Jeff20B

iTrader: (13)

Edit: please scroll down to see the graph. I can't figure out how to post pics in threads anymore because it always says rel=lightbox

So what are we looking at? This is a visual representation of what several boost tunes look like across several Nikkis with different primary venturi sizes. The bottom is a stock 20mm primary venturi with a stock 92 jet tune from the factory and the top is a stock secondary at 28mm with a turbo tune at 140. The four that are close together represent the four hogged out boost prepped Nikkis I built last year, ranging from 24.7mm to 25.6mm.

Notice the angle is the same on all except one line.* This is important because it shows there is repeatability. I think that is one of the requirements for the scientific method.

*The one fluke is on a carb I fixed for Jinkun a month or two ago. Notice the red and green lines. The angles are different. Red is the size (114) of the Holley dominator air bleed that came with the carb. While green is what I would have gone with based on the angle of the other lines with normal stock Nikki style jets.

His carb was ruined by that SVT guy so I had to swap in some correctly hogged out primary venturis. He only wanted 22mm like a Sterling. They ended up at 22.3mm when all was said and done. Then I proceeded to test drive it using a set of the Holley dominator air bleeds as fuel jets that came with the carb. Since then I've discovered these air bleeds should not be used as jets due to the design of their funnel shape. It may work for air, but it is inefficient for fuel and leads to a lean condition at high RPM.

It also might be the cause of problems during high G cornering, leading to stumbles and other problems which Yaw and Sterling would often complain about and tried to come up with baffles inside the float bowls to fix it.

I experienced this with Jinkun's carb and the Holley bleeds but have never noticed it with any other carb using stock designed Nikki or Hitachi jets. Could be the funnel shape?

Notice the graph doesn't go all the way out to 160? Even though we know all modern SA and FB Nikkis came stock with 160 secondary fuel jets. So why didn't I extend the graph out that far to include it? There was no need as we all know they are 160, and it would make the graph wider and show a shallower line. Heck, kinda like Jinkun's 114, but that is comparing apples to oranges. You guys can make your own graph if you want to. The point of this one is to show the repeatability and same angle across primaries and secondaries for boost.

One thing I haven't mentioned yet is about the air bleeds. I've had to make almost all of them smaller than stock, accept for the main 70s and the 46 primary long slows which are part of the transition circuit. I messed with these earlier this year and didn't really get anywhere with it. There will always be a small glitch going from idle to main primaries because Nikki didn't make it easy for us to mod these air bleeds. However when you drive, it's not noticed. So no big deal. Also I've found the stock 70s are the best size for the primary mains. It turns out the next size down at 60 runs glitchy. I drilled a set out to .67mm or .68mm with a precision micro drillbit and it drives fine now. As for the next two sizes up, the rare 80 and the common 90, anything bigger than 70 just kills your tip in. I also don't recommend boosting an SA carb, not just because of the stock 90s but because my experience with them showed a dangerous lean condition at the top of the primary circuit probably due to the timing of when the secondaries open on the SA carbs. I think they open at 70% throttle while the FB carbs open sooner at 60%. Is it adjustable? Yeah, some have messed with it but I haven't. So if you want to boost an SA carb, I won't stop you, but you've been warned.

As for the short slow primary air bleeds, the sizes range from 150 to 190, with 170 being the most common. I fill these with solder, then drill to .35mm and swap into the secondary holes. Then I take out the nickel plated 60s from the secondaries and drill them out to 118 and swap into the primary holes. It brings the idle circuit back into calibration so the mixture screw works again within its range. I've tested slightly smaller sizes and they seem to work as well. Just don't go too small. The smallest I've tried is 105 (1.05mm).

As for the secondary air bleeds, all of them required changes. The mains are usually 140 and that is simply too big. I've found 80 is the perfect size. Anything smaller and it starts to run glitchy as if the air bleeds are too small. I tried several sizes ranging from stock 140 all the way down to .73mm and found .80 or .81 to be perfect.

The secondary long slows needed to be filled with solder. This reduced a secondary delay/bog/hiccup from 1.5 sec average down to 1/4 sec average. Then further air bleed tuning on the mains and the short slows dropped it down to nothing. These long slows are only usable on a stock vacuum secondary carb because they shape the when and where of how and why the secondaries open when they do. But for mechanical secondaries, they aren't needed at all and should be filled with solder. Otherwise you'll have all kinds of problems like I did.

Lastly the short slow nickel plated 60s are only there to break the siphon effect that occurs when you floor it and let off. I was having a major flooding problem using solder filled short slows. It would empty the entire float bowl contents into the secondary barrels and not stop until the level was low enough to suck air, which would break the siphon. It would empty them faster than they could fill at 2.5psi. There would be a pool of fuel in each secondary barrel leading to all kinds of super rich running problems for a while, until it self corrected, which could take up to 1/4 mile sometimes. The fix was to swap in a set of solder filled primary 170s that were drilled with the smallest micro drillbit I had which was sturdy enough to go through solder. .35mm it turns out. This is small enough to not let in much additional unwanted air, because remember with a turbo you have a great abundance of air but it was still large enough to prevent a siphon from getting started.

So there you have it. I've taken these Nikkis further than anyone ever has, and I couldn't be happier with the results.