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Author Topic: CB400SF NC31 problem  (Read 11365 times)
faffi
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« on: September 28, 2014, 03:50:27 PM »

Hello!

My son has a 1992 CB400SF. He managed to fall off at about 60 kph/40 mph, the bike dropping on the left side. Ignition cover got scratched, but not holed.

Since then the engine is misfiring and lacking in power. Between 5000 and 8000 rpm it sounds like it's hitting the rev limiter, but it will rev to the 12500 limiter of forced.

What I have tried/tested:

New spark plugs. No change.
Removed ignition cover, no internal damage found.
Measured pick-up - within spec.
Tried a different battery, no change.
Checked that every carb piston moves freely, which they did.
Checked that ignition timing is correct with strobe - it sits on a mark for 1-4 (no mark for 2-3) but cannot read if it says T or F or anything else.
Checked that the ignition timing advance with rpm. It does.
Checked spark with strobe on all 4 HT leads up to about 7000 rpm, they all fire reliably.
Checked all spark plugs, they have uniform colour, albeit a little dark.
Checked that all 4 headers heat evenly and cool evenly, they do by the touch of a hand and spit test.
Checked voltage drop to the coils, it's only 1/10 of a volt.
Tried to shut off the fuel supply to see if it would run better as things lean out. No change.
Tried to remove air filter, no change.
Tried to remove air filter lid, made it barely rideable.

The engine definitely runs rough and misfire, worst in the midrange, but I am at loss for what else to search for. I wanted to check carb sync,just to see if one cylinder is badly lacking, but I could not find the vacuum port for cylinder #4 - anybody know where it is? The other 3 carbs work, although they could be synced better. Little point doing so when I cannot hook up all 4 carbs, though.

Any suggestions appreciated of what to look for!

Eirik
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CB_CX_NC
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« Reply #1 on: September 30, 2014, 07:57:51 AM »

I'm not familiar with the bike, but just a thought based on CB-1 "workings", could he have damaged the speedo head (or wiring to it) resulting in the 180km/h speed limiter being falsely activated?

Just a stab in the dark based on the misfiring over 5000rpm, as I believe the -1's limiter cuts some ignition pulses above a certain RPM when the limiter is activated by a switch inside the speedo.

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'83 JDM Yamaha 29R XJ750E-II *stolen & stripped* Sad

'95 UKDM Honda PC26 CB500 twin, knackered & tatty winter bike, tyres replaced, running & nearly ready for duty

'89 JDM Honda NC27 CB-1, doing sterling service as usual, soon to hibernate

JDM NC23.7 engine- slow-burn rebuild in "progress"
faffi
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« Reply #2 on: September 30, 2014, 10:15:45 AM »

It does it in neutral as well, and will rev to 12500, albeit reluctantly. Also, the original instrument had the speed limiter disconnected. But yes, the instruments were smashed, but it's the second set he smashes and we had no issues the last time.
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CB_CX_NC
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« Reply #3 on: October 01, 2014, 08:04:06 AM »

Ah right, at least that's pretty much eliminated as  a possibility then.  Smiley Another shot, does the bike have a fuel pump and/or filter? Based on my experience today a blocked filter can cause similar symptoms & a failing pump could do as well I suppose.

It could be that some hidden corrosion got knocked off the inside of the tank/stirred up into the fuel & blocked the filter. Fairly simple to check, just disconnect the pipe from the filter & put a small cup under it to catch any fuel, then turn the tap on & see how well it flows. In my case there was just a tiny drip, enough to allow the bike to run but very badly with misfires & no power.
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'83 JDM Yamaha 29R XJ750E-II *stolen & stripped* Sad

'95 UKDM Honda PC26 CB500 twin, knackered & tatty winter bike, tyres replaced, running & nearly ready for duty

'89 JDM Honda NC27 CB-1, doing sterling service as usual, soon to hibernate

JDM NC23.7 engine- slow-burn rebuild in "progress"
a_morti
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« Reply #4 on: October 01, 2014, 03:08:26 PM »

Ah right, at least that's pretty much eliminated as  a possibility then.  Smiley Another shot, does the bike have a fuel pump and/or filter? Based on my experience today a blocked filter can cause similar symptoms & a failing pump could do as well I suppose.

It could be that some hidden corrosion got knocked off the inside of the tank/stirred up into the fuel & blocked the filter. Fairly simple to check, just disconnect the pipe from the filter & put a small cup under it to catch any fuel, then turn the tap on & see how well it flows. In my case there was just a tiny drip, enough to allow the bike to run but very badly with misfires & no power.
It hasn't eliminated it at all. The bike still thinks it is doing over 180km/h and is cutting ignition back to 5500rpm.

Find the orange/blue wire, give it +12v, your problem will disappear.
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faffi
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« Reply #5 on: October 01, 2014, 04:08:17 PM »

I'll do that and see, well worth a shot. Considering that the two wires - I think there is one orange and one blue/orange by memory - was not connected to anything on the stock instruments (there were nowhere to hook it up, either, that I could find) and no provisions for it - or even the wires present - in the original style cheap chines replicas we fitted. But perhaps there was something somewhere that did supply the blue/orange with 12V somehow that was hidden from sight. But I'll give the wire 12V and see if things are sorted and get back to you with the result - thanks for the tip!
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faffi
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« Reply #6 on: October 02, 2014, 05:53:08 PM »

Thank you, morti - it seems like it did the trick! I didn't get to test ride it, but it started much easier, sounded normal and revved easily to 7000 rpm, which was as high as I wanted to go with a lukewarm engine. So I'm 97% certain it's fixed. Thanks again! (I still have no clue how it was solved with the earlier instruments, but there must have been a bridge of some sort, ensuring power to the wire in question.)
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CB_CX_NC
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« Reply #7 on: October 03, 2014, 11:54:13 AM »

It hasn't eliminated it at all. The bike still thinks it is doing over 180km/h and is cutting ignition back to 5500rpm.

Find the orange/blue wire, give it +12v, your problem will disappear.

OK I stand corrected, but I thought (as did faffi it seems) that the limiter was dependent on the bike being in gear too, sure I've read that on here somewhere. Not the case then I take it?

Either way, glad it seems to be sorted now.
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'83 JDM Yamaha 29R XJ750E-II *stolen & stripped* Sad

'95 UKDM Honda PC26 CB500 twin, knackered & tatty winter bike, tyres replaced, running & nearly ready for duty

'89 JDM Honda NC27 CB-1, doing sterling service as usual, soon to hibernate

JDM NC23.7 engine- slow-burn rebuild in "progress"
faffi
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« Reply #8 on: October 03, 2014, 05:01:00 PM »

Went for a ride today and it goes like the wind  Cool Well, as windy as a 52 hp 400cc four can be expected to go  Wink
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knee-dragger777
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« Reply #9 on: October 03, 2014, 05:49:55 PM »

closure on a problem.  I love it!!
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a_morti
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« Reply #10 on: October 05, 2014, 06:29:05 PM »

Thank you, morti - it seems like it did the trick! I didn't get to test ride it, but it started much easier, sounded normal and revved easily to 7000 rpm, which was as high as I wanted to go with a lukewarm engine. So I'm 97% certain it's fixed. Thanks again! (I still have no clue how it was solved with the earlier instruments, but there must have been a bridge of some sort, ensuring power to the wire in question.)
There is an optical sensor in the speedo whereby the orange/blue has 12v, until the needle gets past 180km/h. Then it cuts the 12v to the ECU. That wire controls cutting ignition to a set level to stop the bike accelerating further - it's to do with the 180km/h speed limit in Japan.
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amilahetti
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« Reply #11 on: April 14, 2022, 06:26:39 AM »

I have the same issue but I can't find the wire in Speedo meter both connectors. Blue orange wire. Plz help 🙏


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a_morti
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« Reply #12 on: April 21, 2022, 10:33:28 AM »

You have a much later bike. Since it has HISS, I guess it's also new enough to not have a speedometer driven by a cable, but electronic. In this case, your problem is not the same and I'm afraid you're not likely to find the answer on this forum.

Also - ask yourself this... Are you going on track? Are you riding on the Autobahn? Then why do you want to ride at 180km/h? In most countries, that'll land you a pretty big fine, if not jail time. Leave it alone Smiley
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