Engine stutter.

[Granty]

Hi all.

I have a 2019 YZFR 125 with 3200 miles recorded and booked in for it's 3500 mile service. I use Esso and Shell petrol although i have used other makes when out in the sticks.

In fourth and fifth gear when accelerating with throttle rolled back to it's max and at 8200/500 revs engine stutters and wont progress. Release throttle a smidge and engine then free revs right up to the red line.

It does'nt do it in any other gear. My first thought was contaminated fuel. However its not a problem in any other gear.

Help please.

Regards.

[fastbob]

Before you start ripping it apart and spending money just try running some fuel injector cleaner through it . I can't recommend a brand I'm afraid because I've never actually had EFI myself but it's got to be worth a go . See what other people think first though . I see that you are booked for a service . I suppose this is a bit naugty but you don't need to mention the problem before hand so if they fix it , good . If they don't , you can take it back until they do .

[Bianco2564]

It must still be under warranty, take it back and tell em to fix it.

[Mississippi Bullfrog]

If you ride it using the throttle more progressively does it do it? My guess is it's a characteristic of a smaller engine struggling with higher loading in higher gears and the EFI mapping not liking the way you're using the throttle control.

The fact that it sorts itself out by backing off the throttle a bit suggests there's nothing major amiss. EFI mapping can be fussy due to the emissions regs the manufacturers have to work with.

Another suggestion would be to check the plug gap. Full load is when any weakness in the ignition will show up most.

[Granty]

Hi.

Thanks for the reply. Next time out will try rolling the throttle progressively with less urgency and see if that works. You have a good point and I can understand the thinking behind it.

Thanks and regards.

[Fozzie]

I've had one of these.

The engine feels quite lethargic compared to other more free-revving 125's. But it loves full throttle.

I did notice it was a little prone to hiccoughs, where if you got it wrong it felt a bit lumpy.

Try different styles and see what happens, but I would have Yamaha look it over for you under warranty.

The only things I would check visually is that the lambda sensor, and air box are correctly fitted. On an old one I knew, the airbox came dislodged due to a previous race boys mod, and it was coughing quite a bit during certain throttle points.

[Granty]

Hi all.

Tried yesterday by rolling the throttle progressively and in fourth and fifth gear it came in again at about 8500 revs and would not go beyond. Closed throttle and re-opened it and revs built up to the red line.

Tried it several times with same result. I'm booked in at the dealers on Friday so I'm hoping to get it sorted. Is it possible for the ECU to have logged it in it's memory as fault. ?.

I'll report back on Friday night.

Regards.

[Granty]

Hi.

Explained to the boss mechanic what was happening. He told me it could be more than one thing that was causing the problem. His bet was that the ECU mapping was objecting to the throttle being given a handful and being held on. Which would explain when eased back a smidge the bike just about took off and revved freely.

They ran the diagnostics and checked for error codes. Nothing found. Checked the Yamaha data base for any mentions and recalls. Again nothing abnormal.

Full service carried out and low and behold when riding the fifty miles back home it ran perfectly. However I'm putting part of the solution down to a change in the way I'm using the throttle.

I've tried building the revs progressively just short of the red line before the up-change and it seems to be working.

I'll make an update if the problem returns.

Regards and thanks.