Right guys this is my first post on this forum, so here goes:

I have a 1989 nc30 running full tyga system, hrc CDI and has been jetted by hm racing to suit setup.

Bike runs sweet on tick over, fuel mixture is spot on basically when accelerating up through the gears everything seems good until I get to 60-80mph and then I just hit a brick wall and the bike struggles to go any faster, when this happens there is a tapping/vibrating sound almost like it is dropping a cylinder ?

The bike has been heavily modified but will list the possible causes to the problem I have:

Standard clocks and speedo cable removed and changed to a acewell digital speedo (being told this May be the problem)
Race wiring loom with keyless start done via immobiliser system
Headlight conversion
Upgraded temp gauge



When the bike was jetted I had the plugs and air filter changed and witnessed the bike hit the limiter in each gear and hit full speed and produced nearly 62bhp but a few days later the problem was back (it was pissing down with rain the day it was jetted so couldn't test properly) please help before I burn the bike !!

It's simply a process of elimination until you find the culprit. It sounds as though your bike is dropping a cylinder or two, there could be a number of causes of this - usually related to either poor spark or no fuel.

Repsolnc30 wrote:Standard clocks and speedo cable removed and changed to a acewell digital speedo (being told this May be the problem)

NC30's have a built in circuit which cuts spark to 2 cylinders over 5000 rpm when the clocks are removed. I imagine this was done so that owners don't simply remove the speedo to bypass the factory 180 km/h speed limit (though there are many other simple ways to bypass it). If it has been running well in the past and you haven't changed anything since then it's not likely the culprit, but read this thread for more info: viewtopic.php?f=24&t=45986

Repsolnc30 wrote:running full tyga system, hrc CDI and has been jetted by hm racing to suit setup

HRC cdi you say? Is it a chinese copy? If so, I wouldn't trust it at all, there are plenty of stories of these units giving trouble and even leading to blown engines. My advice would be to replace it with a known working original cdi, or a quality aftermarket unit (such as Ignitech).

NC30's have a few common issues which can lead to running problems. Electrical troubles can manifest themselves in all sorts of symptoms (e.g. not enough current for spark plugs to operate properly), grab a multimeter and conduct some checks on the system, it's not difficult. There's a good guide on checking the electrical/charging system here: http://www.400greybike.com/docs.htm

The other common issue is that the fuel tap vacuum line becomes soft or perished, which leads to insufficient fuel in the carb floats. The line comes off a spigot under the rear right hand side carb (i.e. cylinder inlet) and runs to the bottom/outside nipple on the fuel tap. Check firstly that your bike has the hose fitted (may have been modified by the previous owner), and replace it anyway - you can't always tell if it's faulty by inspecting it. Read this: viewtopic.php?f=24&t=20113

If everything above checks out then I'd check the spark plugs to see that they're sparking at the revs you have issues with.

I'd be starting with these places to find the culprit. Check each of these and get back to us.

Lastly, I would recommend Rick Oliver for any parts you need, have a read through his stock list here: viewtopic.php?f=22&t=26174

I assumed above that you're running a standard Honda paper air filter and standard airbox?

Repsolnc30 wrote: When the bike was jetted I had the plugs and air filter changed and witnessed the bike hit the limiter in each gear and hit full speed and produced nearly 62bhp but a few days later the problem was back (it was pissing down with rain the day it was jetted so couldn't test properly)

Has it ever run properly on the road??

I assumed above that you're running a standard Honda paper air filter and standard airbox?

Thanks for thr reply cammo will have a look at the weekend just to clarify it is a ignitech CDI bought from RO unfortunately I don't know the history of the bike but it has never run properly on the road the air box is standard as is the filter

Right, since you've never had it running properly on the road it changes a few things...

The ignitech cdi will rule out any speedo removal restriction issues.

My thoughts would be on fuel starvation or induction issues leading to lean running.

Cammo wrote:My thoughts would be on fuel starvation or induction issues leading to lean running.

Wouldn't the the old "rubber flap is missing" chestnut, would it?

Ben.

Ben_OB1 wrote:

Cammo wrote:My thoughts would be on fuel starvation or induction issues leading to lean running.

Wouldn't the the old "rubber flap is missing" chestnut, would it?

My thoughts exactly!

What the guys are talking about above is this thing, it is a rubber flap that fastens under the airbox.



Quite often people (who don't know better) remove this thinking it will improve the airflow to the airbox - hence the carbs. However what this flap does is calm out the air turbulance and provides a smooth air supply to the airbox/carbs. As the turbulance of the air flow gets worse with speed (as per your symptoms) so the air 'fuel mix gets f****d up and runs like a bag of spanners. This would also potentialy explain why they could hit the limiter in every gear on the dyno (no air flow) but it doesn't function in the real world.

Easy enough to check - just pop off the tank and the airbox and have a shuffty. If it is there, move on to the next idea, if not get one fitted and start again

If this flap happens to be missing are they easy to get hold of ?

X2,i need one as well!anyone got one?