ns400r flat spot

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andy b
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ns400r flat spot

Post by andy b »

I really hope somebody can help me with this one..

I have just finished restoring it but other than service the bike (fluids/brakes lines pads/gear oil etc i have not stripped the engine or carbs.. It has a terrible flat spot....The bike revs fine up to 5000 rpm then hits a point where the engine note sounds bogged down (almost like its trying to run on 2 cylinders until it gets past 6000 rpm when 7000 is reached it zings along as it should. First thought was possibly an electrical breakdown but above 6500 it doesn't miss a beat...

When i try to lightly throttle through it on the move it still does it but not as bad as when i try to put it under load at these revs..Changed plugs and got rid of a exhaust leak from the atac cover but still the same. If i raise the revs slowly whilst in neutral it still hits the dull tone at 5000 rpm.

A pal of mine is a mechanic but with no experience of these and does not seem to think it will be the carbs as it is fine below 5000 and the main jets must be clear for the response above 6500.

I really would appreciate any advice on this.. I have heard some do have a flat spot but this feels like it is being dragged back and in all honesty i have had 2 of these before and never had it...

It has standard pipes on and to my knowledge has neveer had race pipes or been jetted..I really dont fancy taking the carbs off especially if this symptom does not require it.

I really would appreciate advice as there are not many good weather days left before the crap weather sets in and cant bear to see it sat sparkling in my garage having not had any fun on the bloody thing and spent the last 18 months contemplating then getting round to doing it !!!...

Thank You.

Mark
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Location: Up Norf

Re: ns400r flat spot

Post by Mark »

Sounds like the electrical side is ok, as you're not getting a missfire. You could check that the ATAC units aren't closing too early, or stuck in the closed position.

At low revs, the ATAC should be open. The idea being to improve bottom end and mid range power. They should close at around 7200 rpm.

Other than that, I think you're into taking the carbs off.

If there's nothing obviously wrong with the carbs, I'd try a crankcase leakdown test to check the condition of the crankseals. Given the age of the bike, it's a possibility.

Keep us posted, hope it's a quick fix.
If it ain't broke, fix it 'til it is.

luggy
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Re: ns400r flat spot

Post by luggy »

some tend to put washers under the main needle a la hrc needle copy, but not done in conjunction with other tecnical changes causes a massive flat spot at or around 5000 rpm/ 6000rpm , maybe you need to take the carbs apart! be careful though , lot of small to tiny parts in there ( usually find them on the worktop, when the carbs are on the bike again, and the fairing ......
gravity is an extreme painful thing !

Mark
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Location: Up Norf

Re: ns400r flat spot

Post by Mark »

I reckon it's going to be a carbs off job too.

ATAC may cause a bit of a flatspot, so worth checking. I wouldn't expect it to be quite as severe as described, but you never know...
If it ain't broke, fix it 'til it is.

Channel Hopper
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Re: ns400r flat spot

Post by Channel Hopper »

Before removing anything, spray the carb inlet manifolds with grease to see if there are any perished rubbers, or leaks.

Then have a quick look at the air filter element and down the carb throats for terminal breakup.

Mark
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Location: Up Norf

Re: ns400r flat spot

Post by Mark »

As Hopper says, filter elements are known to break up and inlet rubbers do perish.

Have you had a look at the plugs? Does it look rich or lean?
If it ain't broke, fix it 'til it is.

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