Carb questions for NC30

[bkk2009]

Carbs have always scared me. My first memory of them was when I was 12 years old and watched my mom's boyfriend and 2 of his friends drink alot of beers and curse and yell at a carb that was sitting on our kitchen table. They tried for hours, with many beers along the way, to get it in working order. I am not sure if they ever did get it right.

So here we are now, 30 years later, and this is my first go so I am cleaning the carbs on my 2 NC30s, following Cammo's carb jetting guide. I am not confident in my local mechanics here in Bangkok, as carb cleanings range from 200-300 baht (6-10 US dollars), and they are really fast about it. Since this is one of those jobs that you can never really tell if it has been done (no returned worn parts, no visual cues of shiny new bits on your bike), who knows if they have ever been cleaned the dozen times I have taken them in for the last couple of years.



Red bike is a fully running bike until last week when she stalled on me while normal road riding. I have had issues with reg/recs and CDIs on both bikes, and the CDI in the red bike was from the black bike. I was waiting for a new CDI from japan. It runs like it has fuel blockage or a vacuum leak – rough idle, non-responsive throttle in all gears and neutral, whether sitting on the side stand or under load while riding. Also, it has sat in the rain lately, and it rains nearly every day here.

The black bike was running but with issues I suspect also relating to vacuum leaks. It would not rev over 8000 rpm in any gear or neutral, with or without a load. So I am changing reg/recs and CDIs on both bikes, cleaning both carbs, and checking over all for vacuum leaks.

There are differences with each bike:



Black bike has a hole in the air box and also no flap

After opening up the carbs, here is what I found:



Draining carb bowls After cleaning both carbs, this red, fine silt or sludge was left



Red sediment everywhere from low quality gasohol in Thailand. Hard to find good fuel.

On the red bike found that the mechanics messed up the mains. The 2 fronts were 118 and 115, the 2 rears were 115 and 118. So I corrected that.

Should I change these to 120 fronts and 122 rears? Also, all emulsion tubes front and back have 5 holes. What effect does running 5 hole emulsion tubes in the front carbs have? I purchased new #35 emulsion tubes and they have 4 sets of 2 holes each, for a total of 8 holes. Which should I put in, 2 sets of 5 small holes, or 4 sets of 2 large holes?

100/105 on the black bike
New emulsion tubes arrived today

On the black bike, the front mains are 100s, the rears are 105s. Both bikes run an aftermarket exhaust. So the black bike is way off.

Should I jump to 120/122s, or something in between? Maybe somebody decided to lean up the carbs for a reason, but wouldn’t the missing flap and the hole in the air cleaner also lean it up, for a very lean running bike? Why would they want to do this?

[Stoozak]

I'm afraid I can't comment on the emulsion tube question, but I can give my 2c on the jets.

As I understand it aftermarket exhausts allow better breathing so bigger jets are needed. I upped mine to 118/120 and the bike goes like a rocket.

I would block off that hole in the air filter.

The other day I had an issue where I could not get the bike to rev up passed 8000 - 9000rpm at wide open throttle. It turned out to be blocked main jets in 2 of the carbs, so you might find that after a good clean and proper jetting your revving problem goes away.

[AdrenalineJunky]

I think the reason for the black bike running badly as you described it, is definitely that the combination of (1) very low octane alcohol-rich "petrol", (2) free-breathing exhaust and (3) ridiculously small main jets, leads to insanely lean running.

Again with the emulsion tubes: more holes = leaner running (see this explanation of emulsion tubes http://www.rotaryengineering.com/faq.html and this diagram of emulsion tubes in a Weber carb http://www.classicandperformancecar.com ... ttors.html)


Why would anyone want it to run lean? Only thing I can think of is because it uses less fuel. But it is bad for the engine to run lean as it runs hotter and that will eventually be a much more expensive thing to sort out than the "extra" fuel would have been...

I was also scared of carbs until I took mine apart. There are some good things on the internet if you want to understand how they are supposed to work, eg http://www.explainthatstuff.com/how-car ... -work.html

I am no expert but I would suggest fitting the standard emulsion tubes if you can get hold of them and then jetting at least 120f/122r to run "track rich" to compensate for the bad fuel and free-breathing exhaust. As a rule of thumb I'd say it's better to run too rich than too lean... Also running rich means you dispose of that awful alco/gas fuel quicker so everyone benefits

Good luck with your bikes!

[Trinirides]

My 2 cents,

Get a new silicone vacuum hose to the fuel tap as heat can cause the older ones to close up, I have had that on one of my NC's with symptoms as you say. Also clean fuel tap.

Carbs off give a goot clean, maybe change screws out for allen head bolts not sure if you can source locally but most NC carb screws are a pain to remove. I would also say change the carb rubbers that hold the carbs to the engine these could have a air leak causing issues. Buy genuine OEM Honda parts but these can take a bit to fit, look for a guide online to help, nothing major but can be a pain to get pliable enough to fit.

Another good thing to do is clean the earth plate on the LHS of the bike situated infront of the coil pack, looks like a V with a engine mounting bolt running through it, amazing how many bad running NC's are due to a poor earth connection.

[bkk2009]

Thanks for all the feedback! Just to update, I have the red bike running, and decided to go 120/122s for both bikes. For the red bike, new CDI, new reg/rec off an R6, carbs all sorted, but the idle adjustment cable doesn't seem to work anymore. I will have to sort that one. The red bike runs good, but different! I can't put my finger on it. Can anyone tell me how running the bike richer will affect the engine performance or response?

The black bike - fargh this bike has been nothing but problems. I haven't been able to get it started after cleaning and sorting the carbs, but also haven't had much time to devote to it the last several months. So I am just going to take it in to have it serviced and sell it.

[Maelstrom]

I have viton seal kits for NC30s and I am also in Thailand.
These are resistant to ethanol blend fuel. Not that I suggest using the stuff.
http://litetek.co/Carb_Kit_Honda_NC30.html
cheers