I have a 2003 F4i with a KN filter, two bros carbon fiber exhaust, and a little over 8k miles.
I picked it up a couple months ago pretty cheap. Here are my issues:

it is getting 28mpg and wants to stall when taking off slowly as well as some stumbling and stuttering. i have replaced the the plugs and one of the coils cuz it was cracked. It helped a little with the surging and hesitaition at low rpm roll on but still gets horrible mileage and some popping from the tail pipe. i had a shop look at it and they said the airbox was cracked and probable cause. it will be replaced shortly. i am also running injector cleaner through it currently.

should i break down and take it to the dealer?

can the airbox cause this?

is it running lean because of the mods?

should i pawn it off on the next guy?

any body?

 

Lets look at the basic maintenance.....last oil change? what type of oil was used? What type of fuel is being used? What is the octane rating of it? Basics of a bike, spark, fuel power.

A bike with only 8,000 miles shouldn't be running that bad. I have 16,000 miles on it with regular oil changes and good fuel and proper maintenance it runs strong as ever.

 

Did you check for insulation stuffedin the air box?
https://cbrforum.com/m_670505/mpage_1/key_/tm.htm

JUST KIDDING..
Check if its the fuel pressure regulator.. can you also smell gas?
https://cbrforum.com/m_231792/tm.htm

 

If your bike is running lean you would be getting fantastic gas mileage, and backfiring especially on decel can indicate unburnt fuel on the exhaust stroke. therefore it appears your bike is very rich. Sounds like a tuning issue or a sensor issue, I would have a dealer check the crank position sensor or other related sensors. and see how the fuel system is setup. if you are not an experienced troubleshooter/mechanic this issue is best handled by a certified honda mechanic, and the repair may be an easy fix that may not cost very much, be prepared to pay a diag fee upon drop off.

 

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could also be the FPR.

 

The owners manual shows the last oil change was performed at 48xx miles by a dealership.
Before i did the plugs it smelled really rich and would puff faint black smoke. Now it doesn't smoke...

It start up easily and idles smooth. If I take off pretty quickly, if feel ok with only a few sputters throughout the rpm range. When trying to take off slowly, or cruise at a low rpm (2k or so) it isn't very happy. The plugs i removed were pretty dirty but not dead. Since it will bog and sputter I am guessing it is running lean? I filled up this morning with 93 octane (OM says 87 OK) and half a bottle of injector and fuel system cleaner, went for a 100 mile ride till the fuel light came on, added 3.7 gallons of 93 octane and the other half of the cleaner bottle and will put some more miles on tomorrow.

The guy at the shop said the cracked air box is most likely the culprit. I ordered one from Ebay, it is at the shop, but havent had time to get by there.

Could the KN and the exhaust cause this since the ECM doesn't correct for O2? Buying this used bike has left a sour taste in my mouth......I just want it to run right.

I had the forks lowered 1.5 inches and had to order the link to lower the rear 1.5 inches (I have a short wife who really want to ride) My plans are to get the bugs worked out, get the wife comfortable on it and get myself a bike later on.

What else, and thanks again for the input.

 

Oh, is there a test port where i can check the fuel pressure? Do i need a special test setup? Like a car, i see the regulator is vaccuum modulated. I am assuming the pressure increases with loss of vaccuum? The manual says 50 psi at idle. Should I take her to the dealer and take it up the A$$ but hopefully it will be resolved?

 

It isn't popping on decel, just during steady state cruising and low rpm roll on. I can ring out 1st and 2nd gear and maybe feel/hear a little something along the way but not very noticable.

 

hmm... 28MPG is not a 'lean' mixture. Just want to make that part clear

 

With your new information given, the fuel pressure regulator (FPR) may be the issue, typically when the regulator goes bad, it doesn't keep the pressure constant. check for a crack in the regulator housing itself, this issue is actually very common on other makes of bikes (won't mention) while your into the fuel system it may be a good idea to clean or replace your fuel filter. good luck! (therefore I concur with FLYOU)