Friday, March 24, 2023

Old Royal Enfield smoking? Easy cure

Smoke from motorcycle exhaust pipe.
Wet sumping causes oil to burn off in clouds of smoke.

 I cured my Royal Enfield of wet sumping by replacing one little part. 

SHORTCUT TO THE SOLUTION: I replaced the worm nut, with its bonded-in seal. That did it. 

It is an easy fix and requires only a screwdriver and a wrench. 

My 1999 iron-barrel Royal Enfield Bullet long ago began smoking when started up, especially after sitting for a few days. Obviously, oil was seeping into the crankcase, turning the Bullet's intended dry-sump design into a wet sump. 

Oil splashed up from the unwanted pond of oil in the crankcase on start up was getting into the combustion chamber and being burned off there. 

Being lazy, I addressed this problem by simply removing the front drain plug below the motor, which drains the crankcase. It was a messy nuisance to do this chore before each ride, but it worked. 

Then, one day, things got suddenly much worse.

On start up there was thick and persistent white smoke from the exhaust that wouldn't stop. My impression was that this wasn't just a case of a pool of oil being burned off, but a steady stream of oil being continually turned into smoke as the engine ran.

Checking the spark plug I found it heavily coated with oil. Obviously, oil, a lot of it, was getting into the combustion chamber.

I drained the sump and measured what came out: about 8 ounces, much, much more than recommended but actually typical for my Bullet, when it has been standing unused for awhile.

Even with the sump just drained, and less than the recommended amount of oil now on board, the Bullet now immediately smoked heavily on start-up and wouldn't quit smoking.

I wondered if the explanation might be in a 2015 post on the Classic Motorworks Forum by Kevin Mahoney, former U.S. importer of Royal Enfield motorcycles. He wrote about a fault in some 1999 Bullets that led to smoking.

Was this my problem?

I wrote an email to Kevin to ask, and he was gracious enough to answer.

"It is HIGHLY unlikely that is your problem," he replied.

"It is more likely to be your worm nut seal. They are a wear item and we used to sell a lot of them...

"It's a little hard to visualize until you've seen it with your own eyes, but the oil that is fed to the crankshaft by the rear pump is separated from all of the oil in the timing chest/cover by the rubber seal in the middle of the worm nut... 

"If the leak outstrips the engine's ability to clear the oil out of the crankcase, oil will be forced past the rings and it will make smoke...

"If you end up with (excessive) oil in the crankcase, then call or email Tim Hirdler and get a worm nut and a timing case cover gasket coming. The worm nut should be replaced from time to time anyway."

A busy holiday season and houseguests interfered, but I eventually ordered the parts from Tim at Western Cycle Supply and installed the new worm nut in my smoking Bullet.

Worm nut held in front of timing chest cover.
Oily, worn out worm nut removed from inside the timing chest cover.

To do this you remove the timing chest cover, the big kidney-shaped slap of metal that forms the right side of the motor and includes the oil filter housing. You must remove 10 flathead screws; be careful to keep them in order: they are different lengths. (I stuck them into holes in a cardboard box, in the shape of the primary cover itself.)

If you're careful, the cover gasket may be left intact and could be used again. (You might not need to remove the motorcycle's exhaust pipe to remove the timing chest cover, but I did, just to get it out of the way.)

Once inside, being careful not to disturb the gear wheels of the valve train, you remove the large hexagonal worm nut.

IMPORTANT: This worm nut is left-hand thread. It's RIGHTY LOOSEY not RIGHTY TIGHTY as you'd assume.

You'll have to keep the motor from turning as you do this. I just put the motorcycle in gear and held down the brake pedal to keep the rear wheel from turning.

Once removed the old worm nut didn't look bad, but its central rubber seal was slightly loose. Was that looseness causing my problem?

Put everything back together. The trickiest part is making sure to mesh the threads of the new worm nut and the oil pump spindle (you'll see the spindle gears peeking out from the cover near the filter housing). You don't want to seal everything up with those gear edges clashing. Gently rotating the motor with the kick start lever (or the rear wheel while in gear) are suggested while fitting.

My next step was to drain the crankcase, to see if the new worm nut was indeed what it took to stop the seepage.  It would certainly be good news if I discovered very little oil in the crankcase in the morning.

When I opened the front drain plug I could have done a cartwheel in joy (if I wasn't too old to be doing cartwheels). Much less oil than usual drained out the crankcase plug.

The Bullet started and, after warming a bit, it didn't smoke. I rode a couple miles and still no smoke!

Very happy.

After the ride the spark plug was dirty, but not oily.

I'm hugely pleased.

ADMISSION: To keep this story short I left out the number of times I experimented with siphoning oil from the tank, draining the crankcase, cleaning the plug, and being discouraged by residual oil continuing to cause smoke clouds on test starts. Replacing the worm nut and its attached seal ultimately did the trick, but it took the motor time to clear up.


  1. Anonymous4/03/2023

    Good info. I endeavour to leave my old motors at Top Dead Centre/piston on compression to stop this happening. But good info thanks

    1. That is a good practice and worked for me for years but ultimately I think the plug did become a problem and not just when sitting (when your technique might help) but while running.

    2. dfarley45044/30/2023

      I have the same problem, but also upon start up, oil gushers out of
      A tube on the left bottom side of the barrel.
      I think this tube initially was for blow by and routed to the air cleaner. But the previous owner routed it to lube the final drive chain, a common practice I think.
      Do you think the warm net seal would solve this as well?

    3. I am no mechanic but my understanding is that the chain oiler you refer to must be routed high up under the seat before going down to the chain. Is that the case? The object is to allow return of the majority of the blow-by to the crankcase, where, if the worm nut is in good condition, the scavenger side of the oil pump returns it to circulation.

    4. Anonymous9/15/2023

      Exactly what happened to my Bullet, thank you so much for sharing valuable information with public.


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