Ricardio Frenz

Howdy boys,
Its been quite a while since I posted anything to the 1st Gen forum. For a qucik update-> I'm running an '84 GS with a few mods: J-Bridge on my 12A, along with a nice gritty Street-ported exhaust. Straight-piped exhaust back to the muffler. Unsprung 4-puck clutch (Yes I know my transmission is crying, no I don't care). Plugged Monolith cooler on the Nikki (That weird tube that went to the exhaust pipe is now capped with a metal plate and gasket). Removed that tube that comes off the exhaust manifold and removed the plastic flap in the breather. And most recently, mechanically converted Secondary butterflies on the Nikki (I followed the mega-thread on Nikki mods but did NOT use a breadtie lol). A few of you guys were kind enough to help my poor soul when I first J-Bridged my 12A back over spring break, and I'd like to ask for some more help.
Here is the problem as of currently:
My 12A is definitely running better. It idles at abut 1700 RPM instead of ~2300 RPM. However, it is still running pretty rough. Idling is spastic, throttle response is still a little sluggish, and occasionally there is backfiring (Although not nearly as bad as there used to be). When I pop the air filter, I can see what looks like some exhaust still backing up in the primaries (But not the secondaries). I'm looking to get her running even better, so any advice as to what I can do (Preferably cheap things) would be greatly appreciated.
TL;DR 12A Bridgeport is running good, but not that good. Any advice is welcome.
Thanks in advance for any help y'all.

Qingdao

iTrader: (3)

Have you tried fidgeting with the ignition timing?

t_g_farrell

iTrader: (1)

Do you have the niki hogged out enough to run this at the right AFR? Have you measured the
AFRs at idle/cruise/WOT? If the nikki is unmodded, the idle cirucit won't have the mixture
right at a normal idle speed to run it. A brdige is going to have a very lumpy idle anyway.

Jeff20B

iTrader: (13)

I'd bet dollars to doughnuts his Nikki just has stock venturis and stock air bleeds. No wonder he's having issues.

Ricardio Frenz

Jeff20B, you replied to my thread back in April about the original bridgeporting . No, I haven't hogged out the Nikki at this point. And yes, I've messed with the timing and gotten it pretty much perfect from the rebuild. Is messing with the venturis the next thing I should be doing? Also, how would I measure the AFR? I don't have money to be buying equipment at the moment. Is there another way to do that?

peejay

1. Put a TPS on the carburetor
2. Install fuel injection, use carb as throttle body
3. Idle at 900rpm


(Disclaimer: My throttle body is a modified Holley 750 with the boosters removed. Way way WAAAAY more flow than a Nikki could ever dream of)

Ricardio Frenz

So seems like the general consensus is:
1) check the ARF. Where should I be for this J-Bridge?
2) possibly bore out the Nikki
3) replace Carb
Anything else to look for to help smooth general operation? Thanks guys!

t_g_farrell

iTrader: (1)

If you don't have an AFR , just do the old skool method of looking at the plug color.

white == leam
tan == ok
brown == rich
black == pig rich

Just remember you have to look at the plugs immediately after running at the desired
rpm. So for idle its easy, start it, let it get warm, turn it off, check color. For cruise you
need to get out there and cruise for a few minutes, then push in the clutch and turn off
the engine (beware the steering lock kicking in if you turn the key too far), coast to side
of the road, inspect color. For WOT same exercise. Takes way more time and effort but
works, has worked for 50+ years.

Getting a bridge to run well is never easy no matter which fuel delivery system you use.
Since you are $$ constrained, I would start with some work on the nikki. Its fairly easy
and cheap to do. Biggest cost is jets and rebuild kit. Jeff has some great writeups and
also some ported tuning experience as well.

Ricardio Frenz

Right, I'll look into that. Sounds like hogging the Nikki is what I might need to look at. Those rebuild kits are around $80 right?

Jeff20B

iTrader: (13)

35, but keep the stock needles and seats and leave your floats alone! Make a new baseplate gasket out of thick paper because the one in the rebuild kits is too thin to work. Use the thin one as a template and only punch the holes you are using. Study and you will see which holes to punch. The brand new accel pump diaphragm can be taken advantage of by doing the accel pump mods that I do. Be sure to read my writeups.

I'm doing a Nikki for a half BP this month. Does that excite you?

Ricardio Frenz

I'll look into boring them out. Until then I'm planning on saving money and looking for a new carb around Christmas, but this Nikki modding should do fine until then. Still torn on the Holley/Edl/Weber carbs, heard lots of mixed reviews on which is the best. I'll have to do some research.

Ricardio Frenz

Hey guys, just wanted to ask a quick question: What kind of CFM's would be good for my J-Bridge? 390? 450? MORE?! Thanks

t_g_farrell

iTrader: (1)

The only other type of carb I would recommend would be a 51 IDA but you will still have to
do the tuning dance to get it right. The big bore 4 barrels aren't designed well to deal
with the rotaries requirements and they sit 90 deg rotated from the direction they are
designed for.

Jeff20B

iTrader: (13)

I'm currently deciding what direction to go on this Nikki. One school of thought suggests going as big as I can while still maintaining velocity. This is due to the reduction of low RPM vacuum BPs are known to have compared to streetports. This means the air flow can sort of stall out at low RPM where the transition circuit is supposed to do its job and carry the engine from the idle circuit into the main primaries. you make the carb to big, and you run the risk of it running poorly. That is to say worse than would otherwise as BPs usually run lousy at low RPM compared to regular side ports.

Another school of though is to go kinda smallish on the primary venturis to guarantee good velocity while making the secondaries big to sort of make up for it. But then I've run into issues with boost and secondaries larger than stock creating a very slight delay. But a good thing about boost is you don't need a huge carb to get good results. You really only need a huge carb when you're NA, but a big carb with boost can help too...

And then you run into the issues common to BPs and other high overlap port configurations. Especially if you put a giant cork in the exhaust system ie turbo. High overlap engines hate backpressure! However, BPs have a lot of exhaust energy compared to regular side ports and could make up for it by spooling the turbo perhaps even at idle. I don't know because I've never tried it. All I have to go on is what others have done. I can also make sure to keep an anti-reversion lip in the exhaust ports like what a factory 12A turbo rotor housing has.

If I do this right, it could turn out to be a very nice turbo engine.

Oh, one more issue. The intake manifold has to have separate runners. You can have this with old school manis only. And then due to the separated runners, you get less total flow through all of them, according to peejay. I admit I don't fully understand it myself. All I know is all 79 and later Nikkis were on channeled manifolds, and all 78 and later manis had channels.

Back in 2011 when I did my first half BP 12A, I used an old school Nikki from a channeled manifold and placed it on a separate runner RX-2 manifold, and it worked pretty well. The power when the secondaries opened was quite nice, but it shows how weak the stock 20mm primaries of this carb were (when secondaries provide a transcendent experience like this, it means your primaries are simply too small). It had an air and a fuel screw like SA Nikkis. The carb was otherwise mostly stock other than mechanical secondaries and I think I did the accel pump mod, and did ok. It wasn't an ideal setup though. Exhaust was too small, stock ignition, no wideband, aluminum flywheel. I was just a noob at it. Tons of room for improvement. Stock flywheel for starters.

I'm starting to think if I were to boost prep a 79 Nikki, which I know how to do now, it just might work. You need an FB top for its shrouded or semi-shrouded air bleed covers (the FB air horn is superior to the SA in this regard). And I think the fully separate air and fuel mixture screws should make idle adjustments a lot more intuitive than the one size fits all approach they did on the FB carbs.

Now it comes down to NA or turbo, and venturi sizes. Separate runner mani = bigger venturis to flow enough while at the same time a turbo = venturis that don't need to be too big. Plus you loose velocity if the venturis are too big. At first I was thinking around 24.6mm but now I'm starting to change my tune based on all my Nikki modding experiences. I'm thinking I can go bigger and not hurt velocity enough to ruin the carb. Heck I just tested the biggest Nikki I've ever done just a few days ago and it ran 99.9% perfect at a whopping 26.2mm. The one glitch it did have was a tiny cough that lasted for a split second which was caused probably by a bubble in the accel pump just working its way out of the nozzle. After that, it was perfect. It even went into boost with zero delay and returned to idle with zero flooding. Perfect. This was all on a 74 ported R5 13B with an S5 turbo and full 3" exhaust. Easy to drive and ran really nicely. Of course the engine has regular side ports and a channeled (I cut them) old school REPU manifold. We know the half BP is going to be a different animal.

So as of right now, I'm thinking I'll use an SA carb, fully boost prepped, and pull the current 24.6mm venturis to go a bit bigger, but not too large. Still debating the size but probably in the 25.0mm to 25.95mm range. I'll probably leave the secondaries stock to avoid any delays when they open, and it will get an FB top. Does all this sound reasonable?

Ricardio Frenz

Jeff, while that's all well and good, I'm not sure what to do with this information. Are you telling me how you would do the Nikki for my set-up? Thats alot of information there. I'm not near ready to work on my Nikki yet, but I'll remember some of those dimensions you mentioned. Also turbo-ing the engine isn't really in my scope of mods at the moment, so I'll be keep the NA.