CB400 NC31 hard start procedure

[untonyto]

I must start with profuse Thank Yous for all your help and feedback with my questions so far.

I am glad to report that thanks to you, my 1992 CB400SF NC31 that sat for two years finally ran today.

It was quite a hassle finally getting it to fire up. First there were many failed attempts where it cranked but wouldn't fire up. Eventually It took two strong men with a high cardiovascular threshold to push it at a run gathering momentum for a hundred meters or so before it fired up. And the first three such attempts failed.

Too much cranking almost drained the battery but luckily it fired up at last and the newly installed Mosfet FH020AA regulator quickly brought voltage back up to 13.1V stable. I owe whoever recommended that upgrade a drink.

Another symptom, when the throttle is pulled, the tachometer rises, but is slow to come down when the throttle is released. Kind of hangs momentarily then descends at it own leisure.

Could the hard start and the sluggish tach be related problems with one common fix?

Also, the motorbike died out on me a few times today while test riding. Eventually the shadetree mechanic I am working on it with almost convinced me that the tank needed to be at least half full because the carbs rely on gravity to get fuel and too little fuel will not have enough pressure to make it to the carbs. So we fuelled up but the starting issues persist.

There are a few other issues but these are the main ones I need your insight on. Thanks.

[magg]

Hanging rpm on throttle closure can be a result of lean mixture in pilot jet system. Have you disassembled and clean the carbies? The fuel tap is controlled by an engine vacuum signal. Is the diaphragm in the tap or the vacuum hose damaged?

[untonyto]

Hello magg, I went back to check the carb mixture settings and they were only one and a quarter turn out. Set them to two and a half, tried the starter and it still would not fire.
Charged the battery off the bike and tried again, no luck.
Took out the park plugs and discovered the plugs were wrong; NGK CPR7EA-9 instead of the expected NGK CR8EH-9. Looking back to since I got the motorbike back from a mechanic, it has never started reliably, and the spark plugs have never been changed. but this was probably masked by the fried Reg/Rec, bad wiring, etc which I have slowly worked through. I can't reliably figure out a list of bike models that use the wrong spark plug for comparison or some sort of rationale for why whoever put them in thought it was a good idea. Meanwhile I have ordered the correct spark plugs.

Will update you all on whether correct spark plugs fix the hard start and mid-ride cut outs.

[cbr400rrn]

Took out the park plugs and discovered the plugs were wrong; NGK CPR7EA-9 instead of the expected NGK CR8EH-9.

Maybe the previous owner sold the bike as his eyesight was failing, as it must be easy getting your spark plugs numbers mixed up

[untonyto]

Maybe the previous owner sold the bike as his eyesight was failing, as it must be easy getting your spark plugs numbers mixed up

No, cb400rn, he's a young man with perfect eyesight, he sold to me to buy a smaller but more modern motorcycle for the reliability.

He was "kind" enough to give me two extra spark plugs when I bought the bike, "just in case". It being my first motorbike I suspected nothing at the time and thought he was just being a good lad. Ignorance is bliss.

I can't really blame him. Out here in Nairobi it's not easy to get parts, one must ship them across oceans. Considering the rarity of spark plugs he probably figured he could use the correct ones on his next bike and palm the wrong plugs off on me. I couldn't have known better, the motorcycle was running when we parted ways, and I was quite nervous as it was my first real ride on public roads. It's a miracle I made it home.

But I digress. Believe it or not I'm still waiting for the spark plugs I ordered on Amazon.

[magg]

The only practical difference between the two plugs is the heat range will not affect starting. Have you got spark when engine is cranking. Do you have fuel flow to the carbies. If you disconnect the fuel hose from the tap and crank the engine does fuel flow.

[MadMotoUK]

Maybe the previous owner sold the bike as his eyesight was failing, as it must be easy getting your spark plugs numbers mixed up

No, cb400rn, he's a young man with perfect eyesight, he sold to me to buy a smaller but more modern motorcycle for the reliability.

He was "kind" enough to give me two extra spark plugs when I bought the bike, "just in case". It being my first motorbike I suspected nothing at the time and thought he was just being a good lad. Ignorance is bliss.

I can't really blame him. Out here in Nairobi it's not easy to get parts, one must ship them across oceans. Considering the rarity of spark plugs he probably figured he could use the correct ones on his next bike and palm the wrong plugs off on me. I couldn't have known better, the motorcycle was running when we parted ways, and I was quite nervous as it was my first real ride on public roads. It's a miracle I made it home.

But I digress. Believe it or not I'm still waiting for the spark plugs I ordered on Amazon.

What plugs do you need? I probably have some.

http://forum.400cc.fr/

[untonyto]

Thanks for jumping in again magg

The only practical difference between the two plugs is the heat range will not affect starting.

I might be placing too much faith in the coming spark plugs after reading all the glowing reviews on Amazon heheheh. They may not be the problem at all, but I'm pretty convinced they were not only the wrong - as in not OEM-specified - but also old and well used by the time I got them. Not too expensive to replace and find out.

Have you got spark when engine is cranking.

All four plugs gave spark when tested against the frame. However I tested them in the full glare of a tropical midday sun so that could be why the spark looked weak.

Do you have fuel flow to the carbies.

I realized by turning the carb drain screws that the petcock in the on position did not flow fuel to the carbs, but having it in the Reserve position worked well. A new petcock is part of the eagerly awaited incoming shipment.

If you disconnect the fuel hose from the tap and crank the engine does fuel flow.

A bit of a story here: I examined the vacuum tube that goes to the petcock and found it permanently sealed within by a previous owner. He had shoved a metal rod in there for an airtight seal. So cranking the engine did nothing for fuel flow at all. A new vacuum tube is also part of the incoming package.

If it helps, the bike still wouldn't fire when there was fuel in the carbs - confirmed by checking the drain screws. I just wonder what's different about a bump start that makes the bike start albeit with difficulty, whereas the starter button just won't do it.

[untonyto]

What plugs do you need? I probably have some.

http://forum.400cc.fr/

Hello MadMotoUK, I already ordered the Iridium version of NGK CR8EH9. It's the waiting part that's got me jumpy. Thanks anyway.

[magg]

When you remove the plugs are they at all wet with fuel. Have you had the carbies off for a clean. Fuel in the float bowl is a plus but the air and fuel jets could still be blocked. No fuel tap vacuum hose with fuel flowe on reserve setting and none on main is odd. Ssounds like someone has played with the fuel tap to remove the vacuum diaphragm and got it wrong. Fuel should fuel freely in either reserve or main settings at all times.