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114gta
New Member
United Kingdom
7 Posts |
 Posted - 23/03/2008 : 09:17:37
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Hi all, new user here. After i got knocked off my Bandit 400 last august, im finally back on 2 wheels with a lovely CB400 which my sister no longer uses. Heres a couple of pics;
http://img.photobucket.com/albums/v489/tob4312/bike3.jpg
http://img.photobucket.com/albums/v489/tob4312/bike2.jpg
http://img.photobucket.com/albums/v489/tob4312/bike1.jpg
Once running, it sounds sweet and rides nice. But, its always reluctant to fire, i have to hold it on the starter for a quite a while before it runs under its own power and then it only runs on three cylinders for a while. If you try to rev it too soon it just dies again. It has brand new plugs, im thinking maybe its a fuelling fault, maybe the carbs need setting up?
Also, it seems down on power. It feels noticeably slower than my old bandit and a super CB400sf version 's' that we have in the family. Surely it should run just the same as the version S, are they not the same engine and power output?! It doesnt seem to come on song until youre at about 10,500rpm, the other we have starts pulling from about 6,000 and just gets stronger at the top end. Mine just feels weak and lacking in grunt below 10,000.
It has one of the kmh-mph mechanical speedo converters fitted so it is no longer restricted, but i hear of others hittng 120mph+ - mine struggles over 100 in 6th, manages about 110 in 4th and 5th. It just feels weak.
Maybe its the fuelling again? Carbs just need sorting out? My only other thought is that exhaust. It looks and sounds lovely, but the center bore of the pipe only looks about 30mm! No idea what exhaust it is, it has a couple of japanese letters on its badge and says 'hand made' under it, and has a 'JMCA' plaque on it.
Any advice would be great - sorry for the essay! |
Edited by - 114gta on 23/03/2008 09:22:29 |
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TektroG
Senior Member
United Kingdom
1009 Posts |
Posted - 23/03/2008 : 09:35:10
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Cleaning carbs out plus balancing sounds like a good idea. Try a compression test maybe? Does the bike backfire?
Check airfilter and airbox are clean. |
I've got 4 words for you sonny: ... |
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114gta
New Member
United Kingdom
7 Posts |
Posted - 23/03/2008 : 09:38:04
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Yeah i think having the carbs balanced is a good start, if it doesnt sort it i'll do a compression test. It doesnt backfire no, runs quite sweet apart from the lacking power.
Thanks for the quick response! |
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BUGIE
Regular Member
Malta
505 Posts |
Posted - 23/03/2008 : 10:05:51
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quote:
Originally posted by 114gta
But, its always reluctant to fire, i have to hold it on the starter for a quite a while before it runs under its own power and then it only runs on three cylinders for a while. If you try to rev it too soon it just dies again.
Before checking out the carbs etc, have you checked the simple things? have you tried starting it with a little gas. Also have you checked the idling speed and tried increasing it to see if maybe its just idling too low hence always cutting out?
quote:
Originally posted by 114gta
It has one of the kmh-mph mechanical speedo converters fitted so it is no longer restricted, but i hear of others hittng 120mph+ - mine struggles over 100 in 6th, manages about 110 in 4th and 5th. It just feels weak.
Why does this mean it's not restricted. Have you tried running it without the convertor as this may be sending the bike into limp mode and so at top end speed/revs it'll run on 3 cylinders |
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114gta
New Member
United Kingdom
7 Posts |
Posted - 23/03/2008 : 10:36:23
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Idling speed is about 1450rpm warm which id have thought was fine? I have to start it with a bit of throttle otherwise it doesnt want to know at all. Trick is to start it with a tiny bit of throttle, hold it like that until the revs start to climb, then give it a bit of choke and close the throttle and it starts to run under its own steam, albeit not on all 4 for a while.
The speedo converter means that the restrictor wont actually come into effect until im doing 180MPH, which is never going to happen! When im doing 100mph the bike only thinks i'm doing 100kmh now etc. Once its warm it runs on all 4 smoothly. It doesnt hit an electronic restriction at the top end, just runs out of puff if you get me?
Thanks for your advice. :) |
Edited by - 114gta on 23/03/2008 10:37:45 |
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BUGIE
Regular Member
Malta
505 Posts |
Posted - 23/03/2008 : 16:57:12
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if the resistance in the speedo cable isn't correct then the bike will go into limp mode at i think 90mph and makes it switch from 4 to 3 cylinders meaning it will feel like it's run out of power hence i would try removing it for troubleshooting).
1450 revs is high for idling speed. Try feeling each downpipe on startup to identify which cylinder isn't firing correctly. you can then try switching plugs around just to ensure it really isn't a plug problem. Also when you replaced the plugs did you notice any unusual discolouration?
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Edited by - BUGIE on 23/03/2008 17:00:25 |
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TektroG
Senior Member
United Kingdom
1009 Posts |
Posted - 23/03/2008 : 18:50:54
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Resistance in the speedo cable?
Surely this is is a mechanical drive off the hub to the speedo, hence putting a mechanical kmph to mph convertor in line will overcome this? Standing by to be corrected (PB1 I'm on about) if I'm wrong.
Surely this is output from speedo as a voltage to the CDI which then cuts in at 180 kmph, but as 114gta says if you've got the convertor fitted, unlikely to be a problem. |
I've got 4 words for you sonny: ... |
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114gta
New Member
United Kingdom
7 Posts |
Posted - 23/03/2008 : 19:08:09
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Yeah i think we can discount the speedo cable idea for certain. It runs fine on all 4 once warm, especially when thrashing it, pulls over 90 fine - its just slower than it should be up there and takes forever to start pulling. Where others seem to start pulling hard at 6k+, mine starts at 10k+. The misfire is only when cold for a minute or so just after starting, although its worse after a couple of days without running. Im pretty sure the missing is fuelling tbh, and the lack of power either that or the exhaust bore, just wanted second opinions!
It bahaved the same on both new and old plugs although its slightly improved with the new ones. Its the cylinder on the end thats reluctant, on the brake lever side of the bike.
Thanks for all input! |
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TektroG
Senior Member
United Kingdom
1009 Posts |
Posted - 23/03/2008 : 20:47:41
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Try running some Redex through it and put in a couple of tankfuls of SuperUnleaded. Check yer fuel filter isn't blocked.
I suspect crap in the carb if its only the one cylinder. |
I've got 4 words for you sonny: ... |
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alexibrow
Settled In Member
United Kingdom
64 Posts |
Posted - 23/03/2008 : 21:38:28
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| If one of the HT leads/coils is breaking down, then this could account for poor starting and lack of power, as well as the slight improvement you noticed the with new plugs, ie the old plug supplied with the faulty lead/coil would get sooted up and provide an even weaker spark. Faulty leads and coils are usually more noticeable when the engine's cold, as well. |
It's not karma, its bikema. |
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a_morti
Senior Member
United Kingdom
1344 Posts |
Posted - 23/03/2008 : 22:41:31
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For something completely different, what is the final gearing on the bike? If the gearing is too short that would allow it to go faster in lower gears, and limit top speed and high speed performance. Wouldn't help your starting question, though.
I'd figure out which cylinder is playing up and then swap the spark plugs over in their pairs to eliminate electricals before stripping the carbs for fun. On older bikes the diaphragms can go hard, or swell, and it's a pig to put everything back together if that's happened. |
WWW.HONDACB1.CO.NR
+ NC29 swingarm ('29 wheels soon), Quill T3 Ti, lots of black, blade rear hangers, beet rearsets, brembo brake. Top mods: BT45's, touring screen & rack with 50L box! Stolen recovered, scumbags best not try again. |
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114gta
New Member
United Kingdom
7 Posts |
Posted - 24/03/2008 : 00:24:58
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Hmm, food for thought. Thanks for all input. Leads/coils is something id managed to totally ignore, ill look into this.
I had the carbs off not long ago as when i got the bike it was a non-runner. I took two apart and they looked mint, started on the thrid and i just thought whats the point as it looked just as mint - just my luck that the only one i didnt look at was the one of the offending cylinder! Doh! Absolute pain in the arse to get them off and on it was too lol, as you said. The CB is not the best bike to be doing this on, as i learnt.
My plan then is to run some high octane and a load of redex through it for a while and see if it cures the missing, ill give it a service including airfilter incase the filter is old and clogged and choking it and ill have a play with the HT leads and/or coils.
I have an automotive multimeter, does anyone know what resistance the leads should read? Anyone know how to test the coils? Are they costly to replace?
Thanks a lot for all the help, its given me something to get on with. :) |
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TektroG
Senior Member
United Kingdom
1009 Posts |
Posted - 24/03/2008 : 02:51:55
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Idle (after 10 - 15 mins running) should be 1300 rpm+/- 100rpm.
Resistance of a lead should be negligible, its more a case of condition and can you see tracking in the dark. Coils are an inductive component. I think I'm right in saying you'll either get continuity or not.
It is possible to get a gauge for health of spark. Vehicle Wiring Products is your friend for most of this stuff. Buy the silicon HT lead I've been told. |
I've got 4 words for you sonny: ... |
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114gta
New Member
United Kingdom
7 Posts |
Posted - 31/03/2008 : 11:01:48
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Ok, little update. Ive just been riding the bike regular, havent actually had the time to work on it recently! But it is starting better now, very rarely doesnt start straight away on all 4 - but it starts from stone cold with no choke, if i give it choke it seems to flood it straight away. Cant be right surely?
Im also starting to learn how it performs. It has a bit of a dead spot between 3-4k, then feels quite pokey for a small engine at these low revs, until you get to about 6k. Here id normally expect a little 4 like this to start to take off, but mine doesnt. In fact, if you give it too much throttle between 6 - 9.5k rpm it actually slows down and starts to die, do you think its running too rich and flooding itself? Once you hit 9,500 it starts pulling ok although not with quite the same grunt id expect. Ive had it pull about 110mph in 4th i think, if i put it up into 5th it cant pull any further, and 6th slows down a touch.
Riding swiftly is very hard work. In top you have to really try to keep it up at 70, but on the slightest incline/headwind it slows down, and like i said above, too much throttle before 9,500rpm and it bogs down. I remember my bandit 400 quite happily pulling in top from much lower speeds and i know the CB should!
Im gonna give it a full service and think i might book it in somewhere with a dyno to get the carbs set up properly. |
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alexibrow
Settled In Member
United Kingdom
64 Posts |
Posted - 31/03/2008 : 19:54:55
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Are you getting any black sooty smoke from the exhaust? If so, there's unburnt fuel getting through to the exhaust. This may mean the mixture's too rich, or it could indicate a poor spark for some reason, resulting in an incomplete burn of the fuel in the cyclinder.
Whip the plugs out after a longish run: what colour are they? Black indicates too rich a mixture (or if on just one plug, could be a faulty plug, coil or lead); white indicates it's too lean; just right is a light tan colour.
You're right to question the mixture if it starts without choke, especially as you've had the carbs apart 
But first things first. Check the electrics over; make sure the leads, coils and plugs are all working properly. Then get the carbs checked over.
One other possibility: an air leak somewhere could cause uneven running and loss of power, for example if the carb rubbers have perished or split (where they join onto the engine).
I wouldn't bother with a dyno unless you're trying to set the bike up after some tweaks - manufacturer's settings should be just fine.
Good luck, let us know how you get on.
Al. |
It's not karma, its bikema. |
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gothy
Settled In Member
United Kingdom
52 Posts |
Posted - 02/04/2008 : 14:07:12
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looks like the exhaust is a yoshimura.
I agree about the speedo restrictor. The converter HAS sorted that out.
just so you know its not the speedo drive cable you check resistance of its the wires on the back of the speedo behind the fake chrome backing.
how many teeth does each sprocket have? |
Honda CB400SF 1995. |
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Project big1 - reluctant starter, lacks power
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