Hi,
I’ve got a problem on my bike which I hope someone else has encountered as well.

When I start it up from cold it’s fine I don’t even have to use the choke or turn the accelerator. It’s got nice even power and everything.

The problem starts when it gets hot It’s got a flat spot when you pull away from a traffic light. If you give it a couple of revs before you pull away it seems to be a bit better. I’ve also found that it has much more power when it’s a cold day then when you run it hot in the afternoons. It never over heats cause I watch the temp gauge closely and the fan rarely comes on.
If you start it when it’s hot you also have to ref it to get it going but like I said it starts perfectly when it’s cold. Another thing is that it starts running unevenly if you idle for a while like at a traffic light.
I had to set the idling a bit faster so it doesn’t stall. It’s very strange know because it’s idling much faster when it’s cold then when it’s hot where usually it’s suppose to be the opposite.
This is a very strange to me because an engine is supposed to run better when it’s hot.
Does any one know what could be the problem? (I don’t want to strip those carbs again unless I know what I’m looking for)

 

i havent had that problem but it sounds like its running to rich.

 

The motor is actually supposed to run better cold, That's why they make cold air intakes for cars. It's not unusual to have more power on a cold day. I know there is a noticeable difference in the power on my 929 on a cold day. I agree with Junior, It seems to be running a little rich. I would get the carbs checked out then once you get them straightened out I would change the plugs. If the bike runs rich it may be fouling out the plugs. That's about all I can think of. The carbs and fouled plugs could be causing all the problems you are having.

 

It makes sense cause over the weekend we drove about 500miles. One of my friends has a GSXR600. Our average speed was about 100mph. My fuel consumption was 28 miles/gallon! (12km/litre) His was only about 40-42. 170km and I was on reserve.200 km and I was empty. Fortunately someone stopped and I coul get some fuel from the bikes on his trailer.
I spoke to another person I know with a 96’ F3 and he usually gets about 42 – 47.
I’m very lucky if I can get it to do 39 and that’s if I never drive it above 4000rpm.
I did give it a service recently and changed the sparkplugs, oil, oil filter.

I think I’m going to take it for a dyno tune and see if they can’t sort it out.

 

You should need the chock to start it for sure....maybe not for long though. But an off idle stumble...its rich on fuel for sure.

There's only four things to check...
fuel/air screw
pilot jet
main jet
needle (look for shims under it)

I don't have a manual handy, but the settings/numbers will all be in there for stock. If you have a pipe or a jet kit thats a different story. Jet kits are no good for the street as they tend to load the engine up....they are really meant for power on the track. A good rejetting job will do just as well.

In stock trim f2 got 55mpg on the highway when cruising at 70mph. My f3 is a track bike only, but my zx6r on the street (with a pipe) does 40-53 depending on speeds. On the track my zx6r gets 27mpg....since the F3 doesn't have a speedo hooked up I have no idea.

 

Did you by the bike used? I ask you this because a previous owner could have put in a jet kit or the ghetto way... enlarge the jets by drilling them out. I agree with what everyone else is saying that it appears to be running rich. Especially after you said what kind kind of mileage your getting. I drove my 97 F3 cross country and I was getting between 55 and 58 MPH. Granted, I never went over 75... but you should be getting way better mileage than what you are currently getting. The only mods I have done to my bike are a slip on pipe and a K&N air filter. I would suspect that the bike is running rich also.

 

Believe it or not the bike was actually worse then this. When I opened the carbs all I could find was that the main jets were in there was the sizes used for the 97/98’ models. I replaced them with the bit smaller ones specified in the manual for the 95/96’ models since mine is a 95’. Didn’t make much of a difference. I think I got about 10 miles more on a tank.

I didn’t even know you can adjust the needles. How do you get them out of the slides?
How would I know if they were adjusted or tampered with because they don’t say much in the manual?
My previous bike had little clips with notches so you could adjust them and the needles were loose. On the F3 it seems that the needles are fixed to the slides?

 

Does your bike have a slip on or a full pipe at all...?

If dropping main jets only gained you 10 miles on the tank I'd look at the needle and pilot jet next. The air screw is also easy to check....if the cap over it was never removed, don't worry about it.

The pilot jet is next to the main, but is recessed in a hole. They can be tight. Its takes a small flat head screw driver....use the biggest one you can fit in the hole to give the best chance of getting the jet out without damaging it. Most jet kits don't touch this part....Even just rejetting you can usually get really close leaving it alone. At most, you might move up a size.

On to the needle....if a needle had grooves, its part of a "jet kit". Jet kit's use their own main jets (and slide springs most times) and the numbers on them don't jive with Keihin/Mikuni jets sizes (and each of those is different too). Some carbs hold the needle tighter than others.

The needle area takes a little time....and patience...some little parts. Work on one carb at a time. Remove the black cap...I turn them upside down on a bench to use to hold the parts. Take the spring out of the way and set in the cap. In the center of the slide you'll see a post sticking up with a threaded hole in the middle. Use one of the screws from the cap to thread into it. Carefully wiggle the post while you pull on it....don't go crazy, there can be little washers and parts are in there. The post will pull out and you want to set that in the cap. Take the slide out of the carb and tip upside down into your hand to catch the needle and all its parts.

If the needle has washers under the head (or clip), set them aside...I'm betting we won't need them. If the needle has grooves (probably 5 or 6), move the clip to a groove closer to the end....maybe the first or second groove....if you don't have a pipe the first groove for sure. If the needle has a fixed head on it, its the original factory part. If there are washers under the head of the needle (or under the clip)....these are called shims. Shimming a needle is a perfectly acceptable way to adjust a carb, but often times people use a washer from the hardware store and they are too thick,sloppy and inconsistant. .020"-.045" of shimming is plenty even on a modded motor and a stock needle.

To reassemble, sometimes the rubber diaphrams swell and need to dry before you can get the edge to sit in the groove so you can put the cap on....leaving them sit for an hour or two will be usually solve it. The perimeter of the diaphram must set in its groove or the slide won't lift.

JB

 

In reply to your question about adjusting the needles, they can be adjusted by shimming. I was having issues with my bike running too lean so I put in shims under the fixed base of the needle to resolve the issue without buying a jet kit. I took out one of the shims from the needles and found a very thin/small shim that was the same as what I had. Just to be sure, I also fitted it over the needle to ensure there was no slop. This essentially does the same thing that the e-clip method does... just a few more pieces. I just put in enough shims for my desired height. I did this years ago, but to my recollection there is only one shim from the factory.

 

Hi, thanks for all the advice. Thus morning it was a bit chilly and overcast. My bike was driving like a breeze no flat spots and lots of power.(wish the weather is like this all the time then I won’t have any troubles with the bike)

Anyway… My bike has a carbon fiber slip on. It’s not a brand I know. Probably came with it from Japan but it sounds excellent. Yes the air screws don’t have there caps on I think they are at 2.5 turns and if I remember correctly I did check the pilot jets and they were standard.The exhaust is burning very dark so I think you’re right it is to rich. I can take of the carbs and check the needles but if you don’t know what is the correct setting you’re just messing around. I usually do services myself and don’t like other people working on my vehicles but I don’t feel like taking of those carbs (again!) One of my friends referred me to a shop that does dino tune-ups on bikes and apparently they are quite good and they have the factory specs on all the bikes. Everybody I spoke to are very happy with them. I’ll close my eyes (open my wallet!) and let them setup the carbs for me. At least they have gas analyzers and stuff. Then I can have peace of mind that it’s setup properly and it won’t burn valves cause if you look closely thru this exhaust of mine you can probably see the pistons cause it goes right thru

But I’m sure your advise will come in handy next time I service the bike cause I can’t afford to dino tune it every time. Any comments or suggestions are still welcome.Thanks