When hot acts like rev limiter? '81 GS750E

[twistedwankel]

My 81 GS750E 16 valve with 42k miles and all stock in excellent repair sat for a couple months this summer while we were away.

I replaced the battery, drained the carb bowls (some very fine particulates in each bowl) and put in new gas with SeaFoam.

Fired right up. Idled fine. Took for a ride and ran great for 1/2 an hour in our 90F summer heat in FL.

Then started "missing"? About 4k in 5th gear it just lost power so I had to give it more throttle to keep up speed @50mph. It could very slowly accelerate. Still acted like it was either starving for gas or just maybe hitting on two cylinders? Within a few minutes the power came back on bigtime and no issues for maybe another 20 minutes. Took it home as was afraid to shut it off. After 10 minutes fired it right up and ran/revved great. Went about one mile and started the same thing over again. Within a couple minutes it ran great again like an on/off switch.

Does this sound like ignition or fuel? I'm leaning towards an overheated coil or ignitor for now? Coils may well be original and they are certainly stock. I've owned this bike for 10 years and never changed the coils or ignitor. Plugs are newer.

I downloaded a couple methods from Basscliff's site for testing the coils and ignitor so I'll be looking into that first.

This is foreign area to me so don't want to miss an elephant looking for a mouse. This bike has been so reliable I'm at a loss BUT it did get me home and it's 32 years old.

Thanks.

Doug

[hjfisk]

Go out and ride it and when it acts up try putting the petcock in the prime position ,sounds like a petcock problem to me.

[lemonshindig]

I am having identical symptoms.

I originally believed that one of the coils was overheating, but they are both cold to the touch when the bike is acting up.

I now suspect that my valves being out of adjustment causes the intake to get hot, and that my old intake boots open up and let air in through tiny cracks.

I'm adjusting the valves, checking compression, and then replacing the intake boots. If all looks good there, I will be checking the ignition system again.

[tkent02]

Might even be a gas tank venting problem. Next time it does this, crack the gas cap for a second and see if it fixes it.

[twistedwankel]

Go out and ride it and when it acts up try putting the petcock in the prime position ,sounds like a petcock problem to me.

Tried this twice and it made no difference. Thanks. I did replace the petcock about 8 years ago Where does the time go?

[twistedwankel]

Might even be a gas tank venting problem. Next time it does this, crack the gas cap for a second and see if it fixes it.

Guess I could do that with the spare key stuck in the cap. Worth a try. It seems to be an increasing problem once it's good and hot. I'll try that tomorrow before I do anything else. Thanks for the input.

[tkent02]

The gas tank actually vents through the keyhole. Sometimes they get clogged with spiderwebs or car wax or whatever. Donno if it is your problem but it's easy enough to check.

[chef1366]

Change the gas

[twistedwankel]

The gas tank actually vents through the keyhole. Sometimes they get clogged with spiderwebs or car wax or whatever. Donno if it is your problem but it's easy enough to check.

Disassembled the gascap which has an amazing number of parts to clean in there and make sure the little holes are clear.

Did all this and it made no difference but needed doing after 32 years. Even loosen cap when it started "starving" and it made no difference.

[twistedwankel]

Change the gas

I removed the tank, dumped out all the gas, removed and tested the 8 year old gas contoller and it worked perfectly. Was totally clean.

Made no difference even tho' I drained the gas out of the carb bowls too.

Put in 1 gallon of fresh gas. No difference.

[twistedwankel]

So this afternoon I took off on the bike and it started acting up in only one mile which is a first.

Turned around and went home. It got better after a couple minutes.

When I got home it started doing it in the driveway.

I put the bike in the garage and removed the tank, dumped the gas and removed/tested the gas valve/filter. No problem with it.

Replaced gas filter that had no issues but replaced it anyhow.

Took apart all the electrical plugs and cleaned and greased them all.

Reassembled and put in clean gallon of gas. Primed. Wouldn't start at all. Wouldn't fire. Crap. I made it way worse.

Finally got it to run by holding full choke and giving it throttle. Would stay running at 3k but would stall at less or if I took off choke.

After playing with it on and off 6 times it finally would keep running at 2k with choke on. Wasn't hitting on all 4 lacking power constantly now.

So apparently I made it worse.

I found an old working spare Excitor that I had from years ago and plugged it in just letting it hang there from the plugs.

It started and ran perfectly!! Revved freely off throttle!! Idled perfectly!!

So I removed the original Excitor and installed the old one. As soon as I was done and buttoned everything up decided to run to gas station and fill up with gas.

When it started it immediately ran for crap again

So I guess in a way I'm back to square one. Will figure out how to test the ignition stuff one by one hopefully with the papers.

At least at this point I'm fairly certain it's not the valves or the carbs.

Thanks for all the suggestions at least I did some required maintenance.

So now I've managed to make it run for crap all the time so that's a good thing!!

[BigD_83]

I keep going back to your original post in the thread, where you say it sat for a couple of months.

That coupled with the bike running on choke only makes me suspicious that your carbs have fouled from the sitting. This idea doesn't sit 100% with me, though, due to the fact that it ran fine for 1/2h the first time you started it up. However, it jives with the "running on two cylinders" feeling.

You did not say (or I have missed it) whether you have cleaned your carbs since it sat for those couple of months.

[twistedwankel]

I keep going back to your original post in the thread, where you say it sat for a couple of months.

You did not say (or I have missed it) whether you have cleaned your carbs since it sat for those couple of months.

No I only loaded it with gas preservative before letting it sit.
Also added Seafoam to new tank of gas when I returned.

Done this many times when bike sat and this is the first time with any issue.

Haven't ruled out all carbs getting torn down yet. Just going to make sure the ignition is 100% now. Too bizarre to ignore it running perfectly/instantly with no choke once an old ignitor was hooked up.

[BigD_83]

That is good. There is probably little chance it is gas varnish clogging things up...

Back to electrical, then.

[bakalorz]

I had crapped up spark plugs do pretty much that ...

an easy way to check is if you can get an old style timing light that clips over the spark plug wires.

don't worry about the timing itself (well, you can check the timing too, but that's not going to give that kind of intermittent problem)

but look to see if the flashes are consistent and equal on all four plugs.

On mine, when the plugs would foul, then the light would be very erratic.

FWIW ... I think it has something to do with ignition system, not gas.