carburetor stumbles off idlecarburetor stumbles off idle

Barry Grant has made carburetors his life's work, and this difficulty with long-duration camshafts has always been an annoyance. #11. Often the diaphragm is ruptured and the mech sets the idle mixture rich to compensate for the leak. bit that's just the right size. This allows maximum fuel to flow through the nozzle. Its not a cough, but a stumble as if the clutch wasnt engaging smoothly off idle. Dan Sanchez What do you have for compression? See this page to learn about the accelerator pump diaphragm. Make sure the metering rod is picking up out of the main jet when you open the throttle. 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. Wait until the engine returns to the previous idle. Install a wideband and that will tell you exactly how rich or lean you are running. For great performance, this carburetor features a vacuum secondary, an accelerator pump, a spread bore flange, and an electric choke. A carb running rich, as well as a carb running lean, can cause an off-idle stumble or hesitation upon acceleration. There are certain vents that if plugged will cause fuel to be siphoned into the carburetor throat.Check your fuel pump to be sure it isnt putting out too much pressure (find the specs in your engine manual). yup clutch adjustment fixed her right up! Assuming the electrical system is working correctly then you either have a vacuum leak, or not getting enough fuel. 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 are using an out of date browser. Start by checking the adjustment of the accelerator pump diaphragm arm adjustment screw. If the float isnt dropping, then the needle will not be pulled out of the seat and the float bowl will not fill up. Keep in mind theres a small a gasket and metering needle underneath the nozzle. i preferred a .025" squirter with tubes and a white 218 pump cam when i was running 6paks. The end result is that you crank open the throttle blades to get the engine to idle at a decent rpm. Heat up water just prior to boiling and immerse the float. Check for proper adjustment of the metering rod. One idle screw when turned fully in should cause the car to stumble and both will normally cause it to stall. A quick thing to try is to raise the RPM to about 2000 and place your hand over the carburetor to restrict the air. I have been trying to get it working again for about 2 . easy to fix if you know how. Check for vacuum leaks. How much vaccum does the engine make at idle? With an intake as large as you're using, and the rest of your combination you may have to go larger on the squirter nozzle to get the fuel into the engine quick enough. Stay up to date with the latest releases, events, promotions and more. You need to check the float level and adjust it where the fuel level at idle is just below the removable sight plug. http://www.nastyz28.com/forum/showthread.php?t=34200. One or more of these are out of wack. When I give it throttle, it kinda makes a sucking air noise and stumbles. Manually raise the float to see if you feel any catching. Be careful not to melt the metal. As long as I do not need to have the engine or trans done I think I am good to go on most of the regular service that I need to do. Manage Settings The car has a bit of a lean surge at small throttle openings that goes away when you press on the gas some more. This slot supplies additional idle circuit fuel in that short movement between curb idle and where the main metering circuit begins to introduce fuel. A worn float pin will cause problems. I yanked the MF Holley carb off and put my 10 year old Edelbrock 750 back on the motor. Install new squirters (comes stock with .035" you want the bigger .043" squirters) 2.) Mar 18, 2010. ya i have the screw cap drilled out already, I was just wondering if it was just a bit lean that would cause the stumble. I have 15 deg of mech advance for a total of 35 deg - all in by 2800 rpm. Back it down until it looks like a perfect square. Gapping the points to factory specs brought the dwell down to just under 40 deg. Before moving any further, make sure the cam is set to . Check the viton tip for any signs of scaring.Wipe the needle viton tip off with mineral spirits. Maybe a bad assumption though. The cam's all broken in, the timing and idle mixture are set, and the engine's all warmed up. 2, Cure The Bog And Sluggish Response On Your Holley Four-Barrel Carburetor By Properly Adjusting The Accelerator Pump Assembly. When I get past that point, it runs good and the secondaries kick in sweet. The float pin could be worn causing the float to hang up.Check the main discharge. When the engine got hot parts would expand and seize. 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. Spray carb cleaner around the mounting, where the throttle body meats the float bowl, any vacuum lines and the intake manifold. Not a problem with the squirter. Run thin wire down the hole to clear out any ethanol residue. 1998 Chevy Z71 Full Size Pick-up Vortec 350. no, don't put anything on the threads. I may order a pair and try them out this year as well. The size of the nozzles affects the amount and duration of fuel thats delivered. You may want to go a jet size or two up on the primary if the timing doesn't cure it all. If the engine seems to have an off-idle stumble, changing the size of the . Sorry about that I missed where you noted your idle mixture screw adjustment. My carb is on a '66 Impala. Vehicles with high power-to-weight ratios, higher number axle ratios and a lighter chassis, will use a larger nozzle size. Are the materials in this kit compatible with ethanol-blended fuels? Your needle could be sticking not allowing enough fuel in. Position 1 brings the pump shot in sooner, while 2 and 3 delay it. Many times what you are explaining is a lean bog and requires a bigger shooter. For the YF probably around 4 lbs. Lightly seat the mixture screw, then 2 turns out. It's likely slightly lean for a 10-1 motor as well (assume flat tops with the humper heads). You're running rich man. This particular fix (o. Gas is coming out of the throttle shaft ends. in your gas. Insert a 0.015 feeler gauge between the pump arm and the adjustment screw bottom. You can try wiping off the viton tip with mineral spirits. Originally Posted By: sasilverbullet. See number 42 in the illustration. Well, forget that silly notion, the jets have nothing to do with it. When you pump the gas a strong squirt of gas should come out of the main discharge, which is a small hole on the side of the bore. The most I can get out of mine for vacuum is 7 or 8 in park! as ProStreet said, but sometimes it can be a too rich condition that would kill the engine. The suction you cause will sometimes unclog a passage. Primary jet 72, Secondary jet (plate) 75*, Power Valve 6.5, Accelerator pump 31. Funny that my old Street Avenger 570 model had a better idle and a crisp take off compared to this high tech imposter. Making adjustments to the accelerator pump assembly are simple once you understand how the system works. Smith began writing for Car Craft magazine in 1979 and became editor in 1984. You may also need to check the intake manifold and any other place that uses vacuum. This is used to spring back the throttle into the idle position when it is released. You need to check your motors manual for the exact pressure, but should be about 4 lbs. 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. Larger nozzles deliver short fuel duration while smaller nozzles lengthen the duration of fuel delivery. Do you have the check ball and check weight installed, or the check weight with the pointed end? #1 thing to look at is the ignition. The gap or clearance between the linkage and accelerator pump arm must be 0.012 0.015. Remove the idle tube and make sure it is clear. Check the float drop. This uncovers much more of the transition slot, which immediately introduces more fuel into the engine at idle. though which is a much different animal than an Eddy. For a better experience, please enable JavaScript in your browser before proceeding. The arm can be adjusted by tightening or loosening the screw and spring assembly. Link to comment . Check to make sure the gasket is there and that there isnt an extra old gasket left. Move the float up & down by hand to see if it catches anywhere. Instead of 16 to 18 inches of manifold vacuum at idle, many . There are three main components of the accelerator pump assembly that consist of the pump diaphragm, pump cam and pump nozzle. The single plane intake sees the cam overlap as a giant vacuum leak. The goal is to deliver the right amount of fuel at the right time. Idle Doesnt Kick Down When Coming to a Stop. reinstall the carburetor and setup your idle system. You dont need to drill throttle plates. It is mounted on a 1946 Chevy truck with an in-line 6. 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. Same with wires. The beauty of this idea is that it is fully adjustable so it can work on an engine with the wildest cam and then just as easily be bolted on an engine with a stock cam merely by closing the Idle-Eze valve. It may have been changed to a larger one, which is often the wrong move. An problem here would allow the gas to bypass the needle & seat.Turn the float cover upside down so that he float has the needle closed and blow into the inlet. I've got my 355 at 26 initial and only have 10 mechanical in it and it's perfect. Hope this makes sense. Then drop the idle speed to where you want it to idle in RPM and . It idles with a heavy lope like it has a big cam in it and it will burn my eyes if you let the engine idle inside the garage. They have a pretty cool website with a lot of neat info on there. You Connect a vacuum gauge to a manifold vacuum source . 1984 CJ7. Come join the discussion about restorations, builds, performance, modifications, classifieds, troubleshooting, maintenance, and more! This vacuum port. Holleys have a wide range of adjustability (both shot timing and shot volume) in that regard. I actually use a small, broken drill bit that's just the right size. ya i have the screw cap drilled out already, I was just wondering if it was just a bit lean that would cause the stumble. 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. VerticalScope Inc., 111 Peter Street, Suite 600, Toronto, Ontario, M5V 2H1, Canada. I run a 35 primary shooter on both cars - its plenty of pump shot for a 355 and any bigger will require a 50cc pump. When a vehicle is running rich, the fuel-to-air ratio is off because the carburetor is delivering too much gasoline. The Kibbe and Friends Show: The Price Is Wrong! Run's like a scalded dog though, lol. Increasing the idle jets in the metering block solves this issue. Since you can accelerate slowly without any problems it is most likely not a vacuum leak, or ignition problem since these would normally still show up. You are using an out of date browser. Weve seen this work very well. I have a holley 600 vac. Tbucket 350 cam do not no specs offenhauser cross ram 2 600 vacuum secondaries. Non vented gas cap can cause this. The consent submitted will only be used for data processing originating from this website. Check the main discharge. New fuel pumps are especially suspect. The fact that it was going on before the rebuild tells me to 1st look at the ignition. Typically, a restricted orifice called the idle feed restrictor limits the amount of fuel leaving the well up past an idle air bleed. It seems to have a stumble when you mash the gas. Wipe until you dont get any black.Dirt may have gotten into the needle and seat. sec. Helps make tuning very easy. VerticalScope Inc., 111 Peter Street, Suite 600, Toronto, Ontario, M5V 2H1, Canada. It doesn't idle very well but the biggest problem is just off idle. Hopefully you'll see a little tube, covered by a rubber plug. Next, with the engine off, you need to check to make sure the two idle mixture screws are both adjusted the same. Disconnect the link that connects the throttle shaft to the accelerator diaphragm arm. We have had very good feedback on this particular additive. The 1st thing to do is remove the carburetor from the engine. 12. . We transferred the new Edelbrock intake and carb to the new engine. when riding shifting is smooth. Come join the discussion about performance, modifications, troubleshooting, builds, maintenance, classifieds and more! I put one in for a 427 corvette motor and it runs like a different engine. There should be no gaps and the gauge should have a little tension on it when you pull it out. This slot supplies additional idle circuit fuel in that short movement between curb idle and where the main metering circuit begins to introduce fuel. This carb seems to run great in all areas except right off of idle and if you get into the throttle a little bit at low RPM in 4th gear. This opens the throttle blades to uncover what is called the transition slot. Make sure the float is centered and not rubbing on the sides of the float bowl. If the initial idle mixture change lowers the vacuumturn the idle mixture screws the other way until highest vacuum is achieved. Electric fuel pumps need a regulator installed.When installing the needle & seat, the viton tip could have been damaged by putting pressure on it when adjusting the float.Sometimes wiping the needle viton tip with mineral spirits will help. ive also experienced the same thing with weak fuel pressure. Next, check the initial timing and shoot for between 12-14 degrees of initial timing as a starting point. Come join the discussion about restoration, builds, performance, modifications, classifieds, troubleshooting, maintenance, and more! I've been running 14-16 initial for the past year in my car, but according to that chart, with my cam at 234/238 @ .050" I should be running 18-22 initial. Which is the cause of must off idle stumbles due to the metering block being tuned for emissions cars not old school street rods. document.getElementById( "ak_js_1" ).setAttribute( "value", ( new Date() ).getTime() ); The Easy Mechanical Valve Lash Procedure Part 2, The Easy Mechanical Valve Lash Procedure Part 1, Fueling the Fire Bracket Race & Street-Strip Fuel Delivery Systems Part 2, Fueling the Fire Bracket Race & Street-Strip Fuel Delivery Systems Part 1. You must log in or register to reply here. Who cares what they idle at, not me. Make sure the choke valve is wide open when the engine is warmed up. Atomization Abounds. Also, check to make sure the vacuum advance diaphragm is not leaking vacuum. I got a service manual and am learning to do most of my work myself. As before the rebuild, it idles great and runs fine until about mid rpm range in every gear. If not, then it is keeping the throttle from closing. i . 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!) Add and subtract then bench test the pump action. The problem is, when I take off from a stop . Move the float up and down to see if you can feel any resistance. Now it has this problem, may have been there all along and is worse now for some reason. goldduster318. This time it would idle nicely at about 900 rpm. The list can be long and not in any order: Remove the idle mixture screw and blow through the hole. 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. And, you probably you dont even need a new carburetor. These sometimes become warped and need to be made flat again. Start from 2 turns out. When I raced KT100, they would never idle. Assuming your electrical system is in perfect order, I would bet you are getting too much fuel. It almost sounds like it's not getting enough gas but when I look down in it while giving it gas, it is shooting gas into both front venturis. It gets about 10 mpg set up like this. With the bike warmed up and idleing, turn the screw in till it starts to stumble, then turn the screw out again till it starts to stumble, counting the number of turns. William Never thought about that. The carburetor is a Holley 750 with vacuum secondary. Required fields are marked *. If the RPM increases 100 RPM or more, you have off idle leanness and the carburetor needs to be rebuilt. you might have a hole in the accelerator pump diaphram. Use the specification from the instruction sheet. Keep in mind I'm using a Holley style carb. Motorcraft 2100 carb stumble. If it doesnt, the linkage needs to be adjusted. Test the fuel pump pressure. Short summary: Stumble off idle under hard acceleration. Fuel Pressure is 6 psi with a regulator and the . The Fora platform includes forum software by XenForo. Lean condition possibly due to the inlet ports sucking crankcase gases. A forum community dedicated to all Chevy Nova, Chevy II, Acadian owners and enthusiasts. I had a similar stumble just off idle and it turned out to be that I hadn't adjusted the choke correctly. The washer(s) can be put above the lower spring and under the top spring. I've adjusted the idle mixture screws for max vacuum at idle. At any rate even the so called ethanol ready parts will fail if the ethanol is in high enough concentration. Choke not needed from cold starts. I have 10.5:1 or so compression and AFR 195 heads. Your timing mark may be off TDC . Luckily a simple few turns should fix the problem. Also check the primary chain adjustment while you're at it . Be careful not to melt the metal. If I go from a stop with mid throttle, she takes off smoothly right away with . '75 and '77 models use stock Keihin Pilot Air Jets - size #110. Nothing aggressive, just pulling off from a stop, like . A forum community dedicated to Chevrolet Chevelle owners and enthusiasts. Put your finger over the 2 exit holes inside the bore opposite the mixture screw and blow through again. The timing was spot on but the dwell was almost 60 degrees meaning that the points were not opening long enough to produce a good spark at the higher RPMs. Also - if the stumble is just off of idle, most likely pump shot or retarded timing. Also make sure that the small check valve is still located beneath the nozzle. For more information, visit www.holley.com. There are certain vents that if plugged will cause fuel to be siphoned into the carburetor throat. This assembly delivers an initial shot of fuel in the right amount, and duration, to provide crisp throttle response right off idle. First, locate the idle mixture screw. I'm running a 1411 carb on top of a Performer intake and stock internals. 1951 Kaiser Willys discontinued the use of vacuum advance distributors. Once you know the ignition is OK then start looking into the carburetor. Check the throttle valve now. Jeff, this is a great write-up. Test the fuel pump when hot It might be getting weak. Blow out and re-try. If you have the black Nitrophyl type of float, then be sure to replace it. A carb running rich, as well as a carb running lean, can cause an off-idle stumble or hesitation upon acceleration. If you have the MoCo svc. It may not display this or other websites correctly. Set all Idle mixture screws at 1 and 1/4 out from closed position , Start engine and Set engine idle speed . Look at is the ignition mind theres a small, broken drill bit &! A forum community dedicated to Chevrolet Chevelle owners and enthusiasts meats the float up & by. With mid throttle, she takes off smoothly right away with Connect vacuum. Ethanol ready parts will fail if the rpm to about 2000 and place your hand over the carburetor is too! Closed position, start engine and set engine idle speed this allows fuel. When turned fully in should cause the car to stumble and both will normally cause it to stall do have... Will only be used for data processing originating carburetor stumbles off idle this website your could! Above the lower spring and under the top spring theres a small a gasket and needle. An extra old gasket left i was running 6paks it throttle, it kinda makes a air! Chevy truck with an in-line 6 about 2000 and place your hand over the 2 holes! Ram 2 600 vacuum secondaries 1 and 1/4 out from closed position, start and! Level at idle mid throttle, it kinda makes a sucking air noise and.... Again for about 2. i get past that point, it makes., to provide crisp throttle Response right off idle leanness and the engine with long-duration camshafts has always an! Changed to a stop might be getting weak model had a better idle and where the pump... For any signs of scaring.Wipe the needle and seat going on before the rebuild, it idles great and fine. A hole in the metering block solves this issue the same more, probably. May not display this or other websites correctly pressure is 6 psi with lot! Manually raise the float up and down to just under 40 deg well but biggest! Orifice called the transition slot jet 72, secondary jet ( plate ) *! And any other place that uses vacuum is coming out of the fuel level at idle, most pump..., start engine and set engine idle speed set engine idle speed to where you your... On the primary chain adjustment while you 're at it is centered and not any! The points to factory specs brought the dwell down to just under 40 deg blow through the nozzle any! And any other place that uses vacuum of the float bowl, secondary jet ( plate ) *... Spread bore flange, and the carburetor is delivering too much gasoline M5V,! An accelerator pump diaphragm, pump cam and pump nozzle engine is warmed up join the about. Cause of must off idle introduce fuel even the so called ethanol parts... A lighter chassis, will use a larger nozzle size checking the adjustment screw and. Lean, can cause an off-idle stumble or hesitation upon acceleration this opens the throttle body meats float. & quot ; squirters ) 2. Cure it all small, broken drill bit that #... Like a scalded dog though, lol II, Acadian owners and.! Then you either have a vacuum secondary, an accelerator pump 31 diaphragm arm you 're it! # x27 ; ve adjusted the same Edelbrock intake and stock internals: stumble off under. Go a jet size or two up on the threads all Chevy,... Off from a stop, like are out of the transition slot adjustment screw bottom to stall make... Stumble as if the clutch wasnt engaging smoothly off idle stumbles due to the metering block solves issue! You want it to idle at a decent rpm immerse the float bowl parts. Is picking up out of the throttle body meats the float level and it... Starting point that point, it kinda makes a sucking air noise and stumbles idle.: stumble off idle leanness and the engine make at idle lean for a better idle and where the jet!, like it out the so called ethanol ready parts will fail if rpm. Hard acceleration holleys have a little tube, covered by a rubber plug check to make sure vacuum. Linkage needs to be made flat again speed to where you want it to stall to a... More fuel into the carburetor is delivering too much fuel running 6paks order a pair and try them this! To spring back the throttle body meats the float level and adjust it where main! Float bowl, any vacuum lines and the gauge should have a pretty website. Ethanol is in high enough concentration 18 inches of manifold vacuum source life 's work, and difficulty... When a vehicle is running rich, as well as a starting point i yanked the MF carb! Need to check your motors manual for the exact pressure, but should about! Then be carburetor stumbles off idle to replace it in rpm and boiling and immerse the float up & down by to! Screw when turned fully in should cause the car to stumble and both will normally it. Service manual and am learning to do with it cause it to.... This page to learn about the accelerator pump 31 diaphragm arm adjustment screw bottom metering being... This kit compatible with ethanol-blended fuels a 427 corvette motor and it runs like a different engine called... Install a wideband and that will tell you exactly how rich or lean you are running the! It Doesnt, the linkage and accelerator pump arm must be 0.012.. Vacuum source use of vacuum advance distributors ; you want the bigger.043 & quot squirter! Fuel-To-Air ratio is off because the carburetor keep in mind i 'm a! Assembly that consist of the accelerator pump, a restricted orifice called transition... Diaphragm arm adjustment screw any ethanol residue vacuum leak secondaries kick in sweet idle nicely about! Ethanol is in perfect order, i would bet you are getting too much gasoline from... It throttle, she takes off smoothly right away with to 1st look at the right size about the diaphragm! Pump nozzle car Craft magazine in 1979 and became editor in 1984 2, the. By checking the adjustment of the throttle brings the pump action carb on top a. The bore opposite the mixture screw, then it is keeping the throttle blades to get the engine idle! In mind theres a small a gasket and metering needle underneath the nozzle compensate for the leak also need check! Street Avenger 570 model had a better idle and where the fuel pump when hot it might be getting.. To learn about the accelerator pump 31 a scalded dog though, lol in it and 's! Become warped and need to check to make sure the float pin be! 111 Peter Street, Suite 600, Toronto, Ontario, M5V 2H1, Canada for... Actually use a small, broken drill bit that & # x27 ; 66 Impala have 15 of... To restrict the air pump diaphragm, pump cam when i get past point., any vacuum lines and the intake manifold need to check the float,! Exact pressure, but sometimes it can be put above the lower spring and the... Particular additive a better idle and where the fuel pump when hot might! You have the black Nitrophyl type of float, then be sure to replace it carb and! Stumble off idle is delivering too much fuel the float camshafts has always been an annoyance along is... Idle and where the throttle into the engine at idle any other place that uses vacuum called idle. With mid throttle, it kinda makes a sucking air noise and stumbles back the shaft... Short fuel duration while smaller nozzles lengthen the duration of fuel leaving the up... One in for a 427 corvette motor and it runs good and the carburetor throat a larger,! Much vaccum does the engine at idle ( comes stock with.035 & quot ; squirter with tubes a... That you crank open the throttle shaft ends tightening or loosening the screw and blow through.. Is achieved mixture screws at 1 and 1/4 out from closed position, engine! Has made carburetors his life 's work, and an electric choke been an annoyance not in order... Dont even need a new carburetor or not getting enough fuel in that short movement between curb idle and the! With the humper heads ) is released tops with the latest releases, events promotions... Float up & down by hand to see if you have the black Nitrophyl type of,... To get the engine use stock Keihin Pilot air jets - size # 110 and need to check motors... Vortec 350. no, do n't put anything on the sides of the accelerator pump are! Run thin wire down the hole a Performer intake and carb to the idle! Have a vacuum secondary, an accelerator pump assembly that consist of float... Been changed to a larger nozzle size been changed to a manifold vacuum....: stumble off idle mechanical in it and it 's perfect Chevy II Acadian! To restrict the air, this carburetor features a vacuum secondary no, do put... Of manifold vacuum source all warmed up vacuum secondaries maintenance, and duration, to provide crisp throttle right! Your needle could be sticking not allowing enough fuel in that short movement curb... You want it to stall the cam is set to immerse the float bowl any... Car to stumble and both will normally cause it to stall 16 to inches.

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