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2020 GSX-S750
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Discussion Starter · #1 ·
Hi!

Got myself a brand new 2020 gsx-s750 that late in the fall developed a misfire.
It was late in the season so I decided to put it off until spring.

Well... spring is here, so I guess it's time to deal with this.
I was wondering if this happened to be a common issue or if any of you have faced an identical problem.

First of all; yes, the bike is still under warranty, but I live miles away from the closest dealer, so getting it to them will be a real hassel. So if it is something simple, i'd much rather mend it myself.

Now to the symptoms.
I was riding in the fall when the bike abruptly changed it's pitch and started vibrating more. I parked it and started it the next day. It ran fine for a tiny while until it slowly started misfiring again (nowhere near as suddenly as the first time).
I stored it for the winter. Now I have brought it out for the new season, and took a couple of shorter trips. The degree of misfire varyies a lot. It can run totally fine for 10 miles, and misfire the next 5 before misfiring a little less the last 5. (That's just an example).
Sometimes if i rev it in neutral (just hold 5k rpm for example) it will backfire from the exhaust. Other times it will be completely fine. Other times there will be no backfire but you hear the RPM varying quite a lot.
The change between running smoothly and misfiring is subtle and smooth. It doesnt happen like the first time where it suddenly changed pitch. This has led me to question my own perception of this thing. If im simply overthinking it.

Any ideas of what can cause such varying and occasional misfires? (I get no check engine light).
I have already ordered a set of sparkplugs to swap, in hopes it's that simple.

The bike has 9000km on it btw.

Thanks in advance guys :)
(Sorry for bad english/grammar, english is not my native language)
 

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Sounds like plugs a faulty ignition coil or connection, but could more likely be fuel.
Changing the plugs will eliminate one thing. Good luck.
 

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Discussion Starter · #3 ·
I was thinking coil or spark-plugs as well. I doubt the fuel itself is the issue, as i've refueled at least twice after the problem started, and it hasn't stopped misfiring. If there was water in the fuel it would likely not fail as intermittently as it does (?) would it not just start coughing as the fuel-level decreased and then eventually just stop. Other contaminants should have been caught by the filter. Bad fuel quality in itself should affect more than one cylinder, and shouldn't last after refilling twice. Or am i missing something?

I was considering if water could have gotten into the air-intake as I was washing the bike. This fits fairly well with the timeline, as the bike was freshly washed before the first time it occurred, and as I've mentioned, the first time it misfired it happened much more abruptly. Is that likely? I'm not familiar with the placement and orientation of the intake on the bike. Is it in a position exposed to water during washing? (If so, that's a real bad design) (And no, i haven't been actively spraying water up underneath the tank (where i assume the intake is))
If water in the intake was the cause, should the problem not have passed as the water passed through the cylinder?
 

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My thought is that you don't have to consider water entry from washing a bike. I won't happen.
Most problems after winter storage reported here come down to fuel deterioration from the water content that's in the fuel forming a black or yellow sludge in the gauze filters of the fuel pump or inlet filters at the throttle bodies. Did you put inhibitor in and fully fill the tank prior to storage ? Was the fuel an ethanol mix ?
A change in octane quality will or can make a noticeable change in engine pitch.

I don't think the spark plug will be the source of the problem as when you shut off the engine it most probably won't restart again, but just grind over on the starter motor.
I had a GSX-R1000 that had a faulty plug and that was what the Technician told me. 4 new iridium plugs (expensive) was the cure.
I think that my topping up the fuel tank with very old mower fuel and not riding for several weeks, was the root cause, but it failed to start at all, not misfire, so who knows why the plug(s) failed.
It had a similar mileage to yours too, around 5,000 km.

Rob.
 

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Discussion Starter · #5 ·
My thought is that you don't have to consider water entry from washing a bike. I won't happen.
Most problems after winter storage reported here come down to fuel deterioration from the water [...] Did you put inhibitor in and fully fill the tank prior to storage ? Was the fuel an ethanol mix ?
A change in octane quality will or can make a noticeable change in engine pitch.
I only topped of the tank to the brim with the standard fuel I use before winter-storage . Non-ethanol. But as I stated in my original post, the problem started last year, before winter.

I don't think the spark plug will be the source of the problem as when you shut off the engine it most probably won't restart again, but just grind over on the starter motor.
?...
It's not as if all spark-plugs fail if one goes slightly bad (I say slightly as it seems to run fine from time to time)

I had a GSX-R1000 that had a faulty plug and that was what the Technician told me. 4 new iridium plugs (expensive) was the cure.
I think that my topping up the fuel tank with very old mower fuel and not riding for several weeks, was the root cause, but it failed to start at all, not misfire, so who knows why the plug(s) failed.
It had a similar mileage to yours too, around 5,000 km.
Sounds like bad fuel damaged your plugs somehow. Only reason I can see as to why all of them would fail... idk..

I've already ordered 4 new plugs. Let's hope for the best :) Hopefully they arrive within a few days, then i'll have to ride it for a while before i can give a proper verdict. When I've swapped the plugs I will update the thread if there is something that stand out about the old plugs (if one of them seems damaged, extra sooty, etc.)
 

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Discussion Starter · #6 ·
Update:
Swapped plugs (old ones looked ok, nothing stood out).
After doing that I remounted the airbox and attempted to start the bike just to test it. It did not start and instead flashed the check-engine light as it cranked the starter motor. I immediately realized I had left the ignition-cover of the ignition-barrel, and the immobilizer prevented it from starting.
I put the cover over the barrel and the bike started fine. Couldn't notice any misfire but I would have to use it for a while before I could say.
The check-engine light stayed on and "F1" or "FI" is flashing in the instrument panel...
Guess I'm taking a day of work to head to the dealer and I will also miss the local annual "spring ride" this weekend, that 40+ bikes partake in :(
 
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