Starts, Idles, Dies with Revs

[Uhlaf]

1981 XS650 Special II
EX500 Carbs, 138/38/JJJ
Stock TCI

Title says it all, so far this season I have 250 fantastic trouble free miles with the ex500 carbs. Plugs where always a beautiful tan colour and I love the way it ran, pulled, idled, started. It was a great beginning to the season.

Went to start up my bike this past weekend for a pretty girl and it kicked and back fired, when I applied throttle the bike would let out a banging choke and would refuse to rev past idle speeds and dies with choke. Pulled the plugs, the are black now. Why is my bike all the sudden running crazy rich?

So far I have;

-Checked battery, full charge and the charging system is functioning
-Carbs where thoroughly cleaned before being bolted to the bike previous to the 250 miles with fuel filter.

What I'm really looking for is for someone to tell me that it's not my TCI box

 

[tonyc]

I would first pull the battery and have it load tested. If there is someone near with a tci box switch and test also check the plug wires,plug,and caps. Good luck.

 

[retiredgentleman]

You could try unplugging the rec/reg unit, and then see how it starts and runs. The TCI pick-up magnet, that is embeded in the alternator rotor may be losing its magnetism after all these years.

 

[Uhlaf]

Questions,
how does the petcock on an 81 function?

-Is reserve gravity or or vacuum fed? Does prime use the same inlet as reserve (same height in tank)?

Why would my plugs be black with TCI problems?

 

[angus67]

Do you have a fuel filter? Could be crap in the pilot.
You don't have a tci problem, because it tried to start.

 

[Uhlaf]

I have a fuel filter. And it does start. I was thinking that the TCI wasn't advancing?

Just came in from a wrestling match with my bike;

Unplugged reg/rec, no change to bike.

Gave the plugs a quick clean up, tested for spark. Hot white. Predictable spark
For whatever reason I unplugged my backup lighting box under my seat, hit the magic button and it started... and REVED!

Excitedly I threw on my helmet, jacket, boots and gloves and jumped on to give her some sweet Shell gas. As I was rolling down my drive way I hit the front brake and the bike died. Started it up, it revved, tapped the back brake. Dead.

Figured I drained the battery starting the bike over and over, on the charger the battery went. Full charge, hit the magic button, starts...no revs.

I'm going to go through the carbs just to make sure no gunk got in at any point. Still open to whatever wonderful theories you awesome people have

 

[tonyc]

Uhlaf I left out possible coil.

 

[TwoManyXS1Bs]

For whatever reason I unplugged my backup lighting box under my seat, hit the magic button and it started... and REVED!

... I hit the front brake and the bike died.
... tapped the back brake. Dead.

... Full charge, hit the magic button, starts...no revs.

... Still open to whatever wonderful theories you awesome people have

Thinkin' out loud here. Don't know about wonderful. How 'bout whacky.

Sounds like electrical.

If the brake lighting power wire (brown) also supplies power to the TCI, then a poor connection upstream of their shared portion could act like a resistor and power to the TCI would drop when the brake light comes on. Fuse block, maybe? Vibration would influence that (no revs).

The scenario is similar if the brake light's ground is shared with the TCI's ground. A poor connection to ground, downstream of their shared connection, would cause the TCI's ground reference voltage to rise during brake light, again producing a smaller power/ground voltage for the voltage sensitive TCI.

Geezz, I hope that makes sense...

 

[Uhlaf]

It's possible TwoMany.

I had something like this happen last year when I was coming back from a long-ish trip. I went to take off back home, rolling down my mothers drive way. Hit the brake and bike died, ended up buying a new battery. (charging system shit itself on my way back in a storm but I know mine is now functioning properly)

I really don't want a new battery, this one is 7 months old, AGM and cost me a pretty penny. I'm going to go and get it load tested. I'm also going to go over my grounds and clean the every living life out of them.

Keep the theories coming gentlemen.

 

[Uhlaf]

Backfires where smoke comes out the unifilters, when I kick it over it kicks me back.

Went through the major connections and grounds and cleaned them up, going to go through carbs tomorrow.

 

[Uhlaf]

This has been resolved;

Mixture screws rumbled loose, stuck in traffic fouled plugs, killed battery trying to start it. In a rush I did a fast charge with unbeknown to me did not provide full charge causing stumble on throttle.

After days of tearing my hair out I replaced plugs and trickled charged bike back to life. Runs even better.

 

[TwoManyXS1Bs]

Mix screws, change plugs, charge battery. Unbelievable.

Seems that issues like this are terribly complicated,

Or numbingly simple.

Go figure...

 

[Uhlaf]

Or...you go for a ride, one of those rides where your bike is suddenly twice as quick and thrice as light, rides where you navigate sleepy country corners perfectly, and you want to pat your bike on the gas tank as a polite and loving "Thank you" for getting you home.

And then the next day you go for a quick ride and find that you're bike starts but won't rev with fresh plugs, full battery, and mix screws where you left them.

I'll be tearing my carbs and hair apart tomorrow.

 

[Uhlaf]

Swapped out carbs back to BS34, starts, nor revs.

I'm thinking I'll get a TCI to diagnose if it is the magic box and if so then invest in a PAMCO.

Quick question, when pulling the BS34 I had to removed my carbs boots and when doing so (I have no idea how this happened) I stripped the bolt threads out of the engine. Anyone know what size bolt goes in there? I need to tap a new one and I'd like to get it right.

 

[angus67]

timesert that sucker and move on. unfortunately, that's a tight area to drill. A 90* drill would work there.

 

[Uhlaf]

Alright, pamco installed. Bike started so fast it actually startled me. And everything was well with the world.

For future reference to anyone else searching these forums with the same problem, the magic black box will fail eventually.

 

[angus67]

my 71 starts so easy with the pamco, it starts halfway thru the kick, nearly hyper-extending my knee at the end. First kick, everytime.