CB400 fuel issues

[chricol]

Hi guys.. I have been reading this forum for months but only now have i made an account and this is my very first post. My name is Chris, I'm 23 and I live in Malta. Pls excuse my English as I know it is far from perfect.

I ride a CB400 Version S which i have had since january and it's my very first bike. I have been riding it almost daily to work (15km round trip) and it runs ok and starts without any hassle in the mornings. However I think that it is running rich due to certain observations I have made.

High Fuel Consumption - 6.6l/100km (35mpg)
Strong smell of rich exhaust (like the engine is on choke) even after a 30km ride.
It has less performance that other 400's I have seen and rode with.

Any attempt to use the choke lever at any time results in stalling the engine and therefore the way to start in the morning is to crank the engine with 1/4 throttle and keep it manually at 2000rm for about 10 seconds after which it will stay happily at 1500 on its own.

Also recently I was experiencing a misfire from one of the cylinders so i removed the spark plugs to examine them and cleaned them. Cylinder no 2 was running very rich and the plug was fouled black. I replaced this with a plug from an earlier set (I had replaced all 4 when I got the bike) and the misfire went away immediately.

I do not ride aggressively so poor fuel consumption is not due to riding. Actually I try to use higher gears whenever I can so that I maximise economy.

Regards,
Chris

[nictecninja]

Hi,
So much to say on this subject don't really know where to start, but I can tell you that I get on average 50mpg / 320klm's to 18 ltrs out of my 93 Superfour fII. Yours does sound very rich!
Hope this helps a bit, Paul.
p.s the Superfour is very sensitvie to plug type & grade.

[chricol]

Hi Paul.

Thanks very much for your reply. I have replaced the plugs and I bought original NGK according to the manufacturer specification so they should be the right grade and heat range.

I am now suspecting an incorrect carb sync. Can this be an issue?

[zen61]

Hi, as a new CB400 owner myself I'm definitely no expert but I would check the float is operational first and the bowl and needle valve are clean. If the float is not adjusted properly or is sticking due to petrol gum it could be allowing too much fuel to be delivered to the carb. On old SU carbs we would just give the fuel chamber a smack with the handle of a screwdriver to temporarily unstick it and if that works on your carb it's only a temporary solution.

I'm not sure if it's possible to drop the bowl on the inboard carbs with them attached as I've never done this. Make sure you have a replacement gasket for the float chamber lid too should you decide to drop the bowl.

That's all I can think of at the moment and I hope it helps.

Cheers...Zen.

[chricol]

thanks zen..

I will try what you suggested. In the meantime I have poured some carb cleaner with the petrol and also bought a carb cleaning spray that should be sprayed into the air intake with the air filter removed. I will see what the outcome is. Had a slightly rough idle again this morning but it was gone when the engine warmed up.

Some forums say you should ride it hard above 8000rpm for more than 10 mins every now and then, but as we have no motorways an an island just 17 miles across from tip to tip, i cannot see how this is remotely possible.

[pynchy]

Certainly sounds like it is too rich.
Start by cleaning the carbs thoroughly, things might be sticking in there due to dirt.

Check the airbox to see if any dirt or other stuff in there.

Check that the air filter is clean, potentially not enough air getting to the carbs and thus making it rich.

[chricol]

I have swapped the filter with a new K&N filter just in case it was the problem. The engine sounds better but still getting the same symptoms. Next I will try giving the carbs a good cleaning with an off the shelf spray through the air intake and a carb sync. If all this fails to rectify my problem, then i'll probably have to take the carbs apart and give them a proper rebuild. I hope no one has meddled about with the jets in the past.

[CB 4]

hi does sound to be a carb problem ,a full overhal of the cabs may be the way to go look for warn float valves and seats ,also look at the floats them selfs for probs and check the float height when rebuilding , would also check the jetting for your location , if bike is an import , also check the mixture screws are set correctly around 2 turns ish from seated in then balance carbs when engine has warmed through,the 4s are great bike had mine amonth own bigger bikes but this bike is just so dam fun regards c

[nictecninja]

Hi Paul.

Thanks very much for your reply. I have replaced the plugs and I bought original NGK according to the manufacturer specification so they should be the right grade and heat range.

I am now suspecting an incorrect carb sync. Can this be an issue?

Hi Chris,
It does sound like you have carb prob's, but you have started down the right route with the K&N filter.
Depending on your exact model your mixture/air screws should be 2&1/2 - 2&3/4 turns out & carb syncing is v important but I suspect blocked jet's by your v,poor mpg.
(when mine was sick as a pig & on & off 3 & 4 cylinders I still averaged 45+ mpg)
hope this helps some more, by the way you can rev her to 12000+ rpm in 2nd & 3rd max 120kph
All the best Paul.

[chricol]

Hi Chris,
It does sound like you have carb prob's, but you have started down the right route with the K&N filter.
Depending on your exact model your mixture/air screws should be 2&1/2 - 2&3/4 turns out & carb syncing is v important but I suspect blocked jet's by your v,poor mpg.
(when mine was sick as a pig & on & off 3 & 4 cylinders I still averaged 45+ mpg)
hope this helps some more, by the way you can rev her to 12000+ rpm in 2nd & 3rd max 120kph
All the best Paul.

Hi Paul,
Thanks very much for your reply. Very very helpful indeed. My model is Version S and I have counted the sprocket sizes as someone said this might be an issue. They are front 15T and rear 46T.

Should I start with a professional carb sync or should I try to adjust the mixture screws first?

Regarding speed, maxing out 2nd edges the speedo over 100kph and the speed limit in Malta is 80kph max lol. I only revved 3rd gear once to rev-limiter and considering I have been riding just for 9months I do not wish too push my luck too much.

Thanks a lot,
Chris