NC21 with NC24 carbs

[homeboy]

Hi everyone

I've been doing some much needed maintenance and i have found few troubling things about my NC21. First there was a broken bolt on the crankshaft holder, luckily it came out easily. Tightening torque should be 12Nm according the NC21 specs i got, but these bolts were tightened at much higher torque so I am not even surprised one bolt was snapped off. Second issue was the valve clearence - almost none at all. So I recommend to anyone who is buying old vfr to check the valve clearence. Next thing i noticed is that the carbs i replaced were slightly different. I bought them as NC21 carburators but it seems they are NC24 (32mm). After I got the bike running again and synced the carbs together the bike still runs as if they are out of sync (holds 1300 almost as it should but the 1300-5k range is unstable - revs to 5k). I am not sure if its caused by the differente size of carburator intake or something else. I already set the mixture screw to 2 and 1/4 turns out. I haven't checked the float level on new carb yet and i will when i get the chance - i forgot to check it when carbs were out. Do you have any other ideas how to fix this issue or what else could be causing it?

Sorry for any typos - I am not native english speaker.
Thanks,
Lukas

[Neosophist]

NC21 carbs do not have the rubber trumpet from the top, the NC24 does... picture will help.

What size is the jets?

You can get either to setup fine.. perhaps you have the airleak from the rubber.. it is sometime very old and broken so the air will leak.

[homeboy]

I will try to get the picturers next week but its indeed carb with rubber trumpet. Jet size is unknown but i will measure them next chance I have. I am not sure if jet size is an issue between 1300 and 5k but since its different carb it will probably need "original" jet size to run properly above 3 or 4k. Rubber leak could be the reason since its NC21 rubber fitted to NC24 carb. Do you know what size is the engine intake on NC21/24? If its the same it could be solved with NC24 rubbers.

[Neosophist]

If the nc24 carbs are setup properly and the rubber is not leaking then they should run ok just by bolting them onto the engine!

new carb rubbers migh be good as nc24/nc30 use same rubbers but nc21 is different...

nc24 carbs should be 115 jet all around.. make sure pilot screw is set properly and float height is ok, also slow jets are all clear.. these 3 things are very important to the rpm range you have problem in.

[homeboy]

Carburator images as promised:


Serial number on the carburator:

[homeboy]

Jets are 112 and 115. Float height is around 6.8mm (+/-0.5mm) on all of them. Gauges show that all carburators are synched. Problem still persists, I think there has to be an air leak around the rubbers. They seem to fit, but its really hard to get the carburator on them. Condition of rubbers is good but since they are for 30mm carburators they might be letting air in.
There is one more issue with NC24 carbs, coil holder won't fit anymore.

[Neosophist]

Jets are 112 and 115. Float height is around 6.8mm (+/-0.5mm) on all of them. Gauges show that all carburators are synched. Problem still persists, I think there has to be an air leak around the rubbers. They seem to fit, but its really hard to get the carburator on them. Condition of rubbers is good but since they are for 30mm carburators they might be letting air in.
There is one more issue with NC24 carbs, coil holder won't fit anymore.

Im pretty sure your carbs are NOT in the rubbers properly, this is causing the front to stick up too much and the coil holder not to fit.

Ive never come across the issue where the coil holder doesnt fit unless the carbs are not seated properly, even with bikes that have had the carbs swapped (although i forgot if the rubbers were changed too...)

this will of course explain your air-leak too if there not seated.

btw two of your float bowls are on backwards... the two that have the drain screws facing outwards need to be changed so all the drain screws face int he middle.

Carbs are always troublesome to get on until you find the technicque, my first ever time i nearly set fire to the bike it was frustrating but now its 2 minute job, even easier if you have new rubbers.

So yea.. i think your front bank isnt in properly hence the air-leak and crap running and coil not fitinng...

[homeboy]

I have taken some new pictures of carb rubbers. As you can see from these, the carb is as close to the rubber as possible. But there is a shadow on one side of the rubber.

I think that the bottom side of the carburator outlet pipe is pushing on the rubber making a gap at the top side (between carbs->rubber). Buying NC24/NC30 rubbers might help. Does anyone have detailed photo of NC24/NC30 rubbers before I order them?

Rear 1:

Rear 2:

Front 1:

Rear again:


Ps: float bowls are all the same swapping them won't change the direction of drain screws. I haven't messed with them so they are as they were when I bought the carburator.

Btw thanks for all the help Neo

[Neosophist]

Ps: float bowls are all the same swapping them won't change the direction of drain screws. I haven't messed with them so they are as they were when I bought the carburator.

Btw thanks for all the help Neo

Yes im confusing witht he nc30 carb which you can make face in or face out.. they look right from memory.

But you have a big gap around that rubber, that looks bad!

have you tried to test for air leaks by spraying "easy start" or something flammable aroudn the rubber when the engine is running? if there is a leak it will get sucked in and you can hear the soudn of the engine changing...

[spikenipple]

Further to the coil fitting issue, I can't seem to get the airbox base back on with the coils fitted. There is a large tray protruding from in front of the front two airbox>carb screws that goes precisely where the coils sit. Do I have the wrong airbox?