Cylinder stops firing when bike heats up

[epytus]

Hi -

GS450


Bike mostly sat during winter. No issues last season. Now after a few minutes of warming up and idling fine, the right cylinder starts sputtering and eventually stops firing after a couple minutes. Will only start firing again when the bike is pushed to 2-3k rpm, but not on low rpm or idle. After letting the bike sit and cool down the cylinder will fire again for a few minutes until heating up.


What I have done -


1. switched spark plugs / confirmed there is spark in that cylinder
2. switched out fuel and added fuel filter
3. cleaned out airbox / intake as much as possible
4. cleaned out carb jets (every time I take off the carb the jets are clogged, even after couple mins of running)
5. cleaned out fuel hoses as much as possible


I did check the compression and it was 150psi on the good cylinder and 120 on the bad one, so I'm wondering if that could cause the issue or if that is a separate problem altogether?


What could cause it to work while cold but then the cylinder start failing after a couple minutes? Is it pulling in dirty fuel and I just need to clean the fuel passages better? Does the carb need to be retuned? Or does this just sound like a compression issue to you?


Any tips appreciated.


Thanks

[Nessism]

Is the tank free of rust and debris?

Have you checked valve adjustment?

[Rob S.]

I did check the compression and it was 150psi on the good cylinder and 120 on the bad one, so I'm wondering if that could cause the issue or if that is a separate problem altogether?

That doesn't sound like a problem to me.

[Big T]

Your jets are constantly clogged?
That's your issue.
You need to clean the inside of the gas tank and then seal it
Caswell's, or POR 15

Oh, and check your valve clearances to determine proper compression
After all that, it could be the ignitor if your bike is 80 or newer

[nate665]

I mean yeah, this...

4. cleaned out carb jets (every time I take off the carb the jets are clogged, even after couple mins of running)

[tom203]

“cleaned out carb jets (every time I take off the carb the jets are clogged, even after couple mins of running)”

this makes no sense.after only few minutes of run time . Is your petcock vacuum hose connected to this right cylinder?

[BigD_83]

What is the material clogging the jets? Rust? Tank liner? Plastic? Metal bits? Gasoline varnish (probably not)?

[roeme]

4. cleaned out carb jets (every time I take off the carb the jets are clogged, even after couple mins of running)

...that right there is your culprit, you'll have to "debug" your fuel system from the carbs right up into the tank. I had similar problems; in my case the carbs wouldn't stop overflowing.
My "journey" is documented in that thread (direct link to post): https://www.thegsresources.com/_foru...55#post2368955

[epytus]

Is the tank free of rust and debris?

Have you checked valve adjustment?

Yes on both. Just found the time to do a valve adjustment this weekend. Same bad results after firing up the bike.

What is the material clogging the jets? Rust? Tank liner? Plastic? Metal bits? Gasoline varnish (probably not)?

I shouldn't have said the jets were 'clogged' when I check them. One jet isn't fully clear but it appears to have some leftover fuel that is sitting in the jet every time I pull it out. Meaning instead of being able to fully see through the jet, it is slightly narrower diameter but you can still see through it. No obvious signs of rust or any foreign substances - just fuel. Not sure if that's to be expected?

...that right there is your culprit, you'll have to "debug" your fuel system from the carbs right up into the tank. I had similar problems; in my case the carbs wouldn't stop overflowing.

Wouldn't the dirt still be there hot or cold? Unless the mere act of cooling down releases the clog. Even if so - would it really do it like clockwork every single time the bike cools down? I also installed a fuel filter between the tank and the carbs so the dirt would have to be in the fuel lines if that is the case. Airbox is already cleaned out.

If it IS dirt and a clogged carb - what about the bike heating up makes the carb dirty? It can't just be pulling in new dirty fuel, because if you just let it sit and cool down it will run on that same fuel.

If it's NOT dirt (or other foreign substance) - does this look more like an internal engine problem?

Thanks

[pdqford]

Swap the coils side to side and see if the right cylinder works and if the left cylinder begins misfiring.

[Maddevill]

Typical symptom of a coil failing is it stops firing when it gets hot.

Mad

[epytus]

Typical symptom of a coil failing is it stops firing when it gets hot.

Thanks for the idea to try - Just checked the coils and they read fine on the ohmmeter both before and after heating up though.

Appreciate the help on this one guys but I'm at a loss to what is wrong.
I spoke to a mechanic about it and they believe the cylinder is trashed due to the 30 psi difference in compression. Basically recommended I just sell the bike considering I only spent $1500 on it...

But I have a hard time believing it since it was running fine before parking only a few months back. And the mechanic didn't actually look at or test the bike himself due to them being booked up with spring appointments.

I guess the issue was around before and the winter tipped it over the edge..? Blows my mind.

[roeme]

The ohmmeter doesn't tell the whole picture. One failure mode of a damaged coil is that the HV side will spark internally when hot (as everything expands, insulation is no longer sufficient). A multimeter however can only tell you about the most serious of damage, as has to use very low current and voltage for measurement.

As suggested by others, you might try swapping the coils as a start.

However, since it does fire again at 2k-3k regardless of heat, a damaged coil might not be the culprit, but rather fuel delivery. Low vacuum paired with a so-so petcock and incorrect fuel level in the bowl...dunno. Because if a cylinder stops firing and resumes later on, that usually results in a mighty bang because of the unburned fuel in the exhaust. You haven't described that, so...sounds like not enough fuel.

Also, provide some pictures of what you're describing as 'clogged', in case you go in there again. It sounds a bit like you're confused by the fuel's surface tension.

I wouldn't mind the 30psi difference too much, at least for now. 120psi is more than enough for those engines to work.

[epytus]

As suggested by others, you might try swapping the coils as a start.

However, since it does fire again at 2k-3k regardless of heat, a damaged coil might not be the culprit, but rather fuel delivery.

Did try swapping and same result. Spending more time with the issue I've noticed that the cylinder gradually fails before any amount of significant heat reaches the coils. Or even the carb for that matter. The cylinder will only work for about 2 minutes until it starts sputtering out at idle. You can raise the throttle and get it going, but as time goes on, it continues to sputter at higher and higher RPM's. Basically it doesn't fire at 2-3k after awhile either.

Low vacuum paired with a so-so petcock and incorrect fuel level in the bowl...dunno. Because if a cylinder stops firing and resumes later on, that usually results in a mighty bang because of the unburned fuel in the exhaust. You haven't described that, so...sounds like not enough fuel.

What you said here about the fuel level in the bowl set me off to spend a lot of time fiddling with different float levels. I did a float adjustment last year because there was too much fuel in the bowl and it was leaking out. I appeared to fix that issue for a few months before winter but it's possible that I had done the incorrect level all along.

But last week I did spend time raising the fuel level, testing, raising or lowering again, testing, etc. When it started to leak from too much fuel I lowered it down again.
Nothing I did changed anything with how the bike fired - it always did the exact same behavior of slowly losing the one cylinder over time.

And yeah it doesn't have the bang. There isn't any obvious vacuum seal problem from the hoses, petcock, or the rubber boots from what I can see. I also took them off and cleaned them.

Also, provide some pictures of what you're describing as 'clogged', in case you go in there again. It sounds a bit like you're confused by the fuel's surface tension.

Here is a picture of what looking through the (pilot jet? - the small one with multiple holes on the sides of it) is like before/after running it for a few minutes. I don't notice any obstruction on other jets.

https://imgur.com/a/Nvs3MWC - link for both pics because it's only letting me insert one in this post

carbbefore.JPG

I'm not certain if that's a foreign substance or the fuel itself doing that. What do you think?

I could definitely see how it would impair the firing of that cylinder, though. Unless it just gets pulled through on every fire.

If that IS the issue - I guess I've just not cleaned it well enough. It just doesn't make sense why that would only make the cylinder not fire after a few minutes, and then work again after the bike sits for a few hours. It'd be there regardless.

I wouldn't mind the 30psi difference too much, at least for now. 120psi is more than enough for those engines to work.

Good to know

[argonsagas]

Carbs seem OK, according to previous posts.
I did not see this issue though:

Each coil fires two cylinders and you have a problem with only one misfiring.

Small break in wires could be culprit as the break would close when cold, and open more when hot and even a weak spark combined with compression could still fire at higher RPMs

Change the plug wires and connector.