Strange Idle CBR900
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Strange Idle CBR900
Hi all,
Im new here, and I have a 1995 (SC28) CBR900
It has been standing still for a while so replaced all fluid and filters, spark plugs etc etc. Brought the carbs to the local dealer to clean them.
Bike runs not bad...but it doesn't run optimal. It is a 100% stock bike (so no aftermarket exhaust).
The problem is that it doesn't idle very good.
If I set the idle RPM to +/- 1500. It runs fine until I touch the throttle. If I rev it to (for example 8k RPM). The bike drops back normally in RPM until +/- 3K rpm. From there it takes a while before it drops back to 1,5K rpm.
My first guess was the carbs or airbox wasn't connected properly. So I checked that, and everything was in good condition (no air leak).
My second guess was the bike runs lean. So I used the air screws/pilot jets and turned them a little bit out (half turn). It seems to run a little bit better. So I gave them another half turn out. But the bike runs crap at idle again.
Anyone got an idea what the problem is?
Im new here, and I have a 1995 (SC28) CBR900
It has been standing still for a while so replaced all fluid and filters, spark plugs etc etc. Brought the carbs to the local dealer to clean them.
Bike runs not bad...but it doesn't run optimal. It is a 100% stock bike (so no aftermarket exhaust).
The problem is that it doesn't idle very good.
If I set the idle RPM to +/- 1500. It runs fine until I touch the throttle. If I rev it to (for example 8k RPM). The bike drops back normally in RPM until +/- 3K rpm. From there it takes a while before it drops back to 1,5K rpm.
My first guess was the carbs or airbox wasn't connected properly. So I checked that, and everything was in good condition (no air leak).
My second guess was the bike runs lean. So I used the air screws/pilot jets and turned them a little bit out (half turn). It seems to run a little bit better. So I gave them another half turn out. But the bike runs crap at idle again.
Anyone got an idea what the problem is?
if the bike has been sitting
your choke needles can be sticking in the open position or you might actually need to re-sync the carbs.
how many miles are on your blade??? where do live?? is it moist there? do you live in a higher elevation.. has the bike always lived there?
sounds funny asking some of these questions but belive it or not it makes a big difference..
how many years have you owend it, when was the last time it ran good? try to put as much info you can.. my everyday rider is a 95 900 so i know the bike well..
jay
also turn your air/fuel mixture screws back to where you found them, they're set there for exactly the stock setup you have.. if your getting to much air into the carbs or getting too much fuel, thats not the way to fix it.. if you have a carb float thats sticking and then gets unstuck you'll never know and you'll be chasing your tail for weeks. "i know from experience". when the dealer cleans carbs they dont always do such a great job.. so long as it starts most of them are happy with that and summitting a bill..
your choke needles can be sticking in the open position or you might actually need to re-sync the carbs.
how many miles are on your blade??? where do live?? is it moist there? do you live in a higher elevation.. has the bike always lived there?
sounds funny asking some of these questions but belive it or not it makes a big difference..
how many years have you owend it, when was the last time it ran good? try to put as much info you can.. my everyday rider is a 95 900 so i know the bike well..
jay
also turn your air/fuel mixture screws back to where you found them, they're set there for exactly the stock setup you have.. if your getting to much air into the carbs or getting too much fuel, thats not the way to fix it.. if you have a carb float thats sticking and then gets unstuck you'll never know and you'll be chasing your tail for weeks. "i know from experience". when the dealer cleans carbs they dont always do such a great job.. so long as it starts most of them are happy with that and summitting a bill..
Last edited by jasonchristopher; Sep 25, 2013 at 12:31 PM.
If you were to put new jets in,
These are the instructions that you would have to gauge carburator troubles.
1. Top end (full throttle / 6k to redline -
Best Main Jet must be selected before starting step 2 (needle height)!
Select Best Main Jet
To get the best, most even top end power (full throttle/after 5000 rpm), select the main jet that produces the hardest pull at high rpm.
If the bike pulls harder at high rpm when cold and less hard when fully warmed up, the main jet is too large. Install a smaller main jet and retest until you find the main jet that pulls the hardest at high rpm when fully warmed up. This must be done first - before moving on to the other tuning ranges.
If the bike doesn't pull well at high rpm when cold and gets only slightly better when fully warmed up, the main jet is too small.
In order to properly tune the midrange and low rpm carburetion, THE MAIN JET MUST FIRST BE PROPERLY SELECTED after 10 to 15 minutes of hard use!
Do not pay too much attention to the low-end richness when you are changing main jets - you still need to be using the main jets that produce the best power at high rpm. You will deal with the low-end / cruise later - after step 2.
2. Midrange (full throttle /4k-6k)
Step 1 (Best Main Jet) must be selected before starting step 2!
Select best needle position
To get the best power at full throttle / 4k-6k 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/4k-6k 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 4k-6k, try raising the needle to richen 4k-6k.
If the engine pulls equally well between 4k-6k 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 positions - you still need to be using the position that produces the best full throttle / 4k-6k 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.
3. Low end (full throttle / 2k-3k)
Step 1 (Best Main Jet) and Step 2 (needle height) must be selected before starting step 3!
Float height (AKA fuel level & How to..)
To get best low-end power, set float height (fuel level) so that the engine will accept full throttle, without missing or stumbling, in 2nd gear from 2.5k to 3k rpm at minimum.
Float heights, unless otherwise specified in the installation guide, are measured from the "gasket surface" of the carb body to the highest part of the top of the float - with the float tang touching but not compressing the float valve spring.
If the engine has a "wet" rhythmic, soggy area at full throttle / 3k-4k rpm, that gets worse as the engine heats up, lower the fuel level by resetting the float height 1mm greater (if the original was 13mm - go to 14mm). This will lower the fuel level, making full throttle / 2k-3k rpm leaner.
If the engine is "dry" and flat between 2k to 3k rpm, raise the fuel level.
Example: change float height from 15mm to 14mm to richen up that area.
REMEMBER, since the main jet WILL affect low speed operation, the MAIN JET has to be within 1 or 2 sizes of correct before final float setting.
Warning: If the engine is left with the fuel level too high,, the engine may foul plugs on the street and will be "soft" and boggy at part throttle operation. Adjust Floats to raise/ lower the Fuel Level.
Base settings are usually given if a particular application has a history of fuel level criticalness. The Fuel level height in the float bowl affects full throttle/low rpm and, also, richness or leanness at cruise/low rpm.
Reference: a bike that runs cleanly at small throttle openings when cold, but starts to show signs of richness as it heats up to full operating temperature, will usually be leaned out enough to be correct if the fuel level is LOWERED 1mm. Check out and RESET all: Needless to say, FUEL LEVEL IS EXTREMELY IMPORTANT!!!
If there are low-end richness problems, even after lowering the fuel level much more than 1.5mm from our initial settings, check for needle wear and needle jet (part of the emulsion tube). Check them for "oblong" wear - the needle jet orifice starts out round!
4. Idle and low rpm cruise
Fuel Screw setting (AKA mixture screws)
There is usually a machined brass or aluminum cap over the fuel screws on all but newer Honda. It's about the diameter of a pencil. Newer Honda carbs have no caps, but use a special "D" shaped driver.
Set for smoothest idle and 2nd gear, 4k rpm, steady state cruise operation. Set mixture screws at recommended settings, as a starting point. For smoothest idle, 2nd gear 4000 rpm steady state cruise , and 1/8 throttle high rpm operation.
Pilot fuel mixture screw settings, float level (but, you've "fixed" the fuel level in Step 3 - which you have already done!)
If lean surging is encountered, richen mixture screws (turn out for fuel mixture screw, turn in for air mixture screw) in 1/2 turn increments.
Pilot fuel mixture screw settings, float level and pilot jet size also affect high-rpm, 0 to 1/8 throttle maneuvers. Too lean, will cause surging problems when the engine is operated at high rpm at small throttle openings! Opening the mixture screws will usually cure the problem.
NOTE: A rich problem gets worse as the engine heats up.
If the throttle is lightly "blipped" at idle, and the rpm drops below the set idle speed, then rises up to the set idle speed, the low speed mixture screws are probably set too rich: try 1/2 turn (turn in for fuel mixture screw, turn out for air mixture screw), to lean the idle mixture.
NOTE: A lean problem gets better as the engine heats up.
If the throttle is lightly "blipped" at idle, and the rpm "hangs up" before dropping to the set idle speed, and there are no intake leaks and the idle speed is set at less than 1000 rpm, the mixture screws are probably too lean: try 1/2 turn (turn out for fuel mixture screw, turn in for air mixture screw), to richen mixture. Be sure there are no intake leaks and the idle speed is set at less than 1000 rpm!
jay
These are the instructions that you would have to gauge carburator troubles.
1. Top end (full throttle / 6k to redline -
Best Main Jet must be selected before starting step 2 (needle height)!
Select Best Main Jet
To get the best, most even top end power (full throttle/after 5000 rpm), select the main jet that produces the hardest pull at high rpm.
If the bike pulls harder at high rpm when cold and less hard when fully warmed up, the main jet is too large. Install a smaller main jet and retest until you find the main jet that pulls the hardest at high rpm when fully warmed up. This must be done first - before moving on to the other tuning ranges.
If the bike doesn't pull well at high rpm when cold and gets only slightly better when fully warmed up, the main jet is too small.
In order to properly tune the midrange and low rpm carburetion, THE MAIN JET MUST FIRST BE PROPERLY SELECTED after 10 to 15 minutes of hard use!
Do not pay too much attention to the low-end richness when you are changing main jets - you still need to be using the main jets that produce the best power at high rpm. You will deal with the low-end / cruise later - after step 2.
2. Midrange (full throttle /4k-6k)
Step 1 (Best Main Jet) must be selected before starting step 2!
Select best needle position
To get the best power at full throttle / 4k-6k 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/4k-6k 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 4k-6k, try raising the needle to richen 4k-6k.
If the engine pulls equally well between 4k-6k 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 positions - you still need to be using the position that produces the best full throttle / 4k-6k 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.
3. Low end (full throttle / 2k-3k)
Step 1 (Best Main Jet) and Step 2 (needle height) must be selected before starting step 3!
Float height (AKA fuel level & How to..)
To get best low-end power, set float height (fuel level) so that the engine will accept full throttle, without missing or stumbling, in 2nd gear from 2.5k to 3k rpm at minimum.
Float heights, unless otherwise specified in the installation guide, are measured from the "gasket surface" of the carb body to the highest part of the top of the float - with the float tang touching but not compressing the float valve spring.
If the engine has a "wet" rhythmic, soggy area at full throttle / 3k-4k rpm, that gets worse as the engine heats up, lower the fuel level by resetting the float height 1mm greater (if the original was 13mm - go to 14mm). This will lower the fuel level, making full throttle / 2k-3k rpm leaner.
If the engine is "dry" and flat between 2k to 3k rpm, raise the fuel level.
Example: change float height from 15mm to 14mm to richen up that area.
REMEMBER, since the main jet WILL affect low speed operation, the MAIN JET has to be within 1 or 2 sizes of correct before final float setting.
Warning: If the engine is left with the fuel level too high,, the engine may foul plugs on the street and will be "soft" and boggy at part throttle operation. Adjust Floats to raise/ lower the Fuel Level.
Base settings are usually given if a particular application has a history of fuel level criticalness. The Fuel level height in the float bowl affects full throttle/low rpm and, also, richness or leanness at cruise/low rpm.
Reference: a bike that runs cleanly at small throttle openings when cold, but starts to show signs of richness as it heats up to full operating temperature, will usually be leaned out enough to be correct if the fuel level is LOWERED 1mm. Check out and RESET all: Needless to say, FUEL LEVEL IS EXTREMELY IMPORTANT!!!
If there are low-end richness problems, even after lowering the fuel level much more than 1.5mm from our initial settings, check for needle wear and needle jet (part of the emulsion tube). Check them for "oblong" wear - the needle jet orifice starts out round!
4. Idle and low rpm cruise
Fuel Screw setting (AKA mixture screws)
There is usually a machined brass or aluminum cap over the fuel screws on all but newer Honda. It's about the diameter of a pencil. Newer Honda carbs have no caps, but use a special "D" shaped driver.
Set for smoothest idle and 2nd gear, 4k rpm, steady state cruise operation. Set mixture screws at recommended settings, as a starting point. For smoothest idle, 2nd gear 4000 rpm steady state cruise , and 1/8 throttle high rpm operation.
Pilot fuel mixture screw settings, float level (but, you've "fixed" the fuel level in Step 3 - which you have already done!)
If lean surging is encountered, richen mixture screws (turn out for fuel mixture screw, turn in for air mixture screw) in 1/2 turn increments.
Pilot fuel mixture screw settings, float level and pilot jet size also affect high-rpm, 0 to 1/8 throttle maneuvers. Too lean, will cause surging problems when the engine is operated at high rpm at small throttle openings! Opening the mixture screws will usually cure the problem.
NOTE: A rich problem gets worse as the engine heats up.
If the throttle is lightly "blipped" at idle, and the rpm drops below the set idle speed, then rises up to the set idle speed, the low speed mixture screws are probably set too rich: try 1/2 turn (turn in for fuel mixture screw, turn out for air mixture screw), to lean the idle mixture.
NOTE: A lean problem gets better as the engine heats up.
If the throttle is lightly "blipped" at idle, and the rpm "hangs up" before dropping to the set idle speed, and there are no intake leaks and the idle speed is set at less than 1000 rpm, the mixture screws are probably too lean: try 1/2 turn (turn out for fuel mixture screw, turn in for air mixture screw), to richen mixture. Be sure there are no intake leaks and the idle speed is set at less than 1000 rpm!
jay
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Thanks for all the replies so far 
Nope throttle cables are fine.
Bike has run 18.000 KM +/- 11184 miles
I live in the Netherlands, it is not very moist.
Yes the bike is bought new in 1995 here in the Netherlands
It ran good about 7 years or more ago.
Local dealer who cleaned the carbs checked the parts. And turned them back to OEM/stock settings.
Nope
throttle cables are fine.if the bike has been sitting
your choke needles can be sticking in the open position or you might actually need to re-sync the carbs.
how many miles are on your blade??? where do live?? is it moist there? do you live in a higher elevation.. has the bike always lived there?
sounds funny asking some of these questions but belive it or not it makes a big difference..
how many years have you owend it, when was the last time it ran good? try to put as much info you can.. my everyday rider is a 95 900 so i know the bike well..
jay
also turn your air/fuel mixture screws back to where you found them, they're set there for exactly the stock setup you have.. if your getting to much air into the carbs or getting too much fuel, thats not the way to fix it.. if you have a carb float thats sticking and then gets unstuck you'll never know and you'll be chasing your tail for weeks. "i know from experience". when the dealer cleans carbs they dont always do such a great job.. so long as it starts most of them are happy with that and summitting a bill..
your choke needles can be sticking in the open position or you might actually need to re-sync the carbs.
how many miles are on your blade??? where do live?? is it moist there? do you live in a higher elevation.. has the bike always lived there?
sounds funny asking some of these questions but belive it or not it makes a big difference..
how many years have you owend it, when was the last time it ran good? try to put as much info you can.. my everyday rider is a 95 900 so i know the bike well..
jay
also turn your air/fuel mixture screws back to where you found them, they're set there for exactly the stock setup you have.. if your getting to much air into the carbs or getting too much fuel, thats not the way to fix it.. if you have a carb float thats sticking and then gets unstuck you'll never know and you'll be chasing your tail for weeks. "i know from experience". when the dealer cleans carbs they dont always do such a great job.. so long as it starts most of them are happy with that and summitting a bill..
I live in the Netherlands, it is not very moist.
Yes the bike is bought new in 1995 here in the Netherlands
It ran good about 7 years or more ago.
Local dealer who cleaned the carbs checked the parts. And turned them back to OEM/stock settings.
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ok, right off the top of my head..
i have a 95 with 13,000 miles and i just had similar troubles and thats why i provided such quick info.
anyway im glad you gave me the rest of that info, if your bike was jeted and piped at one point and then you turned everything back to stock and the bike is past the ten thousand mark then you might need to reshim the valves first.. you have to understand the bike was breathing one way for a long time and now shes been brought back to stock these blades are funny like that.. anyway until you chech your vavle shims i wouldnt look to dial the carbs in until you know for sure their right..
i have a 95 with 13,000 miles and i just had similar troubles and thats why i provided such quick info.
anyway im glad you gave me the rest of that info, if your bike was jeted and piped at one point and then you turned everything back to stock and the bike is past the ten thousand mark then you might need to reshim the valves first.. you have to understand the bike was breathing one way for a long time and now shes been brought back to stock these blades are funny like that.. anyway until you chech your vavle shims i wouldnt look to dial the carbs in until you know for sure their right..
oh and i think the plugs look a tad rich for just being put in..
just my opin though..
again that could be a symptom of the bike not breathing right through the valve side of things.. thats why they usually do a re shim check i believe around 16,000 miles.. just a guess though , might be sooner and if the bike was running a certain way for so many years and then switched back to stock , i think that would be a good place start,,,,
just my opin though..
again that could be a symptom of the bike not breathing right through the valve side of things.. thats why they usually do a re shim check i believe around 16,000 miles.. just a guess though , might be sooner and if the bike was running a certain way for so many years and then switched back to stock , i think that would be a good place start,,,,
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