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Running rough at idle and sluggish at high revs

CBR 1000F "Hurricane" 1987-1996 CBR 1000F

Running rough at idle and sluggish at high revs

  #1  
Old 01-06-2019, 09:56 AM
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Default Running rough at idle and sluggish at high revs

I've opened this new thread in preference to continuing a previous one relating to carb issues as that history is now overtaken by events. The bike's been sat for a couple of months and I now find myself again with what I believe are two separate (but maybe linked) carb issues. Background is that I've cleaned 3 times and synched the carbs previously over the past 6 months and they were running OK two months ago. After being sat for 2 months, on starting her up, i) there's one of the carbs not running to the correct temperature at idle so I suspect there's at least one clogged idle jet - again, but I am also experiencing ii) a major hesitation when revving up to 9000+rpm both in neutral and in gear. When the revs catch up with the opened throttle position it runs kind of OK (but not great) and drops normally back down to idle with no lag. So my question is............could a clogged idle jet(s) be causing BOTH symptoms and hence if I replace with a new set of idle jets (I'm not going to clean them again) could it rectify the hesitation at high revs as well, or is there likely to be a second issue, and if so what? Really getting a little fed up with whipping the carbs off. I know some of you love doing it for pleasure, but I'm not one of those! Thanks for any views. Karl
 
  #2  
Old 01-07-2019, 12:35 AM
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I hate detrimming carbs, for what it's worth. Before going back into them, I'd probably confirm spark and compression

From what I remember, you were pulling small bits of rubber out from behind the pilot jets, right? Is it still #1 that's giving you trouble?
 
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Old 01-07-2019, 03:57 AM
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Hi tentacleslap, all sparks are good and although I've not checked the compression it seems reasonable and there was no issue with it when the bike was running well a couple of months ago. There's no rubber bits, and the problematic cylinder has moved around over they past 6 months each time I've unblocked the pilot jets. It's currently #3. I'm probably going to bite the bullet and change for new pilot jets anyway, and so I'm interested to learn whether that might solve the sluggishness at high revs too. Do you or anyone else have a view on that, or is there something else I should be looking at as well?
 
  #4  
Old 01-07-2019, 11:16 AM
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Higher revs are handled by the main jets. The passages are larger in the mains, which is why they don't usually clog as quickly as the pilot jets

For due diligence, you might want to drain your fuel and check for foreign debris. Ideally you'd want to locate the source of the problem but honestly I wouldn't blame ya for just buying a new (used) set of carbs. The gremlins have clearly had their way with yours

Good luck
 
  #5  
Old 01-07-2019, 11:20 AM
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Oh one more thing is to confirm the pilot screw adjustment. I can't recall if you'd done that previously, but if you notice a problem with one, you could try fiddling with the adjustment at idle. Make sure to adjust idle speed as you go. If it starts to run more smoothly, you'd know you were on the right track
 
  #6  
Old 01-09-2019, 11:23 AM
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The first thing I would do is empty the tank and start fresh. Could be poor fuel, maybe not.
I would start fresh anyway.
 
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Old 01-09-2019, 01:50 PM
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I'm not inclined to buy a replacement (secondhand) set of carbs as there's no certainty they'd be any better. I have never touched the pilot screw adjustment and was running well before so it's unlikely to be related to that. I know there is some light corrosion in the tank which may be contributing to the problem, although I do have the inline filter. I'll be changing the fuel and think it's more likely to be a function of the fuel being stood for a couple of months clogging the pilot jets (again) and that's why I'm thinking of fitting new ones. But does anyone know whether putting in new pilot jets could also solve the high rev sluggishness?
 
  #8  
Old 01-11-2019, 10:55 AM
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Just throwing it out there, but have you checked out the fuel filter? I know you'd installed it recently, but if the tank's throwing a lot of debris at it, it may be clogged up. Cheap part

I don't think pilot jets will make an impact at high revs, so more likely whatever clogged the pilots is also clogging the mains
 
  #9  
Old 01-11-2019, 04:23 PM
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Tentecleslap

It's fair to say the (paper?) inline filter is slightly brown whereas it was perfectly white when bought. It doesn't have any large parts of debris in it though so it's either very fine corrosion products, or the fuel itself that's the problem there. I agree that new (or cleaned) pilot jets are unlikely to solve the high rev sluggishness, but also doubt that the main jets could be so blocked as to be causing the problem as the orifice in those is really quite large. I must admit the combination of symptoms is a bit of a quandary.
 
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