Hi Guys. Greetings from the UK. Carburettor genius needed!! Just finished rebuilding a tasty Ford 302 for a friends Cobra replica. Used all Edelbrock RPM parts (heads, cam, inlet manifold etc) plus lots of other goodies, so a very nice engine. Goes like stink. The carb is a Carter AFB Compe***ion series 625cfm, and for the most part it performs really well. There is however a fairly substantial stumble/bog that occurs when flooring the throttle from cruise. A common problem I understand with the AFB. If you open the throttle steadily all is OK. I've tried adding weight to the secondary air valve, so that it opens more slowly, but without much success, so before messing with attack angles of the blades and adding more weight, I thought I'd join the HAMB and see if there's anyone who's "been there and done it" with the AFB. Any advice or suggestions will be much appreciated.
Make sure the accelerator pump is adjusted properly and move the link to the bottom hole. This always worked for me.
Already done guys. No difference. It's more to do with the transition to the secondaries from cruise, when flooring the throttle. It also does it when shifting gear and going back on the throttle hard.
All the previous suggestions are spot on but the..." when flooring the throttle from cruise...", has me thinking...tell us a lil more about the ignition, dizzy- vacuum or mechanical advance, initial timing, advance settings etc. I am ALREADY ***uming " float level' is OK?, as well as fuel pressure?
okay, so it sounds like there is not enough fuel to cover the opening of the secondaries, pretty rare condition with an AFB given the secondary air doors. I wonder if there is enough fuel in the bowls to have the accelerator pump work properly - what did you set the float height at and which needles and seats are you using?
Sorry, I haven't run across that particular problem myself. A 625 Carter is a good size for a 302; had one on a 318 MoPar that worked really well. An original Comp series AFB generally has fairly rich circuitry; not calibrated as lean off-idle as a modern "performer" AFB sold by Eddy. Some guys running the Eddy (esp. the 750) version of the AFB tend to get a part throttle lean condition - but, I'd be surprised if you were having lean spots... but never know. You could have a rich spot, too. I'd try to keep an open mind and narrow down if the problem is timing or mixture, and if which direction the culprit is off in. A few things come to mind; you may have tried 'em. Some are pretty basic, but WTF... Does the bog change with air temp or engine temp (you probably guess where I'm heading here). Have you changed secondary jets, and did that change the bog? You say you've checked accelerator pump function; did you actually adjust which hole in the linkage was used? Try different squirters? I've found going to different squirters on my twin AVSs made a significant difference in my 383. Have you looked at the plugs? Have you pulled the carb off, and chased all the circuits with a spray can of carb cleaner to visually check that they're all clear? About twenty years ago, I had a 750 Comp that just WOULD NOT run on the main circuit of the primaries. Idle and off-idle was fine, WOT would work if I got it spun up enough to open the secondaries. At the time, I couldn't suss out what it was. In retrospect, I'm sure it was a problem with the air bleeds (circuit leaking, bleeds drilled out, or wrong boosters swapped in) I mention this, because that particular problem didn't respond to any of the above steps. But, if I'd took a fine strand of copper wire and put a restriction into the primary main circuit air bleeds, I bet it'd have run. So if nothing else pans out, you might try a loop of wire in the secondary air bleeds; another 'quick and easy' test. By the way, the newer Edelbrock metering rods have a much larger step between the cruise portion and the tip. I just mention that in case you find you'd like to have the primarys get richer under load w/o going any richer at lower throttle settings. Probably not your problem, just an FYI I thought I'd throw out there. Now, where's your ignition timing at? What initial, how much mechanical, how much vacuum (ported or non)? Can you be having a timing-related bog? You say the engine's basically an Edelbrock Performer RPM catalog setup. I looked up the cam specs: 290/300 at the seat, 224/234 @ .050", .496/.520" lift, ground on 112. So, what's your compression ratio? Reason I ask: if you have a fairly low compression engine such as 8:1 (which oftentimes is lower when ***embled than what is claimed), more initial and less mechanical advance is needed. As I'm typing this, I just remembered a neighbor's car (327 Chev, hydraulic street cam, not unlike your buddy's setup) back in the mid '80's; it had a "notch" when the secondaries opened. Did it with Holleys (multple ones) and Carters. Think it may have been a timing issue now that I think of it. Sorry I don't have a "this is the problem" answer for ya'. Hope maybe something I've said sparks an idea for you and helps you find the bug. -Bill
I have two of the Carter Compe***ion Series 625's (part number 4759s) on my Ford 390, and it runs like a scalded dog! However, I only used the castings. The venturii, rods, springs, jets, throttle shafts, and especially the auxiliary air valve came from genuine Ford Motor Company Carter AFB carbs. The calibration in the Carter Comp Series carbs is a Chevrolet calibration, and doesn't work well with Ford engines. This is why I asked about the carb number. Jon.
O.k., I've got another idea. I am far from an expert carb guy, but I have fettled an AFB or two in my time. One of the first things recommended for true performance use with a Vacuum secondary Holley (bare with me for a moment please) is to replace the secondary spring with the lightest one made, so the secondaries open quicker. When I didn't do this on my current healthy 360 Chrysler, guess what it did? Winner! It bogged badly as the secondaries came into play! Lighter spring installed, and all was right with the world. Could your secondaries be coming in late enough that the primaries are running low on fuel? I remember a very detailed article in one of the Mopar mags from about ten years ago that described this problem with AFBs and the then new Edelbrock version of the same. The prescription was to drill the counter weights on the secondaries to lighten them so they came in at an earlier moment. I'll take some time this afternoon and see if I can find the article. Looked like a bit of trial and error, but the writer swore this was the cure for the same problem described.
Wow guys. Thanks for all the input. Answers to some of your questions. 1. Float levels have been checked and set to recommended specs. 2. Yes it does have the baffles under the float valves. 3. Using the standard needle valves that were in it from the factory. Don't think this is the problem. 4. Bog doesn't change with air temp. Tried larger jets in the secondaries - no difference. 5. Rod on the accelerator pump arm is in the hole nearest the body of the carb. I think that the size of the squirters could be the answer. New rebuild kit and oversize squirters being ordered. 6. Plug colour is spot on. 7. Carb has been completely apart and all p***ages blown through. All clean and clear. 8. Compression ratio is estimated at around 10-1. 9. Running mechanical acvance Mallory dizzy with Hyfire electronic ignition. Initial timing is set at 10º btdc. I have a gut feeling that this will be greatly helped by larger squirter nozzles as it only does it when flooring the throttle from cruise, or when shifting gear and flooring the throttle. If so I'll remove the weight added to the secondary air valve as it's done nothing. I actually replied to all this earlier but my post didn't appear. Hope this gets through safe. Thanks to one and all for the suggestions. Much appreciated.
Jon I think it's you that my friend has been in touch with about the rebuild kit and larger squirter nozzles. He's coming back to you with the carb number. His name is Peter Samuels.
Trouty - when we get the number, we will help if we can. One of the many issues with the Federal Mogul redesigned (now edelbrock) AFB clones is the elimination of the pump inlet check valve; and the use of the slotted pump well. The accelerator pump not only does not function as well, it functions for a shorter length of time before it is necessary to replace the pump. The size of the pump squirter does not increase the amount of fuel from the pump, rather it changes the timing of when the pump fuel is delivered. Larger squirters will still provide the same amount of fuel, but it would be delivered sooner. The size of the pump squirt is determined by the volume of fuel below the pump. This volume is adjustable by changing the hole in which the pump operating rod is placed in the pump arm. This has already been mentioned by ajmopar and thecarfarmer. Changing the hole placement changes the height of the pump in the pump well, thereby allowing adjustment of the pump volume. Jon.
Jon Turns out to be a 600cfm I think. Number is 9605S followed by 1434. Can you supply the rebuild kit and oversize nozzles for that one?
Trouty, this is one of the Federal Mogul clones. We make no parts for them. Supposedly, the e-clone stuff should interchange. Chevrolet calibration. Will need MAJOR recalibration to work well in all facets on a Ford engine. Jon.
OK Jon, so if you were going to recommend a carb for our application what would it be? I'm also intrigued to know how a carburettor knows whether it's on a Chevy or Ford engine. Especially if it's a non standard build like this one. We have the Carter strip kit with all the different metering rods and jets, so one would ***ume that some time on a rolling road would pay dividents with regard to final tuning.
Trouty - if I were going to recommend a carb for your application; I would simply suggest doing some homework, and determine what Ford might have used for a similar engine. If squarebore, then probably an Autolite 4100. Find one that was originally on a 302, rebuild it, and put it on your engine. A better solution, if you are not doing road racing and can find an intake, would be a spread-bore. Carter built some TQ's for some very healthy 302's destined for Australia. Again, a little homework, and look for the appropriate carb. The carb probably doesn't care what engine it is on; but the engine builder/tuner should. Carter did not build 505 different AFB carbs just for the practice. If you read my post above about the two 625's I placed on my own Ford, look at what I changed. The most important changes were the auxiliary airvalves, followed by the metering rods and springs. I don't know if genuine Carter airvalves will fit in the clone AFB's or not; but if you really want to stay with what you have, I would suggest finding a donor AFB from a mid-60's Lincoln, and seeing if the airvalve would interchange. Good luck. Jon.
go to edelbrock and download the manual, it is worth its weight in gold. It will suggest lighter step up springs, cured stumble foe me.
Get all the slack out of the linkage. The ones I have dealt with have slop and the ****erflys were opeing slightly before the acc. pump shot. Juts bend things slightly to get rid of it. Look into the front barrels to get the shot timed to the opening. Maybe this will help.
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