1990 CBR1000F - Kerker 4:1 exhaust. Re-jetted carbs. Bike starts, idles and revs clean and runs extremely strong - Except when I run slow and easy it stuuters at around 2000 rpm as I slowly accelerate. If I accelerate more briskly it is never felt. Any ideas what is the cause?

 

These bikes don't like pulling below 3000 revs, and with a pipe on it you may have accentuated a normal issue -a slight flat spot may be caused by dirty jets. I've been having similar problems and a 2x concentration of injector cleaner has made quite a difference. You won't feel the stutter when pulling hard because the transition from idle jets to main jets happens that much faster.
FWIW.

 

Thanks - sounds like a job for some Techron.

 

Mine, both actually for that matter, are kinda stuttery and flat around that mark as
well Jim but like Shadow says under 3k they tend to be a bit lazy

hope the techron helps a bit

 

I think, ethanol strikes again. These carbureted bikes hate the stuff. Mine at times acts like a 2-stroke loading up and then takes off. I depends upon the pump and and the quality of the fuel.

 

Adding Sea FoamŽ Motor Treatment to E-85 fuel, the recommendation is 2 oz. Sea FoamŽ per gallon.

Sea FoamŽ has been used in Ethanol fuels (10%, 15% and most recently E-85) successfully since their initial introduction into the marketplace during the 1990’s. As these “New” Ethanol fuels become more widely available nationwide, questions have arisen about the use of Sea FoamŽ Motor Treatment in Ethanol fuels.
Sea FoamŽ Motor Treatment has been performing its engine component cleaning function in gasoline and diesel engines since 1940, and now it performs this same function just as well in Ethanol-blended fuels. An average one ounce Sea FoamŽ per gallon of Fuel is recommended in both gasoline and in 10% & 15% Ethanol blends.
Ethanol fuels differ from standard gasoline in a few aspects, presenting potential issues that can be addressed through the use of Sea FoamŽ Motor Treatment.

1. Ethanol tends to strip engine oil from cylinder walls and can be harsh on exposed surfaces & components that require lubrication. Sea FoamŽ is oil, adding lubrication to upper cylinders that will prolong the life of your engine.
2. Ethanol absorbs moisture which needs to be removed from the Ethanol. Sea FoamŽ dries moisture from fuel and helps prevent phase separation.
3. Ethanol blended fuel leaves deposits which periodically need to be cleaned. Sea FoamŽ is a fuel residue and carbon cleaner.
4. Ethanol deteriorates as fast as non-ethanol fuel, the need to stabilize remains. Sea FoamŽ is the best fuel stabilizer around.

You can get it at most automotive retail chains ...oh yeah and Walmart too

 

Also, fuel at such a low RPM is still controlled by the mixture screws. I would try turning each pilot screw in or out a quarter turn to see if the idle speed increases. You want the highest idle (go by sound instead of the tach). And yes, the two inner carb screws are impossible to get to.

 

Shadow's advice is good, Seafoam or some such in the gas may help. I assume you synchronized your carbs after working on them, but if not you might do so.

 

Yep , that seafoam is the best thing I've ever put in my fuel tank since gasoline LOL!! My son's Honda XR80R dirt-bike was spewing gas out the flood overflow tube from sitting 2 1/2 weeks and I removed the fuel line and filled the line with seafoam ,, started the bike for 10 seconds to work it into the float bowl ,, re-attached the fuel line & let it set for 30 mins ,, fired it up ,,and it runs great again . (No more fuel flooding / leak) I bought my 93 CBR1000F from a guy that let it sit for close to a year ,, and it ran like crap ,, seafoam in the carbs,,sat for an hour,,and wwaaa lllaaa ,, its been running great ever since . I put one oz a gallon every other tank full to keep it clean and running good. It's worth every penny I spent on it.
I stand by seafoam , really amazing stuff.
I had a stutter problem about the same as you described ,, I found #3 carb was running a bit richer than the rest of the carbs ,, sync'ed it in ,no more stutter .
Checking to see if the carbs are in sync would be a plus ,, wouldn't hurt ,, and its an easy thing to do yourself . Check out Naga_Thai's HOW TO - CARB SYNC / BALANCE - MANY PICTURES

 

PRoblem seems to be solved.

I put a half bottle of Techron into a half tank of fuel and ran it for around 30+ miles. Stuttering improved almost immediately. I did syn the carbs before, installed new spark plugs and had run Techron before, but this time it seemed to work. As it sits the carbs are now soaking in a arich concnetraton of the Techron which should continue to disove any unwanteds in the carbs.

I like the idea of Seafoam as an upper cylinder lubricant and fuel tretment so I give that a shot as well. I just ordered a stock type air cleaner to replace the K&N, that may richen up the fuel/air mix and further improve the throttle response.

This is my 31st. bike and by far the most powerful and most enjoyable ride. I ordered a Zero Gravity Sport Tour windscreen (will report on how it works when I get it) and would like the handle bars a bit higher other than my right hand getting tingly it the best two wheel ride for the money.