Stumble at speed then won't accelerate

[voodoo child]

Hello,

 

I'm having a fueling problem that appears to be heat related but I'm still diagnosing.

 

The bike:

2021

Acerbis 6gal tank

Acerbis locking cap - vent mod performed (https://procycle.us/how-to/acerbis-locking-gas-cap-venting-fix)

Roll over valve still in place but plugged on the tank side tube.

 

I installed the tank last October, on the 1st long day (400 mile) I didn't notice any issue until I stopped for traffic light.  When the light changed  & I went to accelerate, the bike wouldn't take fuel - almost seemed to be in limp mode.  At that time I hadn't performed the cap vent mod so I thought that was the issue.  I removed the gas cap with the bike still running but the issue remained.  I shut off the bike via the kill switch, cycled the keyswith off & back on, restarted and the issue cleared.  Rode another 45 minutes home with no issue.

 

I modded the cap the next day and test rode it with no issues.

 

I'd ridden another 1000 miles or so with no issue.

 

Last week after getting some gas at a sketchy place in TX the problem returned.  I'd ridden about 1200 miles at that point over 2 days with no issues. 

The problem increased in frequency over the next 2 hours.  During that time, I found that all I needed to do was kill the bike with the kill switch and restart it while still rolling for the problem to go away.  Thinking that the problem was bad fuel, I filled at the next stop (Clovis, NM) added some fuel treatment (SeaFoam) and went on my way.  I think the bike stumbled again once, maybe twice.  I learned that I could feel it stumble at speed (felt like a carb bike when you hit that point where you need to switch the petcock to reserve).  Once I felt that, if I tried to accelerate at anything other than at a most gradual twist, the bike would fall on it's face - like it just isn't getting fuel.

 

The next morning I rode to Roswell, NM and took the bike to a Yamaha dealer (Champion Motosports - GREAT folks BTW).  They found no codes set, no water in the fuel, and the fuel pump was holding 50psi - they told me spec was 37psi (if my memory is correct).

 

Over the next few days the problem only seemed to show late in the day when the temperature was greater than 90 degrees F.

 

I rode 1400ish miles home Sun-Mon-Tues and the issue never returned, but the ambient temperature was never above 74 degrees F.

 

Anyone experienced anything like this?  I spoke with my local Yamaha dealer's head mechanic & he suggested I start with the Throttle Position Sensor.  I'm not opposed to shotgunning sensors but I'd really like to see something diagnostic-wise to give me confidence that I'm fixing it.

 

From the scanner I see a TP(%):

12.9 closed throttle 

80.4 open throttle

 

These numbers are consistent at engine temperatures of 91 degrees F, 172 degrees F, 217 degrees F.

 

I've ordered a bluetooth ODB reader so the I can leave it in place while riding so if the problem recurs, I can look at the data.

 

Any guidance is appreciated, I've searched an searched on the net for anyone with the same problem but my google-fu must be weak.

 

Thanks

 

MT

 

 

 

 

 

 

 

Edited April 24 by voodoo child
words are hard

[Hibobb]

In the last two years, I have had two issues (ATV & Dirtbike) that I swore were fuel related but ended up to be spark-plugs intermittently failing.

I ain't saying this is your issue, but it is a cheap fix to try if you haven't changed your plugs in a while.

Best of luck to you....

[BikeBrother]

Vapour lock?

 

[voodoo child]

1 hour ago, Hibobb said:

In the last two years, I have had two issues (ATV & Dirtbike) that I swore were fuel related but ended up to be spark-plugs intermittently failing.

I ain't saying this is your issue, but it is a cheap fix to try if you haven't changed your plugs in a while.

Best of luck to you....

 

Plugs were changed 2/24/24 at 11,500 miles - 16,136 on it now.  I also did a throttle body sync about that time - it was dead on with no adjustment needed. 

 

58 minutes ago, BikeBrother said:

Vapour lock?

 

 

That's  been my gut feel since it first happened but I don't understand how killing the engine then restarting it "resets" things for a while.

 

Thanks for the replies.....I'm perplexed!

 

 

[bcrider]

IF you have the original fuel tank, put that back on and see if your issue goes away. If the issue still persists at least you've eliminated the after market tank as the problem

[AZJW]

VC welcome to the forum and the look on your face says it all!  My initial thought was some sort of venting issue, i.e. vapor lock like BB suggested as it seems to keep with the theme of higher temps and and symptoms that mimic  starving for fuel.  My dirt bikes have done that when the one way vent became obstructed and it ran poorly with one actually boiling the fuel, not a fun experience.  Curious if you've loosened the fuel cap when this has happened and if so, did you hear the dreaded "whoosh" of air equalizing the pressure in the tank? I can't help but think it's somehow related to the Acerbis tank venting as I've not heard of this with the OEM fuel tank.  I too am perplexed how a recycling of the ignition switch seems to fix it, but maybe the fuel pump is resetting and re-establishing a full fuel flow?

[voodoo child]

8 minutes ago, bcrider said:

IF you have the original fuel tank, put that back on and see if your issue goes away. If the issue still persists at least you've eliminated the after market tank as the problem

 

Yes, I have the tank so I can make that switch.  I started troubleshooting by looking for any reports of issues with the Acerbis tank but didn't find anything related to that.  I have to return to work tomorrow so troubleshooting will begin in earnest this coming weekend.

 

5 minutes ago, AZJW said:

VC welcome to the forum and the look on your face says it all!  My initial thought was some sort of venting issue, i.e. vapor lock like BB suggested as it seems to keep with the theme of higher temps and and symptoms that mimic  starving for fuel.  My dirt bikes have done that when the one way vent became obstructed and it ran poorly with one actually boiling the fuel, not a fun experience.  Curious if you've loosened the fuel cap when this has happened and if so, did you hear the dreaded "whoosh" of air equalizing the pressure in the tank? I can't help but think it's somehow related to the Acerbis tank venting as I've not heard of this with the OEM fuel tank.  I too am perplexed how a recycling of the ignition switch seems to fix it, but maybe the fuel pump is resetting and re-establishing a full fuel flow?

 

The first time it happened I took the cap off & nothing changed.  Several times on my trip, when fueling, I would check the cap to see that it was still able to vent - it was always unobstructed.  I'll most likely order a fuel pump, throttle position sensor & intake air pressure sensor.  Those seem to be the main components of the fueling system.

I don't know what part the idle speed control valve plays at speed.  The name implies it is only for idle  

 

I'm reading thru the service manual to try to find something that will help.

 

Thanks for the replies.

 

[random1781]

Air filter might be worth looking at because it's easy and just to cross that off the list. A shop grotesquely over-oiled my foam filter, and I had some stuttering on acceleration because of that.

 

The "self vented" gas caps from Acerbis are problematic. I'll get bouts of issues typically when either hot or at altitude. If you still have the original cap, it also might be worthwhile to ride a while with that and see what happens.

 

We had an F700GS that died immediately when you let go of the throttle, but intermittently. Pulled my hair out over that one and after a month of troubleshooting just started swapping sensors. The last thing we changed was the TPS, because it was the most expensive and because the OBD and resistance values were nominal, but this was the problem. The variance causing the failure was too small to measure, even for BMW's diagnostic equipment, so keep this in mind.

 

The kill switch connects to ECU, so simply killing the engine and restarting it could reset calibrations. I don't have any specific knowledge of how the ECU works, but, for example, if any sensor's values are drifting over time and the ECU does some sort of reinitialization when you cycle the kill switch, this could explain why it works by just restarting the engine.

[voodoo child]

@random1781

 

Yes, I do have the original cap so I can swap that back on before shotgunning sensors.  I purchased the locking cap without knowing that the Acerbis caps had an issue.  I will probably try that 1st as it is easiest.  

 

The air filter was replaced about the same time as the plugs (Feb @ 11000-ish miles).  I used an OEM filter.   I did that along with the other maintenance items as I expected this trip to be a little over 6,000 miles, it turned out to be less but that's a story for another time  

[Hollybrook]

I was going to make the same suggestion as random1781 - switch back to the standard Acerbis gas cap.  That's a lot easier than moving the fuel pump.