NC24 advice before I take a sledgehammer to the carbs!

[fasttom]

Hi all,

I have a 1987 NC24 which I am in the process of reviving. It was and currently is running on the front two cylinders only. I have run a compression test on the rear cylinders and they both achieve close to 10 Bar, this is in-line with a front cylinder which I tested to compare. Whilst the spark plugs aren't in amazing condition both have good strong blue spark. I mjust confess that I haven't checked the gap but assume all is ok. Carbs have been off twice for a clean, jets removed and soaked for many hours in carb cleaner. I also have checked visually prior to re-assembly to see that the jet holes are visible. Choke linkage bar has been adjusted and I have checked that all chock plungers move in and out as they should. Each time I have run the bike I have removed the rear cylinder plugs to check to see if they are wet (getting fuel) however they don't seem to be getting any . I cannot understand this as the carbs have been cleaned. Short of sending off to be professionally cleaned which I want to try and avoid as you never know how rough they will be and to source new seals are like rocking horse **** or trying to source a set of known working carbs, which again will be near to impossible I am all out of ideas.

Any advice/recommendations highly welcomed. Thanks in advance.

[CMSMJ1]

Chill!

When you open the rear carbs drain screws...does anything come out?

Start simple..floats, fuel lines and so on...

then swap front to rear on the plugs as you know they work.

[fasttom]

Have opened drain screws on rear carb float chambers and plenty of fuel came out so I know fuel is getting to the carbs. I am not sure if the fuel is getting to the chambers, I suspect not as the plugs were dry when removed for inspection. I will try swapping plugs from front to rear tomorrow. Does anybody know if you can source NC24 plugs easily from any auto retailers. I know David Silvers stock them but don't want to wait to get them posted out?

[Maelstrom]

The carb seal kit is the same as the NC30. Honda Part number is 16010-MR8-505. You can use my NC30 kit at http://litetek.co/Carb_Kit_Honda_NC30.html but it does not contain the airbox seal. You could probably source an o-ring for that one. If you are in the UK you can get them from Rick Oliver.

Here is some steps that might help in diagnosing the problem.
Remove airbox.
1/ Squirt some fuel into the rear throats.

2/ It fires and runs on 4 cylinders otherwise go to 5
When you crank the engine over place your hand over the mouth of the rear carbs. This will choke them and should cause a significant amount of fuel to be pulled into the throat.

3/ Fuel is pulled in. You can smell it, see it and hear it. Otherwise go to 4
If it then fires on the rear cylinders you should check for vacuum leaks. Although no one talks about throttle shaft seals I would advise checking them regardless. Also the inlet manifolds. I have just used oil from an oil can to check for leaks around the throttle shaft but I have heard others using engine starter and spraying it at both ends of the throttle shaft. If there is a change in engine speed then you have a leak.

4/ Throats remain dry. It is a fuel blockage.

5/ It does not run on 4 cylinders. It is electrical.

Hope this helps
Blair

[Neosophist]

Have you messed about with the balancing screws? When you had the carbs off did you balance them visually first to get them close?

If there well out there might not be enough suck to pull fuel into the rears.

[fasttom]

Blair many thanks for your diagnostic advice and pointer for the carb seals. I will try to squirt some fuel into the rear carb chambers this afternoon and see where that gets me.

I haven't touched the balancing screws and haven't even looked at them which I suppose is an oversight admittely . What is the easiest way to visually balance the screws, presumably measuring screw height out with a digital vernier?? I know there are three balancing screws will they all need to be set at the same height??

Your advice is appreciated in advance.

Tom

[CMSMJ1]

Balancing screws relay on engine vacuum to ensure correct balance. the quick and dirty method is to adjust them so that the butterfly is open the same amount.

Often called "the drill bit method" - as you can use a pair of 1mm drill bits to use as the guide. Balance them using the 1mm bit instead of a vacuum measurement. It will be close enough..

[fasttom]

Right following the advice given I have taken the following action:

1/ Known working spark plug from front cylinder swapped with back cylinder to observe effect. Rear cylinder did not fire meaning that it is not plugs causing the problem.

2/ Raw fuel squirted into the rear cylinder carb throats, bike temporarily revved up to approx 5k and then gradually settled back to ilde. From the sound of it though it seemed that it was still only firing on the fronts but I could be wrong.

3/ I removed the carbs again and visually inspected the two rear cylinder carb float bowls. Both are clean inside however I notice that there was a small amount of gunk when removing the pilot jet, however, jet holes appeared clean.

In reference to the last posting, I am slightly confused as to what you mean by using 2x 1mm drill bits to visually balance the air screw settings?? where would you insert the drills to balance the air screw settings?

Short of getting the carbs sent off to be professionally cleaned I am rapidly running out of ideas and patience.

Any advice/recommendations of professional carb cleaning companies would be appreciated.

Many thanks

Tom

[Maelstrom]

The plug leads are all in the correct order I hope. I do not own a V4 so I don't know if it is even possible to get them mixed up but it sounds like a possibility. You should get an obvious response when fuel is squirted into the throats.

Lets look at point 2.
How cold are the rear exhaust pipes before you squirt fuel into the rear carbs?
How about after.
You need to distinguish what happens when you squirt fuel into the rear carb throats. 'Maybe it fired' is not going to help you.
Did the rpm increase by just adding the fuel, or did you increase the throttle at that time?

[CMSMJ1]

balancing the carbs - you use the drill bits in the butterfly - wedge thhem open.

You then use the mechanical screws on the linkages that balance the throttle openings.

Nothing at all to do with the air screws that you are thinking of. You do not balance carbs with air screws.