NC35 carb change

[speedy231278]

Aaargh! The bloody order has gone back to estimating shipping next Monday....

[Maelstrom]

You have it correct Speedy. Not forgetting to lower the idle first. The difference will be more than nothing. Air screws are not that sensitive. If you can get it down to half a turn each way on an air screw then you are a legend and your engine is a gem. By the way, you might want to pop the lid on your air cleaner so that you can hear the intake.
You should synchronize first as it is the coarsest of the two adjustments. In other words, you will chase your tail if one butterfly is open more than another.
Assuming you are putting your replacement carbs in, then manually synch the butterflies on the bench first. I have seen a lot of different posts about this but my version is to use rice paper, as in cigarette papers. They will be about as close to perfect as you can get. Then do the mixture screws after you have run the bike. The engine has to be warm of course. If your synch gauges are showing a major discrepancy then something is wrong with them or your engine because you have set your butterflies to damn near perfection already. The trouble with vacuum gauges is that it assumes that all 4 cylinders are in the same mechanical condition, just like the mixture screws. I always used mercury tubes, but they might be banned now. I have nothing good to say about mechanical gauges. Perhaps if you used some that had a certificate of accuracy then I would consider it, but they might cost the planet.

[speedy231278]

Turns out that my order DID get shipped yesterday, and will arrive tomorrow via UPS. However, the reason it still shows pending is because the fuel joint o-ring kit is on back order and expected to arrive with them on Monday. Quite why they didn't hold on to the rest seeing as it's obvious that there's no point in having all the seals for the carbs save for the stuff right in the middle is beyond me. Hopefully they'll go UPS too, otherwsie if they go surface mail I expect the RM will lose the bloody things and/or take ages to deliver and I'll still be waiting for a free weekend to do the carbs the other side of Xmas... lol

So, sync butterflies on the bench first, check with the vacuum gauge, then spend hours chasing round and round the air screws to get them within half a turn. Instructions for the vac gauge say the engine wants to be warmed a bit before connecting, but not full running temp or it will overheat by the time you've finished, and also risk burning yourself when fitting the vac feed screws. I guess common sense would say fit the thing before starting the bike... lol Would removing the air filter lid not affect the carburation? I can't decide if the air flow might be improved or altered, but I guess as the bike will only be at idle or just above, it's not going to be demanding the max airflow anyway.

Yes, the Carbtune is mechanical, not mercury. I don't know if mercury ones are banned or not, however I don't see why it would not be possible to use another liquid dyed a colour that makes it nice and visible, or is the relative density of mercury a useful property? I know that vacuum is often measured in inches of mercury, but for the purposes of balancing we're only comparing levels, not measuring the actual figures. The Carbtune gauge has had lots of positive reviews on various fora and websites, hence me taking the plunge. Will be interesting to see if there is a major difference between the cylinders after the sync on the bench as the engine has a little over 116500kms on it now...! One day I might have to check the compression to see how well it's doing. It feels a little flat, and the carburation is pretty rubbish, hence me doing the carbs. Supposedly this is mainly down to wear on the existing ones, but I wonder how much is the engine being tired. It still has plenty of top end though, and will chug around town at 30 in top.

[Maelstrom]

re Carbtune, sorry I didn't know you had one of those. it should work just fine, I would buy one too. I was talking about those ugly round things

Yes it will be interesting to see the difference between the bench synch and the carbtune. Since the task is to synchronise the butterflies and you have already done that to within the thickness of a rice paper, or less. The manometer is just going to tell you the difference in the health of your cylinders. I would not change my synch based on what the vacuum is. That is for roughly setting carbs that are out of synch to start with.

As you say, popping the lid off the air cleaner won't do squat difference at idle. Don't spend forever on it, your engine might melt. With that mileage if you can get +/-1 turn I would be happy with that. Take note of your settings. If it does not feel right when you ride it then take try setting it back to standard values to see if it is better or worse. The best idle may not yield the best low speed running.