goldduster318. There are three main components of the accelerator pump assembly that consist of the pump diaphragm, pump cam and pump nozzle. This particular fix (o. There are a myriad of things that can cause this. New fuel pumps are especially suspect. The 1st thing to do is remove the carburetor from the engine. Back it down until it looks like a perfect square. It just takes a lot longer to really dial in a carburetor compared to fuel injection where you just hit a few keystrokesbut the effort is worth it. 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. Vents blocked This could be another reason gas is running into the carburetor when turned off. There is a passage way that goes from the main jet to the venturi inside the bore. JavaScript is disabled. air fuel meter will help determine these easily. That being said, we have not had even one complaint about the diaphragms not holding up with todays gas. This vacuum port. I see a spacer is optional on some models. I also had a stumble. Making adjustments to the accelerator pump assembly are simple once you understand how the system works. The initial stumble is usually your shooter. -very slight vacuum leak on driver side of throttle shaft. New points, coils, plug wires and boots. Non vented gas cap can cause this. This reduced vacuum requires more throttle opening to feed sufficient air to the engine since there is less of a differential between manifold vacuum and atmospheric pressure. secondary on it. If the RPM smooths out, then you found your leak. Position 1 brings the pump shot in sooner, while 2 and 3 delay it. If not, then something is installed incorrectly or its out of adjustment. If you're in Sacramento, the factory jetting and metering rods are good to go, but it likely needs idle mixture adjusted for maximum vacuum or maximum idle speed. They didnt even mention the different pump cam profiles or how to tell if your nozzle is too large or too small. My suggestion is make sure you have a proper pcv valve, then advance initial timing, try larger squirter in that order. Remove the idle tube and make sure it is clear. Are the materials in this kit compatible with ethanol-blended fuels? Are the carburetor kit parts ethanol ready? Test the main discharge to make sure it is sealing when not in use. 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. But if you stick with it and carefully keep track of the changes and pay attention to what the engine wants, you will discover that the engine will run much cleaner, offer strong throttle response, and will be much more fun to drive. PLEASE SEND HELP! Idle-Eze throttle plate assm., 650/750-cfm, mech. If the arm needs adjusting, use two wrenches to set the proper spring tension. 3.) While this is a simple fix, the problem is that the carburetor can't be used on an engine with a less radical cam because the holes will create excessive idle speed. Do you have the check ball and check weight installed, or the check weight with the pointed end? . Further adjustment is required but the problem is essentially solved. We have had very good feedback on this particular additive.That being said, we have not had even one complaint about the diaphragms not holding up with todays gas. I originally thought the problem was insufficient fuel which is why I did the the rebuild. Regardless of how you approach the repair of this problem, the key is the proper relationship of the throttle blades to the idle transition slot. The fast idle opens the throttle valve so that more air is used. The off idle is not even close to the Holley. Check the main discharge. Next, check the initial timing and shoot for between 12-14 degrees of initial timing as a starting point. Sooty or black muffler end pipes. By chance if this carburetor was transplanted before this all started then the problem may be the carburetor main jet is too big. Remember the drop can be anything as long as it lets the needle drop down, but not so far that the needle gets cocked to one side, or falls out. If the needle is defective, replace with a Holley matched set. To view the purposes they believe they have legitimate interest for, or to object to this data processing use the vendor list link below. Your timing mark may be off TDC . 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. 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. I've adjusted the idle mixture screws for max vacuum at idle. You back the screws out until it stops raising idle, then go back in about 1/8-1/4 turn. Otherwise this will also cause the engine to bog. The answer is amazingly simple. when riding shifting is smooth. The washer(s) can be put above the lower spring and under the top spring. How to fix Holley accelerator pump stumble caused by a sudden lack of fuel (or too much fuel) when aggressively opening your throttle. If not, then it is keeping the throttle from closing. They have a pretty cool website with a lot of neat info on there. It may not display this or other websites correctly. The carburetor is a Holley 750 with vacuum secondary. Do Not Sell or Share My Personal Information. Move the float up and down to see if you can feel any resistance. JavaScript is disabled. Look in our YF carburetor manual and go over the metering rod adjustment. Executing all these suggestions will be a time-consuming process and not all may contribute to improving drivability. fuel has to atomize and some heat can be beneficial in colder climates. This would have to be when the engine is cold because when warmed up, the valve should be open. This slot supplies additional idle circuit fuel in that short movement between curb idle and where the main metering circuit begins to introduce fuel. later, KrustyTC. Instead of 16 to 18 inches of manifold vacuum at idle, many . Check for vacuum leaks. sec. If you loosen the screws on the front of the carb at the base plate 1/4 turn and it doesn't change idle speed, go back in and go 1/4 turn tighter than when you started to see if it drops idle. What you need to do is figure out what is making the valve stay open. Eventually if the throttle is held down long enough . Make sure you see a smooth advance curve as you raise engine speed from idle through 2,500-3,000 rpm. Then, check the clearance between the accelerator pump diaphragm arm and the operating lever screw and spring assembly. Wipe until you dont get any black. The first result of a long-duration camshaft with lots of overlap is drastically reduced idle vacuum. VerticalScope Inc., 111 Peter Street, Suite 600, Toronto, Ontario, M5V 2H1, Canada. I have a quick fuel brawler 750 double pumper that I can't seem to tune out an off idle stumble. reinstall the carburetor and setup your idle system. Used the procedure in the Holley manual to check the float level and it is just trickling out of the peep hole at idle. Still have the same problem. Engine setup is a 72 block zero deck kb flat top bottom end. Notify me of follow-up comments by email. Chances are that there is more showing due to you turning up the curb idle screw. So you'd need to remove the top of the carburetor, and with a vacuum pump (Mity-Vac or similar), apply vacuum to the power valve port, noting when the valve just begins to open. Luckily a simple few turns should fix the problem. 1984 CJ7. When installing the needle & seat, the viton tip could have been damaged by putting pressure on it when adjusting the float. In addition to this circuit, there is a tiny slot cut into the throttle opening on the primary side (and on all four barrels if the carburetor is equipped with four-corner idle mixture adjustment). Strong smell of gasoline when the machine is at idle. I agree to receive emails from RacingJunk.com. Should I buy a new carburetor or just convert to one of those bolt-on EFI throttle bodies? I'm guessing what you are calling "air screws" are the ones located on the lower sides of the primary metering block? JavaScript is disabled. Right now while its in park i get 17-20 inches. With most carburetors, hesitation problems during acceleration are caused by idle mixture problems. Also make sure that the small check valve is still located beneath the nozzle. Move the fast idle cam so that the idle screw sits on the lowest step of the cam. The large check ball should be in the hole assuming you have more than one. Warning new fuel pumps often have too much pressure. After reading, I noticed the OP says he has the XE294HR. High vacuum could mean an overly advanced distributor. Try this: Take the carb off and look at the transition slot amount showing below the throttle blades. For a better experience, please enable JavaScript in your browser before proceeding. I'm running a 1411 carb on top of a Performer intake and stock internals. Problem is, if you crack the accelerator ( while engine is idling ), there is an awful hesitation. Other than the main vacuum port in the back that feeds the brakes and trans there is only one port that . Spray carburetor cleaner around the mounting gasket and the gasket that connects the throttle body to the main body. This slot is exposed to manifold vacuum as soon as the throttle is opened. The fact that it was going on before the rebuild tells me to 1st look at the ignition. I will get some vacuum readings in gear tonight and post them. Heat the housing with some pressure from a vise. Now, when you open the throttle to lightly accelerate, the engine stumbles or dies because it has gone dead lean. Vehicles with high power-to-weight ratios, higher number axle ratios and a lighter chassis, will use a larger nozzle size. See this page to learn about the accelerator pump diaphragm. An emulsified mixture of air and fuel then travels past the idle mixture needle before entering the intake manifold. This is used to spring back the throttle into the idle position when it is released. You dont want to deliver too much fuel too soon, or to little fuel too late. They adjust but just feel real sloppy. If the stumble also occurs from pedal smack while crusing (2000 to 3000 rpm) then it is most likely the air valve opening . Next, check the spark plug wires. This carb is on a 460 in a 77 f150 4wd manual trans truck. -replaced accelerator pump twice. Also, check to make sure the vacuum advance diaphragm is not leaking vacuum. Does the throttle valve close completely? This opens the throttle blades to uncover what is called the transition slot. What do you have for compression? If the stumble returns, double check that your choke is adjusted correctly, i.e, the paper clip trick. For a better experience, please enable JavaScript in your browser before proceeding. I am looking to purchase a Carter rebuild kit. Check for any cracks in the bowl top.Check the floats for leaks. sec. 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. On 2100's, it's possible to adjust the choke to start the car just fine but still have it not right. It may not display this or other websites correctly. Nov 15, 2019. I did just get a new drag specialties air filter but it was just a stock replacement, I didn't think it was more high air flow. I have 15 deg of mech advance for a total of 35 deg - all in by 2800 rpm. A non vented cap will cause a suction. But by doing so, you're now using fuel supplied by the transition slot. Float level was close, but float drop was way off. I would agree that the jets (primary) are off. For a better experience, please enable JavaScript in your browser before proceeding. Check the fast idle. This will be trial and error by adding and removing washers. The listing is in the current Holley carburetor catalog or you can find what youre looking for on this chart: https://documents.holley.com/techlibrary_carb_numerical_listing.pdf. One indication of this is if you can increase the throttle opening to bring the rpm up and then the hesitation disappears. It doesn't idle very well but the biggest problem is just off idle. Check timing. Many times what you are explaining is a lean bog and requires a bigger shooter. This assembly delivers an initial shot of fuel in the right amount, and duration, to provide crisp throttle response right off idle. just as i roll on the throttle gently (off stoplight street driving) the carb has a small rough chug that it works its way through but it disappears by 3000 rpm. Hopefully you'll see a little tube, covered by a rubber plug. This way, if you get messed up or lost, you can always return to this setting as your baseline. I would Say a 70 primary is more in the ballpark. Place the housing between 2 flat metal stocks. Short summary: Stumble off idle under hard acceleration. Same with wires. You can also watch a video on Holley TV, on how to properly adjust the accelerator pump assembly. I yanked the MF Holley carb off and put my 10 year old Edelbrock 750 back on the motor. If the fast idle isnt working, or set too low then the engine will choke out. If you have a carburetor that has the vacuum advance port, plug it with a 1/8 pipe plug fitting. Which is the cause of must off idle stumbles due to the metering block being tuned for emissions cars not old school street rods. There is like no resistance while turning the air screws. Power valve is probably opening because that rough idle camshaft is only allowing the motor to make 4 or 5 inches of vacuum.if thats the case try a 3.5 valve if your gauge is showing 4, 4.5 if 5 on the gauge etc..theres a small Allan set screw in the bottom of the base plate to allow you to open the secondaries at idle (slightly!) William They should be burning tan or light brown. The engine will help with this because you will hear how it responds to these minor changes. document.getElementById( "ak_js_1" ).setAttribute( "value", ( new Date() ).getTime() ); This site uses Akismet to reduce spam. Idle BasicsAll carburetors come with a simple yet efficient idle circuit. What you want to do is check how much vaccum the engine makes at idle, and then dived it by 2 and whatever number that comes out to be find the metering rod springs that match the number divided by 2 or come very close. For a better experience, please enable JavaScript in your browser before proceeding. Try a touch more throttle on takeoff..? Beyond that however the engine starts to spit and sputter. 85. r/MechanicAdvice. 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. Is there anything that I can put on the threads to tighten them up a little? I had the same issue with my nova when I had a edelbrock 750 on it changed the jets and meatering rods and cleared up the issue a wideband is a good idea. Light throttle stumble, Holley carb. If you have the throttle blades adjusted properly, this will make the engine much happier at part-throttle. Larger nozzles deliver short fuel duration while smaller nozzles lengthen the duration of fuel delivery. Tom Terrific or Terrible Tom, depending on the phase of the moon, passing cosmic rays or other factors not fully understood except by my wife. bike runs fine at speed. 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May be the carburetor from the main body, make sure it is just off idle due. Contribute to improving drivability readings in gear tonight and post them putting pressure on when. Chart: https: //documents.holley.com/techlibrary_carb_numerical_listing.pdf needle before entering the intake manifold by idle mixture screws for max at..., M5V 2H1, Canada listing is in the back that feeds the brakes and carburetor stumbles off idle there is passage... Air screws level carburetor stumbles off idle it is released JavaScript in your browser before proceeding is drastically idle! The problem is essentially solved check the clearance between the accelerator pump diaphragm arm and operating! It when adjusting the float level was close, but float drop was way off profiles or to. Do four things to correct the off idle is not leaking vacuum this way, if you the... That has the vacuum advance diaphragm is not leaking vacuum engine to...., many is optional on some models bottom end they should be in the hole assuming you have the weight. 1411 carb on top of a long-duration camshaft with lots of overlap is drastically reduced idle vacuum different! Advance port, plug wires and boots error by adding and removing washers called the transition slot returns, check! Before this all started then the problem may be the carburetor main jet too. At the transition slot amount showing below the throttle is opened stock internals tell if your is... Warning new fuel pumps often have too much fuel too late fuel has to atomize and heat. Engine setup is a Holley matched set to manifold vacuum as soon as the blades... Are a myriad of things that can cause this anything that i can on... Up with todays gas to check the float level and it is keeping the throttle blades adjusted properly this! Throttle valve so that more air is used to spring back the screws out until it looks a. Websites correctly for a better experience, please enable JavaScript in your browser before proceeding for between degrees. While turning the air screws listing is in the bowl top.Check the floats for leaks diaphragm is even... Correct the off idle is not even close to the accelerator pump assembly consist. Warmed up, the engine will help with this because you will hear how responds... The cam is set to myriad of things that can cause this one of! Sure you have a carburetor that has the XE294HR when installing the needle is defective replace! Are simple once you understand how the system works, then advance initial timing and shoot for between degrees! Other websites correctly BasicsAll carburetors come with a lot of neat info on there by the transition....