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CBR 900RR 1993 - 1999 Honda CBR 900RR

Engine problem - What to try next?

  #1  
Old 03-12-2019, 02:58 PM
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Default Engine problem - What to try next?

I've tried everything I can think of to solve a problem with my completely stock '95 with 5k miles. Issue is: engine starts/idles fine, but instantly dies when given any throttle.

Here's what I've done:
- Cleaned carbs several times, including complete disassembly/soak in carb cleaner/blow through all ports (I could find) with compressed air.
- All jets are all clean/clear - can spray carb cleaner through slow jet ports and fluid comes out freely into carb barrel.
- floats/float valves visually perfect, float chamber is spotless
- jet needles/slides in perfect shape, diaphragms visually perfect condition: no tears/pinholes, seated properly (slides all take about 1 second to descend into place after being raised)
- removed all vacuum hoses (that dealt with tank fume evacuation), capped all vacuum ports to address possible vacuum leaks
- fuel pump flow tested
- carbs synced
- replaced all carb o-rings
- tested with airbox on and off

Starts/runs/idles with choke off. Any application of throttle and engine dies.
Acts as if engine gets no additional fuel as throttle is opened. (float bowls are full)

I'm at wits end... any suggestions what to try next?

Thanks!
 
  #2  
Old 03-12-2019, 05:51 PM
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How many turns out are your pilot screws?
 
  #3  
Old 03-12-2019, 06:35 PM
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Before my first carb disassembly, they were about 2 3/4 turns (which I think is factory for a CA model). I reset them to that initially. I just replaced the O-rings on the fuel/air screws and set them to 2 1/2 turns (just to see if there was any difference.) No noticeable change.
 
  #4  
Old 03-12-2019, 07:20 PM
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I would turn each 1 full turn in more. If it helps, that tells you that you the issue is that area.
 
  #5  
Old 03-12-2019, 07:58 PM
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One other thing, you said it starts without the choke. That may be an indication that the choke circuit really isn't closed all the way. If everything is all square, you should need at least a little bit of choke to start it.
 
  #6  
Old 03-12-2019, 09:19 PM
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Sorry, I was a bit ambiguous....
I meant, it starts with choke, warms, and idles without the choke.
I'm in the process of trying more turns on the mixture screw.

One additional piece of history... the bike sat for approx. 15 years (was running perfect when it went into 'hibernation', but gas wasn't drained). It seems like whatever is wrong, is wrong with all 4 carbs, as it doesn't want to run at all with throttle. (I'd expect it to run, if not great, if it was just 1 or 2 'bad' carbs.)

Only thing I don't know how to test (or if it could be related to the issue) is the air valve on the side of each carb.

Thanks for your suggestions!
 
  #7  
Old 03-12-2019, 10:14 PM
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How is your fuel supply to the carbs?
Are your floats set correctly?
 
  #8  
Old 03-12-2019, 10:54 PM
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Just tried with 3 turns out on the mixture screw. Same result.. stumbles with any throttle. Just for fun, I tried 1 1/2 turns out with the same result. Also, it will no longer idle without choke. Perhaps I'll just keep trying more turns until *something* changes.

The floats and float levels *look* fine..and even across all 4 carbs. ( I have not measured them.) I drain the bowls before opening them, and they appear full. As it's idling, I see the fuel line going to my aux tank "pulse" about once every 3-5 seconds, which shows the fuel pump responding to demand. Fuel hose to the carbs is always full when I pull them off. Fuel supply/feed *appears* to be working.

But the least bit of throttle kills in almost instantly. As I turn up the idle set screw, RPMS decrease (and it begins to stumble).

Don't know if this is significant, but for a moment after it first starts, it revs up a bit (to maybe 2k or so), then immediately settles to a slow (barely stable) idle (with choke). It sure feels like it's not getting fuel above idle, but I can't for the life of me figure out why.
 
  #9  
Old 03-12-2019, 11:29 PM
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Seems like the only thing it could be is the slow circuit is still clogged somewhere (despite my seemingly thorough cleaning). When the butterflies are cracked open, I don't think any extra fuel is delivered. It's not the pilot jets (they are clear). And now it won't run off choke (so only fuel it's getting is from fuel enrichment circuit.) Short of any flashes of inspiration, I guess it's time to dissect them again.
 
  #10  
Old 03-13-2019, 05:22 AM
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Is your bike equipped with a throttle position sensor?
 

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