Manx Club

I have a 1915 with Compufire ignition, dual Webber 40 IDF's, and hydraulic lifters. This engine runs great at full throttle and at idle, but at about 2300 rpm's, has a bad miss and falls flat on it's face. (thats just about highway cruising speed) I may be running to small of idle jets or are the emulsion tubes playing some part in this. I'm not sure what they actually controll but have herd that this could be part of the problem. Any advise would be greatly appreciated. Thank You. Dan Scott

I think this came from Inglese Induction but I am not sure as I saved it some time ago. But its still relevant info even though its very long. THREE CIRCUITS For the sake of simplicity, let's look at the Weber carburetor as having three basic circuits- the idle circuit, the accelerator pump circuit and the main circuit. The idle circuit is comprised of two components, the idle jet and the idle jet carrier. With these two pieces, the tuner can select exactly how much fuel and how much air he wants to provide the engine at idle and during the low rpm operation, while making very fine adjustments to either, if necessary. The idle mixture is delivered as a proportioned mixture whose total volume can be further regulated with the idle mixture screw, which is located on the lower part of each carburetor barrel. On a correctly-jetted idle circuit, the mixture screw on a 48 IDA is never more than 3/4 of a turn out. This will hold true 100% of the time, no matter what anyone else tells you. If you have to go more than that, you'd better heavy-up the idle jet. Even if you get it to idle, going more than 3/4 turn tells you the jet is lean and you're going to have other drivability problems, which brings us to the next part of the idle jet's function. The idle circuit in the Weber isn't just an idle circuit - it does more than that. It is actually the circuit which must carry the engine all the way up to about 2,800-3,000 rpm, where the transition to the main circuit take place. That means if you don't drive over 3,000 rpm, you're only running on the idle jets. After 3,000 rpm or so, the idle circuit is entirely bypassed and no longer has anything to announce. So, if you have a tuning problem that "goes away" after about 3,000 rpm, that tells you to play with the idle circuit. Or maybe the opposite is true. Either way, it's very cut and dried as far as the two circuits are concerned - so isolating the problem is a breeze. One the most frequently experienced "gremlins" with Weber carburetors is a seemingly incurable and very annoying flat spot which rears its ugly head at about 2,200-2,800 rpm. This condition is generally caused by one of two things - you either have the wrong emulsion tube in the carburetor, which is causing a rich stumble due to an under-emulsified mixture at that particular rpm range or the idle circuit is falling off too early to carry the engine up to the point where the main circuit can take over, leaving a "lean hole". In simple terms, the idle circuit is going lean too early. Either condition is easily rectified. In the case of the emulsion tube, there are really only a few which work really well for V8 applications; and if you aren't using one of them it is certainly a big part of the problem. If the flat spot is still there even with the correct emulsion tube, then you'll need to richen up the idle circuit. This is sometimes a tricky area, because the first thing you want to do is throw in a bigger idle jet, but sometimes playing with air bleeds, mixture screws, or choke sizes can accomplish the same thing while sticking with the original jet size. Seeking a little bit of sound advice here can save a lot of time and hassle. The point here is that these carburetors are designed to come off idle and run smoothly all the way up. Your problems can be solved with a little tuning on your own or by relating the symptoms to someone who is knowledgeable enough to help you. Remember, these carburetors will do just about anything you want them to, except maybe wash your socks. The accelerator pump circuit, just like on any carburetor, is responsible for eliminating "bog" and making a passing maneuver without a hesitation or stumble. The circuit also has two basic elements. These are the pump exhaust valve and the pump jet. The pump exhaust is nothing more than a bypass valve and this is located in the bottom of the float bowl. This is the piece that regulates how much fuel you want to make available when you need that pump shot. Putting a bigger bypass hole in the valve allows more fuel to bleed back into the float bowl instead of out of the shooters. The smaller the hole, the more fuel you're making available. You can even put in a "closed" bypass for drag racing, when you need all the juice you can get in order to get those slicks turning. Obviously, there is nothing complicated about a simple bypass system. The duration of the pump shot is varied by installing a larger or smaller pump jet (shooter). Larger pump jets give a heavy blast over a short period, while the smaller ones will give a finer, longer-duration shot. As long as you leave the bypass valve alone, you're still getting the same overall volume. In most cases, the stock pump jets can be left alone. The main circuit is the easy one. This is where you make your power. This circuit has three primary elements you should concern yourself with - the main jet itself, the emulsion tube and the air corrector. You're thinking that's a lot of pairs - usually, it's just a main jet. You know how to "read" what your Webers can tell you on a road test, you wouldn't have it any other way. The capability for fine adjustment is what you pay for. Let's take a look at this main circuit...... The main jet is stuck into the bottom of the emulsion tube and sits in fuel. As the carburetor begins to work, the main jet meters the amount of fuel allowed to pass through it and up into the "main well" around the emulsion tube. Air enters the top of the emulsion tube through the air corrector which meters the amount of air to be mixed with the fuel. The air blows out of the emulsion tube through a series of holes along its length and aerates the fuel that is rising up the well around the tube. This emulsified mixture is then sucked out of the main delivery nozzle as the "depression" in the carburetor increases to the point where it's strong enough to pull it out. This occurs by 3,000 rpm or so, and you're down the road like a shot. Tuning the main circuit for maximum power is something that can be done by a series of road tests and a handful of jets. The simple rule of thumb for jetting Weber carburetors is, if you want to implement a change over the entire rpm range, you play with the main jet. If you want to change the way the car feels at the high end, that's where the air corrector comes in. Also, you should keep in mind that the air corrector is a finer adjustment that the main jet. Example: One step upward in the main jet (richer) equals about the same as three steps down on the air (less air: richer). A change of air corrector would be appropriate; for instance, if the engine pulls strong to 5,000 rpm and then goes flat. This would mean she's going lean on you up top; drop the air corrector three sizes or so, and you'll probably be able to buzz that engine right up to 7,000 rpm. If the motor feels sour all the way up, go one or two sizes heavier on the mains only. No magic! So, tell me, what's so hard about jetting these Webers?

* Make sure the idle jets and idle circuits in the carbs are clean. * install a fuel enrichment valve blockoff plate on each carb if it does not already have them. If the fuel enrichment valves are still on the carb you have the potential to suck dirt into the idle circuit. * Here is the final jetting in my 1914 after a couple hours of dyno time, how does it compare to yours? Idle: 50 Emulsion tubes: F-11 Air correction: 200 Main gas: 150 Venturies: 36mm This engine runs so smooth I can idle down to 1200rpm in 4th gear step on the gas and pull away without a hicup, get 27mpg on the highway and run 12.51 at the dragstrip.

Dan, Here is a previous post that I made sometime ago. I was experiencing the same problems that you appear to be having with your 1915CC Motor on my original 2007CC and was becoming very frustrated until I purchased the Redline Weber Kit and was able to experiment with many different options until I found the right configuration for mine. Besides the installation of the Redline Weber kit the other most important thing that made a substantial improvement in the operation of my engine was the addition of velocity stacks on the carbs which are inside the air filters. I installed 2" stacks (2) per carb. which made as much performance improvement in addition to the weber rejecting and modifications. Since making the modifications from the Redline Weber kit and added the velocity stacks to the tops of the carbs things have been screaming ever since and most importantly.... no more problems with hesitation. Then again, my fuel economy also dropped considerably but I would much rather have performance over economy any day. Just my :2cents: worth. Besides the initial throttle on acceleration this same problem was also most evident at highway crusing speeds and then when adding light pedal on acceleration. Things would just get crazy with the engine wanting to cough, spit, and sputter every time until I put my foot into it and until such time as I made the final re-jetting changes, installation of the .00 fuel pump bypass valves, and the addition of velocity stacks. This engine has been running great ever since. HESITATION ON LIGHT THROTTLE -- PROBLEM SOLVED..!!! OK....Redline Weber Re-Jetting Kit #702 IDF-2 Dual Carb. Installation - from Start to Finish only took a little over 1:45hrs. to complete. That's removing air cleaners, Weber 2" Velocity Stacks, Cross-bar Linkage, Tops of both Carbs down to where you need to install the components from this re-jet kit, installation of all components, Reassembly and Carb Re-Sync to Engining running again. I think I held my breath for the better part of this installation. I must admit, I would have never thought that this kit would have made such a performance difference but after driving my Manxter for 15 miles now after re-jet kit completion I am nothing but IMPRESSED! Should have done this some time ago. Installation was not that difficult and I was actually able to install everything without removing either of the Weber carbs from the intake manifolds while the engine was still mounted in the Manxter. Engine Specifications: (2007CC DUAL PORT) CYLINDER HEADS: 044 MAGNUM PLUS cylinder heads with 42 x 34mm S/S valves Dual SUPER REV Springs, Chromoly Retainers and Sure Grip Locks, 42mm Intake, 37mm Exhaust. Racing quality silicone maganese-bronze valve guides & hi-tech valve seats, Performance-prepped three angle valve seat machining. PISTONS & CYLINDERS: FORGED 90.5mm pistons and cylinders Precision matched and balanced. CRANKSHAFT & CONNECTING RODS: 78mm COUNTERWEIGHTED crankshaft precision balanced and micro-polished. Connecting Rods - balanced, rebushed/sized and matched. CAMSHAFT & VALVE TRAIN: #110 High Performance cam, 400 degrees lift / 284 degrees duration and micro-polished. HIGH Performance Chromoly Pushrods, Heat-Treated Rocker Shafts and Rockers. FLYWHEEL: Lightened and Balanced 12lb. 12 Volt 200mm. High Tech silicone flywheel oil seal. Kennedy Clutch 200mm High Performance kit. OIL SYSTEM: Heavy Duty high capacity oil pump, HD oil pressure relief valve, Finned aluminum valve covers. CARBS: Dual Weber 40 IDF with Cross-bar linkage. IGNITION: PERTRONICS CNC machined FLAMETHROWER-II distributer with GM style distributor cap and internal electronic module, FLAMETHROWER-II High Output Coil, Pertronics 8.5mm plug wires. EXHAUST: Meyers Sidewinder Exhaust (Hot-jet coating) In tearing down the Weber 40 IDF carbs that originally came installed on the 2007cc engine, and which I have been having this "Hesitation on Light Throttle", I found that they must have just installed an "Out of the Box" set of carbs as the internal components were all stock sizes and as follows: Original Idle Jets: .50 NOW -- .55 Original Pump Jets: .50 NOW --.55 Original Fuel Exhaust/Bypass Valves: .55 NOW -- 00 ZERO Bypass All other internal jets, venturi's, and valves are still stock sizes. These .00 Zero Bypass Valves, in the Redline Weber kit, keep the fuel, that is normally being partially shunted back to the pump, and applies all of it to the carb. I believe that this was the biggest difference in getting rid of the hesitation on light throttle. I must also admit that even changing out the Idle Jets themselves from .50 to .55 made a considerable difference as well once I got the Air/Mix screws adjusted correctly. These Idle Jets were what I did first a couple of days ago just to see what they did in performance. Not bad results but still had a very slight hesitation and still wanted to increase carb performance a little further. There is definately a LARGE improvement with everything from the kit installed now and no more hesitation. Certainly worth ever penny for this kit. This 702 IDF-2 kit also comes with .60, .65, and .70 Idle Jets so I can still experement a little more with these if necessary and they do not require having to dismantle the carbs again to change out and install different sizes for further testing. Right now---my engine is running better than it has since the beginning and I'm going to stop for now and evaluate a little more before doing anything else. Well those were my findings and engine specs as they were and are now and from my Redline Weber Re-Jet Kit installation and velocity stacks. The Manxter is runnning so good now I'm leaving again to enjoy the rest of the day in it.