Passing this along.

I got my 2004 Road King back from my local HD shop where they installed an S&S cam gear kit. All was good and I took it on a long ride up the CA coast. Checked oil along the way and at a thousand miles it had used a quart of oil.
I’d never had a problem like that so figured it was something the shop had done wrong. No oil leaks and nothing coming out of the air cleaner.
I pulled the plugs and the back was OK but the front cylinder was black and sooty. So on the advice of a long time Harley guy I replaced the plugs with NGK and changed the oil, replacing it with synthetic.
800 miles home and no oil used and the plugs were white and ashy.
To get some piece of mind I had the dealership do a leak down test and it showed 9% on the front with 15% on the back.
The answer appears to have been a bad plug. Not firing all the time allowed oil past the piston into the firing chamber.

Sound reasonable?

 

Are you saying you think you ran for 1,000 miles with the engine only firing on one cylinder?

Don`t you think you would have noticed you were down on power by a little bit?

A bad spark plug was not the problem.

Did you check the oil level at the beginning of the trip?

 

Your 04 like mine probably uses the plug to detect spark knock (ping). On your bike, it's actually best to use the plug with Harley written on it. There probably made by NGK since they are identical but the resistance is way different then the NGK.

I ran both and could never see the difference but I drove easy. I made a promise to myself to never WOT or get rid of the Harley. (I still have it). My son occasionally borrows it and told me it pings occasionally on a little grade if he doesn't down shift.

I never heard it but I am also old and 46 years in a Shipyard dulled my hearing.

I am with Dan on the plug. They lost half a dip stick from full to add when they removed cam case cover. High speed uses more oil. Old oil goes faster.

If that bad boy is not leaking, get it hot, and top it to just the full line. That gives you reserve for trips. Check it till you see all is good.

PS.. .. Welcome from Hampton Roads Virginia. ___

Just in case you have not done both of these two important items!

Be sure to add an Avatar of your favorite shot in the USER set up for the small picture you will see in the top left corner of your post.

And most importantly a Signature Line in your USER set up for your bike model, year, and all the letters that Harley recognizes it with.
(See mine below my bike picture.)

This enables others to know your exact model if you ever ask a question or another member interested in it may ask you a question about it.. ..

 

How many miles on the motor? Agree with Dan and Ripsaw but would add that the 15% leak down is why the front plug was black and sooty and expect the new plug will look that way at some point. Any leak down more than 10% would indicate, to me any way that rings or valve guide seals or both would need replacing soon.

 

In my opinion leak down gages are quite variable in 20% or less leak down in what they mean. They are really good for telling you if it's rings or valves. (you listen) . In my opinion, if it has few miles and it says valves, one can sometimes get away with just heads as long as you don't disturb the piston and cylinder. If a lot of miles, you do it all. The darn problem about a Harley is it little expensive to do bottom in and really hard to check it accurately.

One really needs to use a screw-in compression gage with intake wide open. Then look what the service manual says if you want to know compression.

 

There are a number of ways to impact the result of a leak down test. However, if the testing apparatus is quality and the test performed as it should be, the results will be accurate.

 

So how would 15% leak in the rear cause the front to be black and sooty? I'd expect it to be whiter as the front with less leak should be combusting better.

 

If he's off 6% between cylinders with a leakdown, what is allowable if you compare it to the service manual of more then 90 PSI and no more then 10% variation. Leakdown in service just say 10 is good, 15 indicates problems.

Interesting enough, I always see rear as the lower on a Harley.

Most really good engines I have did leakdown are around that 10%. What PSI compression do you think 15% leakdown represents?

 

Max, I was typing from memory and I recalled the 15% leak was the front; remember, us old guys have those brain farts from time to time. So, I don't know why the front was black and sooty at 9% leak down but the front was not at 15% leak down. There could be other reasons but I would be just speculating as there is nothing in the OP that might explain that. I am pretty sure that the oil loss was not related to spark plug choice.

 

Not sure I understand the question; 15% leak down is 15% at 185psi or 200psi. I don't accept 10% leak down as good and where it was coming from would determine whether or not to take any corrective action but OK to just keep riding until it gets worse; say up to 15%. The all bore 107 in my '02 FLHT registered 7% front and 9% rear after 40K hard miles which I was OK but not happy with. When I built that motor, forged pistons were set up a bit on the loose side so I was not really surprised. I have not had a reason to check leak down on the 98" motor in my '05 FXSTD at 212psi but being a fairly new build, I would expect 5% or less on both cylinders.