Spark plug leaks

David Toll

David Toll

Reliving my youth?
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Bike's giving me a great deal pleasure but I am at that tenuous "rebuilt engine" stage of the project, consequently I am all over the old girl after every run. I have noticed what appears to be a leak around each plug. I thought this oil was coming from the cam seals so I replaced them both but the situation persists. The plugs seat, screw and tighten down OK but they do exhibit a bit of a "wobble" during the installation process. I know they should not be overtightened so I fear I may have to rethread the plug holes using either Helicoils or the Time Sert kit. Which do you recommend?
 

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I've done tons of helicoils.... never used the other, so I can't offer opinions. Was just gonna point out that the plug holes are steel and quite durable... even if they wobble a bit. Unless one's actually buggered, I'd just follow The Beetles and let it be. ;)
 
Just reread your post David.... You're not getting an oil leak from the plugs. Were there that much oil in the cylinder to leak out, you'd be smoking worse than an RD350.
 
OK Jim, I'll look at I again but there is no oil behind the points and advance overs. Don't know how well I tightened the head bolts under the spark plugs - hard to get to as I recall. Could they leak oil?
 
Valve covers and head gasket mating surface seem OK, (visually at least). Definitely a leak from the right plug. Cleaned everything up and ran it for a few minutes before this photo. The threads look pretty average in the bore. A lot of oil Jim, as you say and the plug thread has a liberal dousing as well but there is no smoke, even under rev. The right cylinder is running though it doesn't seem quite as powerful as the left when I do the plug wire off bit. I bought a tool that fit in on the head bolts under the plug. Couldn't torque them but they are now pretty tight. I'm leaning toward a rethread. Any warnings of a "here be dragons" nature??
 

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David Toll said:
I'm leaning toward a rethread. Any warnings of a "here be dragons" nature??
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David, there is no oil there to leak. On the other side of that plug is the combustion chamber. Containing, in order... a fuel/air mixture and spent exhaust gasses. There is no oil to speak of that can leak out the plug hole. You're answer lies down another path.... ;)
 
That's what I was considering as unusual Jim. This stuff looks like oil, seems to dry like oil, coats my plug like oil but there really can't be any oil in that chamber unless my rings were failing. I'll do another compression test but the fluid appears to be dribbling down from the plug. If it's not oil, it must be fuel that's not being ignited. Is that possible? I will wipe the motor over with clean tissue to see if I can locate another source of leak but either way, that thread appears to be leaking something.
 
I have seen oil around the plug area before and assumed the cam seals. But I eventually traced its origin to a slightly loose oil feed banjo on top of the rocker cover. Maybe??

Edit: It seems impossible for oil from the banjo to get to the spark plug area, but there must have been sufficient turbulence in my case for that to happen. The obvious route to drain from the banjo is straight back towards the carbs.
 
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I think the banjos up there are OK Paul but I appreciate the fact that you're considering the issue. Dust it down heh Max? That's quite a left wing plan actually but, cover the motor in powder and then heat it up? Is that asking for a tempura 650? I may get to that idea but, not yet.
Cheers
 
David
The spark plugs threads don't seal like a pipe thread they just hold the plug tight in the hole. The washer on the bottom of the plug does the sealing. If you have had the plugs in and out a bunch of times the washer may be your problem. Try new plugs. Like Jim says there is no oil in the combustion chamber to leak out. I've seen plugs loose in heads and no oil leaked out.
 
David, this is a long shot but it did work on an old Jeep once. Get into a very dark place and fire it up. I found one plug showed light flashes where it should not. (I also found the plug wires were leaking electricity as evident from a flickering light show in several spots. The Plug had rust around its head and the washer had deteriorated from that. New plugs and new wires made the old rig run wonderfully.

I know this is not what you are seeing, but looking in the dark may show where or not the compressed air is pushing out around the plug. A compression check may not find this issue as it is with the plug and not the jug.
 
If you put the plugs in and torque to 14ftlb and the threads are holding then nothing wrong in that department. If still concerned about the threads run a thread chaser down to clear the threads and correct any irregularities. 14ftlb is not very tight but it is the plug specification and will seal a new plug just fine, even an old plug.

When the engine is hot it is very hard to see where a minor leak is coming from because the oil viscosity is very low so only a smear will be left on the surface. I think you will have to try Max Midnight's suggestion or variants of the process e.g. give the bike several short runs with each followed by dusting. The dust will stick to the oily surface and you should be able to follow the progress of the leaked oil with each dusting. If you do not like talc then corn flour should also work. An air compressor would be useful to gently blow away excess dust.

I see you replaced the cam seals, but this only seals between the advance/points housings and the cam shaft. I wonder if the other seals (gasket + O-ring) behind the advance and points housings are doing their job. It may also be that one of the 3 screws holding the housings in place is loose?
 
David Toll said:
Dust it down heh Max? That's quite a left wing plan actually but, cover the motor in powder and then heat it up? Is that asking for a tempura 650? I may get to that idea but, not yet.
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Your choice of course but if it isn't obvious where where the oil is coming from it saves the guesswork and time wasted with the wrong 'guess'.
 
Max Midnight said:
Your choice of course but if it isn't obvious where where the oil is coming from it saves the guesswork and time wasted with the wrong 'guess'.
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Agree on the powder treatment. One slightly different idea is that foot powder spray for Athletes Foot.
Here is an example that is sold in the State but other countries probably have it by other brands.
https://www.cvs.com/shop/cvs-health-athlete-s-foot-powder-spray-prodid-334067
The advantage of the spray is you can shoot it with a little more precision than tossing a can of talc at the engine.

Or if you want to be more professional you can buy this stuff:
https://www.aircraftspruce.com/catalog/topages/metlchek2.php

That is a developer used in crack detection kits. I'm sure Jim has used it in the past!
 

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