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EFV-8 Club Forum / General Ford Discussion / carbie flat spot

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Posted By Discussion Topic: carbie flat spot -- page: 1 2     37RAGTOPMAN 03-16-2010 @ 8:35 AM reply profile send p.m. Senior Posts: 1963 Joined: Oct 2009 its funny nobody mentioned a different camshaft, multi carbs and modified cams work together,and also a depending what year engine you are working, maybe a little rework in the ignition distributer might be of advantage also,so the spark advances quickly along with increased RPM, you might be able to rejet the PAIR of 97's so they work better, to do it right you have to do the whole ball of wax, my 3 cents worth 37RAGTOPMAN Years ago a friend of mine asked me to take a look at 1936 Phaeton, why it had no power, it idled with no problems. I checked the carb it said 81 on the side of it, I knew what the problem was, the venturi is a lot smaller than a 97 carb, changed the carb and no more problems. Stroker 03-11-2010 @ 2:12 PM reply profile send p.m. Senior Posts: 1460 Joined: Oct 2009 I don't know why your 4-banger guys are happy with dual 97's unless they have lightened their flywheels. Reducing rotating mass allows the engine to rev quicker, which in turn makes it's apatite for air/fuel increase faster, which minimizes the "dead spot". It may be that given that a Model B crank only weighs about half of what a V8 crank does has the effect of speeding up the throttle response. In line with Supers comments on plenums, the attached image is of an early design 2 carb manifold that should illustrate the point that not all manifolds take kindly to blocking off one carb. I still am of the opinion that the Stromberg 81's are the way to go. Even though they are not common in Australia,they can be found in the US. Appearance wise, they look just like a 97, except the venturis are scaled down to .81 inches. Holley also made a .81 inch version, but these are very scarce even in the US. The whole idea is to get the air moving faster through the venturi so the venturi can function to draw fuel and mix it with air. Carb "risers" are often used to improve low-end torque, as they effectively make the intake tract longer, which "tunes" the manifold to respond better. Adding "velocity stacks" to injector tubes is often used for the same effect. The ultimate example of exploiting long induction paths were the Cross-Ram manifolds used on Chrysler wedge motors in the 70's. supereal 03-11-2010 @ 10:10 AM reply profile send p.m. Senior Posts: 6819 Joined: Oct 2009 The reason I mention common plenum is that the stock intake manifold feeds four cylinders only from each side. If you restrict the carb at the top, it will act as a choke. If you want to see the effect, just run with the choke partially on. You will soon see black exhaust smoke from an over rich mixture. The venturi size determines the cubic feet per minute capacity of the carb, which must be matched to the engine to achieve volumetric efficiency. Base restriction will increase manifold vacuum, but will reduce the venturi effect by hampering flow and thus the overall efficiency of the carb. ians34 03-10-2010 @ 11:45 PM reply profile send p.m. Member Posts: 10 Joined: Oct 2009 Thanks fella's. I am talking about restricting the choke diameter at the top, not using a restrictor at the base. Has anyone used the manifold risers that fit between the carbie base and the manifold? they are about 2 inches thick from what I can work out, was the principle of these to increase air speed? would they help or aggrevate the problem? How do the 4 banger guys get on with twin 97's. thier capacity is even smaller. Any tips would be greatly accepted. there are not a lot of flathead experts in Western Australia. None I think! Stroker 03-10-2010 @ 8:05 AM reply profile send p.m. Senior Posts: 1460 Joined: Oct 2009 Super: I think the issue with his Fenton two-pot is exactly as you state: "If the manifold has a plenum". I'm not familiar with this particular manifold, but I found an example on line that would indicate that it may. The manifold that I am most familiar with is the old Edelbrock "Super". Blocking one carb off on that manifold would have the engine running on four cylinders. supereal 03-10-2010 @ 7:44 AM reply profile send p.m. Senior Posts: 6819 Joined: Oct 2009 A good many "for show only" multiple carb setups have the extra carbs blocked off to make the engine run properly if the manifold has a common plenum. We used to insert "drive away" plates in some instances. These were made to restrict speed for vehicles that were driven, instead of trucked, to dealers years ago. They are similar to the NASCAR restrictor plates in use today. ians34 03-09-2010 @ 11:29 PM reply profile send p.m. Member Posts: 10 Joined: Oct 2009 thanks for your reply, These carbs are for show only!On the topic of venturi size, I have a Falcon GT which has quad downdraft webers and to assist with bottom end performance I machined up some smaller choke tubes which worked fine. If I restricted the initial venturi diameter on the strombergs at the top of the choke tube do you think that would work. The carbs you fellas are talking about using (v860) I have never seen avialable in Australia. supereal 03-09-2010 @ 8:27 AM reply profile send p.m. Senior Posts: 6819 Joined: Oct 2009 I agree with Dan that progressive linkage is most often used on triple carbs, as the engine usually won't run well, if at all, without it. As I said in a previous post, carb capacity must be carefully matched to engine displacement and the usual or maximum engine speed for the application. This is called "volumetric efficiency". If you are going to choose a replacement carb, it is better to err on smaller, rather than larger, capacity. Two important effects of overcapacity are fuel starvation and poor atomization of fuel. As carbureted engine size increased, the metering rod was introduced to compensate for fixed main jets. Our old fixed jet carbs just are not suitable for multiple applications without modification. The usual reason for multiples is for the "look". At our shop, we recommend a mechanical secondary four barrel for good idle, smooth acceleration, and extension of the power band. Unless you have experience in carb modification, it is likely you will create a bigger mess than a "flat spot". BrianCT 03-09-2010 @ 6:09 AM reply profile send p.m. Member Posts: 95 Joined: Oct 2009 Forgot to mention that I once had a similar problem [same symptoms] and it turned out to be a clogged jet in one carb. BrianCT 03-09-2010 @ 6:06 AM reply profile send p.m. Member Posts: 95 Joined: Oct 2009 From my experience , the 81's are the way to go on a 221", but they are getting to be expensive.

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