Won't start when hot-coils?

[Buffalo Bill]

After an hours ride on hot days (80º+) bike has to cool down (30 min.) before it will restart after being shut off for 5 min.
I think it's the coils heating up from engine heat, should I replace them? OEM or aftermarket?
Bill

[argonsagas]

That could be it, but it may not. Too little information.

What exactly occurs? What symptoms does it give you?

[p_s]

All coils require more voltage to spark when hot. New coils may need less voltage & mask the problem, which might be fine. I would pull the headlight fuse, connect your battery to a car battery, see if anything changes, and then get out the multimeter.

[SqDancerLynn1]

Will it push start ? When it gets hot pull a plug cap & check for spark (have a spare plug)

[DMorris]

I had a VW that did that years ago and it did finally turned out to be the coil. The coil case was cracked, allowing the coil to ground out intermittently.
It would start cold but was a crap shoot when warm.

The key would be to measure for shorts to ground or open windings when you have the problem because it could be points/ignition problem too.

[Buffalo Bill]

Today I pushed it.

This is a '81 GS550T with all OEM intake, carbs and ignition, MAC 4>1 exhaust. Bike is tuned great with 2 washers under the carb needles.
Today I was parked at a house on a hill for 20 min. When I came out I could get just a couple fires, but not sustained running. I tried it with the headlight switched off.
After a few waits and tries I rolled it down the hill to get speed up then popped the clutch. It started but ran rough for some miles, probably because the plugs got fouled from the failed attempts to start it.
When I got home, I tried to duplicate the issue, but it was cooler around here. After a 10 min. wait the bike started up again but a little slowly.

[TheCafeKid]

Running too rich will cause similar symptoms when the bike is hot. I think perhaps you may have a problem there since you've shimmed the needles. Where are your adjustment screws set?

[Buffalo Bill]

Too rich?

Running too rich will cause similar symptoms when the bike is hot. I think perhaps you may have a problem there since you've shimmed the needles. Where are your adjustment screws set?

I set them with the help of a Colortune, 3 turns out.
The carbs and engine did not feel hot to the touch, just warm. I tried the choke of course and that did not produce a fire, big mistake and probably fouled the plugs.
Thanks.

[Griffin]

I had an intermittant stumble/hesitation/die problem for a while on my old '81 GS750E, which only manifested itself when the bike was good and hot. It drove me nuts until it happened in my driveway after being out for a ride. I immediately took readings on the plugs caps, and found that one of the coils had infinite resistance through the wires. Pulled the caps off, the wires still read infinity. It was also REALLY hot to the touch, considerably more so than the other coil. An hour later (after cooling off), it read fine.

I replaced the coil with a used one off my GS700 parts bike, and never had a problem again.

I suggest doing what I did immediately upon the problem manifesting itself. You might get lucky and find a simple fix like I did.

It definitely does sound like an electrical heating/shorting/grounding problem, at least.

[atchbo]

Do these bikes get vapour lock like old cars? I've heard that some old carbureted cars used to evaporate the gas out of the carbs when really hot, and refused to start on the first crank. After re-introducing vacuum to the carb bowl, the gas would re-enter and it would work on the second try. That's why there's a vapour pressure spec in the quality standard for gasoline. Just a tangential 2 cents.

This might be why on some old bikes you had to 'tickle the carbs'.

[Buffalo Bill]

Gravity.

Do these bikes get vapour lock like old cars? I've heard that some old carbureted cars used to evaporate the gas out of the carbs when really hot, and refused to start on the first crank. After re-introducing vacuum to the carb bowl, the gas would re-enter and it would work on the second try. That's why there's a vapour pressure spec in the quality standard for gasoline. Just a tangential 2 cents.

This might be why on some old bikes you had to 'tickle the carbs'.

Because bike carbs are gravity fed, the gas is always topped off, so no vapor lock. My bike carbs were only warm and they had plenty of gas.
Bill

[kerrfunk]

My bike behaves similarly... doesn't like to start when it's real hot, and might even stall at a stop if it's hot. 5-20 minutes of cooling off and it's okay.

But I'm a noob and don't know what "coils" means. Or testing spark or all that other fun stuff.

Yet.

[TheCafeKid]

I set them with the help of a Colortune, 3 turns out.
The carbs and engine did not feel hot to the touch, just warm. I tried the choke of course and that did not produce a fire, big mistake and probably fouled the plugs.
Thanks.

Ok, Ive TRIED a colourtune...they're just about worthless IMO. They'll get you in the ball park, but thats it. The BIKE will tell you where it wants to be. 3 turns out on a stock set up is IMO kinda on the rich side, plus, you've richened up the transition from pilot to needle as well with the shims...

You need to set the mix screws based on LISTENING to the bike. For starters, set them all back at 2 turns out. Then, one at a time, with the bike warm and idling off choke, turn the screws SLOWLY (allow time for the bike to compensate for your adjustment, a second or two at least) a quarter turn at a time. When the idle speeds up, and your next turn doesnt speed it up any more, STOP, and back it off a 1/8 turn and leave it. Then move on to the next. Another clue as to whether or not your pilots are rich: Again, while the bike is warm and idling on its own, blip the throttle. If the bike hangs a little before coming back to idle, its a little lean, but if it drops BELOW idle RPM, and then returns to idle, its rich.

IMO, you MAY have a coil issue, but knowing what your carb set up is, I think you're a little on the rich side. And the hotter it gets, the worse it will rear its head...

[Buffalo Bill]

Useful info.

My bike behaves similarly... doesn't like to start when it's real hot, and might even stall at a stop if it's hot. 5-20 minutes of cooling off and it's okay.

But I'm a noob and don't know what "coils" means. Or testing spark or all that other fun stuff.

Yet.

Thanks, even without tuning knowledge your input is useful. Best 'O fun when ever you do take up the tools of ignorance.
Bill

[Buffalo Bill]

<–?–>

You need to set the mix screws based on LISTENING to the bike. For starters, set them all back at 2 turns out. Then, one at a time, with the bike warm and idling off choke, turn the screws SLOWLY

Thanks Kid, but which direction, in or out?
Bill