Lexthepilot

To start from the start,

I was out for a drive in my 1981 Rx7 with a stock ported 12a, factory nikki carb(Rats nest removed as per RX7 carls tutorial, Air pump and ACV also removed), racing beat air filter housing(and filter) and racing beat streetport exhaust(Most recent install at roughly 200kms ago). All was normal and as usual the car felt great. upon pulling up to a stop on my way home she sputtered, idle rpm surged and spat around 400 rpm for a few seconds, then it stalled. I managed to get her started but it took some coaxing with the accelerator pedal. Once running I had to keep the accelerator down slightly to keep it running. If I let it drop down to idle at all it would sputter and stall. Once back at my shop I went through my usual checks to rule a few things out(I am a ticketed auto mechanic but have next to no experience with rotaries).

First I pulled and inspected spark plugs, all appeared fine with exception to the front rotor having fuel soaked plugs. Next just to rule it out I threw my spark tester in the end of each lead and cranked to verify a strong spark, all good there. While I had the spark plugs out I felt each of the apex seals to verify they had similar spring tension, All ok. Next came a compression test. All I have at my disposal is a traditional style compression tester so I removed the schrader and checked the initial spike up on both rotors. Both rotors spiked between 58-62 PSI consistently so I moved on. Before I continue I feel it necessary to admit that I was trained in the age of fuel injection and the extent of my carburetor experience is rebuilding the carb on my snowmobile. That being said I had comfortably ruled everything else out so I began research into the carb. My first step was to run through the idle adjustment procedure that Sterling posted on this forum. It had zero affect on my idling problems at all, however; it did allow me to notice that after running the car up to operating temperature if I shut it down and looked in the air-horn the front primary had a pool of fuel sitting on it(Unfortunately I cant recall if the secondary was also swamped)

At this point I finally had a specific concern to research, A little reading informed me that there are sight glasses for the float levels(how handy) so I ran back into the shop and took a look. The car had been shut off for roughly 30 minutes by this time. The front glass looked full but may have also been empty, either way it had no interface, the rear glass showed an interface halfway through the sight glass. After significantly more research I pulled the air-horn off the carb to take a look. My problem is that I didn't find anything tell-tale. There is no significant build-up in either float bowl. Neither of the needles look to be gummed up or feel restricted at all in their bores, both needle seats appear fine(they look to be some type of silicone or rubber on the needle then brass in the housing). Both floats look even with the air-horn set upside down and both floats still float at roughly the same height when held in a dish of gasoline(I slid them onto a pin punch and laid it across the rim on a bowl of gasoline)

Any help or theories you may have to offer would be greatly appreciated

Thanks,
Garry

Jeff20B

iTrader: (13)

Float bowl vent solenoid might have failed.

DivinDriver

iTrader: (15)

You, sir, must have some very skinny fingers.

+1 on Jeff's comment; other possibilities are a bit of debris stuck on the needle that has since moved on (does your carb have the inlet & needle screens in place?) or a needle that got hung up somehow.

Lexthepilot

HA! no I used a 1/4" drive extension to push on the apex seals, not ideal but I made it work. I jumped 12V to the vent solenoid and it actuates properly. I tried just holding a finger over the return line and putting some lung pressure on the supply line and I don't appear to be getting any leakage at all past the needles so I'm assuming it was just a little piece of garbage lodged in the needle. Hopefully going to re-assemble today and see what she does. Thanks for the replies guys.

Garry

Lexthepilot

Just an update:

So I reassembled everything and fired her up. Once I got the idle adjustments dialed back in she ran great. front float bowl was a little above half and the rear was bang on. Went out for a drive and everything felt great, my idle felt smoother than it ever had before and the light throttle surging I had at 50km/hr before was gone. When I pulled up to a stop in the garage at home she sputtered and stalled. With the key on I looked into the carb and both primary and secondary jets for the front rotor were just streaming fuel. I gave a few taps on the top of the carb with a small hammer and it sealed up. This leads me to believe it is just a sealing problem with the needle. I ordered new needles and seats so I will give that a try when they come in. Also I am going to be ordering a wideband O2 sensor to help me monitor if I am getting AFR fluctuations while driving, I would prefer to not burn my car to the ground if possible....

Garry

DivinDriver

iTrader: (15)

This happens a lot with new needles, too. Usually goes away at break-in, or you can gently burnish the outer corners of the needles with a smooth metal object (like the back of a spoon) to speed the process.

Cookboy

iTrader: (10)

Good idea, DD. I use a green scotch brite pad. With which I've been scrubbing spoons and everything else for 35 years.

NCross

iTrader: (18)

You didnt cap off the fuel tank vent did you? The line that goes to the charcoal canister?

Lexthepilot

No the fuel system hasnt been modified at all as of right now, thats not to say that the vent isnt blocked in some way but I would think if that were the case it wouldnt have sealed up for me when I tapped the carb. I am going to lightly coat the new needles with light oil and sit and lift the floats manually for an hour or so before installing to try and speed the break in process... I will let you know how it works out once the parts come in. Thanks for the help guys

Garry

Jeff20B

iTrader: (13)

One way I found that "burnishes" the needles, for lack of a better term, is to blow a little compressed air into the fuel inlet nipple. Be very careful. It causes the floats to bounce up and down and starts to break in the needles. You can also compare one float to the other and see if there are any obvious differences. This so called procedure, if you want to call it that, allows the bowls to fill with little to no hang ups first time, every time. If they do tend to stick, it usually goes away after a short time.

Another thing that helps is to have an easy way to turn the fuel pump on or off from the engine bay so you can see whether the carb is over-filling before it has a chance to pour (flood) into the engine. You probably won't do this mod, but it is a good idea to have a thicker wire going to the fuel pump with a relay because the stock wires are too small.

NCross

iTrader: (18)

I usually lightly rub the needles with 2000 grit sand paper. Oil might make them even stickier. If anything dip them in Acetone or mineral spirits.

bjhines

Good read, Thanks for the info from various sources on various ways to accomplish smooth inlet and float operation. I was having some issues with fuel pressure at 3PSI with my Nikki overflowing. I changed floats and used a tack-hammer to burnish the leading corners of my float needles.

Jeff20B

iTrader: (13)

Do yourself a favor. Leave the stock seats alone. Don't remove them to swap in new seats because all new seats are machined incorrectly where the bore is oversized and has concentric rings from a dull tool. They make thousands of these, and they're all bad.

Think of it this way. When a brand new part has a larger ID (inside diameter) than an old used part, stick with the old used part. A larger ID causes the needle to get a little crooked. Then add the rings and the needle will always have a tendency to want to flood.

Why new seats are included in all aftermarket rebuild kits, I'll never know. Probably because they mass produced thousands. And they're all bad.

Jeff20B

iTrader: (13)

bjhines, the Nikki prefers 2.5psi. There is some bad info out there suggesting 3 or even 3.5psi. Ignore it. Stick with 2.5psi.

bjhines

I was wondering if the stock return line restrictor was used by you guys with an aftermarket pump.
I burnished the needle's sharp edges. They seem to work fine while running. I noticed that the float bowls appear to overfill initially until the car is running. I have simply told the other drivers to always start the car if they turn on the electrical switch.

Jeff20B

iTrader: (13)

Mine tend to overfill first too. This happens with new needles in OEM seats for at least 100 miles. I can't say the same for aftermarket seats because I stay away from them, but I know they are way worse.

As for the return line, I block it off at the carb's 1/4" nipple. I use an aftermarket Mallory 4309 fpr and an MSD 2225 fuel pump. The fpr has a return on it which is plumbed at 3/8" under the car and only squeezes down before it goes into the tank, which seems to work, while the send from the pump is only 5/16" as it goes up to the fpr. This is correct according to the instructions that come with the fpr.

My alligator clip up in the engine bay allows me to "babysit" a carb with fresh needles so I can kill the fuel pump relay's ground if the carb is overfilling. Then I fire it up and reconnect the ground from inside the car and let it run until it's warm enough to idle, then I hop out and look at the float bowl windows. They are usually in the middle.

The overfilling is usually from new needles and will persist for 50 to 100 miles or so. I can either fire it up right away like you said, as engine vibrations keep the needles centered so the bowls can't flood, or I babysit it by disconnecting and then reconnecting the fuel pump relay. It depends on how badly it wants to flood. I gotta tell you aftermarket seats are the worst for this.

t_g_farrell

iTrader: (1)

I went back to original needles and seats to finally fix this for good. Unless theres corrosion or pitting
theres no real reason to replace them at rebuild time. I have yet to see a worn needle or seat on
any carb thats not been mistreated.

DivinDriver

iTrader: (15)

/wishing for the three millionth time that SOMEbody would re-start manufacturing grose jets for these carbs. I'd pay well for a set.

Jeff20B

iTrader: (13)

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