NC21 - Advice/feedback required on running issues

[FastMart]

Hello All,

Thanks in advance for any feedback or suggestions regarding the below.

Bike was running sweet as a nut last year and was garaged with storage prep from November till April.
After going through the usual checks after storage, start-up.... The bike is running crap..!!!

Symptoms
Using full choke the bike starts and picks up to higher revs as expected and once it starts to warm the revs drop off slightly and the choke can be turned off where the bike then idles ok and revs up as expected crisp and clean.

After the engine starts to get warmer the idle starts to drop away and the bike cuts out.

The only way to start the bike at this point is to hold full throttle and crank the bike until it fires (30 secs), however it then seems to sit on low revs and does not burst into full throttle for anything from 30 secs to over a minute.

Blipping the throttle the engine will keep running, however easing off the throttle it will again drop off and cut out.

It did also seem to hunt slightly when revving high before dropping back to low revs then cut out. (spraying rubbers did not appear to show any air leaks)

Obvious action was to carry out a full clean of the carbs.

I have removed the carbs and carried out extensive cleaning of all components/passages using ultrasonic cleaning, carb cleaner and compressed air (All components removed first)

Slides removed cleaned, diaphrams checked for tears, leaks, checked needle (single washer fitted on all)
Slide Springs checked for correct length, slides checked for vacuum/operation after installing using hoover.
Choke plungers removed, cleaned, checked and adjusted set-up for full closure and opening.
Low speed needle valves removed, cleaned, checked. put back and set to 1 1/2 turns as was when last running, also tried 2 turns. no joy.)
Starter jets flushed through with carb cleaner, passages checked with micro fine wire and compressed air (No blockages)
Main jets/emulsion tubes removed separated, ultrasonic cleaned, carb flushed, all holes clear. Checked for correct jet installed. (112 Rear/110 front)

New needle valve and seat have been fitted (NRP)
New slow/pilot jet has been fitted (#35) (NRP)

All fuel pipes and link pipes have been cleaned and are flowing through.
All Floats checked for left & right flat/level top and bottom, Floats re-installed and bowl level checked at 6.8mm
All butterflies have been bench synced and are moving in harmony as expected.

Carbs re-installed fully into manifold rubbers, clamps tightened fully.

Re-installed air filter, fuel tank, fuel pipe and vacuum pipe. (Fuel tap checked for flow prior to installing)

Bike re-started again using choke and exactly the same symptoms as previous.

Also tried with temporary fuel source with vacuum pipe blocked....No change.

Knowing how critical cleaning is on these carbs, I of course carried out the same cleaning process another two times and issues still remain.

Note: The valves were checked/set as required this time last year, bike had probably done around 300 miles dry riding.

Spark plugs are iridium NGK-CR8EIX and were fine when last running.

With my carbtune pro connected, balancing only required minor adjustments to get a level around 18.2 with tick over adjusted to around 2000rpm for this and then backed off to 1300rpm.

Questions
Any ideas on possible causes for the above issues ?

Cheers
Martin

[Neosophist]

did you pay attention to the choke levers on the carbs, there are 4 of them, one for each carb, operated by a cable pulled arm on the top of the carbs.

Perhaps one or more chokes are not backing off when you close the choke off?

this would cause the rich / flooding issues you describe

how do the plugs look? black?

really sounds like over-fuelling.

only other thing i can think of causing such a rich symptom is a leaking float... if the chokes seem to go off propelr (looking at the actualy mechanisim on the cabrs, cable dont mean much)

put the cabrs on the bench and use a separate high up fuel bottle to gravity feed the carbs.. fuel shouldnt leak out the carbs.. its hard to tell when they are on the bike as the fueltap is vaccum controlled.

[FastMart]

Hey Neosophist,
Thanks for your response, much appreciated.
I did check all four choke plungers and the adjustments of the rails/lever after I cleaned and re-installed them.
All appeared to be opening and closing fully.
I will however check them again as advised and also carry out the gravity feed test.
Will also check the plug colour/state.
I will let ya know when I get chance to do this.
Thanks again
Martin

[FastMart]

Hey Neosophist,
I drained the oil and....petrol in the oil...Very thin and watery
Whats your advice moving forward ?
Cheers
Martin

[FastMart]

Just seen this explanation on another post which makes sense when there is a leaking fuel tap,etc.
Which may be a possible cause for my issue.
Next step is to check the fuel tap and carb using gravity as you advised.
Thinking back I did have the same symptom as below at one point with a hyrdo lock in the past.
------------------------------------------------------------------------------
1) The diaphragm in the fuel tap isn't sealing properly;

2) when the tap is in the "ON" position and the engine is running, the engine sucks some liquid fuel directly from the tank into cylinder 3 via the vacuum hose take-off in the cylinder 3 inlet manifold;

3) when the engine is running some of this excess fuel is burnt in cylinder 3 but some (most) of it makes its way past the piston in that cylinder and gets in with the oil;

4) the circulating oil/petrol emulsion gets everywhere as the engine rotates;

5) since oil and petrol don't mix, and oil is denser than petrol, once the engine is turned off the emulsion starts to separate as the oil sinks to the bottom of the block/sump and the petrol floats on top of it;

6) The petrol has nowhere to go except up through any tiny gaps between the piston rings and the cylinders, so eventually it will end up on top of the pistons in the combustion volume, locking the engine.

CREDIT----->AdrenalineJunky for this explanation.

[Neosophist]

Hey Neosophist,
I drained the oil and....petrol in the oil...Very thin and watery
Whats your advice moving forward ?
Cheers
Martin

Hey fella,

looks liek you are making progres..

yes.. thats the most common expalnation for why you get fuel in the oil... overflowing carbs leaking fuel into the engine.

the carbs have float pins in them (sorry if teachign you to suck eggs)

much like a ballcock in a toilet... when the tank (float bowl) is full then they shut off the fuel until the level drops.

This is quite important in a carb (float height setting) as the level of fuel in the bowl also has an affect on the low speed jetting as the air pressure in the carbs sucks the fuel up into the engine.. too high / low and you suck the wrong amount of fuel and get shit running.

Hondas have a vaccum fuel tap, which means that when the engine isnt running, regardless of tap position then the fuel shouldt flow (a secondary safety system) that prevents fuel leaking in carb failing and in a crash scenario as if the engine is off fuel stops flowing (a jammed open carb in an upturned bike can leak fuel until tank is empty for example)

So you have at least 2 problems that need fixing here.

1. Fuel vaccum tap...

its part of the tap itself, its well documented and you can get rebuild kits (3rd party) do a google search for nc21 tap rebuild kit.

They have come down quite a lot in price now...

You need to check this first.. it might be working and your draining just the fuel from cabrs / lines.

Example Test procedure:

switch tank off and Remove tank... be careful.. if the fuel tap itself besides the vaccum tap has failed it may leak fuel.

connect a small hose to vaccum hose on tap (when removing tank good idea to replace vaccum hose if its not been done in a while.. its cheap and only needs doing every 5 years or so and if it goes when riding will cause a world of pain.. been there

place tank over bucket and switch tank to on / reserve.. there should be no fuel flow.

with tank still in on / reserve suck on the small vacum hose and fuel should flow.


Many times people bypass the vaccum tap if its old / failed instead of rebuildilng... its there to hepl you out in an accident, some people just turn the tap off when parked up incase the bike falls over / carbs fail but you cant often do that in a crash, since its relatilvly trouble free when repaired i always go for a fix rather than a bodge save it coming back to bit me in the ass.

Your discretion how you proceed with that.. but it does have its merits when working (its lasted 30 years already!)

if the tap is leaking even in the off position gonna need a whole new tap.

Carbs:

These need fixing anyway.. a leaking float bowl isnt somethign than can be fixed by turning tap off at engine off.. it will mess up the fuelling for the bike and can lead to ruined plugs, ruined oil and a tow truck home.. too much fuel in engine can lead to ruined engine... float valves are not cheap, sometimes a good 2nd hand set is cheaper but with new components going to last the age of the bike.

However, might not always be the float valves that have failed, time to investiage.

1. the floats are plastic that float.. one or more could be stuck with crap stopping them rising and shutting fuel off.
2. the float pin could have some shit in it from the tank between the pin and the seat causing a bad seal.

I test carbs by getting them on a bench.. use a decent length hose with a fuennl and brim it with gas.. look for the carbs that are overflowing fuel... can take an hour if its a slow leak or 2 seconds if its a bad one.

while the carbs are off good idea to clean and check settings on all carbs anway (note original settings first)

mark any carbs that are leaking so you can double check later.

remove float bowls and clean any shit out.. remove float and inspect for damage, inspect the float pin seat.. i usually clean these out with a cotton bud to remove any crap.

make sure float heights are set correct and floats all move nice... dont mix up float pins.

reassmble carbs and bench test again for leaks (this will also fill the float bowels with fuel and aid starting)

[FastMart]

Hey Neosophist,

Thanks for the latest information and advice.

Progress report so far today,

Fuel Tank Tap
I checked the fuel tap and it appears to be working fine.
(No fuel flow when switched off, ON or RES)
(Fuel flow when set to ON - Vacuum applied through vacuum pipe)
(Fuel flow when set to RES - Vacuum applied through vacuum pipe)

Vacuum Pipes/Breathers
Vacuum pipe to cylinder clear and free flowing, tight fit to connections.
Tank breathers are all free flowing

Carb Floats / Needle valve/seats
Checked all floats again and they all appear to be set-up correctly.
Float heights are 6.8mm
All floats free moving freely
No dirt/gunk,etc present in bowls
Needle valves and seats were purchased new from NRP ony this week and they do seal off the seat inlet when down, when up air passes through the seat into the bowl. (Blowing through main fuel feed pipe to test this)
Gravity feeding the carbs and there appears to be no signs of fuel leaks at this stage. (I have left this set-up to be sure)

Spark Plugs
All spark plugs were as expected....Black sooty.
While plugs were out I carried out a basic compression test, results below.
Cyl-1 = 12.5kg/cm2 (178 psi)
Cyl-2 = 13.5kg/cm2 (191psi)
Cyl-3 = 12.5kg/cm2 (178psi)
Cyl-4 = 13kg/cm2 (182psi)

Oil in fuel
I measured the amount of oil/fuel drained from the sump and there was almost 4 litres

That is as far as I have got so far.

Other than feedback on above....A quick question....
Are the slide needles different on the NC21----Front/Rear ?
Does the NC21 slide needle need only the single washer or does it also benefit from an extra 0.5 washer ?
Cheers
Martin

[FastMart]

Neosophist,

Update on gravity feed testing.

Left overnight and the level in the funnel dropped by a small amount. (Evaporation??)

There was no fuel in the catcher bowl below carbs and I could not see any leaks anywhere.

Any further advice on the above and previous report details?

Cheers

Martin

[Neosophist]

Strange one there then.

Everything seems good from what you've said.

Black / sooted plugs deffaintly show its been over-fuellin at some point.

Could have had a sticking float bowl that has corrected itself... soemtimes it happens when fittign carbs.. a float bowel will jam open.

you can test this after fitting carbs by doing a gravity feed to them for a while.. if your fuel starts draining away rapidly leave carbs in place and tap on the bowles with the handel end of a screwdriver, this jolt is often enuogh to dislodge a sticking float.

if it were me my next step would be to wire brush the plugs up, refit the cabrs and try it again.

[FastMart]

Hey Neosophist,

Yes...I hate Bill gates specials

Thanks again for your feedback.

I will be cracking on with this again over the next few days and will let you know how it goes.

Playing it safe---------> I will certainly do a gravity feed once the carbs are back on the bike before moving on and this will also help with starting

Tank flush, fresh fuel, tap tests again and then......Ignition

Cheers

Martin