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Boost prepping Nikki secrets revealed

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Old 05-12-15, 03:24 PM
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Arrow Boost prepping Nikki secrets revealed

The other thread turned into a conversation with too much speculation. Too hard to find info etc.

Assuming you've stripped and cleaned the carb, pull the OMP nipples, then pull the boosters and primary venturis. Hog out the venturis to 24mm keeping the shape as accurate is possible. Warm up the main body and freeze the venturis to remove/install. If they fit loose upon reassembly, use some blue loctite.

Cut the booster supports and do the airfoil mod. Slightly expand the hole with a tapered waxed center punch. Be gentle and check the fit in the carb. Get them centered.

Fill the holes where the altitude compensator was with JB-weld or quicksteel, or block off with an aluminum plate. 1/4" thickness to reuse stock screws and some kind of gasket. 12A gasket sets come with this gasket.

Get a .465" drill bit (118 in metric or 1.18mm). Drill these parts:
*secondary slow air bleeds: nickel plated 60s
*accel pump nozzles (two holes) and banjo bolt (three holes)
*primary 92 fuel jets

Fill these parts with solder:
*the small 40 air bleed under the air horn (remove it first, fill from threaded side so solder doesn't fall into your carb should it ever come loose)
*primary slow air bleeds: brass 170s (they range from 150 to 190)
*secondary long slow bleeds

Remove choke flap valve and rod from air horn. Fill both holes with quicksteel. Clean well first for good adhesion. Keep fast idle linkage.

Remove float bolt vent solenoid:
81-82 easily comes out. Block off with a 1/8" alumium plate and a gasket in the rebuild kit using orignal screws.
83-85 requires the nylon plunger to be ground down with a dremel stone until it is destroyed enough to remove. Reinstall the solenoid without spring or steel rod.

Fill float bowl vent nipple with quicksteel or block off with a 5/16" vacuum cap (cheap rubber deteriorates so keep that in mind).

Remove both fuel rail banjo bolts. Get rid of both filter strainers (restrictions). Make sure steel fuel tubes are straight and true, bending if necessary. Do not damage the threads in your air horn. Reinstall banjo bolts with new aluminum crush washers supplied in rebuild kit.

Remove stock seats CAREFULLY using a vice grip gently but firmly gripping the body of the seat and a light tap with a hammer. One hit is all it should take. Don't bother trying to use a big screwdriver because the factory tightens these so hard it will damage the brass when you go to unscrew it. Do not crush the seat with the vice grip. Remove the stock tiny filters (restrictions):
81-82 have a short soldered brass filter that sometimes comes off and stays up inside the air horn. Dig it out.
83-85 have a long plastic filter that easily comes off.
Reinstall stock seats with new crush washers from the rebuild kit. Use your big screwdriver.

ABSOLUTELY DO NOT use the aftermarket seats that come in a typical Hygrade kit. They're all machined incorrectly and lead to instant flooding. You've been warned.

New needles in hygrade kits are good quality and seem to work after a few minutes of run time in OEM seats.

Check floats against factory specs. If the carb has never been molested, the factory settings are perfect. If some monkey has been in there with a wrench, good luck with that.

Flip the cast iron baseplate over and study the secondary linkage. Weld the linkage or use a wire tie. I trust a weld more than a wire with boost.

Buy a gasket paper set from your local auto parts store. The blue paper is thick and works well as a base plate gasket that won't leak. The charcoal gray paper is used in the accel pump mod.
base plate gasket:
The new one in the kit is your template. It is very thin and cannot be used for its intended purpose! Only punch holes for the circuits you will actually use. Study it and you'll easily figure it out. This saves time and is less likely to leak.
accel pump gaskets:
Use the new long travel accel pump diaphram in the Hygrade kit as your template. Trace six gaskets and use a quarter (US and Canada) for the about 1" circle in the middle. Get four longer screws of the exact thickness of the stacked gaskets and same thread pitch. It is very important to get this part right. Chase the threads in the four screw holes as I find they're usually pretty tight. This lets you get away with screws that are slightly too long. Only one hole, upper left corner, is blind. Be careful! 4mm x 0.70

Cut out a strip of 16 gauge steel and drill two holes to fit an 8-32 screw at 1 1/4" or 1 1/8" apart. Bend slightly in two places. File one hole to an oval to match the orignal accel pump lever arm. Use an 8-32 screw with a lock washer and a 11/32 nut, or a nylock nut to attach new strip to stock arm. Maybe a flat washer or two to get the spacing right. Pull long spring slightly and reinstall 7mm nut on pushrod. Check for binding and other problems. The travel should end when the throttle is fully opened.

The boost prepping mods are basically done at this point. If you've rebuilt these carbs before it gets pretty easy from here. If you've not rebuilt one of these before, you're a noob and will require additional info which won't be covered here. So... search noob!

Bolt the main body to the base plate with the new thick blue gasket paper.

Primary side:
Install the solder filled 40 air bleed in the air horn with red loctite.

Install the 118 drilled fuel jets in the primary side.

Install the 118 drilled nickel plated 60s in the primary slow air bleed holes. This recalibrates the idle for use with large primary venturis and boost.

Primary main air bleeds should be 60 or 70. Don't use 80 (rare) or 90 (common in SA carbs) as it doesn't work correctly with boost.

Do not mod the stock 46 primary long slow bleeds as they are part of the transition circuit and don't amount to much if you mess with them.

Accel pump:
drop a steel ball and the thin brass weight in the accel pump housing area. Wrap the long thick brass 30 screw with teflon tape on the threads and install. Stretch the accel pump spring just a little bit to account for the extra gasket thickness. Don't overdo it. Be sure to also install the accel pump nozzle and banjo bolt with a steel ball and thick brass weight under it.

Secondary side:
Install the main 140 air bleeds. These can be modded later if a delay or hiccup is experienced when the secondaries open.

Install the solder filled long slow bleeds.

Install the solder filled short primary slows in the secondary holes.

The stock 160 fuel jets are a good place to start. If they are pig rich in boost, think of it as a base tune for safety that can be leaned out once you start tuning.

Be sure to reinstall the OMP nipples. Tap them to the boosters and use a box cutter blade as a spacer for a tiny gap. I don't recommend premixing but you're welcome to do it.

Install site glass windows with new gaskets and rubber seals.

Install float bowl jet plug bolts with annealed copper crush washers. Use a propane torch only until they just start to glow. Then let air cool.

Install throttle stop screw and mixture screw. Remove any unneeded linkage from the stack such as cruise control (remove linkage but keep the nickel plated pivot spacer it rides on) and hot start assist (cut off ends but keep the circle of steel). Keep the emergency return spring because it stabilizes the shaft apparently.

Block off all vacuum nipples with vacuum caps or some stock vacuum line with a small piece of a broken drill bit or something shoved inside. OEM vacuum line lasts virtually forever. Can't say the same about cheap rubber vacuum caps. Pro tip: If 84-85, it had an extra solenoid on the secondary vacuum box with a nice OEM vacuum cap. Put this cap on the vertical nipple on the air horn at the altitude compensator flange area. It will fit under most carb hats.

Now the top (air horn) can go on. Choose your screws from the pile of hardware you made during teardown. There are enough screws from a typical FB Nikki teardown to cover everything. Test the screws in the holes and chase threads if necessary. 5mm x 0.80 I find only one or sometimes two holes require chasing.

Important! Cut off the little spacer section of the old choke bimetal bracket before you throw it away. This must go under the throttle return spring bracket for all three screw holes to align correctly.

All the pictures are in the other thread, mostly on page one so be sure to look through it.

This thread won't cover your carb hat or fuel system, charge pipe, type of turbo etc. That's for you to figure out.

This thread sums up a year of research and testing. It has been condensed down to one post. And you guys thought boost prepping a Nikki was hard.

Last edited by mar3; 05-13-15 at 02:29 PM.
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Old 05-12-15, 07:57 PM
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Should just sticky and lock thread!
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Old 05-13-15, 02:25 PM
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The motion is seconded but it will stick around for a month and then head to archives.
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