Sticking carb slide?

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jchesshyre
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Sticking carb slide?

Post by jchesshyre » Fri May 25, 2018 12:14 pm

Since I replaced my carbs I've had an issue where just occasionally the idle speed hangs at just under 2,000 rpm instead of 1,100 where I've set it. On the road this happens sometimes when coming to a stop soon after using high revs (not that common a scenario but occasionally occurs e.g. between multiple traffic lights on a dual carriageway). At standstill it happens fairly consistently if you give a handful of revs and snap the throttle shut (noticeable during carb synching, and when it does this the front carb shows higher than normal vacuum).

When this happens, the revs do not drop back to normal on their own as you'd expect them to with a lean mixture. The only way to get them back down is to do a few more rev-ups until they fall back down to normal idle speed.

As a (probably) related symptom, the bike sometimes feels a tiny bit funny when riding and going from large throttle openings to closed or nearly closed - again, not something that I do often but of course sometimes is necessary - and will hesitate slightly when coming off the power in these scenarios.

Does this sound like a carb slide sticking? As I say, when vacuum gauges are attached and this thing happens the front cylinder shows higher vacuum, so I think I can rule out a sticking throttle cable or linkage otherwise presumably both cylinders would be affected?

*edit* I do have one lift hole blocked in the front slide, but currently have no extra washer in the rear slide, just the stock washer in both. Carbs are other wise standard.

*edit2* only other thing it occurred to me could be responsible is the air cut-off valve for the front carb?

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E.Marquez
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Re: Sticking carb slide?

Post by E.Marquez » Fri May 25, 2018 2:56 pm

jchesshyre wrote:
Fri May 25, 2018 12:14 pm
*edit2* only other thing it occurred to me could be responsible is the air cut-off valve for the front carb?
Or diaphragm not seated correctly/fully under the cap, or a hole in the diaphragm. Though both of those are usually a constant issue, not a come and go one.
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VTRDark
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Re: Sticking carb slide?

Post by VTRDark » Fri May 25, 2018 4:33 pm

The one lift hole blocked and diffrent from the rear will give an uneven reading between the two. I would start with removing the airbox and visualy inspecting the slides. Pull them back and see if they spring back smoothly. While you at it check all the throttle linkage and give the cables a lube. But it might be too lean causing it to hang. Does it hang and then drop or litrally stick and not drop at all. You say replaced the carbs. Make sure they are seated correctly and that there are no air leaks as this can cause this too as well as allready mentioned a badly seated diaphram.
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jchesshyre
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Re: Sticking carb slide?

Post by jchesshyre » Fri May 25, 2018 9:09 pm

VTRDark wrote:
Fri May 25, 2018 4:33 pm
The one lift hole blocked and diffrent from the rear will give an uneven reading between the two. I would start with removing the airbox and visualy inspecting the slides. Pull them back and see if they spring back smoothly. While you at it check all the throttle linkage and give the cables a lube. But it might be too lean causing it to hang. Does it hang and then drop or litrally stick and not drop at all. You say replaced the carbs. Make sure they are seated correctly and that there are no air leaks as this can cause this too as well as allready mentioned a badly seated diaphram.
No, it doesn't hang and then drop (which I know is s symptom of a lean idle mixture), it hangs and doesn't drop until I've revved it a few times, when it then falls cleanly down to the proper idle speed without hanging at all.

I'll definitely take the airbox off tomorrow and look at the slide operation. I do always check this after I've had the carbs apart but we shall see...

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tony.mon
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Re: Sticking carb slide?

Post by tony.mon » Sat May 26, 2018 3:44 pm

Suggest you slacken off the throttle cable half a turn to see if it improves it.
Also worth checking that you haven't got a hose rubbing on the carb linkage rod, holding it open a little.
It's not falling off, it's an upgrade opportunity.

jchesshyre
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Re: Sticking carb slide?

Post by jchesshyre » Sun May 27, 2018 5:35 pm

tony.mon wrote:
Sat May 26, 2018 3:44 pm
Suggest you slacken off the throttle cable half a turn to see if it improves it.
Also worth checking that you haven't got a hose rubbing on the carb linkage rod, holding it open a little.
Checked both these things – still doing it.

I've also checked for correct and smooth operation of the slides and they seem fine. I've even unblocked the lift hole in the front slide just in case – no difference in the issue.

I also checked to see if the high idle went down again on its own and it didn't after a good few minutes. It only does so after an indeterminate number of rev-ups.

When the high idle is happening, I can see that the main throttle drum (if that's the word) is still sitting completely against the throttle stop. It's hard to see all of the linkage on the LH side of the carbs with everything in situ but I can't see anything that would be stopping it operating properly either when I had the carbs off or with them in place.

Could this be a float height or float needle issue in one of the carbs? I have a spare set of floats and (new) float needles I could try.

As I say I also have the issue of slight hesitation under large throttle openings at low-mid revs, and if changing from large to small and back to large throttle openings. I feel the issues are related as they both started at the same time (when I swapped my carbs).

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8541Hawk
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Re: Sticking carb slide?

Post by 8541Hawk » Mon May 28, 2018 4:02 am

I would check the sub-filters and all the passages to the back side of the diaphragm.
If the filters are clogged of if some debris is blocking the ports or hoses, it could cause a "vacuum" lock on the slide.
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tony.mon
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Re: Sticking carb slide?

Post by tony.mon » Mon May 28, 2018 2:36 pm

You could try popping the tops off the carbs and measuring the spring lengths.
Specs are in Haynes.
It's not falling off, it's an upgrade opportunity.

jchesshyre
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Re: Sticking carb slide?

Post by jchesshyre » Mon May 28, 2018 6:52 pm

Well, I've solved one problem but not the other, so I think they're not related.

I took the carbs off again and had a closer look at the throttle linkage, and discovered that the nut at the front of the linkage bar on the LH side is threaded and won't tighten fully. This means there is a few mm of play in the linkage.

I have swapped these carbs for another set that I had as spares and the problem has gone. Yay! Funnily enough after fitting this I found that the idle speed and synchronisation were both spot on already.

Actually, I don't think a sticky carb slide would cause a hanging idle speed. When I was playing with the slides with the bike running before I took the carbs off, I noticed that opening one of the slides while the throttle is shut has no effect - and then realised that of course it doesn't, since the fuel is being supplied downwind of the throttle butterfly by the pilot circuit until the throttle is cracked open.

The other problem, the hesitation that sometimes happens at large throttle openings, is still there. Seeing as it is not only still there but exactly the same as before changing these carbs, I can (hopefully) say that it is not a carb issue. It feels like brief fuel starvation so I'll be looking at the fuel tap.

If it's not that I suppose it could be an ignition advance issue? I hope it isn't but I suppose that's where I'd have to look next (as I doubt it's a carb float issue seeing as swapping the carbs hasn't affected it at all). TPS is set to 500 ohms but what else can be wrong with the ignition?

Fingers crossed it's just a dodgy diaphragm or something in the fuel tap.

*edit* ...checked the fuel tap - absolutely fine. As is the filter. As is the tank vent. What next? :roll:

jchesshyre
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Re: Sticking carb slide?

Post by jchesshyre » Tue May 29, 2018 11:03 am

tony.mon wrote:
Mon May 28, 2018 2:36 pm
You could try popping the tops off the carbs and measuring the spring lengths.
Specs are in Haynes.
I will try this as well. Were you thinking to see if they're tired, or to check that they're the stock springs? They definitely are the stock ones but I will measure them to see if they're still in spec.

Interestingly, I've noticed the WOT hesitation that still (sometimes) remains happens most often either at 3500-4500 rpm or 6500-7500 rpm – am I right in thinking that these are both areas where the carbs are transitioning between different circuits (pilot to needle, then needle to main)?

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tony.mon
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Re: Sticking carb slide?

Post by tony.mon » Tue May 29, 2018 1:44 pm

Tired, but also to make sure that they're fitted properly.
It's not falling off, it's an upgrade opportunity.

jchesshyre
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Re: Sticking carb slide?

Post by jchesshyre » Tue May 29, 2018 6:32 pm

I may have found my issue...

Because the hesitation seems to be related not just to revs but also to throttle openings, I thought I should check that my TPS is definitely working properly. So I did the test in the manual of disconnecting the TPS, holding the revs at 3k rpm, and then plugging the TPS in, which should cause a rise in revs. Nothing! I've just ridden around a bit with the TPS disconnected and it feels exactly the same. So I suspect I have an issue with the wiring between the ICU and the TPS (the TPS itself tests fine), probably from when I had a crash a year ago and had a new subframe etc. I think I was away from the bike for long enough (just under two months) not to notice there was anything wrong! This is the kind of problem I hate having to sort out, breaking into/tracing wiring looms. I'll get the multimeter out tonight to test those connections.

Does anyone have any tips for removing the ICU connector? I always feel like I have to force those kinds of connectors.
Last edited by jchesshyre on Wed May 30, 2018 9:25 am, edited 1 time in total.

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VTRDark
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Re: Sticking carb slide?

Post by VTRDark » Tue May 29, 2018 8:09 pm

I know what you mean about those connectors. Flat head screwdriver or trim clip remover to help prise them off helps. Place the blade in the grooves of the bit your supposed to depress with your finger on to prise and remove the big connector and then for the smaller White one on the converter unit use the screwdriver to pry back the little tab and although I am not a fan of pulling on wires in this case it seems the simplest way.
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jchesshyre
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Re: Sticking carb slide?

Post by jchesshyre » Tue May 29, 2018 9:08 pm

Just performed a full test of the TPS via the ICU connector (as described in the manual) and it passed the test with flying colours. And the continuity between the ICU and TPS is all good.

So WTF next I wonder...

...have ordered new inlet rubbers. At 77k miles they could probably do with replacing, and my issue does feel like brief leanness, so I want to rule them out as a cause. I went with these https://www.wemoto.com/bikes/honda/vtr_ ... ng_rubbers rather than the Honda ones...hopefully they're not sh*t.

jchesshyre
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Re: Sticking carb slide?

Post by jchesshyre » Wed May 30, 2018 9:56 am

tony.mon wrote:
Mon May 28, 2018 2:36 pm
You could try popping the tops off the carbs and measuring the spring lengths.
Specs are in Haynes.
Can't see this anywhere in Haynes or Honda manual...

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