Calling all carbed pelicans...

[jorian]

I have a new-to-me 79 SC with 46mm Webers. Throttle response is great BUT there is a bit of a stumble @5500-6000RPM.

Not sure if it is fuel delivery or spark or something else. Before I get into it, any advise on where to start? TIA.

[Elombard]

I dont know about this specifically but I would find out what all your jets, venturies, emulsion tubes etc. are and then call PMO, tell them what you have and what the use is for the car and get there recomendations. I have always thought I would go this route.

Then I think you will need to get an exhaust analyzer to really fine tune it.

I am sure there will be loads of good suggestions on the inital set up from the board.

[randywebb]

you can rule out the idle jets and the transition holes

[zumwoll]

If I recall my last SC , it had a cut-out (governor) rotor which started cutting in the hi 5k range.
If that is not the case, does it stumble from there on? My experience are two fold.
The first is to test your plug wires for continuity. If these are the original plug wires they could now be crapping out at hi rpm. I have been fooled a few times by this. Second scenario is of course the main jet. What I used to do was with my jet drills is slowly increase the size of the main jet until the problem went away HOWEVER this takes a lot of patience and you have to be sure it is fuel delivery other wise things are only going to get worse. Have you checked the four fine screen filters that are in the fuel line that attach to the carb bodies? IMHO this is not a task you want to do on a dailiy driver cause being in a hurry is not going to git er done. WHen this old engine builder showed me how to do it I was amazed at his patience, but the results were awesom. I had a little outlaw speedster that after doing this would pull the highest peak in the Cascasdes in 4th where before I had to downshift. Paying an old timer who knows Webers for an hour of labor might be a very good investment if none of the obvious stuff works. I have been fighting the carb battles for years and it is never fixed. It is always a work in progress regardless of what MotorMeister says!

[A Quiet Boom]

What fuel pump are you running? If you don't have enough pump you'll run out of fuel up there, I did.

Dynojet Fun!

[randywebb]

good idea to be sure of the ignition before getting deep into the carbs...

[jwetering]

The responses above are all spot on Jorian. For the record Jorian is using Weber 46 IDA on a 3.0L with CIS pistons, S cams and Euro high compression heads. It's not his daily driver, that's what the motronic 3.2 Carrera is for.

Check the wires and obvious ignition bits like plugs, cap, and rotor first. Then the fuel delivery system. IIRC, that rig of yours is running a CIS high pressure pump choked down to 3 psi at the fuel rail...that's lower than the recomended 3.5 psi. Maybe a big deal, maybe not, but I'd sure like to see how it runs with a little more fuel pressure. If that all checks out, then a carb teardown is indicated to make sure all the passages are clean. I've rebuilt my 40 IDA a few times now and can certainly help you with this. It's not hard, just time consuming (days not hours) and you'll need access to compressed air. If after all that the problem persists, then try fooling with the jets. Did it always stumble at high revs? If not then it's for sure not your jetting. Richard Parr at PMO is the go-to guy for jetting recomendations IMO. I sent him my specs, he made recoemndations that I followed, and when all is running well engine behaves like it's fuel injected.

My carbs have never really given me any grief in terms of transition from idle to main circuit, and only once did I lose power at the top end. In that case it was bit's of crud in one of my emulsion tubes and one of my main jets. It is possible to live happily with weber carbs, but they aren't the kind of things you just set and forget that's for sure.

My *guess* is that it's either wires, or low fuel pressure, or crud in you main circuit.

Talk to me on the weekend. Markus is coming over Saturday night to help me bleed my brakes.

[A Quiet Boom]

I'm trying to understand 46mm Webers and S-cams on an SC with CIS pistons. Aren't the carbs a little big? And how are you running S-cams with CIS pistons? I'm running a stock SC long block with backdated exhaust, custom muffler and Zenith's with 36mm venturis (34's would be better) and making good power. I've replaced the junk Facet pump with a WAY TOO LOUD Holley pump and regulator and now my AF checks out at the top end. I'll be heading back to the dyno soon to see what kind of difference the new pump makes.

Do check the ignition like others have suggested, especially in the dark on a humid night, you might be surprised.

[jwetering]

The fact that the 46 IDA are too big has been acknowledged, but that was how the car was set up when Jorian bought it. The s-cams are claimed, not confirmed. The CIS pistons are assumed. I can't recall what the venturies are...I think they were 38, or 36. That's one of the limiting factors of the 46 IDA isn't it? That you can't run small enough venturis?

A little off topic, but can anyone explain why 46 IDA are too big for a 3.0L? Let's say you could find a set of 34 mm venturis to fit, how does the bore of the carb body affect the airflows?

[A Quiet Boom]

Actually I agree with you. 34-36mm venturis would be better with 46mm than 40mm per weber recommendations for venturi to butterfly size. Even better if the heads are ported to match IMO.

Hmmm, let me see.....spare set of Zenith's, two milling machines, two lathes.... make some throttle plates....46TIN Zeniths? LOL I'd be better off starting with a block of 6061

[2.7RACER]

So what is the venturi size? Main jet? Emulsion? Air Correction?
I agree Jorian would be better off with some 40's and 34mm venturi's. 36mm for racing.

[jorian]

I'm out of town with little web access. Thanks for all the replies. The venturi's are 36mm and as Jasper mentioned the S cams are claimed but not confirmed. The car was running beautifully until about a week ago when I noticed a slight stumble at 2000RPM. In the past few days it seems to only lose power at 5500-6000. It doesn't seem intermitent but it has moved up the in RPM so ignition does seem like the logical place to start. The fuel line doesn't seem to have a dial to set pressure but rather a metal o-ring that crimps the fuel rail - is this normal?

If the motor was rebuilt with a higher redline, would that explain the larger 46MM size? A high pressure fuel pump would require a correspondingly larger volume of air, no?

Thanks again for the info - love this board.

[randywebb]

metal o-ring that crimps the fuel line was an older way to regualate pressure - not sure re the _rail_ The rails I've seen are metal and not crimpable except by Godzilla. If that's what it is - any crimping anywhere -- I'd toss that and get teh gauge and pressure regulator.

Yes, it might have had 46's and then been detuned.

re the benefit or lack of same up a few posts -- I'd like to see a cross-section of the airflow area. Maybe there is a space where the venturis don't fill out the bore.

[A Quiet Boom]

jorian,

Fuel pressure has nothing to do with carb operation per see. Pressure and volume are there to ensure the float bowls are kept full even at full demand like high rpm at wide open throttle. Carbs actually use vacuum to pull the fuel up through the emulsion tubes to mix with air and atomize at the booster venturis.

Whatever you do I recommend putting a proper regulator and pressure gauge on that fuel system, that metal o-ring setup sounds flaky at best.

Here is the order I use when working on an engine problem,

First I check the ignition, is the timing correct, are all the plugs firing, are the cap, rotor, CDI box, coil, plugs, and wires in good working order etc.

After checking all of that then I look at the fuel system, I first check for obvious problems like vacuum leaks, broken or missing parts and pump pressure and output (I forgot to follow my own rules on my car for the fuel pressure) then I check float level, carb side to side balance, cylinder to cylinder balance, accelorator pump circuits, check for clogged passages or jets etc.

You can spend hours chasing a carb problem that is actually an ignition problem. One more thing, before adjusting and setting up carbs it's a good idea to check valve lash and timing.

As for 46mm carbs on a 3.0, it is my belief that with 34 or 36mm venturis they will work better than 40mm carbs. This is because the recommended venturi size is around 80% of the throttle bore size, any higher than that and the signal to the boosters get's weaker and weaker and fuel atomization suffers. The problem however is port size, late 3.0's have 34mm intake ports while early 3.0's have 39mm intake ports so the ports themselves become the restriction which will weaken the signal to the boosters. At a minimum I'd recommend blending the intake ports to the manifolds and the manifolds to the heads for best performance.

I'm running 36mm venturis in my 40mm Zeniths so my venturis are 90% of my throttle bores, I'm sure this hurts my midrange and possible my top end. Your 36mm venturis and 46mm throttles put you at 78% which should be excellent for atomization of the fuel provided the intake ports aren't screwing it up. I've ordered a set of 34mm venturis just to see what it does to my HP and torque curves as well as part throttle drivability. I've got a light throttle stumble in mine that while barely noticeable is still annoying. This occurs between 2000 and 4000 rpm when cruising, I think the signal to my main jets is so weak that the car is slightly lean at that point but I haven't confirmed it as I don't trust my single wire O2 sensors attached to my AF meter. I'm gonna install heated O2's and see where the AF really is in this range. I highly recomend an AF meter with carbs and I can supply part numbers to do it on the cheap using regular 3 or 4 wire O2 sensors available at any auto parts store and an inexpensive AF gauge/meter.

So, thoroughly check that ignition then look into the carbs but please put a fuel pressure regulator and gauge on it. Right now you have no idea whether it's running the float bowls low from lack of pressure and volume. If the floats run low the engine leans out and will stumble, this will effect the mains and the idle/transition circuit.

[Wayne 962]

In general, I recommend 40s for 3.0 and 3.2 engines, with the 46s for 3.2 race engines. Experience seems to indicate that the 46 carbs are just a bit too big for the 3.0 and smaller engines.

That said, you cannot run S cams and stock CIS pistons - the valves would hit the pistons. You must have something else in there...

-Wayne

[jorian]

Quiet Boom,

Thanks for the comprehensive reply. I'm really looking forward to getting at the car next week. I'll start with the ignition and then fix the pressure reg issue. I would also like the part #'s for the AF meter guage. PM or email at joriand AT shaw DOT ca.

I bought the car with all documentation EXCEPT the engine rebuild info so all we have several best guesses as to what is really in there.

Thanks again for the replies.

[A Quiet Boom]

Jorian,

I have the numbers at my shop and I'll be there tommorrow so I'll post them here sometime in the afternoon. I've got one meter on my car with a switch to check a sensor in each bank. Having an O2 in each bank really isn't necessary unless you want to seriously fine tune it. I find setting all the mixture screws at the same number of turns from seated postion works best on my car.

As for my car's stumble/surge I think I've narrowed it down to either a vacuum leak or trouble with the transition circuit. I believe it's only one cylinder and now I need to determine which one.

I wanted to add one more thing, my experience with carbs on high performance engines has always been that they run the smoothest when they are a little rich at idle. This is because it richens up the transition circuit as well. I've seen and heard a lot of carbed cars that pop through the exhaust on decel, mine does not, not ever.

Also the idle and transition circuit are still active at WOT, despite what others say. I've seen it firsthand with my car on the chassis dyno. My first time there I didn't have any main jets with me but I did have a larger set of idles. The first pull was way too lean (later found to be a warped intake manifold not sealing) but I still wanted to get a baseline for the car. I went from 0.60 idles to 0.68 idles and the car richened up about 0.5, enough for it to be in the safe zone for a dyno pull but still too lean for best power.

I've been going nuts, trying to relearn everything about carburation I knew 8 years ago when I drag raced a Mustang. You think you got troubles, try tuning a 1000cfm 4bbl carb with a large plenum manifold, 2" race headers and a full race cam on a 410" small block engine to be drivable on the street. I wound up drilling and tapping the idle air bleeds for little jets so I could fine tune the idle and transition, took a lot of time but I got it. IMO the individual runner setup for 911 carbs is much more easily tuned for drivability and big power than a plenum type system. IR systems also have much more even fuel distribution and sharper throttle response. Be patient and write down things as you do them, in the end you'll be very happy with your carb setup. My basically stock SC engine will easily pull from 1500 all the way to the redline if I mash the throttle and it never feels like it's being lugged down if I run around town at 2000 rpm or so.

[Steve@Rennsport]

Jorian:

You've received a lot of good information and the benefits of other people's experience here.

I would simply like to offer that Weber's are incredibly sensitive about fuel pressure, float levels, and dirty fuel. One should never begin the jetting process or troubleshoot running problems until one has 3.5psi of regulated fuel pressure, precise float level settings using the vials with the engine running, and really effective filtration down to 10 micron.

Its impossible to overstate the importance of all of the above,...

Good luck, Sir.

[A Quiet Boom]

Originally posted by Steve@Rennsport
Jorian:

You've received a lot of good information and the benefits of other people's experience here.

I would simply like to offer that Weber's are incredibly sensitive about fuel pressure, float levels, and dirty fuel. One should never begin the jetting process or troubleshoot running problems until one has 3.5psi of regulated fuel pressure, precise float level settings using the vials with the engine running, and really effective filtration down to 10 micron.

Its impossible to overstate the importance of all of the above,...

Good luck, Sir.

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