Boxerworks.com

Hi everybody,

I've been off the forum for a while, between the forum issues and a surprising bike issue I got out of the habit of checking in.

But now I have a tale to tell.

I put a fresh tune-up on my 75/6 and take a little trip, round sunset, on the Los Angeles freeway system. My new digs are up in the hills so the first 10 miles are down hill all the way.

As soon as the road levels out I begin to get an occasional miss, like I'm out of gas and need to switch to reserve. But I have lots of gas and now the miss comes with a loud bang. It seems to be a real backfire. Pulling over 5 lanes of crowded traffic is not something to be done quickly, but suddenly everyone is slowing down and making room for me. I am confused. Pleased, but really confused.

I get over to the shoulder and discover why everyone made room. Remember I'm driving at about sunset, well it is almost full dark now and the bike is literally spitting fire out of the left pipe. 2 foot long tales of flame are coming out of my exhaust system. Bits of burning carbon are adding glowing orange sparks to the spectacle and there are two bands on the muffler glowing red hot. Each is about 2" wide and they look very much like standing wave patterns.

No wonder everyone made room.

When I got it home I found three issues:
1. The timing had slipped to very, very retarded.
2. The left head seemed to have lost all the torque to the cylinder. It was as if the head gasket (pretty new) had shrunk by several thousandths on the ride.
3. (no surprise) there was lots of carbon buildup on the left exhaust valve. It still made a good seal, however.

Sorry, no pics from the incident.

I'm going through the bike to do a lot of deferred maintenance and I was thinking about lapping the valves. Any opinions out there on that? I was thinking about doing a real top end rebuild, but Brown's wanted $800 or so for new valve guides and valves and that is beyond my wallet right now.

She burns a little oil through the valve guides, and I was going to put new rings in. The measurement on the exhaust valve seat I pulled was just over 1mm (the square part that Clymer has you measure as an indicator of wear)

I'm trying to figure out how to do the fire spitting thing on command. Really clears out the traffic.

Good to be back

Dave R.

DaveBBR wrote:I'm trying to figure out how to do the fire spitting thing on command. Really clears out the traffic.

Me too, I like that trick

This is almost exactly what happened to me today.

On the 5 south in Burbank, the bike seemed to lose power like out of gas...full tank..
I'm coasting and try downshifting and popping the cluth to restart. BANG! smoke..shuts down again.
I manage to coast to the side....no shoulder on the freeway there due to construction. I manage to push it past the barriers to where some shoulder is and not get ran over in the process...THANK YOU GOD

I try to restart. starts fine... I put it inot gear, engine shuts down. I try it again...bug BANG! black smoke. shut it off and called for a tow.

Any ideas where to start? The heads were redone about 700 miles ago. The bike has been running great recently.

I would check for ignition first and then fuel. Having fuel in the tank means little if it doens't get into the head.

I had something similar happen out near death valley off the 395 about 4 hours from LA. I had hit about 105mph and then let off the throttle and the bike sounds like it was exploding. after a minor heart attack, i realized i was getting fuel and fire then concluded it was my timing due to all the backfiring. Took off the engine cover and saw that the sensor for the dyna ignition came loose. Trying to time a bike while on the side of the road as 18 wheelers blow all your belongings into the desert isn't fun... but i got the bike running, made it to my destination and then dialed it in as best i could until i got home.

I'll check out my Dyna leads. I found that the brown lead that goes to the coil may have been intermittently shorting against the tank. I haven't confirmed that it was the problem but I insulated it and the bike is now running fine. Both sides are getting fuel, good flow to the carbs.

The scary part was not having a shoulder on the 5 freeway to pull off on with traffic bearing down on me. Between that the mad push to get past the construction barricades where there was a shoulder to get off the road, my heart was pounding.


(the worst part, my friend on his 1940 WW2 BSA M20 and I were supposed to meet to go to the Hansen Dam Brit Bike ride. He was doubtful that his bike would make it without breaking down. His made it there and back fine, mine didn't....the Brits beat the Germans again)

I might have found the problem, meaning that my bike is running fine again (but it was running fine beforehand too).

There is a lead from the Dyna III tat goes to right coil. That coil has 3 lugs and it looks like they may be shorting out on the tank. There are wear marks on the coil body from the tank. The tank is seated correctly but there is little clearance from the coils. MAYBE that lead was shorting on the tank? If so, maybe shorting to ground would mess it up and cause the engine to stop.

I have insulated the connections and it is working fine but I've had the bike for 5 years and this hasn't been a problem in the past. Not sure why it would happen now.

BoxerTwin wrote:
I have insulated the connections and it is working fine but I've had the bike for 5 years and this hasn't been a problem in the past. Not sure why it would happen now.

Perhaps it wore its way through the paint in that time?

Major Softie wrote:

BoxerTwin wrote:
I have insulated the connections and it is working fine but I've had the bike for 5 years and this hasn't been a problem in the past. Not sure why it would happen now.

Perhaps it wore its way through the paint in that time?

quite possibly. It's real tight under the tank there. Should it be riding on the side of the coil? the tank is seated properly in the mounts. It seems the coils should be about a 1/4" - 1/2" lower.