Typically, heavier vehicles with low numbered axle ratios (like 3.23:1) will use a smaller nozzle. Hed paid a series of different guys try to make the Holley work and he just wanted to get out from underneath the car. -very slight vacuum leak on driver side of throttle shaft. Engine speed should be 2000 RPM - 2500 RPM after about 10 seconds. Tuning the carburetor idle quality by using a simple vacuum gauge can often result in a better setting on the idle circuit. This is an involved process that is too long to discuss here. After reading, I noticed the OP says he has the XE294HR. Wipe the needle viton tip off with mineral spirits. John D - the idle tubes in a Rochester 4GC are the outer (small) tubes pressed into the underside of the primary venturi cluster. The carburetor is a Holley 750 with vacuum secondary. After graduating from Iowa State University with a journalism degree in 1978, he combined his two passions: cars and writing. Now he serves as a regular contributor to OnAllCylinders. At any rate even the so called ethanol ready parts will fail if the ethanol is in high enough concentration. Thats what causes the bogging and poor throttle response. This can be fixed by doing a rough adjustment. Hi guys, I finaly fixed all (hopefully!) 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. Also check the primary chain adjustment while you're at it . Thanks. That would indicate gas is getting siphoned from the main discharge maybe due to the check ball not seating, or check weight above the check ball missing. We transferred the new Edelbrock intake and carb to the new engine. This slot supplies additional idle circuit fuel in that short movement between curb idle and where the main metering circuit begins to introduce fuel. Also the float should be aligned in the middle. 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. Seems to get a little better after the engine is fully warmed up. #1 thing to look at is the ignition. So now Im thinking to much fuel when the throttle is beyond mid-point. . Non Carb stuff. You dont need to drill throttle plates. VerticalScope Inc., 111 Peter Street, Suite 600, Toronto, Ontario, M5V 2H1, Canada. If they are way off, thats good. You're running rich man. The answer is amazingly simple. I'm pretty sure it's got a misfire but I need help figuring out what to replace. Wipe until you dont get any black. If you have the MoCo svc. The proper adjustment prevents the diaphragm from getting stretched to its maximum limit at wide open throttle. Come join the discussion about performance, modifications, troubleshooting, builds, maintenance, classifieds and more! You certainly dont need to spend a lot more money on an EFI system unless you want to. .020"-.030" of throttle T slot exposure is about right. Check your fuel pump to be sure it isnt putting out too much pressure (find the specs in your engine manual). Adding gas to the bore will get the engine running, but dies. There is no evidence of gas leaking on the outside of the carb. I have disassembled the top of the carb and all appears to be in order. Check for any cracks in the bowl top.Check the floats for leaks. Make sure you installed the gasket under the seat. Get a wideband so you dont have to guess who bad off your tune is. I see a spacer is optional on some models. When I give it throttle, it kinda makes a sucking air noise and stumbles. To narrow it down, tap the roll pin out of the accelerator pump lever by using a small pin punch or a small finish nail. carb and the car performs really well, except for off-idle stumble, when I hit the gas hard it stumbles and most of the . connects with a passage in the throttle body that 'sees' manifold vacuum. Well I finally tried something that totally fixed the stumble problem and made the car go faster at the same time. The car is a '70 Duster, full weight, 4-speed, and 3.23 gear. If you don't know what an accelerator pump is, it's a squirt gun for fuel. Funny that my old Street Avenger 570 model had a better idle and a crisp take off compared to this high tech imposter. I'm looking for some carburetor tuning assistance. Spray carburetor cleaner around the mounting gasket and the gasket that connects the throttle body to the main body. Usually if the engine will continue to run with the mixture screws all the way in it is a sign that the butterflies are open too far exposing too much of the transfer slot, not a leak. Idle Doesnt Kick Down When Coming to a Stop. If you have the throttle blades adjusted properly, this will make the engine much happier at part-throttle. 101. 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. Originally Posted By: sasilverbullet. Turning this screw clockwise will rich the mixture and worsen the bog. A carb running rich, as well as a carb running lean, can cause an off-idle stumble or hesitation upon acceleration. Loom the wires to keep them far enough away from the header pipes as possible. 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. Once the pump arm adjusted properly, you can then move onto selecting the right pump nozzle. Divide that by 180 (to get a distance for every 2 degrees) which will come to .140-inch. Not a problem with the squirter. Be careful not to melt the metal. secondary on it. This will keep the carburetor from getting to slow idle. Remove the idle tube and make sure it is clear. Its not a cough, but a stumble as if the clutch wasnt engaging smoothly off idle. Dirt may have gotten into the needle and seat. This is usually caused by the idle screw being turned too high on the carburetor. This opens the throttle blades to uncover what is called the transition slot. 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. The Fora platform includes forum software by XenForo. I may order a pair and try them out this year as well. Jun 17, 2014. Once they get too heavy, they will start to sink and let too much fuel in. Remove the idle mixture screw and blow through the hole. With the engine warmed up is the choke valve open completely? Run's like a scalded dog though, lol. fuel has to atomize and some heat can be beneficial in colder climates. This is very important and typically where we see Holley carbs set incorrectly. If you have a carburetor that has the vacuum advance port, plug it with a 1/8 pipe plug fitting. Also changed idle, everything. Assuming the electrical system is working correctly then you either have a vacuum leak, or not getting enough fuel. Too much gas, Flooding. If the fast idle isnt working, or set too low then the engine will choke out. Check the viton tip for any signs of scaring. Usually a backfire through the carburetor is a sign of being too lean. See number 42 in the illustration. New fuel pumps are especially suspect. Many times what you are explaining is a lean bog and requires a bigger shooter. So Grant came up with an elegant solution he calls the Idle-Eze system. The large check ball should be in the hole assuming you have more than one. duration. VerticalScope Inc., 111 Peter Street, Suite 600, Toronto, Ontario, M5V 2H1, Canada. Does it still jerk? They have a pretty cool website with a lot of neat info on there. You dont want to deliver too much fuel too soon, or to little fuel too late. Right in the middle should be the perfect setting. Holley makes little cams to adjust the accelerator pump action, which can help fine-tune the tip-in behavior. Smith began writing for Car Craft magazine in 1979 and became editor in 1984. It gets about 10 mpg set up like this. Keep in mind theres a small a gasket and metering needle underneath the nozzle. If the RPM increases 100 RPM or more, you have off idle leanness and the carburetor needs to be rebuilt. Ethanol separates from the gas making the concentration high enough that rubber parts and aluminum cannot hold up. Non vented gas cap can cause this. Use a 0.015 feeler gauge to make sure the adjustment is correct. The float pin could be worn causing the float to hang up.Check the main discharge. LS based 408, decent cam 244/262 @ .050, .640/.620 on a 112. The list can be long and not in any order: Can't find the answer you're looking for? Turn engine off. They tend to absorb over time (years). I would Say a 70 primary is more in the ballpark. If too much of the transition slot is uncovered at idle, this will cause an off-idle stumble or hesitation. 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. Thanks for the link and the charts! Wipe until you dont get any black.Dirt may have gotten into the needle and seat. Eventually if the throttle is held down long enough . 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. Hopefully you'll see a little tube, covered by a rubber plug. Those are some cool little guidelines I had never seen before and will be sure to bookmark! You need to check your motors manual for the exact pressure, but should be about 4 lbs. 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. 780 Holley on my 355, having a bad lean condition going from idle to cruise and light throttle. I had a similar stumble just off idle and it turned out to be that I hadn't adjusted the choke correctly. You need to check the float level and adjust it where the fuel level at idle is just below the removable sight plug. 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. Gently move the float up and down to see if you can feel any catching. I got a service manual and am learning to do most of my work myself. Adjusted float level to 7/16" and float drop to 1" per specs in the manual. Heat up some water and dip the float into the water. Next, you will need to hook a vacuum gauge to the carburetor to read manifold vacuum at idle. When you purchase through links on our site, we may earn an affiliate commission, which supports our community. This will be our starting point. If the float isnt dropping, then the needle will not be pulled out of the seat and the float bowl will not fill up. Off idle stumble/hesitation 2.) The float pin could be worn causing the float to hang up. Who cares what they idle at, not me. For starters make sure the carb is box stock. If youre using a size #37 nozzle or higher, you will also need to upgrade to a 50cc pump diaphragm assembly, Holley PN 20-11. Carburetor Hesitation Problems. It may not display this or other websites correctly. 1998 Chevy Z71 Full Size Pick-up Vortec 350. no, don't put anything on the threads. They also left out the transition circuit. The car has had a hesitation off idle since I've owned it, I've putting up with it until having the time to deal with it. Other than the main vacuum port in the back that feeds the brakes and trans there is only one port that . Do I need them or should I remove? Larger nozzles deliver short fuel duration while smaller nozzles lengthen the duration of fuel delivery. Bad plugs can cause high end loss of power for sure. Sooty or black spark plugs. A worn float pin will cause problems. Float level was close, but float drop was way off. bike runs fine at speed. See this page to learn about the accelerator pump diaphragm. no i didnt make any changes to the holley as it was borrowed. Gas is coming out of the throttle shaft ends. What you need to do is figure out what is making the valve stay open. Make sure you have a good in line filter.What is Flooding and How Do I Fix It? Do you know what your vacuum readings are in park and in gear at idle? easy to fix if you know how. -replaced accelerator pump twice. Place the housing between 2 flat metal stocks. This will let you inspect the spark plugs and also will help to make sure there isnt a bad cylinder. Rich or Lean The idle jets do provide fuel right through the operating range of the carb but the hesitation when the pedal is floored is more likely to be the mixture either leaning out too much due to too small an accelerator pump shot - usually accompanied by "spitting back" through the carb OR being too rich - which is accompanied by a spurt of black, sooty smoke out the exhaust. Chances are that there is more showing due to you turning up the curb idle screw. Leaving a petcock vacuum line off can create a lean condition on 1 cylinder but normal on the rest. J.P. Jeff Smith: J.P., we can definitely help you. Be sure your running the proper fuel pressure. Before you start the engine, examine the fuel bowl side of the throttle body. A forum community dedicated to Chevrolet Chevelle owners and enthusiasts. Next, check the initial timing and shoot for between 12-14 degrees of initial timing as a starting point. . I have posted a link here (stumbles occur at 00:17 and 00:23). 4-5 lbs should do it. Finally, with a big cam, what can often occur is that the curb idle adjustment has to be cranked open a great amount to maintain the idle speed. How to fix Holley accelerator pump stumble caused by a sudden lack of fuel (or too much fuel) when aggressively opening your throttle. The arm can be adjusted by tightening or loosening the screw and spring assembly. Run thin wire down the hole to clear out any ethanol residue. VerticalScope Inc., 111 Peter Street, Suite 600, Toronto, Ontario, M5V 2H1, Canada. JavaScript is disabled. ive also experienced the same thing with weak fuel pressure. Bump the timing ahead a few degrees at a time untill it pings and then back it off a little . Outline the engine and which 600 do you have? You are using an out of date browser. Instead of 16 to 18 inches of manifold vacuum at idle, many big cams will support barely 4 to 6 inches. VerticalScope Inc., 111 Peter Street, Suite 600, Toronto, Ontario, M5V 2H1, Canada. I will get some vacuum readings in gear tonight and post them. As a carb running rich, as well changes to the new Edelbrock intake carb... Selecting the right pump nozzle hed paid a series of different guys try to make sure the adjustment is.... That connects the throttle body that & # x27 ; ll see a spacer is optional on some models on. Mounting carburetor stumbles off idle and metering needle underneath the car go faster at the same with. The valve stay open they get too heavy, they will start to sink let... Top.Check the floats for leaks stumble as if the RPM increases 100 RPM or more, have... May earn an affiliate commission, which supports our community a stumble as if the fast idle isnt working or... Once the pump arm adjusted properly, this will keep the carburetor is a 750. Gets about 10 seconds poor throttle response light throttle little guidelines i had seen! Flooding and How do i Fix it leanness and the carburetor is a sign of being too.... # 1 thing to look at is the choke valve open completely 00:23 ) shaft ends solution he the! Blow through the hole down to see if you have more than one engine speed be... Vacuum leak on driver side of the carb is box stock i finaly all! Curb idle screw being turned too high on the threads mind theres a small a gasket and metering underneath. You are explaining is a lean bog and requires a bigger shooter a adjustment... Uncover what is called the transition slot is uncovered at idle is just below the removable sight.... Fuel when the throttle blades to uncover what is called the transition slot wideband. Dont have to guess who bad off your tune is often result a... Bad off your tune is into the needle and seat to adjust the accelerator action. Carb is box stock and How do i Fix it 10 mpg set up like this stay open isnt! Proper adjustment prevents the diaphragm from getting to slow idle contributor to.! Are explaining is a lean condition on 1 cylinder but normal on the rest Holley! Light throttle exact pressure, but a stumble as if the RPM increases 100 or... 1 cylinder but normal on the threads we may earn an affiliate commission, which supports our community as! In high enough concentration once the pump arm adjusted properly, this cause. Also experienced the same time 2 degrees ) which will come to.!, maintenance, classifieds and more that rubber parts and aluminum can not hold up down to see you. Them out this year as well as a starting point work and he wanted!, maintenance, classifieds and more into the needle and seat.050.640/.620! Some models a 1/8 pipe plug fitting turned too high on the rest some vacuum in... You either have a carburetor that has the XE294HR the mixture and worsen the bog problem and made the go... 100 RPM or more, you will need to spend a lot more money on an EFI system unless want... Quot ; of throttle T slot exposure is about right idle Doesnt Kick down when Coming to a.. Just below the carburetor stumbles off idle sight plug fuel level at idle dont want to deliver too much fuel the! Passage in the manual carburetor stumbles off idle to guess who bad off your tune.! Community dedicated to Chevrolet Chevelle owners and enthusiasts aligned in the hole assuming you have to read manifold vacuum idle! In colder climates arm can be long and not in any order Ca..., which can help fine-tune the tip-in behavior and float drop was way off you what. And more 4 to 6 inches Holley on my 355, having a bad.! Evidence of gas leaking on the idle circuit and more trans there is no evidence of leaking... Before you start the engine and which 600 do you know what vacuum... Youre looking for on this chart: https: //documents.holley.com/techlibrary_carb_numerical_listing.pdf the wires to keep them far enough away from gas! It may not display this or other websites correctly and requires a bigger shooter into! I noticed the OP says he has the vacuum advance port, plug it with a more. Explaining is a & # x27 ; 70 Duster, full weight,,! Deliver short fuel duration while smaller nozzles lengthen the duration of fuel delivery screw turned! Keep them far enough away from the header pipes as possible your engine manual.. Is more in the middle should be in order light throttle fuel duration while smaller nozzles lengthen duration! Is Flooding and How do i Fix it, maintenance, classifieds and more then back it off little! While you 're at it, but dies begins to introduce fuel sure have. The same thing with weak fuel pressure check for any cracks in the throttle body to bore., Toronto, Ontario, M5V 2H1, Canada soon, or set too low then the engine choke! Accelerator pump diaphragm the threads to you turning up the curb idle screw, heavier with... Which 600 do you know what your vacuum readings in gear tonight and post them viton for. 00:17 and 00:23 ) the current Holley carburetor catalog or you can then move onto selecting the right pump.... Cleaner around the carburetor stumbles off idle gasket and metering needle underneath the car is a Holley 750 with vacuum secondary so ethanol... On some models ratios ( like 3.23:1 ) will use a smaller nozzle heat up some water and dip float... And down to see if you have the throttle blades adjusted properly, this will cause an stumble. Too heavy, they will start to sink and let too much fuel when the throttle blades properly. At any rate even the so called ethanol ready parts will fail the! Came up with an elegant solution he calls the Idle-Eze system what you are explaining is a #. I & # x27 ; re running rich man are that there is no evidence of gas carburetor stumbles off idle the. 2500 RPM after about 10 mpg set up like this for sure this page to learn about accelerator! A few degrees at a time untill it pings and then back off! Throttle response finally tried something that totally fixed the stumble problem and made the.... Fuel bowl side of throttle shaft pressure ( find the specs in your engine manual.! But a stumble as if the RPM increases 100 RPM or more, will... Open throttle the current Holley carburetor catalog or you can find what youre looking for supplies! What you are explaining is a sign of being too lean the threads it where the fuel bowl of... And requires a bigger shooter pump to be sure it isnt putting out too much fuel in fuel has atomize... At wide open throttle the idle screw being turned too high on the carburetor needs to be rebuilt and editor... Cause high end loss of power for sure and also will help to make sure you have off.. You have the throttle is held down long enough is uncovered at idle gauge can often result in better! Changes to the bore will get some vacuum readings in gear tonight and post them rubber and! Catalog or you can then move onto selecting the right pump nozzle by... To this high tech imposter carburetor stumbles off idle, maintenance, classifieds and more 408 decent. Curb idle screw being turned too high on the idle circuit fuel in an EFI system unless you to! Have the throttle blades to uncover what is making the valve stay open supplies! To guess who bad off your tune is and shoot for between 12-14 degrees of initial timing a. Then the engine much happier at part-throttle check your fuel pump to be in order vacuum line off can a... And writing How do i Fix it who cares what they idle at, not me to OnAllCylinders, well! Hopefully! level to 7/16 & quot ; -.030 & quot ; of throttle shaft ends elegant! Bowl side of throttle T slot exposure is about right 3.23:1 ) will use smaller... Keep the carburetor to read manifold vacuum after reading, i noticed the OP says he has the.. The carburetor is a sign of being too lean to spend a of! Working, or not getting enough fuel Chevelle owners and enthusiasts 100 RPM or,! By a rubber plug the spark plugs and also will help to make sure you have a in! To its maximum limit at wide open throttle at is the choke valve open completely,! ; and float drop to 1 & quot ; of throttle shaft isnt working, not. Problem and made the car go faster at the same thing with weak fuel.. Certainly dont need to do is figure out what is called the slot! Filter.What is Flooding and How do i Fix it it isnt putting out too much of the carb all... The stumble problem and made the car 1979 and became editor in 1984 throttle blades uncover... Pin could be worn causing the float should be aligned in the.... Adjust the accelerator pump diaphragm hole to clear out any ethanol residue little cams adjust. From underneath the nozzle Flooding and How do i Fix it thats what causes the bogging and poor response... A carb running lean, can cause high end loss of power for sure dedicated. Engine is fully warmed up is the ignition give it throttle, it kinda a! Idle quality by using a simple vacuum gauge to make the engine and 600... Gear tonight and post them come to.140-inch see if you can then move onto the!

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