Hey guys, just have been working out some bugs since I got things back together and I've noticed a fairly bad stumble on moderate to heavy acceleration. Stumble is about the only word I can think of to describe it, the engine basically feels like it runs out of fuel, and if I pulse the pedal, power kicks back in. I'm running an edelbrock performer carb, with a see-through fuel filter assembly and when blipping the throttle under the hood it looks like there are some air bubbles (some fairly big ones) coming in from the pump side of the filter. I asked a mechanic buddy about it and he didn't seem to think it was unusual, but it doesn't seem right to me. There is always fuel in the filter area so I don't think it is a low fuel pressure issue. Does it seem strange? I was thinking there might be a cracked line somewhere that is allowing air in. I haven't noticed any leaks anywhere, so I would assume if there is one, it would be between the tank and the pump. Am I looking in the right area? Suggestions? Oh it doesn't seem to stumble when rev'ing in park. Only when I'm on the road.
I have the exact same problem, Edelbrock carb too! You would think it would do it in park also, but for some reason it's only when it has a load in drive from accelerating. No answers yet. ????
On Qjets....it is the air blade opening speed on the secondaries that causes the bog....did you try tightening the torsion springs for the air blades?
I think Buick Rat is onto something. I am not saying that the carb is not the problem, but, typically, when troublshooting, I doscover ignition issues, which must be addressed before condemning fuel delivery. Check the plugs, wires, etc, as typically, if the engine is not fully warmed up before another cold start, the plugs become fouled to the point of a mis-fire. I suggest an HEI, or an aftermarket ignition, as the points type ignition was iffy at best. If the problem is fuel related, it is likely the result of mis-calibration if all of the circuits are performing properly. It is quite likely that, if the air cleaner has been modified, the incoming air, being too dense for the carb, may not meter fuel correctly, causing surge, hesitation, stumble etc. It should be noted in one of my previous posts, that fuel chemistry is changing constantly, and this will directly affect metering as well. Start with new plugs, and wires, and then note carefully when the stumble occurs. If it is during warm-up, for example, emission controls, such as the air cleaner, manifold heat, as well as the choke calibration may be the cause. We'll go from there. Ray
"I'm running an edelbrock performer carb" "I have the exact same problem, Edelbrock carb too!" The Edelbrock Performer series carburetors are basically reworked AFB clones. Keep in mind that the AFB carburetors were obsoleted in 1967 due to ever tightening emission standards. Anyhow, they lack any adjustment for the opening rate of the secondary airflap. The accellerator pump is a POS, and the spring under the pump is WAY too stiff, overcoming the spring on the pump. (what were they thinking there?) I have extensively dyno and drag stripped tested them, and they are far and above the WORST carburetors we've ran to date. They are however very easy to tune for primary side operation, with plenty of parts support, etc. They seem to be OK on some dual quad set-ups, and set-ups with less than optimum power to weight ratios, ie, heavy cars with small cid engines, low stall converters, low numerical gearing, etc. (One of the units that wouldn't work at all on my car was sold to a local guy, for use on a late 70's 4 X 4 Chevy truck with a pathetically anemic 350 engine, 3.08 gears and huge tires.....he LOVES it!) There are always a few who come rushing out of the crowd to defend them, as they may just have a combination of parts that one happens to work flawlessly on. We consider the lack of any adjustment for the secondary airflap a BIG showstopper for those carburetors. In actual testing on my 600hp 3760lb car they are absolutely USELESS. Any attempt to go to full throttle in any gear or any rpm or vehicle speed results in the engine "puking" all over itself. This hesitation/stumble/bog is worse at low rpms and low vehicle speeds, not as bad at higher rpms/higher vehicle speeds. In any case it can NOT be tuned out, without replacing the weighted secondary airflap or modifying it, or reworking them for a serious shot of fuel thru some sort of secondary POE system, etc. I also tested the new Thunder Series Edelbrock AVS clones, and they worked a LOT better, as we were able to quickly tune the opening rate of the adjustable secondary airflap. Even after drag strip tuning an 800cfm model, it still ran .08 seconds and almost 2mph slower than my recalibrated 1977 Quadrajet. The Edelbrock Performer carburetors have other "issues" and built in problems, but overall they are decent units if you just want a dirt cheap carb to get to back and forth to car shops, swap meets, and ice cream shops. We don't work on them here, recomend them for serious HP use, or even allow them in the shop.......Cliff
Thanks for the feedback, I'm going to have a look in the garage and track down a few q-jets we have stored. Likely will need to be rebuilt though. I'll ask around local, but if that fails, is there anyone on the board who may be interested in going through it? Cliff, if I went the other route and bought a carb, would you 'recommend' the AVS, or a different brand all together? Or is rebuilding the q-jet the best option? I'm not opposed to a new carb since I was going to buy a bigger one anyway, something around 750cfms. As for the ignition, I have an HEI installed already, accel cap, MSD 8.5mm wires, and brand new plugs. One thing I have left to do is play around with the springs in the dizzy. The stumble is there with or without vacuum advance as well. The stumble does seem worse on very hot days when the engine is fully warm.
I'd just have Cliff do it. He has all the high quality components that can handle todays ethanol blends that you'd probably have trouble finding eslewhere. Doubt many have more q-jet knowledge, either. http://www.cliffshighperformance.com/quote.html Devon
Hey Craig, I don't mean to take over your thread but maybe some of my questions will help you out too. I have a 67 340, all stock except for the Edelbrock Performer 1406 carb, that has the same hesitation problems. This car came with that carb and I don't have the original one. What would be a good carb for this engine???
Cliff, sounds like you know your stuff and from what I've experienced with this junky Edelbrock, I believe you!
I simply try to post usable/accurate information as I know it to be. I test EVERYTHING, and do my best when carb testing to tune whatever carburetor we are using exactly for the application. I don't recomend the Edelbrock AFB clones at any level. The "Thunder Series" AVS clones are OK. They are ugly, wide, and the user may run into clearance issues with some "Ram Air" and other factory high performance intake set-ups. My first choice for anything street driven is the Q-jet. Simple, easy to tune, and plenty of them around. They will run right with any Holley or Holley clone if you put a garden hose on them for a fuel supply, but a really fast car should have a good fuel supply system anyhow......Cliff
First let me say I am not a carburetor guru, I know little more than just to push the pedal to go. So I offer this free, and my free advice is worth every penny you pay for it. I bought an Olds 65 330 that has a new Edelbrock carb, I had a stumble anytime it came off idle after it warmed up a little, from a stop, from 10mph, anytime you let off and gently get back in it. Someone told me to richen the idle circuts some. I protested, he insisted. He says they have trouble transitioning from idle. I tried it and it worked fairly well. It definitely helped. I don't know why, but it helped. Try it it's free and easy. Best luck, Mike
The Edelbrock AFB clones are easy to tune on the primary side, and they offer plenty of good advice and a host of parts as well. No problem at all getting them to idle well, and be smooth right off idle, light part throttle, heavy part throttle, etc. When I say they "stumble/hesitate/bog" or cause the engine to "puke all over itself", I'm talking about quick/heavy FULL throttle application of the secondaries. If you experience this problem, the airflap has no tension spring, not adjustable in any way for opening rate, and there is nothing you are going to do to solve this problem that I know about.....Cliff
I have to agree with Cliff; - my experience with Edelbrock clones on Buicks and Chevs is much the same. They aren't worth the money or the hassle. Do what he says, junk the Edelbrock and buy a Q-jet.
About 8 or 9 years ago we obtained two Edelbrock 750 cfm AFB clones for testing. We had plans to do an upcoming magazine article for HPP and the Popular Hot Rodding Engine Masters edition. I spent countless hours messing with the Edelbrock units and never could get the "stumble" out of them. They pulled really good after the engine recovered from "puking all over itself", but the stumble was not to be tuned out with any part from their kit or setting of the carburetor that I could come up with. We went on with the carb testing, and to my surprise the 1977 Pontiac q-jet outran the custom tuned Holley 4781-2 850 DP carb it was tested against, dyno and at the dragstrip. That same carb has outran every other Holley or Holley "clone" we've had on all three of the engines used to power my car since, dyno and at the track. The dyno shop operator tells me to never touch the q-jet, it is spot on everyplace, as I have over 1200 documented runs dialing it in. The "high performance" carbs we bolt on to outrun it, just don't have a chance! I set up the q-jet about 15 years ago, and have never had any problems with it in any area. I have pulled the top off a few times to test needle/seat sizes, different floats, and our new accellator pumps, etc, but have never had to pull it down for any reason. It sits for weeks and sometimes months without being used, dries up, etc, and when placed on the engine it roars to life and runs flawlessly everyplace. My car is a 1973 Pontiac Ventura, with 74 GTO hood and Shaker assembly. It is currently powered by a 455, 11 to 1 compression ratio, small hydraulic roller cam, RPM intake, 1977 Q-jet, stock HEI distributor (welded up for a positive stop for the mechanical advance only), TH400, tight 10" Continental converter, 3.42 gears. On DOT tires in full street trim thru quiet exhaust it runs low 11's just over 120mph! Race weight with driver is 3760lbs, "stock" suspension and no power adders.......Cliff http://www.youtube.com/watch?v=t9eWgLd0q-U PS: I attached a link to the final round in the "King of the Hill" race at the Pontiac Nat's in 2006. These are the last two cars on the property. We had already beat up on everyone else to get there. At that time we were running a 455 making right at 514hp, the new engine is closer to 600hp. We've made it there on several other occassions, and just haven't managed to win all the banannas. Not the cars fault, it's deadly, probably more to due with the dummy behind the wheel!
Thanks for the info, I'm no carb expert so this is the kind of stuff I was looking for. I definitely agree that the stumble I am experiencing can be described as "puking all over itself" since it loses all power. The way it acts makes me think if I stay in the throttle, that it will just die out completely. Usually need to feather the throttle a bit to get it back. I'll be visiting your website Cliff, thanks again! :beers2:
