Occasional full-throttle hesitation

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tony.mon
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Re: Occasional full-throttle hesitation

Post by tony.mon »

The battery wouldn't give rise to the problems you describe.
I'd be thinking about the carb vacuum slide lifting side of things.
The engine vacuum lifts the slides- that's based on the whole vacuum line and carb diaphragm system being in good order - and the lift holes in the slides.
Have you drilled out the lift holes?

If you haven't already, just borrow a a set of carbs and swap over, if the problem goes away it at least narrows it down to carbs.
It's not falling off, it's an upgrade opportunity.
jchesshyre
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Re: Occasional full-throttle hesitation

Post by jchesshyre »

tony.mon wrote: Thu Mar 05, 2020 8:33 am The battery wouldn't give rise to the problems you describe.
I'd be thinking about the carb vacuum slide lifting side of things.
The engine vacuum lifts the slides- that's based on the whole vacuum line and carb diaphragm system being in good order - and the lift holes in the slides.
Have you drilled out the lift holes?

If you haven't already, just borrow a a set of carbs and swap over, if the problem goes away it at least narrows it down to carbs.
Yeah I thought as much re. the battery!

The lift holes are completely as standard now. I did have one blocked in the front slide but removed the epoxy the other day (and prefer it without actually as I said). I changed to some different slides the other day in case one of the diaphragms was faulty but this made no difference.

Is the vacuum relating to the air-cut system connected to the main vacuum system for the carb slides? These are the one part of the carbs that I didn't test/examine when I had them off the other day.

I do have a spare set of carbs so will give these a go.

I think the RH fuel hose was holding the rear choke open by a miniscule amount as on my commute in this morning I thought the off-idle response felt slightly cleaner...but it may have been my imagination (or might just be because it's a nice cold morning today...) and it hasn't sorted the stumble on acceleration!

I'm also wondering whether it's just a brief bit of leanness that I could iron out with an extra washer under each jet needle (currently just have the standard one under each)...although to me with my mediocre/intermediate carb knowledge it feels as if the problem happens as the fuelling is transitioning from the needle to the main jet.

I guess I should also check the float heights.

My air filter has 11,000 miles on it so I should probably try a new one even though I'd normally wait till 12,000.

[edit] further reading of the Factory Pro tuning guide suggests that it actually is needle height not float height that would be affecting this range, if carb tuning is the issue here:

2. Midrange (full throttle /5k-7k)

Step 1 (Best Main Jet) must be selected before starting step 2!
Select best needle clip position
To get the best power at full throttle / 5k-7k rpm, adjust the needle height, after you have already selected the best main jet.
If the engine pulls better or is smoother at full throttle/5k-7k in a full throttle roll-on starting at <3k when cool but soft and/or rough when at full operating temperature, it is too rich in the midrange and the needle should be lowered.
If the engine pulls better when fully warmed up but still not great between 5k-7k, try raising the needle to richen 5k-7k.
If the engine pulls equally well between 5k-7k when cooler as compared to fully warmed up, the needle height is probably properly set.
Do not pay too much attention to the low-end richness when you are changing needle clip positions - you still need to be using the clip position that produces the best full throttle / 5k-7k power in conjunction with the main jets (Step 1) that produce the best power at high rpm. You will deal with the low-end / cruise next.
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Pete.L
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Re: Occasional full-throttle hesitation

Post by Pete.L »

jchesshyre wrote: Thu Mar 05, 2020 9:47 am I'm also wondering whether it's just a brief bit of leanness that I could iron out with an extra washer under each jet needle (currently just have the standard one under each)...although to me with my mediocre/intermediate carb knowledge it feels as if the problem happens as the fuelling is transitioning from the needle to the main jet.
Aftermarket pipe and a K&N filter often cause this type of hesitation with std carbs. Normally its most pronounced after steady cruising and then wacking the throttle open but at about 3-4 thou rpm. If I remember rightly a 0.5mm washer was the recommended cure.
Also check all the vacuum hoses. I was doing a bit of work on a mates Storm last summer and because of the age of it, the pipes had started to deteriorate and I found several splits when investigating why I couldn't balance the carbs properly.
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jchesshyre
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Re: Occasional full-throttle hesitation

Post by jchesshyre »

Pete.L wrote: Thu Mar 05, 2020 11:10 am
jchesshyre wrote: Thu Mar 05, 2020 9:47 am I'm also wondering whether it's just a brief bit of leanness that I could iron out with an extra washer under each jet needle (currently just have the standard one under each)...although to me with my mediocre/intermediate carb knowledge it feels as if the problem happens as the fuelling is transitioning from the needle to the main jet.
Aftermarket pipe and a K&N filter often cause this type of hesitation with std carbs. Normally its most pronounced after steady cruising and then wacking the throttle open but at about 3-4 thou rpm. If I remember rightly a 0.5mm washer was the recommended cure.
Also check all the vacuum hoses. I was doing a bit of work on a mates Storm last summer and because of the age of it, the pipes had started to deteriorate and I found several splits when investigating why I couldn't balance the carbs properly.
That's exactly it Pete, it is definitely after steady cruising whenever this happens. My silencers aren't standard (Fuel Exhausts) but I do run with baffles in. Air filter is standard (well, HiFloFiltro). But I'll definitely see what it's like with an extra washer under each needle, although last time I tried this I found it too rich at cruise (exacerbating the richness caused by slightly worn needle jets).

I will also double check the vacuum hoses. I made my fuel tap manual so that the vacuum hose that was going to that is just hanging down ready for carb synching; I fitted a brass spigot with a rubber cap over it to this hose so there are a couple of failure points there to check. RH inlet stub just has a rubber cap over it which again I'll double check for splits etc.
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Pezed
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Re: Occasional full-throttle hesitation

Post by Pezed »

Hi. I'm not sure if this is going to be of any help to you, but my bike recently developed the exact same problem! It was fine, then I changed the front cct, now I have this hesitation occasionally when I open her up. Thing is, I took a chance and only changed the one cct, and I only unshipped the tank for access and disturbed nothing else. My tubes and rubbers are also old, and I now suspect that disturbing them has likely opened up a tiny crack in one, causing this snag as the pressure changes when opening. Since I have not disturbed anything else whatsoever, it's likely that your problem with the exact same symptom, is not any of the other work that you have done. Not guaranteed of course, but it may pay you to concentrate on the hoses before delving too deep elsewhere.
I hope this helps and would deffo like to hear back either way, likely save me diagnosis time too!
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jchesshyre
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Re: Occasional full-throttle hesitation

Post by jchesshyre »

Pezed wrote: Thu Mar 05, 2020 6:03 pm Hi. I'm not sure if this is going to be of any help to you, but my bike recently developed the exact same problem! It was fine, then I changed the front cct, now I have this hesitation occasionally when I open her up. Thing is, I took a chance and only changed the one cct, and I only unshipped the tank for access and disturbed nothing else. My tubes and rubbers are also old, and I now suspect that disturbing them has likely opened up a tiny crack in one, causing this snag as the pressure changes when opening. Since I have not disturbed anything else whatsoever, it's likely that your problem with the exact same symptom, is not any of the other work that you have done. Not guaranteed of course, but it may pay you to concentrate on the hoses before delving too deep elsewhere.
I hope this helps and would deffo like to hear back either way, likely save me diagnosis time too!
Thanks for this! Which hoses do you mean in particular? I disabled the vacuum function of my fuel tap so the hose on the rear cylinder inlet is just capped off and used for carb synching. I *think* when I did this I used some new hose but it's on my list of things to check!
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Pezed
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Re: Occasional full-throttle hesitation

Post by Pezed »

Without looking anything up, and from my piss poor memory only, I disturbed four pipes. Feed to each carb, vacuum from the inlet, and one other, so likely a tank breather? More knowledgeable people could probably finesse these down to a likely culprit, but I'd be surprised if a case couldn't be made for any of them really. I intend, when it's warmer, to remove the tank again and replace the lot 'cos they were all perished and therefore suspect. I'm not gonna faff about changing one at a time and test ride to see if that worked, not just because of the work involved, but also because it doesn't happen every time anyway, as you know. I'm unsure whether the culprit will be a full blown crack from inner to outer wall, or merely a weakened portion collapsing under suction and causing a temporary restriction. I can't really suggest anything more specific to you unfortunately, I was cutting every corner possible to get mobile again toot sweet because I need the wheels and have roadside facility only- ie cold and wet! Sorry I can't be more useful.
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jchesshyre
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Re: Occasional full-throttle hesitation

Post by jchesshyre »

No problem, I appreciate the input! I've checked the fuel tank vent hose and the fuel delivery hoses and they're good. The fourth one is just the fuel tank overflow hose.

On my journey home tonight I tried to really get a clear sense of how to describe the issue. What follows may contradict some things I've said above but I have also been chasing an HT leak which is now resolved and will have been causing some misfiring at lower revs. It seems this remaining issue is confined to the upper revs. I'm not sure if it is related to whether I've just been cruising at a steady speed, but really it actually only seems to happen as the revs pass through 7000 rpm under full throttle. It doesn't happen every time but maybe 2 out of 3 times...it just feels like a brief but very noticeable hesitation as if the engine's momentarily struggling for either air, fuel or spark, then acceleration continues. It's pretty consistently at a point between 6 and 8k rpm (let's call it 7k hahaha). I should add that otherwise the engine feels absolutely great and just as perky and grunty as it always has done, which is great considering it's on 103,000 miles.

Does the above description narrow it down at all? I think actually it's not a lean needle issue nor a vacuum issue given the revs it happens at.

I'm thinking I should try a new air filter. And could a failing coil do this – fine most of the time except for full acceleration around the engine's peak torque zone?

This is really annoying me now and also we're literally just about to have a baby, in the next week or two, and I know I'll struggle to find the time to work on it for some time once he or she is here!
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Pezed
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Re: Occasional full-throttle hesitation

Post by Pezed »

These intermittent type snags are often tricky to nail down. You have other considerations because of the amount of things that have been done, in addition to reasonable fault examination and diagnosis. If I was approaching this problem cold, I'd say that the symptom was electrical, because of the, albeit momentary, seemingly total loss of power, just for an instant. That's what happens to mine. My upper body lurches forwards, then back again, it's that pronounced. However, this started the very same day that I had the tank off, then on again. I'm convinced that my problem is related in some way to that operation, but I can totally see why you question other things. I can only wish you good luck with it. I'm living with mine until I change all the pipes. If it doesn't disappear then I'll be well miffed! 😊

Good luck with the sprog. 👍
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Re: Occasional full-throttle hesitation

Post by jchesshyre »

I went to start taking my carbs off again just earlier, and on closer inspection my air filter is f---ing filthy! In 17 years of riding bikes I've never really felt a massive difference when I've changed air filters every 12,000 miles, or a small improvement at most. But my current one does have 11,000 on it so I'd be changing it soon anyway. I guess we have had a lot of rain recently but as I say I've never known this happen before on any bike I've owned.

When I think about it, presumably as an air filter gradually clogs up it's the engine condition requiring the fastest increase in total airflow (which I presume is somewhere around 7k rpm, full throttle, on the Firestorm) suffers first with fuelling? It certainly could be said to feel like it's briefly gasping for air when it happens.

If it did turn out to be just the air filter I will hide my head in shame at writing all this speculation about electrical or carb faults all because of needing to change the bloody air filter!
Last edited by jchesshyre on Thu Mar 05, 2020 11:26 pm, edited 4 times in total.
jchesshyre
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Re: Occasional full-throttle hesitation

Post by jchesshyre »

Pezed wrote: Thu Mar 05, 2020 8:50 pm These intermittent type snags are often tricky to nail down. You have other considerations because of the amount of things that have been done, in addition to reasonable fault examination and diagnosis. If I was approaching this problem cold, I'd say that the symptom was electrical, because of the, albeit momentary, seemingly total loss of power, just for an instant. That's what happens to mine. My upper body lurches forwards, then back again, it's that pronounced. However, this started the very same day that I had the tank off, then on again. I'm convinced that my problem is related in some way to that operation, but I can totally see why you question other things. I can only wish you good luck with it. I'm living with mine until I change all the pipes. If it doesn't disappear then I'll be well miffed! 😊

Good luck with the sprog. 👍
Thank you :D
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sirch345
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Re: Occasional full-throttle hesitation

Post by sirch345 »

jchesshyre wrote: Thu Mar 05, 2020 11:14 pm I went to start taking my carbs off again just earlier, and on closer inspection my air filter is f---ing filthy! In 17 years of riding bikes I've never really felt a massive difference when I've changed air filters every 12,000 miles, or a small improvement at most. But my current one does have 11,000 on it so I'd be changing it soon anyway. I guess we have had a lot of rain recently but as I say I've never known this happen before on any bike I've owned.

When I think about it, presumably as an air filter gradually clogs up it's the engine condition requiring the fastest increase in total airflow (which I presume is somewhere around 7k rpm, full throttle, on the Firestorm) suffers first with fuelling? It certainly could be said to feel like it's briefly gasping for air when it happens.

If it did turn out to be just the air filter I will hide my head in shame at writing all this speculation about electrical or carb faults all because of needing to change the bloody air filter!
Fingers crossed it is just the air filter :shh:

Yes, all the very best with the new baby when he/she arrives,

Chris.
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Re: Occasional full-throttle hesitation

Post by jchesshyre »

sirch345 wrote: Fri Mar 06, 2020 8:11 pm Fingers crossed it is just the air filter :shh:

Yes, all the very best with the new baby when he/she arrives,

Chris.
Thank you : )

Baby hasn't arrived yet, and I'm very relieved (though embarrassed) to say that a new air filter has sorted the issue! I can't believe I didn't try the most obvious things first, but as I say I've never ever encountered performance issues as a result of taking the air filter up to its full 12,000 miles replacement interval, so I didn't even think to check it.

Touch wood, the bike's running reaaally well now, despite its 103,000 miles. It even wheelied in 2nd gear when I whacked the throttle open at 6000 rpm while trying to get the misfire to happen – I've only ever managed to get it to wheelie off the throttle in 1st gear until now :lol:
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Pezed
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Re: Occasional full-throttle hesitation

Post by Pezed »

That's great!
I'm pleased that you have found your solution, and I'm certainly gonna pull mine out for a good look, possibly tomorrow, even though I can't quite see how it would cause the problem that I have. Best news for you is now being able to concentrate on the missus and new arrival, I'm sure they'll be delighted! 👍
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sirch345
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Re: Occasional full-throttle hesitation

Post by sirch345 »

jchesshyre wrote: Sat Mar 07, 2020 3:43 pm
sirch345 wrote: Fri Mar 06, 2020 8:11 pm Fingers crossed it is just the air filter :shh:

Yes, all the very best with the new baby when he/she arrives,

Chris.
Thank you : )

Baby hasn't arrived yet, and I'm very relieved (though embarrassed) to say that a new air filter has sorted the issue! I can't believe I didn't try the most obvious things first, but as I say I've never ever encountered performance issues as a result of taking the air filter up to its full 12,000 miles replacement interval, so I didn't even think to check it.

Touch wood, the bike's running reaaally well now, despite its 103,000 miles. It even wheelied in 2nd gear when I whacked the throttle open at 6000 rpm while trying to get the misfire to happen – I've only ever managed to get it to wheelie off the throttle in 1st gear until now :lol:
Excellent news :clap:
It's funny how quite often it's the simplest of things that can catch us out,

Chris.
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