Oneiros

Hi guys, here is a guide on how to do my custom mechanical secondaries mod. This will get the secondaries opening much earlier than the common wire tie method, which has two benefits: more power earlier in the throttle range; and a much smoother transition from primaries to secondaries. The transition is smoother because in most cases the accelerator pump will still be squirting fuel down the barrels when the secondaries start to open, avoiding the common stumble or bog from a lean mixture.

Tools required are just the basics like a drill, hacksaw, files, etc. First up take the carby apart and remove the throttle body section. These are the parts to work on:



The blue and green tabs need to be attached to each other so they move together. I use metal work solder, other people have used wire ties etc. Whatever you find convenient. Make sure the secondaries still move freely after doing this.



Next up is the red part. This needs to be bent so it is straight, and drilled. I usually drill the hole in the groove that is left from the bend, this is about the right area.



Then cut the excess off and file it back. I recommend leaving this step until after you have tested it as outlined below.



Now comes the time consuming part. The yellow rod needs to be bent to fit in the new hole on the red part. That's easy enough. Put the parts together (including piece #136) and hold the rod in there by hand. With the other hand open the throttle (if you can't hold it all together use some disposable bits of wire to secure the yellow rod). Notice that when the primaries are fully open (straight down) the secondaries probably aren't - if they are you got lucky. Otherwise the yellow rod needs to be taken out, bent again, and put back in to test the throttle synchronisation. Repeat until you get the primaries and secondaries both perfectly straight when fully open. It will probably take a lot of testing and bending back and forth but you will get there eventually.



Depending on which model of Nikki you have, the yellow rods come in a few different shapes. The one shown above is different to most because it attaches to the red tab on the opposite side to normal. But the principle is the same.

You also may find that no matter what, the rod doesn't fit. In this case you will have to drill another hole further from the center on the red tab. The position of the hole controls how early the secondaries will start to open, within certain restrictions (the rod has to fit properly and not hit other tabs on the throttle shaft).

If you want to be safe rather than sorry, you can do a mock-up of sorts before drilling the red piece. Put the yellow rod through the green tab, and hold both throttles fully open. Move the rod over the red tab and it should line up with the right spot. You may have to bend the rod before doing this. Then close the secondaries and find how far open the primaries are when the rod lines up with the same spot again. This will show the transition point.

Lastly you may have to bend the blue tab so that it hits the throttle body when the secondaries are fully open, this is to prevent both throttles from going past that straight up/down point.

Good luck and if you have any questions or problems ask away. This one does require a bit of effort and thought, so I have tried to highlight the potential problem areas for you guys, but I've done so many of these I may have missed something.

peejay

That a lot of work compared to removing four screws, removing the diaphragm spring, reinstalling the diaphragm cover, and installing the spring into the glovebox...

DarkDrakeX

They did a write up on the whole spring thing. The spring isnt the only thing that opens the mech secondaries. Making it actual mechanical secondaries is the best way. I only havent messed with secondaries cuz im afraid if i take the carb off itll never run again (i have terrible luck with these nikkis)

Kentetsu

Can't imagine the benefit of opening the secondaries even earlier...

LizardFC

iTrader: (1)

I can definitely see the benefit to part throttle response by having them open earlier. Gotta be careful with this though... if you set them to open too early, they'll be open while you're cruising around town or down the interstate and kill your gas mileage.

Nice post, thanks for the detailed write up.

Oneiros

Yup. This mod gets them opening around half throttle, which allows plenty of room for cruising on the primaries. The limits of where you can drill the hole on the red tab prevent them opening too early.

On a side note I did a different mod a while back to get the secondaries opening together with the primaries. Bottom end pull was amazing, I imagine fuel economy would be terrible though (I didn't test it for long enough to find out).

peejay

Um, there's an interlock on the primaries that prevents the secondaries from opening before the primaries are open a certain amount.

All this "mod" is doing is welding the secondary shaft to the interlock.

While, on the other hand, by removing the secondary diaphragm spring, any time there is air moving through the primaries (which tends to happen when the throttle plate is open to any degree) then there is vacuum on the diaphragm and it is holding the secondary throttle against the safety stop. So it works exactly like taking apart your carb and welding the shaft to the linkage except, you know, it takes a hell of a lot less effort. And it's reversible in case you find that you get a bog if you don't know how to drive smoothly.

But what do I know, I only did this stuff 12 years ago when I had a stock SA to play with.

Oneiros

OK so maybe you didn't read it? The whole point of this particular mod is to get the secondaries opening earlier than the usual mod which is simply wire-tying or tack welding those two tabs together. Yes the tabs are joined but there is more to it than that. There is a lot less bog on the transition this way (if handled properly there is none).