If your car or truck is equipped with a Holley four barrel carburetor and stumbles upon acceleration, more than likely the carburetor needs an adjustment to the accelerator pump assembly. If you know where your air screw is at on your carb turn it counter clockwise in 1/4 turns until it revs perfect. The proper adjustment prevents the diaphragm from getting stretched to its maximum limit at wide open throttle. This will be our starting point. This time it would idle nicely at about 900 rpm. See number 42 in the illustration. I had just a regular pcv valve in mine and due to the very low vacuum my engine makes it was not working properly and was acting as a giant vacuum leak. 4-5 lbs should do it. Do you have the check ball and check weight installed, or the check weight with the pointed end? Helps make tuning very easy. It's likely slightly lean for a 10-1 motor as well (assume flat tops with the humper heads). Adding gas to the bore will get the engine running, but dies. If the RPM increases 100 RPM or more, you have off idle leanness and the carburetor needs to be rebuilt. Use the specification from the instruction sheet. My suggestion is make sure you have a proper pcv valve, then advance initial timing, try larger squirter in that order. 3 days ago. When you purchase through links on our site, we may earn an affiliate commission, which supports our community. I would agree that the jets (primary) are off. If you crack the accelerator from about 1200 rpm, there is no hesitation. Not sure , but tried several times to set the 4 corner screws with very little positive results. This article leaves out a bunch of detail. I followed your advice and worked the ignition system. Increasing the idle jets in the metering block solves this issue. Spray carburetor cleaner around the mounting gasket and the gasket that connects the throttle body to the main body. ive also experienced the same thing with weak fuel pressure. For a better experience, please enable JavaScript in your browser before proceeding. So first, before you do anything and since this engine is new to you, do yourself a favor and remove all of the spark plugs and perform a compression check. The most I can get out of mine for vacuum is 7 or 8 in park! Jun 17, 2014. with a 4.5 pv, then built a 700 dp with the stock 6.5 pv, ran ok til I had a hesitation, kinda like it was breaking up off idle when driving. This should not be happening. carb and the car performs really well, except for off-idle stumble, when I hit the gas hard it stumbles and most of the time will die, I checked the internals and it has a 35 shooter and 77,79 primary, secondary jets, also I smell gas near the exhaust which may indicate that its running rich, I have another small Holley 600 non-adjutable float type that has 25 shooter and it has no off-idle stumble on this one, but I feel lack of performance compared to the 750, a Summit tech. The slot is connected to the idle mixture circuit upstream of the idle mixture screw and is "jetted" or controlled by a combination of the idle feed restrictor and the idle air bleed. If the stumble turns to bog its the idle jets. Air:200. Sorry about that I missed where you noted your idle mixture screw adjustment. Terrible stumble off the hit . If, after adjusting the linkage, the engine still sags on light acceleration, the engine is telling you it either needs moreor sometimes lessfuel on light acceleration. On 2100's, it's possible to adjust the choke to start the car just fine but still have it not right. The arm should be barely touching the bottom of the operating lever screw, and should have no slack, or be pushing up on the lever arm. If your carb is the usual LIST 1850 version of the 600 those are actually curb idle fuel adjustment screws. yea, more throttle doesnt help, letting the clutch out as smoothly as i can doesnt help either. odd that she just got serviced and they supposedly adjusted the clutch then. After reading, I noticed the OP says he has the XE294HR. A compression test has all 6 cylinders within 10% of each other averaging 115 psi. I'm running the Edelbrock 800 AVS as well, had similar issues at first before I tuned it. Join. Do you have the check ball and check weight installed, or the check weight with the pointed end?Dirt may have entered the carburetor after it was rebuilt. Well, I am at a total loss. (5) Adjust float to specifications. I'm looking for some carburetor tuning assistance. Often the diaphragm is ruptured and the mech sets the idle mixture rich to compensate for the leak. Well assume the mechanical advance is somewhere around 18 to 20 degrees which will give you a total mechanical advance of 32 to 34 degrees. Did you test the float when rebuilding? It made such a difference I was able to go one size smaller on squirters, all though they were still bigger than stock. Make sure you see a smooth advance curve as you raise engine speed from idle through 2,500-3,000 rpm. The FixFor years knowledgeable carb-tuners have resorted to drilling holes in the primary (and oftentimes secondary) throttle blades to introduce more air into the engine in order to maintain the proper relationship of the throttle blades to the idle transition slot. Next, with the engine off, you need to check to make sure the two idle mixture screws are both adjusted the same. Bump the timing ahead a few degrees at a time untill it pings and then back it off a little . To bring the curb idle back to the ideal spot of uncovering about 0.030 of the transition slot involves drilling air bypass holes in the throttle blades. Hi guys, I finaly fixed all (hopefully!) And, you probably you dont even need a new carburetor. It gets about 10 mpg set up like this. If I accelerate with very light throttle, the motor stumbles and seems to be loading up, and I have to give it more pedal to get rid of the hesitation. Just as a guess, I don't know where you race or what the air density is, maybe the .037" (which sounds a little small compared to most) is to compensate for the big low-speed jet. Ive seen crud build up below the jet causing a restriction. Checked the idle mixture screws and idle screw, car idles perfectly. Check your fuel pump. This is indicative of a problem in the carburetor. The Doug Roe book on Rochester carburetors is helpful in nomenclature. To modify an existing carburetor, all you have to do is disassemble the main body and baseplate and drill a hole through the carb stud hole, and you're ready to reassemble the carb. You're running rich man. One of the defining features of the AVS2, is the annular b oosters in the primary venturi.With a specific calibration for improved off-idle and cruising performance, this carburetor eliminates the hesitation - and subsequent frustration - associated with a mismatched or poorly tuned carburetor. Leaving a petcock vacuum line off can create a lean condition on 1 cylinder but normal on the rest. though which is a much different animal than an Eddy. secondary on it. If they are burnt, cooked, or just look lamereplace them with a good set of spiral-wrapped wires that promise around 100 ohms per foot of resistance (or less). Install new squirters (comes stock with .035" you want the bigger .043" squirters) 2.) Non vented gas cap can cause this. For a better experience, please enable JavaScript in your browser before proceeding. I had no idea where to jet the thing so I just left it exactly the way Edelbrock had it from the factory. Fuel Pressure is 6 psi with a regulator and the . Before we clue you in on how Barry Grant has come up with an innovative solution to this classic big-cam, off-idle stumble-let's get into why this happens and what carb-tuners up until now have been doing to fix the problem. Adjusted float level to 7/16" and float drop to 1" per specs in the manual. Lightly seat the mixture screw, then 2 turns out. Jun 11, 2009. Off idle stumble/hesitation 2.) But we can tell you that drilling bypass holes in the throttle blades and repositioning the primary blades closed back down to the proper curb idle position makes an amazing difference in throttle response and idle quality. Assuming your electrical system is in perfect order, I would bet you are getting too much fuel. hesitation during light throttle acceleration750 holley. Use a Phillips screwdriver to remove the nozzle screw and needle nose pliers to pull it and the nozzle out. Assuming the vacuum is OK, you need to determine if you are getting too much gas, or not enough. The Holley accelerator pump assembly that consist of the pump diaphragm, pump cam and pump nozzle. Fuel Delivery, How-To and Tech Tips, News, Tech, Tech Tips (Image/Holley). If the intake manifold/head surfaces are not perfectly square and parallel or if the intake gaskets are not properly fitted to cover the ports on the interior side of the manifold/head interface, this can happen. Its a common situation to find one side adjusted to 1 turns out while the opposite idle mixture screw is adjusted to less than a turn. You can hit the throttle hard enough to pump fuel into the engine and launch with tires spinning and everything's great-but you can't seem to execute an easy, light-throttle leave from a stoplight.Arrrggghhh! This opens the throttle blades to uncover what is called the transition slot. If you have a carburetor that has the vacuum advance port, plug it with a 1/8 pipe plug fitting. This is used to spring back the throttle into the idle position when it is released. This means that the carburetor is flooding and when this happens the gas runs down the carburetor and out the 1st place it can, which is the throttle shaft. Accelerator pump nozzles come in a variety of sizes. Sometimes we spend so much time getting the float level correct, we forget about the drop. If it isnt coming up far enough it would restrict the fuel. They also left out the transition circuit. If the float isnt dropping, then the needle will not be pulled out of the seat and the float bowl will not fill up. Not a problem with the squirter. Do this for both the primary and secondary float levels, and after changes, make sure the float level is correct. A non vented cap will cause a suction. You then tweek the idle rpm speed with the ignition base timing and the air flow thru the PCV valve internal orifice at idle to get more or less idle air flow and idle rpm speed. A carb running rich, as well as a carb running lean, can cause an off-idle stumble or hesitation upon acceleration. Add and subtract then bench test the pump action. Blow out and re-try.Gas could be coming in the bowl around the seat. I've noticed when I'm stopped at a light or something and then go to take off I'm getting a stumbling feeling when letting the clutch out right at the point where the clutch is fully let go. Cure The Bog And Sluggish Response On Your Holley Four-Barrel Carburetor, By Properly Adjusting The Accelerator Pump Assembly, How To Tune The Power Valve In A Holley Carburetor, How To Choose A Regulator For Carbureted Vehicles, The Inside Scoop on Deb Halls Detroit Speed Built 1971 Pontiac Firebird. , where should I look into first? Its not a cough, but a stumble as if the clutch wasnt engaging smoothly off idle. Any leaks will bubble. Look at the tip of the needle. Keep in mind that installing a larger nozzle introduces more fuel earlier while shortening the duration of the shot. Take the carb off and look. Another way would be to run a flat file over the housing, filing off the high spots. Look closely at the float and float pin. Learn how your comment data is processed. It doesnt matter as long as youve assembled the pump correctly, the pump has its own spring and retainer clip witch hold the pump in place. Any bubbles means its leaking. 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