Occasional full-throttle hesitation

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jchesshyre
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Occasional full-throttle hesitation

Post by jchesshyre »

I'm getting a momentary hesitation as the revs pass through roughly 7k which has started happening recently. It's only on full or nearly full throttle hard acceleration. It doesn't happen every time but it's often enough to be noticeable and annoying!

My carbs and air filter are all stock (except for one blocked lift hole in the front slide) and I have Fuel silencers with baffles in.

It feels to me like a very brief bit of fuel starvation. My fuel tap is modified to not be vacuum-operated so it's not the diaphragm in there.

I'm guessing I should look at the carb diaphragms for cracks and also maybe whether either float needle is sticking slightly. What's the best way to test for that? And is there anything else I should check?

I recently replaced my HT leads with stranded copper core wire which gave a really nice improvement. During this swap I tested the coils which are fine, and the spark plugs are recent, so I don't *think* it's an ignition issue.
Roger Ditchfield
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Re: Occasional full-throttle hesitation

Post by Roger Ditchfield »

Fit a pre 1997 CBR600 electric fuel pump and all your problems will disappear :D
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AMCQ46
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Re: Occasional full-throttle hesitation

Post by AMCQ46 »

I had a similar problem which was tracked back to the fuel tap diaphragm failing and restricting fuel flow.

but that wasn't rpm specific, it was most noticeable when a longer hard acceleration was draining the carb float level faster than the tap could flow to refill... and then I was experiencing fuel starvation.

new diaphragm and all fixed
AMcQ
jchesshyre
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Re: Occasional full-throttle hesitation

Post by jchesshyre »

Thank you very much both of you for your replies. The issue has been solved by richening the pilot mixture by half a turn, which I'm quite surprised by considering this was happening most often in the upper midrange (but when transitioning from closed to large throttle).
jchesshyre
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Re: Occasional full-throttle hesitation

Post by jchesshyre »

...I spoke too soon...issue is still there, but just sometimes. I can't quite work out exactly what triggers it but I think it's when I give the bike a large and relatively prolonged handful of throttle after coasting or cruising for a bit. It just briefly hesitates and then continues accelerating nicely. It's sometimes up near peak torque area (c. 7k) but also does do it lower down around 4/5k sometimes as well.

I'm opening the carbs tonight to check the slide diaphragms and the float valves, and I'll also check the HT lead connections as I have recently replaced these and don't recall this happening before I did that...although it might have done :problem: I will also check the sidestand switch – my bike does have 102,000 miles on the clock and I had a misfire issue with a high-mileage CB500 which turned out to be caused by this, although it was only when I went over bumps whereas the current problem doesn't seem related to bumps etc.
jchesshyre
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Re: Occasional full-throttle hesitation

Post by jchesshyre »

Just updating this as I've tracked down the issue, finally. Sometimes it's necessary to spend some time with a problem getting acquainted with how and when it manifests... The cause is a bit embarrassing, but:

Recently I replaced my HT leads, and failed to notice that the sleeve over the HT lead needs to be fitted over the stub on the coil where the lead emerges, to seal it from moisture. The rear coil is fairly well protected from moisture ingress, but the front one is less so, and it was this cylinder that was misfiring. To get by on my commute in to work today in the pouring rain I doused the front coil with WD40 where the lead emerges and the misfire disappeared, and tonight I will dry it all out properly and install the sleeves how they should be installed :shifty:

Thanks for the help with this and hope maybe this thread helps someone who makes the same stupid mistake I did (it was overall *well* worth replacing the HT leads though, even if I'm only now going to be able to fully enjoy the benefits!).
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AMCQ46
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Re: Occasional full-throttle hesitation

Post by AMCQ46 »

glad you got there in the end … good result
AMcQ
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sirch345
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Re: Occasional full-throttle hesitation

Post by sirch345 »

jchesshyre wrote: Tue Mar 03, 2020 11:28 am Just updating this as I've tracked down the issue, finally. Sometimes it's necessary to spend some time with a problem getting acquainted with how and when it manifests... The cause is a bit embarrassing, but:

Recently I replaced my HT leads, and failed to notice that the sleeve over the HT lead needs to be fitted over the stub on the coil where the lead emerges, to seal it from moisture. The rear coil is fairly well protected from moisture ingress, but the front one is less so, and it was this cylinder that was misfiring. To get by on my commute in to work today in the pouring rain I doused the front coil with WD40 where the lead emerges and the misfire disappeared, and tonight I will dry it all out properly and install the sleeves how they should be installed :shifty:

Thanks for the help with this and hope maybe this thread helps someone who makes the same stupid mistake I did (it was overall *well* worth replacing the HT leads though, even if I'm only now going to be able to fully enjoy the benefits!).
:lol: I found that bit funny, as it's absolutely true :thumbup:

I'm glad you got there in the end :clap:
At least you're man enough to admit it :wink:

Chris.
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Pete.L
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Re: Occasional full-throttle hesitation

Post by Pete.L »

I had a similar problem with a RF900 many years ago.
It would misfire after about 5 miles only on cold rainy days. You could stop for 10 minutes and the problem would clear itself.
After much mucking about it turned out that water was penetrating up the HT lead into the coil. The warmth would dry it out and then you could move off again.
A quick blob of silicon onto the end of the HT lead and then rescrewing them into the coil and I never had the problem again :thumbup:
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jchesshyre
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Re: Occasional full-throttle hesitation

Post by jchesshyre »

Thanks guys. I spoke slightly too soon, although it turns out there are two separate issues.

It was dropping out on the front cylinder in the rain and this was indeed water getting into the coil/HT lead interface. Entirely my own stupid fault as mentioned above in that I hadn't put the sleeve right over the stub on the coils when I replaced the HT leads. Have rectified this, including adding some silicone grease (and I also splashed out on some new OEM spark plug caps for good measure) and I'm quite sure that next time it rains I'll have no problems...

However, the slight hesitation that this post was originally about is still there! It just a brief hiccup (sort of quite a smooth blip rather than an abrupt hesitation though) in otherwise very strong and smooth acceleration when I'm giving it beans. It doesn't happen every single time though, just maybe half the time I'm giving it a handful and it's generally in the mid or upper midrange of the revs (though doesn't seem restricted to a particular area of revs, I don't think).

I think as Roger says above it is a fuel flow issue of some kind. I'd rather not fit a fuel pump unless I really get desperate, as surely these bikes are designed without one in mind so it shouldn't really be necessary? My setup is absolutely stock other than some Fuel Exhausts silencers which I run with the baffles in.

I've recently opened the carbs and can't see any problems with the float valves or any jets. I thought it might be a faulty slide diaphragm but I swapped both slides with a good spare pair I had and it made no difference. I might re-check the pilot jets and also open the tank and check the fuel filter and tap. Just to reiterate, I've made my tap manual so it's not an issue with the automatic/vacuum functioning of that.
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Pete.L
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Re: Occasional full-throttle hesitation

Post by Pete.L »

That's a shame.
My only thought when I first read your post was it could be a sticking carb slide (i've had that in the past) but I see from your last post you have already changed the diaphragms and slides.
What about the petrol tank breather. Does the problem appear to be dependant on how much fuel you have in the thank? Any sucking sound if open the fuel cap?
My new ride is a bit of a Howler and I love to make her Squeal
jchesshyre
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Re: Occasional full-throttle hesitation

Post by jchesshyre »

Pete.L wrote: Wed Mar 04, 2020 12:18 pm That's a shame.
My only thought when I first read your post was it could be a sticking carb slide (i've had that in the past) but I see from your last post you have already changed the diaphragms and slides.
What about the petrol tank breather. Does the problem appear to be dependant on how much fuel you have in the thank? Any sucking sound if open the fuel cap?
No, it happens whatever the fuel level. I've also checked the vent pipe is clear and tried running with it disconnected (so just the tiny stub of hose is attached to the tank) and there's no difference. As I say I want to look at the fuel filter and tap and check for something blocking the fuel tap as the next thing to try.

I also changed my front carb slide, from having one lift hole blocked, back to the stock two holes, in case this was the cause. It wasn't (although it actually feels nicer with both slides having two lift holes so I'm leaving it like that!).
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popkat
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Re: Occasional full-throttle hesitation

Post by popkat »

Couple of things it could be.
1. At your high mileage the cam chain will have stretched a lot, the cam timing will be out from when it was new, could be close to a tooth out now.

2. Carb needles and emulsion tubes, both can wear causing odd carburation problems.




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jchesshyre
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Re: Occasional full-throttle hesitation

Post by jchesshyre »

popkat wrote: Wed Mar 04, 2020 5:33 pm Couple of things it could be.
1. At your high mileage the cam chain will have stretched a lot, the cam timing will be out from when it was new, could be close to a tooth out now.

2. Carb needles and emulsion tubes, both can wear causing odd carburation problems.
.
These are great suggestions but I don't think it's either of these as:

I put manual CCTs in at 37,000 miles and have only needed to adjust them once in the 66,000 miles I've done since then and this adjustment (where I reduced the free play between the sprockets by about 1.5 mm) was only 6,000 miles ago! So hopefully this indicates that the chains aren't too badly stretched?

I installed new OEM needles 30,000 miles ago and when I inspected the carbs the other day the needles look pristine. I replaced the carbs at the same time as I put new needles in with some off a low-ish mileage bike. I do have a mild amount of oval wear to the needle jets but this mainly causes richness when cruising at about 3000 rpm, when the needles are just lifted. The hesitation I'm getting usually happens up around 6 to 7k rpm.
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Re: Occasional full-throttle hesitation

Post by jchesshyre »

Well I've just checked my fuel filter and tap, and both are what I can only call pristine, so they're not the issue.

Two rather tenuous ideas have come to mind:

1. I noticed that my RH fuel hose was routed incorrectly, and this is how I've routed it the last few times I've removed the carbs, namely over the rear choke cable instead of under it. The fuel hose was therefore pushing the rear choke cable down a fair bit – it could be that it was doing so far enough to very slightly open the rear enrichening needle (and I've even convinced myself that now the choke knob action feels smoother and easier since I've rectified this) – I'll see on the way to work in the morning whether this was the cause, but I'm pretty doubtful (although have read plenty of threads about the choke cables being routed wrongly and causing funny issues like this.

2. My battery is three years old and I'm basically expecting it to fail before long (it might have done already if this winter had been colder). I tested it and it's showing 12.4V, after an hour's ride. If the battery's dodgy can this cause a slight misfire when the engine's under full load, or would I get starting problems before this?
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