Poor Throttle Response, Its Complicated ;-)

fjahandari · 08-22-2008, 3:33 PM

Ok help me out here.

I have a 97 cbr1100xx (carbs not FI). Its mostly stock except basic induction/exhaust mods including Factory-Pro Stage I Jetkit, Factory-Pro Ignition advancer along with a set of Jardine Ti slip-ons. I’m using OEM Honda airfilter (I expect to change it to OMG airfilter next week). Bike has 22,000 miles on it and generally in good shape, adult kept, took granny to church on Sundays… and all that good stuff.

So here is the problem I was having BEFORE I started the carb clean and synch:
I have been having this irregular performance problem. When started w/choke she comes right up and after a few seconds I can roll it off and it will idle pretty well thereafter. However, when I get on the bike and roll the throttle (w/choke off of course), it can feel a bit timid on the throttle with an odd bike-body/frame/seat of the pants vibration that “feels like” cylinder/s are not all running at same rpm. Throttle response is slow and the bike dumps a bit; it is just not crisp. When, I start moving and accelerating I feel this inconsistency through say 4000 rpm, after which it is not as noticeable if any. I feel it particularly prominent when I’m driving in slow-mid speed traffic around 3500-4000 rpm.

So I tore the carbs and all to do a full cleanout and synch service. I removed the gas tank and air filter. Before removal of the carbs I used an Auxiliary fuel tank and ran the bike in my garage. I noticed Cylinder No. 3 does some weird things like make off clunking noises every once in a while. At the time, the No. 3 carb slide was also not moving up and down with the other three; and I’d get sprouts of air/gas mist coming out or a few small bangs and mini-explosions from immediately below the carb no. 3 engine intake. There were also clunking sounds in the motor that were very unusual. However, these all only happened with the tank off, before I had cleaned the carbs or completed any synchronizing. I also looked inside the tank and it is perfectly clean as is the fuel filter.

So, I continued and removed the carbs, disassembled them, cleaned them with carb cleaner (nothing was noticeably dirty at all!). I put the carbs back together and continued by synchronizing all with each other. They were not synched according to Honda specs to be 20mm of hg; either way, this matter is relatively miniscule in my opinion. I put fresh gas in the tank, put everything together checked all fuel and vacuum lines, started it up, let it run to operating temp and set the idle to around 1100-1200 rpm. A note - the bike has not been dyno tested as a baseline; and I don’t have easy access to a local dyno to test anything.

After all this work was completed I went for a ride and things were darned good next to perfect. Bike ran smooth, no problems all over; very smooth and linear throttle response. I said yaay… we’re ok.

Less than 10 hours later, I started the bike to commute to work. Choke start was normal. I rolled off the choke, perhaps a tad early (its 85F ambient temp outdoors) and immediately it was sputtering and not working right at all. I pulled over, pulled the choke and let it run for 15-20 seconds, turned it off and started to ride. Well, I’m getting similar vibes in the lower RPM range, and a sense that I’m not getting smooth and consistent fuel/air flow to all cylinders. Also, throttle response is not as crisp and liners as it should be. So, wise ones… what do you say? Let me guess:
1- Do a few dyno runs to be sure it’s not “all in my head”
2- Completed a compression test on all cylinders 1st then do Dyno run
3- Get a kayak
-->FJ

XXNUTZ · 08-22-2008, 5:58 PM

You might want to check to see if you getting fuel to the carbs. I had a similar problem, the petcock diphragm was split and intermittently cut off fuel flow to the carbs. I put a new assembly on from Honda and no more problems. Just a thought here. Cheers.