originally Posted by Dave Martin
For many years I was a plumber, I learned early on that where you see the water ain't necessarily where it is coming from. Working on the second law of plumbing (1st cold always on the right, 2nd [***] flows down hill, 3rd Friday is payday) I would be looking ABOVE the spark plug hole, it is probably just collecting there in sufficient amounts to be detected.

Allan, yes the dye is showing up there.

Dave, It is dry all around and above the spark plug hole and the dye only showed there. I think there is a helicoil in that spark hole, so there may be some opportunity to wick up under pressure.

Looking to investigate the valve guides.


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'67 BSA Lightning--A collection of Helicoils held together by matching numbers
Re: A65 Oil Leak at the Carb?
Redshirt #803834 04/03/20 10:34 am
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I’ve got a couple of heads with bad left plug threads. One I think is from standing water. The other has a good thread but leaks around the plug. I always thought it was from unburnt fuel. Compression was always lower on that cylinder when turning the engine over. Could be oil.

Is the bike smoking at all? Blue smoke?


Now let’s all have a beer beerchug

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (undergoing restoration)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

Re: A65 Oil Leak at the Carb?
Redshirt #803843 04/03/20 1:06 pm
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I agree with your assessment of "still not making sense". If the oil was coming UP from the combustion chamber, it would be by definition combusted, unless that cylinder was not firing. Given that your initial problem was that it was apparent at the carb, and finding it at the plug is new, then the leak MUST be higher up. The trick is finding where and HOW the oil gets to where you are seeing it. I cannot see by what route bad valve guides would result in oil at the plug, or indeed at the carb flange (unless they were REALY bad).

Re: A65 Oil Leak at the Carb?
Redshirt #803854 04/03/20 3:33 pm
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Just a thought, have any of the valve cover studs been replaced with bolts? or even has a stud been put in with some dirt in the hole and the casing cracked a tad.

Re: A65 Oil Leak at the Carb?
Dave Martin #803867 04/03/20 5:39 pm
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Originally Posted by Allan G
Is the bike smoking at all? Blue smoke?

Originally Posted by Dave Martin
I agree with your assessment of "still not making sense". If the oil was coming UP from the combustion chamber, it would be by definition combusted, unless that cylinder was not firing. Given that your initial problem was that it was apparent at the carb, and finding it at the plug is new, then the leak MUST be higher up. The trick is finding where and HOW the oil gets to where you are seeing it. I cannot see by what route bad valve guides would result in oil at the plug, or indeed at the carb flange (unless they were REALY bad).

Originally Posted by Dave Martin
Just a thought, have any of the valve cover studs been replaced with bolts? or even has a stud been put in with some dirt in the hole and the casing cracked a tad.


Allan, the whole investigation began when I couldn't get the left side to idle right with the pilot air adjust (new Concentrics). On (lumpy) idle, faint traces of grey smoke were there. On revving, I'd get a burst of smoke. That's how I began to consider a linkage between the oil mess on the outside and the possibility of oil contamination into the chamber being related.

Dave, On this head the spark plug hole bisects several layers of cooling fins and opens to the spaces between them. So oil at the plug hole moves back along the lowest open fin to exit near the bottom of the carb flange. I watched that progress with my own eyes after I had everything clean and dry. Seriously, there's no oil showing except in a very narrow ring around the base of the plug. Nothing from the middle and nothing from above. The dye is there so I know I'm seeing only new flow.

As for how can bad valve guides = oil bubbling up around the plug, well it wasn't easy to come to that possibility. Rings are new, properly installed and gapped in honed cylinders. Compression is 180 on both sides, so I'm confident in my rings. But I have oil in the chamber (the smoke and the bad idle). Having removed the carb and checked the health of the seal and flange as well as the cleanliness of the intake tube, I'm satisfied that the oil is not entering the chamber via the carb or the carb flange. So, valve guides are left. That cylinder has a helicoil in it. I'm guessing that there is enough slop in the installation of the helicoil that the fuel and oil coming off the valve is being force wicked (along with some fuel) up the plug helicoil. I've snugged down the plug to tighter than I'd like to see if the rate of leakage is impacted, but I have not run it yet.

I plan to run it this weekend to reexamine the leak, then possibly pull the head for a valve exam.

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'67 BSA Lightning--A collection of Helicoils held together by matching numbers
Re: A65 Oil Leak at the Carb?
Redshirt #803874 04/03/20 7:15 pm
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Hmm ……. will be interesting to see what happens at the week end. I cannot see how the Helicoil would have any effect, however sloppy it is. The threads on a spark plug take no part in it's sealing, it is a straight forward flange seal with all the work being done by the compression washer. If both pots have the same pressure it suggests that one of them is not leaking.
This is going to come down to "when you have eliminated all possibilities, what ever remains, however improbable, is the truth". In my, admittedly limited experience of UV leak detector dye can be misleading especially if not observed with a strong UV light in an otherwise completely dark environment. Small seepages can be missed and just gathering places seen.
My guess is that the "oil leak" is a red herring, and the lumpiness and whiffs of smoke are down to the rings not being fully bedded in yet.

Just looked back and noticed you said GREY smoke, not blue. Isn't grey smoke usually unburnt (or rather incompletely burnt) fuel rather than oil? How are the sparky bits?? As I said in a different post ….. 90% of carburetor problems are the spark plug.

Lord I am bored, second day into a 24hr complete curfew due to last 7 days (but will be more!).

Re: A65 Oil Leak at the Carb?
Redshirt #803876 04/03/20 7:22 pm
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+1 Dave, if the rings are not yet bedded in you will find it can be a pig to idle, even my head with low compression on one side still ticked over nicely.

By the time a helicoil has been fitted into a spark plug hole there isn’t much washer available to sit on the head so it can (and will leak) a “timesert” or other form of solid insert is the best way to repair a spark plug thread.


Now let’s all have a beer beerchug

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (undergoing restoration)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

Re: A65 Oil Leak at the Carb?
Redshirt #803878 04/03/20 7:24 pm
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Hi, sometime on a bsa over the years people swap a lot of things back and forth. One that I noticed is heads. Early heads had the small bolt holes in the back. You can uses these heads if the hole is enlarged to fit the larger head bolts in rear. I have had bsa's that leaked or weeped slightly in rear by either car, sometimes left, somtimes right. Upon dissasembly could find nothing wrong. After talking to a few guys over the years I discovered that many of these heads were not drilled out on a jig, many people drill them out on the ground and get them a bit crooked. Sooooo, you sometimes get a weep from that rear headbolt hole. Just sayin, take a look at the rear holes in head and see if that old trick was done long agao.

Re: A65 Oil Leak at the Carb?
Redshirt #803884 04/03/20 8:50 pm
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If oil is coming out of the spark plug threads I would expect blue smoke from the exhaust.
Oil leaks are tricky, it might be oil working its way from the head joint up a stud then blowing into the plug depression,
Other places that can cause weird leaks, rocker cover centre studs, the nut should bear down on a sealing washer of Al or fibre, plain steel washers will leak oil . usually onto the carb flange ,

The head gasket oil drain holes can wick oil to the joint , this can be fixed by fitting o rings into the gasket drain holes.

If the guides are worn the most obvious tell is to have someone follow you, get the revs up then close the throttle, if the guides are passing the pipes will smoke blue smoke on a trailing throttle.


71 Devimead, John Hill, John Holmes A65 750
56 Norbsa 68 Longstroke A65
Cagiva Raptor 650
MZ TS 250
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Re: A65 Oil Leak at the Carb?
Redshirt #803888 04/03/20 9:33 pm
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Take the manifold off and have a good look at the back side of the intake valve. If it's leaking down the guide you should see some evidence on the back of the valve. More likely to be the intake than exhaust but still possible. Were the guides replaced? If not done correctly oil could be wicking down the outside of the guide. Still having a hard time understanding significant oil wicking up the plug threads.


1978 Bonneville T140E
1974 Trident
1970 BSA Thunderbolt
1971 Norton Commando
1972 Norton Commando
1973 Norton Commando
1974 Norton Commando
2018 Kawasaki Z900RS


Everything will be alright in the end. If its not alright, its not the end.

Moderated by  Allan G, Jon W. Whitley 

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