3 two barrel carbs on a 250

[Anonymous]

what are the easyest to tune 2 barrel carbs to tune, and I can run 3 of them for an individual runner setup?

I get 130 cfm per carb, that is a total of 390 cfm at 4500

I want manual choke easy too tune simple and reliable

 

[Anonymous]

You would need an individual runner setup if you really want to run 3 carbs... Start by hacking off the current intake and building a new manifold from scratch.... 3 DCOE Webers would be nice $$$$

Why not just fit a single Holley or Weber to the existing manifold? MUCH easier. You could be modifying another head for tripples if you really want to try it.

 

[xctasy]

The tune and acceptability is based on getting the venturis sized correctly. To get power from 3300 to 4500 rpm, you need 22 to 30 cubic inches inches of engine for each square inch of venturi.

(The chart I used is the Weber sizing chart, and the rules with IR set-ups are critical)

The maximum size of total venturi area for an Isolated Runner 250 is 11.3 sq in total ( 39 mm each, or 1.55" diameter).

The minimum size of total venturi area for an Isolated Runner 250 is 8.3 sq in total ( 34 mm each, or 1.33" diameter).


The carbs must fit this critera. The throttle diameter can be between 1.5 and 1.9375".

In practice, the ideal is the 500 cfm Holley 2300 carb. The 350 is too small to sustain operation...it's like a DCOE 38 with a 30mm venturi, way too small. The 500 is ideal. It has a 1.375" venturi diameter in each barrel ( like a DCOE 43 with a 35 mm venturi)

The maximum indicative peak jet size is 1/20 th of the venturi diameter. If you have a 1.375" veturi, you need a 1.375/20 thou jet----> 0.069"

According to the IR gospel all motorbike and race car engineers learn from:-

Maximum start is a call size jet of 67 on a 500 Holley, and the power valve has to be set down to the 4.5 mark. The Chrylser Valiant, with tripple DCOE 45's ran six 57 thou jets as a start point on a 280 hp gross engine with about 300 lb-ft. Engine only was supposed to rev to 5000rpm, with maximum torge down at 3000 rpm, but could hit 6000 rpm with ease!

I'd suggest going way low, at about 53 jet size start.

The set-up then has to be set up like the factory Weber 45 DCOE set-up.

You must change the idle correction circuit. The power valve channel restriction sizes can stay as they are.

The only other option is the rare Weber DCD/DCNF carbs.

I want you to know that all independent runner systems work by simple physics. All the details on venturi size and start jetting must be correct to begin with. As an off road carb, the Holley isn't perfect, but its cheap and easy to alter.

For years, the sizing has been based on secrete knowledge from those who had the books. What I've suggested is to run the common Holley, and treat it like an Italian IDF/DCOE.

The Chrysler slant jobs have run carbs like this. I'm convinced this is the cheapest unless you can source the Weber down draft carbs.

 

[MustangSix]

To use individual runners on a 250 the minimum carb size would be three 500 cfm Holleys. Triple 45 DCOEs would flow close to 1500cfm in a constant flow situation.

In an IR system each cylinder only "sees" one venturi. That "250cfm" of carb capacity is only used on that intake cycle. The entire 42 cubic inches of that cylinder (in a 250) has to inhale thru that one venturi, but not in the space of one minute, but in a much, much shorter period of time.

 

[Anonymous]

what brand makes 500 cfm 2 barrels that are the easyest to tune, and are reliable, and are not the most expensive ...

I can make new manaflods that is not a issue .

what can I do about float issues on steep inclines. can I modify the floats or should I just by military type floats or the holley bugy spray.


do engines run good like this when they are tuned?

 

[Anonymous]

you say that holleys and webbers are the best between cost and effiency?

if i know how to tune carburaters how long should this take to tune this system?

i just make adjustments between carbs in common right?

 

[xctasy]

you say that holleys and webbers are the best between cost and effiency?

if i know how to tune carburaters how long should this take to tune this system?

i just make adjustments between carbs in common right?

Yes, the recalibration tune up is done on all three, with no change between centre and end carbs.

I'm looking at this myself.

Job 1:

a)There is some data on squirter capacity per pump of the accelerator from the Weber DCOE book. I'll find it. All you have to do is copy the milliters or cc's per 10 squirts to equal to the specs for a 260 Z or Aussie Valiant E37/38/48/49.

b)Find the plastic cam position which allows the correct accelerator pump action in cc's. Holley and Weber have very accurate information here, and I've been privy to it from books just like everyone else has. It is the primary means of controlling correct progression.

c) ensure the squirter jet is sized to copy the Triple DCOE calibrations for aftermarket Datsun Z or Aussie Valiant E37/38/48/49's.

d) float level stays stock

e) The linkage must use as many stock Holley bits as posible. The linkage bar must be one peice, and then it must link to the standard Holley's 1.25" bolt hole. The linkage carrier must be rose jointed, and of stiff steel and very, very resistant to any torsional bending. The carb throttle turns through 80 degrees, and is linked via a pushrod on the 6.3 mm ball to the linkage. It turns through 80 degrees, and at half the throttle turn, the pushrod must be at 90 degrees to the throttle arm. This then joins a similar 1.25 out 6.3 mm ball, and the stock acceleartor runs the set up. With this, the total movement of a cable throttle pedal is 1.25" at the top of the pedal.

I've found that with mutliple carbs, you must have a long throttle travel, and it must allow you to hit wide open throttle without your legs submaringing into the floor boards. It's best to have a fairly high pedal, with a lot of lost leverage. This takes the weight off the throttle, and allows you to relax. There is three times the load than a stock Inter with 345 and 2-bbl, so you gonna have to gear it down with more pedal movement.

f) the throttle stop position is based on the centre carb. The best option is to adjust this as the leader, and then ensure any alterations to curb idel position or wide open throttle position are able to be followed on the out carbs. If you fail here, the tuning is worthless. Note that no torsional stress can be placed through the Holley throttle rod wihic holds the throttle blades...it isn't strong enough. (Trust me...I'm at least alive to tell the tail!)


Job2: Once you've met these pre conditions, totally forget about idel and progression from cruise to wide open throttle, and work on this:

Just sort out top end and mid range fuel air calibration on a dyno with no alterations to the carbs. Use the 4.5 power valve. Tune with a common set of 53 to 58 jets, and see where the fuel air ratio sits with an exhast gas analyser. The power valve limits the peak size of the jetting by about 6 sizes in this application. The power valve should remain, because it can richen the set-up at very low vaccum and aid power delivery. The fact that it won't idle is totally irrelevent at this stage.

The time cost for the Job 2, jetting, is about 6 hours tops, in conjunction with ignition advance optimising when the right jeta are founds. Adjusta-jet main jet conversions on the 2300 carb don't allow you to run below a 58 main jet size, so your stuck with starting small at 53, and then going up in stages, with quite a cost in gaskets. But it's heaps cheaper than a Weber kit.

Job 3:Once that is done, then the idle circuit can be sorted without compromising the power delivery. There is a vast amount of information on this, and the circuit just needs a brass restriction to sort it. There is inforamtion on this in the old 1980 SA Holley book by Dave Emmanuel. It is suggested that no alteration is needed unless it doesn't work. In this case, it will need altering to suit because one 500 is designed to serve one 400 cube engine, not one 42 cube cylinder. There is info galore here, so I won't go into it.

I'd say it would take about 4 or 5 separate days of 3 hour strip and updates and checking to do, slowly restricting the air bleeds and idle ciruit tailoring, but once its done, it'll work perfectly. This can be done by yourself, following the procedure documented. The actual restrictions need to be made by a jewler or exceptional fitter. They are best made of brass rod, and then you can start small and open them out to suit. I'm gessing about 8 hours of you own time sorting the options, and about 2 or 3 hours of jewler time, plus material cost. A few will get bent so make up 12 brass correctors which are drilled out to a internal diameter of 16 thou, and fit in the ~ 26 thou stock hole. There are two ways to attack it, by drilling out or restricting. Can't remember how long the brass pieces are, I'll get that data to you.

Final work is experimenting with close limit jets, if you can get them. This will allow very accurate calibration of the carbs.

This general IR Weber process of tuning is intimated in David Vizards Mini book in the Weber section, and the SA Weber carb book. It works because it focuses on getting the common faults all IR carb installations sorted out first.

Lastly, the Holley shouldn't be placed end on like we are suggesting, but there is no other option.

If you run into problems, then David Vizard has a totally cheap solution to fixing the matter, and its about 200 bucks of electric fuel pump, and some race tank foam filler. Three old Holley jets, and the use of a stainless steel stand pipe to scavange the float bowl, and you are there. You could almost tilt over on its door handles, and it would still work.

I've given details on that before in the Aussie section, about page seven from memory.

Can't help anymore than that.

Regards,

Dean

 

[Anonymous]

I have a 1976 Holley book

 

[Anonymous]

heres an idea, how about connecting the throtel blades together into one piece all at the same angle so that way there is only one idle adjustment but you can still have six venturis or one throat per cylinder.

 

[twentyover]

Actually, that was my thinking, but on a port injected log head. Rather than a throttle bore and blade per cylinder (and there's a great article in recent Practical Performance Cars about building injection throttle bodies for individual cylinders), three throttle blades on a common shaft, centered more or less between 1-2, 3-4, & 5-6. Should take care of the flow retriction at end of manifold, use a single TPS and megasquirt.

Unfortunately nore imagination than money right now

 

[xctasy]

Note that no torsional stress can be placed through the Holley throttle rod wihic holds the throttle blades...it isn't strong enough. (Trust me...I'm at least alive to tell the tail!)

The Holley thottle shaft isn't a good quality item at all. They wear, and when you use old carbs, it just goes form bad to worse.

The option is to follow this. The Strombergs on this race wining GM Torana has all the stress on a stock rotating lever, with 4 inch rods pulling the thottles open. A very simple, easy to tune system. The curb idle and wide open throttle positions can be indexed very accuratley with this system.

Here is a US spec TR6 linkage. Webers turn through 78 to 80 degrees just like Holleys. This is the only kind of throttle system to use , in my humble opinion.

Here is a 26 Ounce, um, 260z aftermarket intake

Here is a big Healy with Webers. Note the cable operated carbs, with link rods.

Pat Braden, "Weber Carburetors", HP Books, 1988, covers the philosophy of running IR systems, and it is very easy to follow if you have an interest.

 

[xctasy]

Yet more details of the linkage ideas, stolen from the Slant Six forum and Ebay...

You'd have to be very smart on an early Stang or Falcon. (The brace bars would interfere with the rear most carb.)


A couple of notes. Here is Webers venturi sizing for Port on Port induction systems.

I've done a litterature search on the ratio between Cubic inches of cylinder per square inch of venturi.

Webers and Dell Ortos have interchangable venturis, but Holleys can only be up sized in 62.5 thou, or 1.6 mm intervals. You could expoxy it down, and emery it if you were patient, and the finish the diameter.

Generally, though, your stuck with either the 43 mm 500 cfm or the 30mm per bore 350 cfm.

Three 30 mm 350 carbs will only serve

a 144 engine (394 cc per cyl) reving to about 6000 rpm (rev limit 6600)

a 170 engine (464 cc per cyl) reving to about 5500 rpm (rev limit 6100)

a 200 engine (545 cc per cyl) reving to about 5000 rpm ( rev limit 5500)

a 221 engine (603 cc per cyl) reving to about 4600 rpm ( rev limit 5100)

a 250 engine (682 cc per cyl) reving to about 4300 rpm (extrapolating back down the graph, rev limit 4700)

Three 43 mm 500 carbs will serve

a 144 engine (394 cc per cyl) reving to about 8000 rpm ( rev limit 8800)

a 170 engine (464 cc per cyl) reving to about 7200 rpm ( rev limit 7900)

a 200 engine (545 cc per cyl) reving to about 6500 rpm ( rev limit 7200)

a 221 engine (603 cc per cyl) reving to about 6200 rpm ( rev limit 6800)

a 250 engine (682 cc per cyl) reving to about 5800 rpm (extrapolating back down the graph, rev limit 6300)

Independant runner, port on port systems are based soley on venturi size, and if you are keen to use domestic carbs rather than exotic Italian stuff, you've got to use Latin logic systems.