Poor running at low <3000 rpm

[kingarthur83rd]

Hi Guys

Im just doing a little troubleshooting on my bike as it has a little niggly problem.

I have a CBR400 RRL NC29. Stainless exhaust, mechanically everything else is standard bar the derestricted CDI unit

what I'm noticing is that the bike under load has very poor uptake under 3000 rpm (sometimes even failing to tick over / idle and intermittently cuts out.

on the pipe its fine, soon as you hit 3000 rpm it revs clean and gets the power down as it should.

Now, so far I have cleaned, set up and balanced the carburettors, cleaned the fuel filter, reconditioned the fuel pump (although that wasn't related) the bike is using new plugs and caps. The coils have been tested and test absolutely spot on (resistance comes back at 15.1 ohms, tolerance is 13-17 ohms, props to the haynes manual), and due to a rather embarrasing moment at the MoT station a new battery. This leads me to believe the fault lies with the CDI unit.

My first question is, have I missed anything?

Secondly, if the CDI unit is faulty, How is the NC29 restricted for the japanese market? a lot of places say its restricted by the CDI unit, but ive seen in a few places that apparently there is a sensor in the speedo that once the bike hits 112mph / 180kmh it stops the CDI unit, restricting it to that speed.

any info on the poor running would be great as its just doing my chuffin box in trying to figure out whats wrong im almost at the point where i might do something silly and pay someone to fix it

Its not fair that a beaut of the CBR400 spends so much time in bits for so long

[GF Racing]

Please Post a picture of your exhaust headers ... do you have A or B spec carbs.

Cheers

[dunkenb]

Fit a blue kmh to mph speedo convertor to get round the restriction.
is the problem at or just below 1/4 throttle? Probably blocked slow jets or the little intakes for the fuel/ air ratio
(just by the main inlet on the carb under the black plastic cover that holds on the trumpet)

[Davez29]

If you try everything but dont come up with an answer suspect your stainless pipes. The ones i got from ebay dealer sandy bike spares, show the same problem. I tried most things but its never fully cleared. I just over rev if needed. Sounds crap and the clamps have been re welded three times as well. The worst thing i ever bought my bike!! Anyone got any oem R model headers gimmi a pm.

[kingarthur83rd]

If you try everything but dont come up with an answer suspect your stainless pipes. The ones i got from ebay dealer sandy bike spares, show the same problem. I tried most things but its never fully cleared. I just over rev if needed. Sounds crap and the clamps have been re welded three times as well. The worst thing I ever bought my bike!! Anyone got any oem R model headers gimmi a pm.

Funny you should mention that, me too. it was a fair price and since fitting them ive notice a horrid rattle c. 5000rpm but clears after 6000rpm. (and guess where I do all my pottering about. c.4000-8000 rpm :/ )

In fairness it does make a little sense, as I changed the engine and carburettors (not related) due to a faulty gearbox and I really didnt have the motivation time or interest to strip 2 engines just to swap out the gearboxes. the mild steel headers were quite happily rotting away, so it made sense to swap the engine and pipes at the same time. Oddly enough, this is when I started to notice it (but with everything new/unknown to me, plugs, carbs, filters, exhausts, gaskets etc) so I began to troubleshoot.

anyway, i'll get the CDI / caps tested just for peace of mind. Delkevic do a set of headers for the NC29 on ebay now for £190 which I may need to buy... eventually.

[Davez29]

I have read that the delvic ones have a similar problem around mid range on some bikes. I think the only good pipes seem to be the original ones. Maybe Graeme France can advise you better as he seems to do a sucessful set. Although the headers pictured on his site seem to be a delvic picture, unsure.

[kingarthur83rd]

Please Post a picture of your exhaust headers ... do you have A or B spec carbs.

Cheers

A or B spec carbs??? I dunno, what's the difference?? its an RR-L (1991) machine.

The exhaust itself is a unit from Sandy Bike Spares I bought a couple of years back. I didn't really notice the power low down was missing until I took her for a scratch to work (stuck in traffic on the daily commute) and with the british weather being as it is... not a lot of miles has been done since they were fitted.

Pics of the exhaust to follow, once shes back on the bench.... again :/

[kingarthur83rd]

Guys

Many thanks for the support on the site.

Offline i thought it best to try and nail down if the fault was mechanical or electrical. After a casual peruse over google, it turns out the head office / workshop of Quill exhausts is right on my doorstep (how did i miss that!) anyway, i took it for a potential dyno run to figure the fault. the rather helpful chap at quill took one listen and stated the issue was fuelling.

With more modern fuels, when they evaporate, they can leave a gummy-like residue which if left untreated can block the carbs up through the 2nd idling jet, causing the bike to run like a bag of spanners at low revs. the 2nd idling jet is too small for the human eye to see though so its nigh on impossible to check ifits blocked. "in these cases the only way to clean out the jets is the use of ultrasonic cleaning, as it will break down the residue to clear the jet." - but having sit down and think about it, the science does make sense.

after limping the bike home, having said 'fuelling' i took a look at the headers, and sure enough, barrels 1, 2, 4 all had blue-ing on the manifolds, barrel 3 was just starting to blue.

so, begrudingly and at a cost of 60 beer tokens, (soley due to the fact I dont have an Ultrasonic cleaner (although ebay is about to change this)) the carburettors have been ultrasonically cleaned. carbs now back on and shes running perfect. revs clean and free, no flat spots, and best of all will engage drive at pretty much tickover without coughing and dieing!!



Hopefully this post will get to some of the fair weather riders upon us, so they wont need to

oh and GF racing: I might need to speak to you re: carburettor rubber inlet manifolds, mine are looking a wee bit manky and tired after 20 years on the bike! ^_^