Twin Weber carb setup advice.....

petex19

petex19

True Classic
I've had my BVhead work done for a couple of years now and have been playing around with jetting (Mains and Air Correctors) only for my DCOE carbs and am very happy with the results but my car lacks consistency meaning it goes like a bat out of hell but not all the time as it seems like my plugs are fouling so I replace the plugs and it runs amazing again but I find I'm changing plugs every 100miles or so depending on wheter it is city or highway driving I'm mostly doing. I would like it to run optimum all the time without costing me money for plugs so often.

The machine shop that built my head and set it all up installed an O2 bung on my header since it never had one and he connected a wide band Air/fuel device to his laptop and set it up accordingly and I spoke to the guy who did the work and he said to bring it to him and he will give it a supertune which I have no idea how much will cost me or what it entails.

What I've tried.....

Without touching the air fuel screws on the carbs as I don't want to make things worse. I purchased a good range of both Main and Air Corrector jets and have tried various jetting and recently removed my MSD Blaster 2 ignition coil and re-installed the factory coil with resistor as I thought that might help. It runs just as good with the factory coil but now is backfiring more frequently. With the MSD coil it would hardly ever backfire.

I guess what my question after all this talking is are other dual carb owners out there buying Air/fuel gauges and do they work and are they easy to use and get good results. All I've ever been told is that you have to buy a wide band if you want any accurate results.

'PeteX1/9
 
I bought one

I bought a wideband gauge, but just don't have the upgraded engine in the car yet. Your fuel levels (rich or lean) matter a lot and you can't tell where you're at without a gauge. Too lean, you can burn things up. Too rich, you can foul things up.

I'm sure the carbs are balanced, but having the gauge to check that as well is a nice thing to have.

Dyno time can be expensive unless you know someone. I ended up spending $160 for my dyno time and didn't even get to swing the cam to find more HP.

Seat of your pants tuning (not dyno tuning) can have nice results. Just need the tools to check adjustments.
 
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Carb Tuning.

I ran a single DCNF for over 25 years all tuned by feel and ear.

In the old days we had leaded gas and tail pipe colour was a big indicator of rich or lean. O2 sensors had not yet made it to production cars. You didn't say what camshaft you are running. That will matter alot as well.

What you must keep in mind is that you are trying to obtain a 14.7 part air to 1 part fuel ratio. What you must also keep in mind is that the Carb does this by mixing air through a restriction or venturi by way of a jet. The barrel size allow more or less volume to flow to the engine. Put the stock 1500 carb on the car with it's vacuum actuated secondary and the car will run, mostly like a sick dog but it will run. Increase the barrel size or add more barrels and the engine will preform better as it is flowing more air and fuel.

There are 3 cicuits in the basic Carb. One circuit is for idle, it uses a fixed jet and a needle valve that adjusts the mixture at idle. The throttle plates are closed and just at the edge of the plate in the barrel are small holes that allow the fuel to be drawn into the engine at this high manifold vacuum state. At idle the main jets do not do anything and it is possible to remove them with the engine running. If the engine will not idle with the main jets removed then the thottle plate is open too far and you are using the main jet circuit to supply fuel to idle the engine. You must increase your idle jet size then accordingly.

The second circuit is the main jet circuit and the fuel is mixed in an emulsion tube that has an air correction jet on the top and a main jet on the bottom. The tube feeds through holes in the side the centre of the venturi in the main barrel. The main jet restricts the amount of fuel entering the emulsion tube the air correction jet adjusts the amount of air entering the tube and adjusts the pressure or lack of in the emlusion tube. If the air correction is small then the pressure drops in the emulsion tube and more fuel is drawn at a faster rate through the main jet and into the carb barrel. This where the balancing act occurs. A large main jet on the bottom and a large air correction jet on the top will not flow as much fuel if the air correction jet was smaller.

The 3rd circuit is for the accelerator pump jet and it dumps more fuel into each barrel to compinsate for fast opening of the throttle plates when vacuum drops off and the fuel air ratio goes lean for a second or two. If the engine bogs down bump up the jet a size.

Where to start, a wide band hand held O2 sensor will be of great advantage, but like I said years ago that wasn't around.

Judging by what you are telling me, you are running rich. Way rich. But you must first get the car to run on the idle circuit and if you have a long duration camshaft, idle will have to be rich and this where a wide band sensor will steer you wrong.

On the main jet circuit leave the air correction alone and go down a jet size or two, or just increase the air correction to lean out the mixture. Now go for a run come back and pull a spark plug and look at the colour, if it is black, then go leaner again. If it is going ash colour, the you must richen the mixture. Go in small steps and take the car for a run. If you are running real rich I have seen the oil pan level go up because the engine oil is full of fuel washing past the rings.

Jetting runs can be fun and frustrating, take notes and expect your problems to be a bit compounded because you have 2 carbs instead of 1. A flow meter will help you syncronize your throttle plates, but in the old days we used a length of copper wire to set all the throtle plates the same. By slipping the wire down the barrel and just letting it touch the throttle plate and barrel wall on one carb then adjust the linkage the same on the other carb.

Anyway there is some seat of the pants how to that I used years ago, I am sure someone will tell me I am full of crap, but if worked for me, it was fun at the time to do.

TonyK.

Grimsby Ontario Canada.
 
The cheap way...

Get yourself a bottle of propane, like what the plumbers use. Not MAPP gas.

With the car running, open up the bottle so a little bit of propane is taken in by one of the venturies.

If the car increases rpms, you are too lean. If it boggs down, you are too lean.

Hope this helps

Eric
 
If it blows up, start over

LOL Just kidding, but I've never heard that one before and I grew up pretty deep in "dem dar hills".
 
DCNF set up

What kind of driving do you do? Are you a rev it till it begs for mercy guy?

Or just want something easy to drive with good sounds when you open it up? If you just want a car that drives easy, I would seriously consider buying a set of the smallest venturis you can find for the 40s, probably 28mm. Then make sure you have F36 emulsion tubes (source: PBS). Main jet size (4x venturi size) would be 110-115. Air correctors about 190. Idle jets 45 - 50. (Source: Pierce Manifolds recommendation) This combination should give you 30MPG highway.

With the 32mm venturis that are probably in there now, you need 125 main jets and 205 air correctors to start with. Probably 50 on the idle jets.

MikeR ('86X, former 128 owner w/36 DCNFs on a 1500)
 
Nice information....

That was very informative thank-you.

My camshaft is a PBS SX1 which is quite mild but has great duration and lift. I believe it is 27-52 if I'm not mistaken.

'PeteX1/9
 
Carb setup....

My driving style is spirited street driving:) I rarely go over 5000rpms so no I do not redline it to death. My Webers are 45DCOE's and yes I do have 32 venturis and am currently running 145 Mains but have a full set from 120 to 145 and am running 200 Air Correctors. I am familiar with the formula so I should probably be running 130 Mains which I have aswell. Emulsion tube is F11 and Idle jet size is F8 55.

In the write up TonyK did he mentioned that larger Air Correctors will lean up the mixture so is larger meaning higher in number because I thought Air Correctors worked opposite Mains meaning as Air Corrector numbers like 190,195,200....get larger the Air Corrector actually gets smaller unlike Main jets that get larger with the larger number.

I hope I'm not to confusing.

'PeteX1/9
 
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Why 45's....

I know it's weird to reply to my own thread but I'm sure like many others over the years have told me "Why 45DCOE's get 40's or 36's ?" Well no reason other then I was only 20 years old and had just bought my copy of Project X1/9 and this is what I read so why not:


"The ultimate carburetor setup for the 128-X1/9 engines, from a performance standpoint, is a pair of 45 DCOE side-draft Webers. Unfortunately, however, they won't fit in either the 128 or X1/9 cars without modifications to the car for clearance. In the 128, the heater inlet box must be removed or at least cut down for clearance. The X1/9 installation requires cutting clearance into the rear trunk area for the carburetor air horns and filters. The box member, across the top between the engine bay and the trunk, must be cut out along its bottom for clearance. Then a piece of sheet steel must be welded back in higher up to box in this area for structural integrity. Customers who have tried both the 40 downdraft carburetors, and the 45 side-draft installation, say that the extra performance of the 45 side-drafts easily justifies the extra effort required for installation. In addition to allowing somewhat better breathing at high speed than the downdraft carburetors, the DCOE carburetors are also less sensitive to cornering, etc. The same comments about low speed fuel economy apply to these carburetors as well as the twin downdrafts, of course." -By PBS

So I bought them and have the cam and header and shaved BVhead with matching ported intake manifold and lightened flywheel to complete the setup.

'PeteX1/9
 
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It sounds like the mixture at idle is way too rich which is causing the plugs to foul, then clear up after running at higher RPM for a time.

Take the X to a good shop with a chassis dyno and exhaust gas analyzer to figure out the jetting changes required. You'll discover jetting charts are only a initial guide and actual jetting depends on your specific engine if good results are desired.

If you're not running the engine in the X much above 5,000 RPM, there is little justification for twin carbs as a single carb will deliver better performance under 5,000 RPM due to the higher flow velocity at lower engine RPMs. If the desire is to extract as much power as possible from that X engine, then it's going to require using far more than 5,000 RPMs. To get a proper higher power X engine, it requires a complete engine concept, cylinder head, cam, compression, pistons, con-rods, intake, exhaust and etc.. all must work together to get good results. What is desired from the X engine must be considered before the engine build. A race engine is different from a performance street engine is different from a endurance race engine.

I have no reservations what so ever to spin the engine in the 88' just past 8,000 RPM when desired and this is a street car. If the engine is properly built and healthy, it's not gonna break.

Word of caution about the stuff written in the PBS books and etc.. Some of it is very dated.. keep in mind much of this was written 20+ years ago and some things have changed. They did not have Electronic Fuel Injection systems available today and their suspension theory for the X is IMO, flawed. Use what was published by PBS as a guide and ideas of what has been done. What they wrote is by no mean the "Bible" on how to make a faster x1/9.

Bernice

petex19 said:
My driving style is spirited street driving:) I rarely go over 5000rpms so no I do not redline it to death. My Webers are 45DCOE's and yes I do have 32 venturis and am currently running 145 Mains but have a full set from 120 to 145 and am running 200 Air Correctors. I am familiar with the formula so I should probably be running 130 Mains which I have aswell. Emulsion tube is F11 and Idle jet size is F8 55.

In the write up TonyK did he mentioned that larger Air Correctors will lean up the mixture so is larger meaning higher in number because I thought Air Correctors worked opposite Mains meaning as Air Corrector numbers like 190,195,200....get larger the Air Corrector actually gets smaller unlike Main jets that get larger with the larger number.

I hope I'm not to confusing.

'PeteX1/9
Click to expand...
 
Pete! 5,000rpms?!?

Ok, from one Fiat owner to another. Go out and drive your car for awhile and shift it at 7k for a bit.

You are short shifting your car. Stop it or else the entire Xweb will have to meet up at your place simply to smack you around then take turns driving your car! (If we could get Tony out of it!)

You have been warned!

Seriously, don't hesitate to drive your car to its potential. With your modification, you are missing out on some of the best areas of it.

Eric.
 
Honestly....

Seriously, don't think for a second that I wouldn't love to drive it like I stole it but it attracts so much attention because it is so loud that at 5000rpms you can hear me from three blocks away for sure.:)

On a track absolutely but on the streets the police would have a field day with me if I shifted at 7000rpms

So knowing my jetting what do you recommend? Is my Emulsion tube okay(F11) and my idle jet (F8 55)?

I re-installed again my MSD ignition coil and the backfiring has gone away and I can install smaller mains like 130's with my 200 Air Correctors and that should work good.

'PeteX1/9
 
Pete, the exhaust can be free flowing AND quiet....

So that you can use all of your RPM range. What is the exhaust like on the car?

For an upper end of the motor like you describe, if you fit a 2.25" or 2.5" DynoMax muffler - Super Turbo is a bit quieter, UltraFlo is straight through - to a header, you will have a setup that is extremely free flowing, yet not noxiously loud. Certainly a lot quieter than rice boy cars. And when you open the throttles up, you get that nice Italian motor song, not just noise.

The trick is a high quality muffler with some volume inside. SuperTrapp mufflers have long since been de-bunked for either high flow rate or quietness. Bullet style glass packs are small and burn out. A number of guys on the forum have built really nice looking setups - single outlet, dual outlet, exit left, exit right, even two mufflers.

The system on my 1300cc car is as I describe above in the 2.25" inch size using a Super Turbo muffler. It has a nice rumble at low speeds and plenty of flow for the top end. Chris Obert's Dallara racer has a long tube header with a 2.5" Magnaflow muffler (same thing as Ultraflo). People always ask why it is so quiet, being as it is a 1600 stroker, 45 DCOEs, big valves, cam, etc. The rejoinder by us was always, "It don't have to be loud to be fast."

Steve Hoelscher published some photos of the setup on his Championship winning race car. Do a search there.

Limiting your car to 5000 RPM is like running it on 3 cylinders.

Ciao,
 
My exhaust...

My exhaust is a PBS header which is welded right after the collector to a 2.25" pipe which goes to a "Raven" highflow single in dual out muffler which was intended for a '87 Camaro and that's it. My total exhaust pipe length is probably less then 2 feet after the collector which I don't know if that makes a difference but my car rumbles like a V8 at idle and when you homp on it, it screams. I've never made a video yet but will have to so you can hear it.

'PeteX1/9
 
Plug gap ?

Try closing the gap in the plugs up a bit . It might be your ignition is a little weak for the gap you are running. I have a BMW that is sensitive to plug gap. I have to set it at .028 for it to run right . Just a thought. Play around with the gap a little and see what happens.
 
Bernice is correct

The carburation sounds like it's too rich. Opening up will only temporarily solve your problem as the plugs will foul again. Like the lady said everything must be dialed in and matched to have the carburation work correctly. Dual carbs is over carburation for city driving no matter what you do to them but is very streetable when adusted properly.

A single carb is proven more efficient for city driving unless you drive on the freeways alot, track etc. A good tuner will start with the air ratio for each venturi then set the mixture screw.

Tyrone
 
Very Interesting...

That's interesting that you mentioned plug gap because I just did exactly that a couple of days ago and the car does run much better since I did.

I had my plug gaps at .028 according to my Haynes Manual but when I went to a local auto parts store their computer had the gap listed for an X1/9 at .025 so I went home and adjusted the gaps and the difference is noticeable for sure.

'PeteX1/9
 
More plug gap stuff

petex19 said:
I had my plug gaps at .028 according to my Haynes Manual but when I went to a local auto parts store their computer had the gap listed for an X1/9 at .025 so I went home and adjusted the gaps and the difference is noticeable for sure.

'PeteX1/9
Click to expand...

Take a look at http://www.artigue.com/fiat/serviceletters/pdf/000-36.pdf from Brad Artigue's amazing service letter collection... The difference between the resistor and non-resistor plug types is striking.
 
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