Early Ford V-8 Club Forum

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

Reply to Discussion | New Discussion << previous || next >>

Posted By Discussion Topic: carbie flat spot -- page: 1 2     ians34 03-08-2010 @ 2:38 AM reply profile send p.m. Member Posts: 10 Joined: Oct 2009 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 reply profile send p.m. Senior Posts: 6819 Joined: Oct 2009 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 reply profile send p.m. Member Posts: 10 Joined: Oct 2009 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 reply profile send p.m. Senior Posts: 1460 Joined: Oct 2009 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 reply profile send p.m. Member Posts: 98 Joined: Oct 2009 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 reply profile send p.m. Member Posts: 98 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. 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". 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 03-10-2010 @ 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. 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.

<< previous || next >>

PAGE: 1 2

NOTE: YOU MUST BE A REGISTERED USER AND BE LOGGED IN TO POST (and reply to) messages in this forum. If you are a first time user, please click the CREATE A NEW ACCOUNT in the masthead above to register and Log In. After that, all you do is LOG IN to enjoy using this site.

DISCLAIMER: The V-8 Club does no independent testing of any of the opinions, thoughts or suggestions presented in the website on the Forum, in the Tech Tips section, or any section. A reader should consider the website to be a forum wherein differing solutions to a particular set of circumstances may be discussed. Ultimately, the selection of an item for an individual's vehicle must be based upon the independent study of the vehicle owner in consultation with people in the hobby and restoration experts.

EFV-8 Club Forum Home | Back to Home Page | Contact the Webmaster

Copyright © 2009 - EFV-8.org
Powered by < CF FORUM > v.2.1