Runs fine for 100km then loses power, frustrating!

[Centurion85]

1990 NC30 23000km Jap import

I've fitted new R6 reg, new battery (both tested and good), new plugs, new air filter, oil change, cleaned the carbs following the jetting guide on here and fitted new main jets.

Before doing all this I'd just got the bike and it ran ok for about 200ish miles before losing power and seeming to only run on 3 (think it was the front right one that stopped, number 4 in the Haynes).

While doing the carbs I noticed that it had 114 main jets fitted all round, one of the 5 hole emulsion tubes had been fitted to the same front right carb/cylinder.

Reset the pilot screw to 2 turns out, checked the float heights were 6.8mm. Fitted 120f/122r mains.

Only thing I didn’t do was synchronise the carbs as I don’t have the vacuum gauges

I also noticed that some biff had tried to make a ram air intake. He’d cut the rubber flaps off the air box and fitted a small piece of aluminium just above the radiator which I’ve now binned but the rubber flaps are still missing.

Fitted it all back together, started no problems , slightly erratic idle but think that’s because I didn’t sync the carbs.

Rode it for about 100km, seemed ok however there was a big flat spot at about 11k/12k when at full throttle. Just as I pulled up at home it lost power and seemed to be running on 3 again (not sure which ones not working this time)

The bike also seems to run very hot, temp gauge sat between red and the line after 80c most of time (was riding in crappy London traffic for a while)

I’m a bit frustrated and stuck with what to do next. Waiting for the bike to cool before trying to get the plugs out to check what colour they are. Am I better off just taking it to a garage and saying fix it?

Cheers for your help

Fedup NC30 owner!!

[CMSMJ1]

don't stress - can be fixed.

so - you checked the charging recently? it is not failing to charge ad slowly getting to a point where the battery cannot keep up? Do you run with your lights on?

Hot - well, they do run hot. Does your fan work? Does it overflow into the header like it should do? Have you got an understanding lady friend and can put the rads through the dishwasher before you clean them properly?


From your description it is the last part of the charging system you have not yet checked - the genny. Get the details to chekc it and see what it says - it is easy to do (check Haynes for the process - but basically A+B, A+C and B+C with the AC probee - see what you get output wise. then test the same three ways for resistance)

Come and tell us how it looks....we'll sort it !

[Centurion85]

Ah thanks for the fast reply!

I forgot to mention that the generator was checked and is ok. The battery charges at about 14v, I don’t think this is down to a lack of power as the bike turns over and runs if you hold the throttle open a little, when the battery died before the reg was replaced it wouldn’t even turn over. The ride today was without lights and the battery output is still good.

While I was riding down the road it was very jerky, as if the fault was intermittent. Maybe the cylinder firing and then stopping and then working ok. As if there’s a connection being broken by bumps/vibrations. Though I don’t understand why that happened right at the end of the journey after running fine. That was when the bike was at its hottest mind so maybe it’s a heat thing?

[CMSMJ1]

could easily be a split vacuum hose or an old one that is getting hot and collapsing causing fuel starvation.

Can you see the hose - is it old and scabby looking? If it is a clear and stiff tube it is old and can get hot adn collapse, causing starvation.

[Centurion85]

Which hoses are those? There are some which join between the diaphragms/top covers and then lead to a little box at the back of the air box. I noticed a hose comes from just below one of the inlets on the engine and connects to the fuel tank. I'm not sure what others there are?

[Centurion85]

I think I've fixed it... I switched the fuel to reserve and strangely enough it seemed to work... I know, I'm a bloody fool!

Not used to these manual reserves, last bike had a light that came on.

Cheers for your help though, if it turns out that I've not fixed it I'll let you know!

[CMSMJ1]

you need one from cyl 3 (the rear right) that leads to the outermost spigot on the fuel tap.

[Lucky Tampon]

While I was riding down the road it was very jerky, as if the fault was intermittent. Maybe the cylinder firing and then stopping and then working ok. As if there’s a connection being broken by bumps/vibrations.

This is exactly the problem i'm having, wish it was as simple as switching to reserve for me Good to hear you've sorted it though.