Nikki Carb mod(not mechanical secondaries)
 

So this is probably on the site somewhere already, but I figured i would share what I learned(makes me feel special).

So I was on the hunt for a carb mod that didn't have the words "mechanical secondaries" involved.

Came across this on Ausrotary.

Similar results to mechanical secondaries, but without having to worry about re-jetting to deal with that flat spot.

So you take the Secondary vacuum diaphragm, open it up, take the spring that's in it(its about 45mm long IIRC), Cut about 1/3 of the spring off, stretch it back to roughly 45mm to make the spring stiffer. **Make sure to flatten the cut end so it seats properly to avoid any damage to the diaphragm!** Put it all back together and take it for a drive.

You should notice that the secondaries activate sooner than before, and without the annoying flat spot, and it's a smooth transition just like stock.

Takes about 5 minutes to do, and there is a noticeable difference.

Here are some pics I stole from ausrotary.

Vacuum Diaphragm:
http://i6.photobucket.com/albums/y23...0mod/carby.jpg

Attachment 723637

Attachment 723638

Attachment 723639

Hope this isn't a re-post or anything.

Enjoy.

-Colby

This is the exact mod we use for our SR Nikki carbs except we take a little more out of the spring... Great write-up! :icon_tup:

Nice, pretty good mod if you want to keep the vacuum secondaries. Thanks for posting it.

Awesome little mod, butt-dyno has confirmed!

wish i would have known that! does that also solve the "they do not always open" issue too?

Maybe, maybe not... that diaphragm gets very brittle over the years and has a tendency to tear, which is not going to help you no matter how much you cut the spring. It's also a vacuum-dependent system, so if you don't have it hooked up right or if your carb is not pulling adequate vacuum (as with a vac leak) then it can also fail to operate. On a Nikki that is in good shape and fully functional, this should work really well.

Does anyone sell replacement diaphragms? I haven't seen this included in any rebuild kits.

Black dragon maybe? I don't have their product book with me right now but will look later.

GP Sorenson kit includes the diaphram

IMO playing with the spring is way way better than mechanical secondaries. you can take the spring out completely too

yeah! i actually still have that diaphragm, kind of thinking about re-verting to that and testing it on the butt dyno.

You guys are tempting me to try this... I like my mechanical secondaries though. Also, will this system work at all under boost?

D'oh! I did all this... except stretching the spring back.:doh:
So the secondaries came on - but only halfway - when I didn't need them anyway, and the kick of full secondaries was hardly noticeable.

What's with that Nikki? Looks totally different than mine.

this nikki has been stripped down a lot. maybe thats why it looks different?

I did some research into this mod a while back and came across a couple of other things preventing early opening of the secondaries in the stock vacuum operated system.

- Vacuum is ported from both primary and secondary venturis. So under moderate throttle, there will be vacuum drawn from the primaries, but leaking out the secondary port. Until there is a lot of vacuum through the primaries, then the secondaries will begin to partly open, and then continue to open fully once there is vacuum through the secondary venturis.
- A tab on the secondary throttle linkage that prevents the secondaries opening completely until the primary throttle has moved a certain amount.

Modding the spring will give you a SLIGHT improvement, because spring tension is not the only factor controlling the vacuum secondaries.

It isn't difficult to close the secondary vacuum port - pull the secondary main venturi out and rotate it . And then modify the linkage tab to allow the secondaries to open earlier. IIRC this was pretty powerful, but of course having those secondaries open on part throttle around town chews up fuel, which is why I didn't pursue it. I also never had the chance to test whether the secondaries were opening completely, with the vacuum signal being weaker than before now that there is no vacuum being drawn from the secondary venturi.

Personally I prefer a more detailed mechanical secondaries mod, where I re-drill the holes on the tabs where the linkage arm connects, and bend the arm to suit. It takes time to get them synchronised but is well worth it. The secondaries open earlier, and in most cases while the AP is still pumping fuel down the barrels, resulting in a very smooth transition.

Because I was bored, I decided to try this mod before I hit the track last week. Butt dyno said it did nothing. Felt no different on the street. I hit the track with no other changes besides cutting this spring to 2/3 its original length and giving it a stretch. Same car, same track, same tires, same tire pressures. Track temp may have been a hair warmer, but not by more than a few degrees. Both days were cool late afternoon 2 weeks apart. Anyway, I'm always improving my driving and this outing I shaved off 2 seconds from my lap. Was it because of this mod, or because of improved driving lines?

App: Harry's Lap Timer
Sensors: default iPhone
Red: old best reference lap 2:04.66
Gray: new personal record lap 2:01.91
Area under the curve is where the time was gained (ignore the whacky spikes)

First thing to note is that there is literally no gain on the front straight or top speed (last part of the lap). One could argue this mod won't really affect a high RPM pull since the secondaries should already be open. I exit the final turn around 4,500 rpm in 3rd and pull to the buzzer around 7,000, shift to 4th and pull to about 6,500 before hitting the brakes.

But what if this mod really affects midrange instead? Unfortunately I don't spend much time below 4,000 rpm on this track, nor would I want to. Any place my RPMs are low coincide with corners that I sacrificed some entry speed on my line to get a better exit speed, the proverbial late apex.

My conclusion is, I'll welcome the faster laps, but I'm giving myself more credit than this mod. At least I know it doesn't make things worse or change the behavior of the carb in a negative manner. Maybe next outing I'll remove the spring entirely and see what happens then.