bored venturis
 

I just bought an extra carb for my stock port engine to rebuild and modify. I have heard boring the venturis yields a power gain, but at the sacrifice of low end power. I'm not too worried about low end power but I still want it to be streetable. So how much is too much? How far should I bore them out? Do you know of any other modifications to a carb that yield power?

If you want to run a modified Nikki carb, send it to Paul Yaw. You will spend close to $300, but I'm willing to bet that nobody knows these carbs better. This is his web site http://personal.riverusers.com/~yawpower/

Ryan

I want to send it to yaw, but i dont have the $ yet. I am rebuilding it anyway, and I have access to a machine shop for free :D. Anyone know anything about boring venturis or other methods of increasing airflow?

my nikki has just got the secondaries bored as far as they can go and bigger jets and air bleeds it makes big difference when they come in.

good idea bored venturis cant suck as much afuel at low rpm, but mor eair andfuel at high rpm secondarys ore only open at hight rpm thanks man

I have been looking at modified Nikki carbs and seehing advice on what is good for a street machine.

The problem is most home modifies can only report on their limited experience, which is sometimes incorrect as it relies on opinion such as it felt much faster

In contrast professional tuners with their equipment can really tune a carb. Once they have found the magic numbers they can repeat it with possibly minor improvements over and over again. This is why for example Paul Yaw after doing 300 can be so good. Why should they pass on their secrets, experimental time costs money and they have to get their money back somehow.

There is general agreement that
1 do not touch the primaries.
2 bore out the secondary venturis, and have bigger jets and air bleeds.
3 change so the secondaries come in earlier.
4 the ideal air flow is about 425cfm
5 the carb must have a good air filter

There is some agreement that you shold go to mechanical secondaries.

When it comes to numbers for the above the suggestions are
1 open out the secondary by 2mm
2 start with a 196 jet and see
if that works

Rob Golden from Pineapple Racing gave good advice, you always have access to a dyno, the road! In my requirements use the 50-70mph time, and rejet as necessary. He also sells adjustable jets to make tuning easier

From the song of Tom Leheur{?}on achieving academic success, plagarise, plagarise but call it research.

Hmm, I thought I remember seeing that he modifies the primary venturis, boring them out to 24mm. As for air flow, I don't think that's the ideal so much as the limitations to what you can do with the stock carburetor and still get a semi decent fuel signal. Air filter... well you'd be stupid to not run one :)

http://personal.riverusers.com/~yawpower/strtcrb.html details what is done to the carbs

http://personal.riverusers.com/~yawpower/carbtech.html in the middle he explicitly states he enlarges the primaries

http://personal.riverusers.com/~yawpower/niktest.html for gits-n-shiggles is the tuning of one of the carbs

so i should bore the primaries to 24mm and bore the secondaries 2mm farther than stock and start with a 196 jet? Any other opinions? what about air bleeds etc.? thanks for all the help.

I wouldn't touch 'em. Venturis are an aerodynamic restriction and it would be far too easy to destroy them by not putting the proper contour on them. So unless you're an aerodynamicist and a machinist and have the proper kind of machinery to do it (nothing hand-held) I wouldn't attempt it unless it doesn't bother you if you get bad results.

And yes, you'll need to re-jet... you'll also have to play with air bleeds, as changing the venturi size and shape will dramatically alter the vacuum signal getting to the booster. To play with air bleeds you need to modify the stock ones so that screw-in bleeds can be used...

If you still want to try, go for it, but beware that it's not as simple as bore 'em out and stab bigger jets in. IN fact that will probably lead to worse performance than stock.

so you are saying that i should just rebuild and clean ,etc it, give it mech secondaries and leave it at that. It will be on my extra carb so i wont need to worry too much about screwing up. I just don't want to do something i can't fix. Where can i get adjustable air bleeds? can they be fabricated?

...If you gotta ask, then you're not ready!
 

Before you go screwin' around with venturis, you need to know carburetor basics. There are "home modifications", and then there are the type that require a bit more than , "Oh...Maybe I'll try that...".

The venturis have an hour-glass profile to them, and if the neck of that contour is'nt kept perfectly in line with the booster venturis, your whole flow dynamic will be fucked up, and your result, well...you might as well have hit it with a hammer.

If you want to play, then that's a different story. Just don't go expecting miracles from your experimental surgeries on your only carb. So if it's a spare carb, then go ahead.

First, learn everything you can about how and why a carburetor works. Do searches, study Yaws site, yaddayadda.

You need to make sure that the neck is in the correct place on your new venturis, as this is the most important part of it...shape, location and diameter. Yes; I said "new venturis", because you should remove the old ones, and make a few copies from aluminum. Make them identical, and turn each one down on the ID. The OD should be slightly smaller than stock so they will slide back into the carb body easily. Then you can experiment with the ones you've made by removing them like the Demon carbs.

To get them out, you'll have to remove the booster venturis. Everything is cast from crap pot-metal, and it's all very soft, porous, and cracks very easily.
The booster venturi is pressed into the body. To remove it, remove the airbleed, and find a bolt with EXACTLY the same thread pitch. Insert the bolt, and pull out, wiggling it from side to side AS LITTLE AS YOU CAN GET AWAY WITH! You need to wiggle it, but you don't want to deform anything.

The venturis should be pressed out from the top. Lathe a piece of polypropylene to just under the venturi diameter. (Wood will work)
soak them down with penetrating oil and let them sit awhile. Then tap them out through the top with a hammer and the plastic/wood.
Go easy, and do only one at a time.
They go back in by inserting them through the top, the same direction as when you pushed them out. You're never pushing on the thin part on the bottoms.
Be sure that your replacement venturis, or the originals (whichever you're re-installing) are free of scratches on the outside. Use 600 paper wrapped around a ruler to make a fine file to take them out. (go easy- it's soft!)
When rebuilding one of my carbs, when I put one of the venturis in, I cracked the entire center of the whole housing! That's how crappy the body metal is! And I was being careful.

There are step air bleeds, too. Inside the holes are little plastic jackets. Don't lose em. I've always put them back, so I don't know just how important they are, but carbs can be finiky if things are missing.

I do alot of machining to the carb body itself (only a couple of carbs, mind you), but I use threaded dowel that I made on my lathe, and cut plugs from it to put in all the jet holes so no chips find their way into the circuits and fuck things all up.
And if you bead blast, be sure to tape everything up, too.
I machine the extranious plumbing off the carb body with a Foredom flex shaft, and then I mill out the air horn. I'm currently working on float bowl covers that are individually screwed on, and a seperate airhorn, and I'll be able to drop in venturis much more easily.

Adjustable bleeds and jets...I plan on doing bleeds, but from everything I've read, the fuel jets are "what they are"...I.E, if you go trying to install a 'micro' needle valve in them to adjust flow, you'll impede the flow in such an inconsistant mannor, you'll chase your tail forever. It's not just the orifice diameter that's important. Apparently, the shape of the face of the jet is important, too. (A bit too in depth for me to understand quite yet.)

I've been playing for two years, trying to get a carb to do flips for me. I do it because I love to tinker, and metal is my gig. But man oh man, am I glad I bought a Yaw carby!

I hope you stumble onto something really cool, and post it here!

:)

man sounds like a project thanks for the help guys

hey nieanderthal, i mailed the carb this morning and they said it would be there thursday. I changed my name from 81dragrx7colorado to piston pounder!.

After reading Sterling's post and Paul Yaw's description, it is easy to see why many modified carbs provide little improvement.

On the point of increasing the primary venturi, Paul increases the size from 22 to 24mm. A couple of others say they do not touch the primary until the car has been streetported to provide better drivability and fuel consumption in traffic.

I will wait to a buy a Nikki on ebay and have Paul modify it. They are difficult to buy in Australia and it would save freight.

Are there any differences in Nikki's? Is one year better than others? I only ask cause if I am to send one to Yaw which one should I get( I probably already have it)?

Anyone?:confused:

Yeah what about your motor first.

Paul, I'm sure you can find someone with the know-how to modify your carb. I'm running a modified Nikki as well and it's had both the primary and secondary venturis enlarged. It's also had the air horne machined to increase flow and the throttle butterflies and shaft have been modified to provide less restriction. My intake manifold has also been ported out and matched to the intake ports on the engine. With this combination my 12A extend-port made around 160rwhp with a relatively quiet exhaust system.

Revhed, Thanks I will let you know how it goes.

One of the Victorian experts[?] was very strong on not touching the primaries unless you wanted a race oriented vehicle with big porting. He said boring out the primary venturies had been all the rage some years ago in the RX2/3 crowd with race pretensions. The theory was you lost streetability when just using the primaries in city traffic conditions. It is interesting that the Hitachi have 27 bore and 22 venturi, 2 mm up from the Nikki and 2 down from Yaw's mod. {early I was inaccurately given stock gen 1 as 22mm]

Congratulations on 160rwhp, that is about 185 at the fly which is Yaw's figure for a good extended port engine, even better if its measured in Australia! I read your performance data on another thread, along with your modifications somewhere else and it sounded like the way to go for what I have in mind. I am incorporating most of your suggestions in particular the cold air ducting. This time last year it was 46 in Wilcania, and when I shifted to a turbo 4wd on the 600 km dirt to White Cliffs and Tibobuura [where it was 48 in the shade] the turbo really suffered.

On another point, is your MSD system a good mod on the Gen 1, it seems to have its supporters. But a search on MSD gave many Gen 2 throwing it off? I am always a bit worrird about the reliability of electronics, I drove nearly 600,000 km in my 'bush basher' XF Ford wagon as all the later more electronic models were as reliable as a politican's election promise. Ironically after surviving several roo and emu strikes, the XF was written off in Canberra hit while parked by a girl leaning over to adjust her radio. It was still drivable but not economic to repair, while the offending vehicle and my wife's three month old Fairlane, parked in front, were totalled. There must be something about mid-80s cars which made them special!

I read on paul's site that the later first gens carbs were easier to modify. The older ones had a larger air horn but he said that there was something about them that made them hard to mod good luck

Paul will only do his modifications on the 81 and later Nikki carbs. He also says the 79 and 80 manifolds are better than the 81 and newer. So you want a 79/80 manifold with a 81 or newer Nikki.
Plan on spending close to $500.00 by the time you are through.
hanman

So for $500 I could get a YAW carb/manifold or for $300 more a weber or holley set up. Which would you go with? For daily driving? For performance/autocross?

I assume the YAW carb would be a better street carb and the weber for ultra power.

what do you people think.

The "Yaw advantage" is that he achieves an almost flat powercurve throughout the RPM range. I can stomp on it at 3000 RPM, or 7000 RPM, and get the same blast of power from it! He calls this streetability, and it makes perfect sense.
I think the Weber is probably the best for high RPM all day long racing...but then again, that's what it was designed for.

500$ for a Weber?? You can find used webers on the net for 150$ or less. Used intake manifolds are also available for cheap if you're willing to shop around.

If you can find Weber 48IDA's for $150 let me know 'cos I'll buy them all up and sell them for $600-800 like they normally go for... :)

Used my friend... used.