I have a 1973 Buick 455 in my 67 Special. The engine has a moderate rv cam. I'm running a performer intake,1407 750 cfm (mechanical) carb & mechanical pump. Yesterday I had the opportunity to mash it and stay on it for 1/4 mile or more. It went flawlessly and continued to accelerate the entire run. I stopped and turned around and made another pass. This time it sputtered heavy around 2500 rpms in each gear. The sputtering has been problematic since I bought the car. My first thought is not enough fuel but a 750 should be enough carb. It runs fine messing around but it's a coin toss of what's going to happen when I floor it. I welded o2 sensor bungs on my pipes already, just have access to a wideband right now. Any ideas?
Just for clarification, is the 1407 some kind of Holley or Carter or Edelbrock carb? I'm not familiar with that number as is related to quadrajets. Devon
750 isnt enough carb. 800 qjet stock. whats the timing at. sorry i dont know much about edel carbs. i would try a holley 850 if i were you
The 750 may not be enough for a particular max hp effort, but even so it's in no way related to the symptoms described. The only symptom lopping off 50 cfm will show will be on the dyno or the track timer. Devon
Another thought, the lifters could be pumping up and not allowing the valves to seal. If the engine will again pull hard after allowing the car to sit for awhile, that could be a problem.
I'm becoming convinced that 750 is not large enough for my engine. Although I've read that a 650 could be tweaked enough to show favorable results. The proof is that 800 cfm Q-jets were factory. It stands to reason that I need a larger carb. Not sure. Should I just toss a new one on? Can fuel pumps be intermittent? Could be "a" problem or "the" problem. Very interesting thought. Can you elaborate?
The Edelbrock carburetors are not as good as a Q-jet. They cannot be tuned as easily to be seamless in the transition from primary to secondary operation. Usually, fuel pumps give you plenty of warning before they go. Problem is, once you get into 12 second power, they just can't keep up. Your pump may be fine. Usually, the car noses over when there is a fuel delivery problem. I just mentioned this because a friend of mine was having a problem, and we think it is the lifters. It runs fine, then he romps on it, and it starts running like it has 3 cylinders. He lets it sit for a few hours, and it's fine again. Could be other things, we don't know yet.
I wasn't sure that you realized that the Edelbrock AFB clone used the counter weighted type of air valve, which really isn't adjustable at all. Not unless you grind on the weight, but that is not reversible. That is probably a big reason why they are not as seamless as a Q-jet, and will have some sort of bog/hesitation when transitioning from primary to secondary operation. The AVS carburetors have an adjustable air valve like a Q-jet, but they are still not as good as a Q-jet IMHO.
right. thats why when i see an edelbrock i dont touch it. to confusing for me. qjet is enough to make my head spin.
Stop blaming the carb just because of its name. For a while these motors had a 750 as standard equipment and one year even had a 2 barrel. Everyone hangs on the few years that they were 800 and assumes anything less will make the engine explode. I've seen some extended runs cause the coil to overheat and start making a high rpm missfire until it cooled off. That's the reason the later remote mount coils had heat sinks on them. I'd try checking for loss of spark durring one of these sputtering episodes. It deffinetly sounds like either fuel starvation or ignition loss. The fact that it only happened on the second run means either something is overheating or something is slowly breaking down and can only tolerate short bursts.
im not saying that the 750 part is making it run like. that. but have you ever met an edelbrock carb expert? probably not. because they are throwaways. you dont work on them. and i wonder if he is pulling vacuum at wot causing timing advance to far. a holley or qjet would be easier to work right or troubleshoot. thats all im saying. could be vapor lock, also.
The symptoms indicate a fuel delivery issue, not a carburetor problem in general. The Edelbrock Performer series carburetors are poorly cast cheap AFB clones. They have several design flaws built into them, which Edelbrock hasn't completely figured out, and finally got smart enough to produce a version with an adjustable airdoor (Thunder Series AVS clones). They are by far and above the WORST carbs I've ever tested here. We don't want any sent here, and as a general rule don't touch them. They are easy to tune and have great parts support. The BIG showstopper is lacking any adjustment for the opening rate of the secondary airflaps. If it stumbles, bogs, hesitates, pukes all over itself (most common problem we've seen with them) when you go quickly to full throttle, NOTHING you can do about it. Also keep in mind, that the AFB design was absoleted in 1967 because it would not pass emissions. They are just not very efficient units right to start with. They are very simple and easy to work on, but really don't fill any gaps or serve as a decent replacement carb for any of the later engines using spread bore intakes and Q-jets....IMHO......Cliff
Something I'm learning the hard way with the two I have...one on my Skylark and one on my son's Dart. They don't last all that long before they get wear in places like the throttle shaft bores, requiring repair at ridiculously low miles, like 30k. Hard starting hot...don't get me started. Then again, you couldn't GIVE me a Thermo-Quad. :idea2: I've never found one to work right, no matter how I tweaked on it.
I obtained a custom Thermoquad a few years ago and was quite disappointed in it. It was a new carb, factory built by Carter as a "high performance" unit. I put it on my old engine right on the dyno, and it was way off in power production. The A/F ratio was actually pretty good, not sure where all the power was hiding at? We backed up the dyno testing with street and track testing, and sure enough, it was down on ET and MPH compared to my q-jet. TQ's are a PITA to take apart and tune, and lacking in parts support. Same deal with the Edelbrock AFB and AVS clones. Who wants to remove the top of the carb to change jetting? With the Q-jet, I take a full set of custom machined secondary rods to the track with me, and can change them in seconds. I NEVER remove the top, or change the primary side set-up. I still remember my early days tuning the leaky Holley carbs. Things have gotten better for them with modern non-shrink gaskets and much better accl pump assemblies. They still have at least a dozen places to leak below the fuel level, so count on one of them showing up sooner or later. I shelved all my Holley carbs quite a few years ago, and even sold one of them recently. It was prepared by an "expert", really cool looking unit with all sorts of custom features, 4 corner idle, adjustable idle bypass air, removable airbleeds, etc. The q-jet made more power on the dyno and ran quicker at the track, even though it's ugly and not supposed to be able to run as good!:TU: .....Cliff
