race nc30 carb issues

[rigsby]

Hi there,

Getting my nc 30 prepped for the coming season first race is in 3 weeks time so im running out of time. After purchasing the bike which seemed to run fine on its paddock stand when viewing didnt run as nice on the throttle, although the bike would rev around to 13000 it just had no guts there was no surge of power. I decided to fettle the bike so re shimmed valves so they are all now in spec did a compression test and although not the best at roughly 170 psi each cylinder was at least within its limits. I checked carbs and noticed they were running 127.5 mains on rear cylinders and 125 mains on the front as its got a stock airbox etc i decided to down jet to 117.5 rears and 115 fronts. I tried the bike this afternoon and it was running exactly the same as before the re jetting has had no effect. I then removed tank and removed the airbox cover so i could check for air leaks on the inlet rubbers etc, so i fired it up and this is where i noticed that the slides were not lifting, even when i opened the throttle and the revs went up to 10,000 they still didn't budge, if i manually move the slides with my finger they will push all the way up and then return to its idle position but with the engine running they dont want to lift. I reckon it had the same issue initially and this is why the rejetting has made no noticeable difference. I know i need to check the diaphragms for splits and to make sure they are sitting right but is there any other explanation for them failing to move and also at what rpm should i expect them to move i was guessing around 3000 rpm, any help greatly received as i am running out of time.

Thanks

[TessierAshpoolSA]

Huh. this might be why mine is running like a piece of shit. I will run it up on the stand at the weekend and have a look at the slides.

No help to you sorry.

I did have a chat to a mechanic about it and if the diaphrams are stiff they are NLA from Honda, but David Silver Spares put me onto these guys:



NRP Carbs and Diaphragms
7 Park Street
Off Cheetham Hill road
Manchester
M3 1EU

Telephone: 0161 832 8646

Email: andy@nrp-carbs.com

http://www.nrp-carbs.co.uk/shop/index.p ... yword=NC30



I also read that if you take the diaphrams out of the carbs and soak them in petrol it will soften them up. I havent tried that but that is the cheapskates way forward - I am in sunny New Zealand so the "send the carbs to them to put new diaphrams on" option is not an attractive one to me.

[rigsby]

Hi,
Thanks for reply, i investigated further and two of the diaphragms werent seated properly but they seem so small in dia. i think they have shrunk so im going to soak them and try and work them before attempting to re seat them hoping they stay in the grooves, also i noticed the plastic T piece that joins the two carbs together should have rubber collars to aid sealing between the carb bodys and the t piece, these t peices join the chambers where the diaphragms sit, i have cured this by cutting sections of electrical heat shrink then shrinking it around the join, mind you i dont think this will help as the pipes are connected to the airbox lid so even with the airbox lid off and filter out you should still see the slides move as the throttle is opened. I will try again later, thanks for info for replacement carb parts.

Thanks

[Neosophist]

yeah weird one there..
they should move at least a little.

https://www.youtube.com/watch?v=N3hkTaXcI2A


tried putting your hand over the intake while its running.. the change in pressure should pull the slideup fullly, will probably flood out the motor and youl lhave ot move your hand quickly to see as the spring will push it back down.

[skinnydog0_0]

I had this issue with one cylinder on my HRC NC30 carbs.
I took the slide out sprayed carb cleaner down the air air tubeand refitted - hey presto works.

But as you say i think its probably most likely that the diaphrams are not seated properly- they can be a pain to get just right.

[rigsby]

All sorted now, after soaking the diaphragms they were much more supple. I soaked 3 out of 4 and refitted, i then checked the operation by pushing the slide up and observing them as they returned, the one that was slow was the one i didn't soak. I then removed it and soaked it in fuel over night, refitted it and they are all returning at the same speed. Fired it up with the airbox off and hey presto they were lifting, so back on with the tank and a quick test round the block and it pulled well all they way through the rev range so at least i have a bike that is performing as should just need a little more power i will fit ram air in the near future.
Hopefully it wont start misbehaving as the diaphragms start to stiffen again!
Thanks for peoples input.