Yet-another odd, intermittent problem

[cdpruden]

So I've run into yet-another-odd problem with my 78. It's got about 57k miles on it.

Several mornings in a row, it ran perfect all the way to work (35 miles) -- and perfect all the way home -- until I got off of the freeway, when, as soon as I slowed down, it would stall out on me. It wouldn't restart for at least 30 - 60 minutes -- and I'd usually end up draining the battery trying to start it.

Last week I took it to work anyway, and it did this on the way into work.

I don't think it's entirely related to temperature, as earlier in the summer, I rode it to and from work on hotter days, too.

Last weekend, I had it out in a nearby parkway after changing the oil, and noticed it would stall when shifting up -- that momentary letting the RPMs fall on a shift did it. I can keep it alive for a bit longer by keeping the revs way up, but...

It's very odd that once it cools down, it fires up and runs perfect. There doesn't seem to be any rhyme or reason to how hot it gets, although I suppose that on all of these times that it's stalled, I'd been revving it high, and when I rode it *very* gently (no freeway speeds, no acceleration), it seemed to be okay... that might just be a red herring, though.

Sometimes after it stalls, for the first few minutes, when cranking, it will fire on what sounds like 1 or 2 cylinders, not enough to start. Oh, and gas tank cap has been opened to test, vent isn't closed off. Works perfect on the freeway for as far as I want to take it.

I've been leaning towards coils for this, but the thing that gets me is it seems like it should start on two cylinders, even if it isn't running well.

It's very odd that it fires up just fine, just like normal when cold, and after sitting for a while, it will fire up just fine as well.

Any thoughts? I tried bypassing the ballast resistor, with no luck. Does the coils seem like the next thing to try? or pickup coils? what else could cause this?

[prometheus578]

"Odd chance, but known to happen.

OK, here's the game.
Rules are simple... and you probably already have the playing peices.
Before starting the game, you need to pick which token you'll be moving around the board. Sort of like playing Monopoly... 'cept I'd advise you to pick the token that looks like a spark plug, not the "Top Hat".
(You should have an old spark plug lying about.)
Anyway... here's how to play:
Next time it stalls and doesn't want to start up... Pull your spark plug token out of your pocket.
Now... ignoring the #'s 5 and 6... roll the dice.
Let's say that it came up "3".
This means that you should pull the number three spark plug wire, insert your token, and carefully laying it on top of the head, crank the engine.
If, when cranking, you see a spark, then you can pass go.
Continue to roll the dice till you've checked all the spark plug wires in this manner.
If they all spark... you've won the game!
(You've also ruled out "lack of spark" as being the problem with your bike, but that is of secondary importance.)
Being a winner is all that matters.
Now then... let's suppose that all cylinders are getting spark... but the bike still doesn't run.
When was the last time you checked your valve adjustment?
Sadly, there's no game for this, as it takes a little effort.
As the engine heats, the valves expand.
Metal expands when heated, this we know from watching "The Discovery Channel", and also because I say it does.
Now, one recalls from reading "ze book uf Instructionnes", that when checking valve clearance, this is done with the engine cold.
"Why", you ask?
Because hot valves expand... and expanded valves have less of a clearance than when they're cold. (Also because hot engines are a bitch to work on)
So.. in an extreme case... Let's say that you haven't checked your valves for a while... and your valve clearances are very small, or almost non-existant.
Oh, the bike will still run, no doubt.
Now.. let those valves heat up... say... by riding to Walmart to laugh at people... and then back home again.
The valves have expanded, you've lost some valve clearance... and if the clearances where wrong to start with, you may actually now have some valves being held partially open.
Open valves means poor idle. Open valves mean bad compression. Open valves mean also not enough suction to draw fuel up from the carbs.
See where I'ma goin' wid dis?
So...
this may be, after all my babbling, to not be the cause of your runability issues, but look what we've accomplished.
You checked your valve clearances, which is always a good thing.
(a compression check would be nice, too)
And, you got to play a little game to pass the time... while you sit around waiting for the bike to restart.

[prometheus578]

Check for vacuum leaks around the carbs and manifolds, too.

[Toolmaker Tim]

Carburating

Hey Prom, I dated a girl like your Suzie. The only difference was I cracked a funny while see was sipping on her straw and it sprayed out thru her nose. Is that carburating or turbocharging?

[prometheus578]

"I would'a choked her!"

Dating's always a ticklish situation.
She sounds more like one of those carbs with a "tickler" choke system.
Pressing on that plastic bubble full of fuel provided extra fuel for starting.
So... You pressed her buttons.. and she sprayed.
Simple.
She's a carburated model.

[redbone]

check tech

your prob sound alot like one i seem to rememberr reading about int the "tech tips" section. try looking in that link under 'coils' or coil replacement' somethin like that. anyway [if memory serves] someone has an article in there about the coil heating up, not working, then cooling after 15 mins. and working fine. hope this helps.

[randy]

After coils, next likely suspect would be ballast resistor. When it gets flakey, you losse ALL sparks.

[psycoreefer]

Re: "Odd chance, but known to happen.

Originally posted by prometheus578


...If they all spark... you've won the game!

...Being a winner is all that matters.

... Metal expands when heated, this we know from watching "The Discovery Channel", and also because I say it does.

...Now, one recalls from reading "ze book uf Instructionnes", that when checking valve clearance, this is done with the engine cold.
"Why", you ask?
Because hot valves expand... and expanded valves have less of a clearance than when they're cold. (Also because hot engines are a bitch to work on)

.. let those valves heat up... say... by riding to Walmart to laugh at people... and then back home again.

Possibly the funniest post I've read in a while, all my co-workers are looking at me funny cuz I'm laughing. Good job!

[cdpruden]

Re: "Odd chance, but known to happen.

Originally posted by prometheus578
OK, here's the game.
Rules are simple... and you probably already have the playing peices.
Before starting the game, you need to pick which token you'll be moving around the board. Sort of like playing Monopoly... 'cept I'd advise you to pick the token that looks like a spark plug, not the "Top Hat".
(You should have an old spark plug lying about.)
Anyway... here's how to play:
Next time it stalls and doesn't want to start up... Pull your spark plug token out of your pocket.

Thanks for the suggestions -- I neglected to remember that I've even got a spark plug tester that can check that...

It certainly seems electrical, since it's a very sudden thing, not like it's running out of fuel or anything.

I adjusted the valve clearances last year, I'd guess 4 or 5k miles ago... they didn't need much adjusting then, so I was thinking they should still be reasonable. It seems like if it was the valve adjustment, it would wind up running poorly, rather than just die suddenly, right?

I'll probably try to ride it around the block after work or something and see if I can make it happen that way... then 2 cylinders (in a pair) means coils or pickup coils, 4 means ballast resistor or ignition, right?

[prometheus578]

Valve were just an EXTREME example, but worth mentioning on the odd chance.
Your valve clearance values should still be OK.
Electrical:
For coils, they would fail cylinders #1 and #4, or #2 and #3. That is the pairing.
Yes, ballast resister would claim all cylinders.
Fuel:
#1 and #2 cylinder would mean one petcock. #3 and #4 would mean the other.

[rickmc2000]

Yet-another odd, intermittent problem

My bike did the same thing when I first started restoring it. I found and fixed a broken wire in the vacuum advance. Problem went away for a while but came back. I found missing insulation on plug wires so replaced coils. Problem went away for a while, but again, came back. used contact cleaner to clean connectors on wiring harness. Same storey. Found that if I put pressure on the plug on the TCI Unit. Replaced TCI, and haven't had another problem since. All of these problems were real, all intermittant, so take your pick and dive in. If you haven't already, get to know Andreas and stock up on some parts to use for troubleshooting. Hope this help.

[cdpruden]

Thought I'd wrap this one up; it was a bad wire to the pickup coils (both had breaks, amazing it was running at all.) I wanted to have this set last year, but work ate all my time.

It would work fine for a while, but then when I got off the freeway, the vacuum advance would let up, the timing plate would drop back down to where it normally sits, and that would be enough to break the connection.

The wire was bad far enough up in the pickup coil that I couldn't just splice in a new lead, but I managed to get a set off fleabay.

Had her out today, and with that and the new coils (Mike's XS), she runs nice.

Thanks for all the suggestions and help.