Knucklehead plug issues

[PSANTORA]

I have a newly built motor that keeps fouling the front plug. I have changed the condenser, coil, points, plug wires and plugs. I 've done a leak down test and the the cylinders are showing 6-8 percent, just what you would expect for a new motor. Exhaust is tight, no manifold leaks, the timing and points setting has been checked and re-chacked several times each. I have gone thru the carb and see no issues there. The float level has been checked and re-checked. The valves tappets have been checked and re-checked. Yet within 1-2 minutes of running the front plug is fouled with black soot, definitely not oil fouled. There is never any problem with the rear plug, just the front one. Does anyone have any suggestions. Phil

[23JDCA 808]

Run a hotter plug in the front cylinder. I did this years ago with an old Toyota.

[T. Cotten]

Phil!

You declare you have no vacuum leaks, but I predict one to the rear cylinder.

Please bubble-test as discussed at http://virtualindian.org/11techleaktest.html.
Running tests only show huge leaks.

With Knucks, it is even possible to suck air beneath the spring cup and guide, and only bubbles will display exactly the location and extent of leaks.

Good luck!

....Cotten
PS: Did you try switching the plug wires at the coil, to see if the fouling swtched cylinders?

[bobdo]

Phil,
I'm following this closely, I'm having the same problem.

[PSANTORA]

I made a set up similiar to the one Tom has in his Virtual Indian link and found leaks on both the front and rear sides of intake manifold. My question is why would only the front cylinder foul the plug and why so quickly.

[T. Cotten]

I made a set up similiar to the one Tom has in his Virtual Indian link and found leaks on both the front and rear sides of intake manifold. My question is why would only the front cylinder foul the plug and why so quickly.

Phil!

A carb can only be adjusted for the weakest cylinder, which seems to be the rear, in your case.
Leaning out to clean up the front would be too lean for the rear to run.
But a cylinder can run quite rich, until the plug fouls.

No vacuum leak is acceptible.
Even the tiniest leaks affect performance, and may eventually become destructive.

....Cotten

[ricmoran]

Tom,

Do you think these vacuum leaks were common occurances decades ago? Any idea when people started doing the vacuum testing?

Do you recommned any frequency for renewed testing over the miles ridden?

[T. Cotten]

Tom,

Do you think these vacuum leaks were common occurances decades ago? Any idea when people started doing the vacuum testing?

Do you recommned any frequency for renewed testing over the miles ridden?

Ric!

Yes, but you could always get a fresh assembly from the dealer.
(Although '47 bronze manifolds often had casting porosities to begin with!)

Please note that the problem often is not just at the ferrules.
Severe torquing over the years has often distorted the spigots, or has loosen'd the nipples themselves, or egged the rivets within them.
The vagaries of six+ decades of being treated like a farm implement add up, along with the simple wear of the brass upon the spigots from expansion and contraction.

Please note also, that the nipples cannot be tested with the manifold installed, as the nuts cover the seam.
They can easily be tested with a large rubber stopper with a nipple through it, of course.

Pressuretesting with bubbles was probably first described by Heron of Alexandria, around 150 AD.
(H-D was aware of it, as noted on page 103? of the "48-57" service manual collection.)

I recommend testing while assembling,
and whenever a tuning concern arises,
and before disassembly if a real problem occurs: headgaskets, valves, pistons, etc. do not burn themselves.

....Cotten

[Bruce T]

Have you checked the sparkplug seat on the rear head to see if it leaks? My 46 had this problem. Also, I know a guy who had exactly your problem with a fowling front plug. He did all things discussed above. But with no change to front plug. Eventually he swapped his M35 for an M74B and both plugs are now perfect?? Cheers Bruce

[ricmoran]

Ric!

Yes, but you could always get a fresh assembly from the dealer.
(Although '47 bronze manifolds often had casting porosities to begin with!)

Please note that the problem often is not just at the ferrules.
Severe torquing over the years has often distorted the spigots, or has loosen'd the nipples themselves, or egged the rivets within them.
The vagaries of six+ decades of being treated like a farm implement add up, along with the simple wear of the brass upon the spigots from expansion and contraction.

Please note also, that the nipples cannot be tested with the manifold installed, as the nuts cover the seam.
They can easily be tested with a large rubber stopper with a nipple through it, of course.

Pressuretesting with bubbles was probably first described by Heron of Alexandria, around 150 AD.
(H-D was aware of it, as noted on page 103? of the "48-57" service manual collection.)

I recommend testing while assembling,
and whenever a tuning concern arises,
and before disassembly if a real problem occurs: headgaskets, valves, pistons, etc. do not burn themselves.

....Cotten

Tom, apology forgot to thank you for your reply....and by all means do the vacuum test!

[PSANTORA]

I've replaced both intake nipples and eliminated the leaks and the front plug still fouls. Started the bike and sprayed WD40 anyplace I thought it may be sucking in air with no change. I have a rubber ducky float set at 1/4", is it possible the float is set too high? Phil

[T. Cotten]

I've replaced both intake nipples and eliminated the leaks and the front plug still fouls. Started the bike and sprayed WD40 anyplace I thought it may be sucking in air with no change. I have a rubber ducky float set at 1/4", is it possible the float is set too high? Phil

Phil!

The WD-40 trick is never conclusive; That's why we bubble-test.
You did do a final bubble-test, right?

.....Cotten
PS: Your float is set properly.

[PSANTORA]

I installed the set-up as your earlier email recommended, did the soapy bubble test with no bubbling.

[T. Cotten]

Phil!

You might try soapy water around the sparkplugs themselves, and kick it through.

Otherwise, we must start suspecting plug wires and stuff.

....Cotten

[PSANTORA]

I tried spraying WD 40 around the plugs with the bike running with no change. I also changed out the wires, plugs, and coil.
Phil