| ians34 | -- 03-08-2010 @ 2:38 AM |
|
Good evening all. New day new problem. I have fitted a pair of 97's which I rebuilt on a Fenton manifold. Bolted them onto my 34 21 studder, engine starts well on choke and will rev well on choke. however as it warms up and I take it of choke, she flat spots almost to the point of stalling when I give it a rev. Also the idle drops of. If I leave some choke on she seems ok. I thought that the carbs could be running to lean. I have wound out all the mixture screws 2 full turns which to my thinking is too much, and the idle is better, but the flat spot remains. any clues?
|
|
| supereal | -- 03-08-2010 @ 6:38 AM |
|
That problem is not unusual with multiple carbs. It can usually be traced to a loss of vacuum due to excess air handling capacity which, in turn, affects both the main jet and power jet operation. This causes fuel starvation, evidenced by the "flat spot. That is why using the choke seems to help. Once the engine gains enough speed to increase manifold vacuum, the problem moderates. Multi carb manifolds were designed for high speed applications, and always have caused low end problems. Progressive linkage is one remedy, to restrict venturi capacity until vacuuum builds. When you start swapping jets and power valves, you open a whole new set of conditions.
|
|
| ians34 | -- 03-08-2010 @ 3:48 PM |
|
Thank you for your response, how does a progressive linkage work? Is it something I can purchase of the shelf?
|
|
| Stroker | -- 03-09-2010 @ 5:46 AM |
|
Progressive linkage is generally used with 3-carb set-ups. The center carb opens about half way, and then a sliding rod connected to a long bell crank picks up the end two carbs which have shorter bell-cranks that open completely during the last half of the center carb throttle travel. Edelbrock, Vintage Speed, and others make progressive linkage set-ups. These are seldom, if ever used with a 2-carb manifold, as the mixture distribution within the manifold becomes un-even except at full throttle. You might improve your throttle response (eliminate dead spot) by dropping down from Stromberg 97's to Stromberg 81's. The smaller venturis on these V8 60 carbs will keep the velocity (and thus the fuel delivery) higher during quick throttle openings. As a general rule, adding more carbs with more venturi area will affect throttle response unless you have increased the displacement of the engine. Increasing the stroke on your accelerator pump (winter position) may help, but generally the only solution is to learn to apply the throttle gently especially in high gear, so as to not "get ahead" of the available airflow passing through the venturis.
|
|
| BrianCT | -- 03-09-2010 @ 6:06 AM |
|
From my experience , the 81's are the way to go on a 221", but they are getting to be expensive.
|
|
| BrianCT | -- 03-09-2010 @ 6:09 AM |
|
Forgot to mention that I once had a similar problem [same symptoms] and it turned out to be a clogged jet in one carb.
|
|
| supereal | -- 03-09-2010 @ 8:27 AM |
|
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".
|
|
| ians34 | -- 03-09-2010 @ 11:29 PM |
|
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 | -- 03-10-2010 @ 7:44 AM |
|
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.
|
|
| Stroker | -- 03-10-2010 @ 8:05 AM |
|
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.
|
|
| ians34 | -- 03-10-2010 @ 11:45 PM |
|
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!
|
|
| supereal | -- 03-11-2010 @ 10:10 AM |
|
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.
|
|
| Stroker | -- 03-11-2010 @ 2:12 PM |
|
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.
|
|
| 37RAGTOPMAN | -- 03-16-2010 @ 8:35 AM |
|
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.
|
|
| EFV-8 Club Forum : | https://www.earlyfordv8.org/forum |
| Topic: | https://www.earlyfordv8.org/forum/viewmessages.cfm?Forum=18&Topic=903 |