Carburetor Transfer Slots - Most Common Questions Answered
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- Опубліковано 4 чер 2022
- One of the most common questions I get asked about setting idle on a carburetor, its the inability to get it to idle below 1000 RPM. Almost 100% of the time, its the transfer slots giving people fits. We'll discuss what they are, how they work and most important - how to correct that high idle issue.
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Chevy 348 with huge cam and Edelbrock 1405. Only runs at 34 degrees timing at idle. I had to drill 4 5/32 holes in order to back that throttle off.
You are a good teacher. Many thanks for your clear and detailed explanations.
That has been the goal all along. Provide all the details of WHY, instead of just doing something and giving no reason behind it. Thank you for validating that. Appreciate you watching and the comment.
great video you covered some very important stuff there
I’ve been binge watching your videos the last couple weeks. With your help I’ve gotten my big block in my square body running like a Swiss watch. I’m not kidding it’s like a factory driving truck that’ll snap the 40’s loose with a blip of the pedal.
Thanks a million brother. You’ve been a big help!
Nice! Glad to hear that!!
Great video! I'll add that not only do carburetors and ignition timing go hand in hand but ignition timing also goes hand in hand with ENGINE COOLING especially at idle!
It can, but todays ethanol added fuels make that extremely difficult. The alcohol has no noticeable effect other than the need to advance timing, which leads to hotter idle temps. There’s a give with every take!
Thanks for the great info, nicely done!👍
Thanks boss! Appreciate it.
Great video very helpful
Thank you this was very interesting and helpful.
Cool! Glad you found it helpful!
Fantastic video! I was missing that timing aspect with my 1970 GTX.
Like peanut butter and jelly! Timing always goes with carb tuning. LOVE the GTX. Best looking b body car the Mopar folks made!
I believe Uncle Tony at UTG once said, you can do the air bypass on the throttle plates by splicing a fish tank bleeder valve between ported and manifold vacuum. That way you don't modify the plates and then you have an adjustable ball valve to dial in exactly the right amount of air for that rowdy cam. I'm still on a stock motor, I can't wait to have a motor that gives me transfer slot problems. 😏
Great video, thanks for showing!
Get eye drops and plenty of spark plugs.
I had that problem of non effect idle circuit adjustment with a single barrel Rochester carb. It just baffled me but there's no adjusting a transfer circuit on it. After seeing this I'm wondering if I either have a timing issue or a sticky throttle. This was an excellent video thanks
Brilliant. Thanks for fixing my issue.
Glad we got it figured out!
Great video
I have a 383 stroker motor
With ARF 75cc heads
Comp cam 292h 292/292 and 501 lift
a Holley 86770 carb
MSD Ignition and distributor
Your video answered a lot about the mixture screw problems. But I can get the idle set and idling good but when I rev it up and it comes back down to idle it stumbles and try to die. I can give it a little gas it’ll clear up. Also after it warms up and I shut it off it’s very hard to restart. Any suggestions ?
at 3:55 in your video... wow the Holley transfer slot is more than double the height of the AFB or AVS...
You got me anxious to see that new carb that addressed this issue, great video as usual keep them coming
It will be sometime this month. Eager to get my hands on it and show it to you!
Hi there, I had to drill 2 holes 3/16” on the primary throttle blades, to get the transfer slot just barely showing a square, on edelbrock avs650 cfm with the Tfs stage 1 cam, now it idles and the mixture screws do respond to tune.
Poor choice to drill holes.
I have a holley brawler 600 vac secondary on an Lt1 with stock cam, intake and headers. When i am off throttle its pulling 25hg vacuum and pops and farts until i knock it out of gear (manual). My 02 goes lean to 18/1 and even though its not under load its annoying. It idles fine, ive adjusted it a bit rich, it cruises fine 13/14/1, if i stomp on the gas i get a good shot of fuel, no pops or bogging. Every other circuit is fine. If say i sit in neutral and gently raise rpm and hold it at say 1500 it goes lean. Ive tried silly big jets, adjusting idle screws all sorts. I just cant stop this transitionary lean spot, but only not under load, under load its fine. I havent tried adjusting the secondary idle position to maybe expose the secondary idle slot a bit more, but other than that i'm stuck. My timing is a good 15degree base timing, altho it is possible its not advancing enough as im using an msd with springs so obviously different curves.
I have a 72 Pontiac Trans Am 455 HO with a Quick Fuel Slayer 750. Auto transmission. Took the original Quadrajet off because of an off idle lag. Had it rebuilt but still lagged. Quick fuel does the same thing. I've added an AFR gauge and am running about 13/1 at idle. 850-900 idle rpm in park and going to about 650 in drive. Car wants to die if I come up to a light and try to accelerate from a stop. Falls flat and stutters for about a second.....if it doesn't die. Once through transition though, runs strong. But off of idle, I basically have to fool the carb a little by forcing the RPM's up a tad before accelerating with a quick stab and release on the pedal. Changed accelerator nozzle from .032 to .035. Tried numerous timing settings with little help. I'm actually running a Progression Ignition digital dizzy and have added additional timing immediately off of idle. Not sure if cam is stock. Neighbor seems to think its been upgraded just from the sound of it. Not sure. I'm considering going with a smaller nozzle (.028) just to see if it helps. Today I backed the accelerator pump adjuster screw off about .030 just to see if it had any effect. None! Thinking I want to drill holes in throttle plates as a last resort. Totally frustrated and running out of ideas. Hoping you might have some thoughts. Thanks! Great video by the way!
Should note also, intake is a spread bore design. I have a spread bore to square bore adapter plate (1/2”) installed under the carb. Also, I moved stock pink accelerator pump cam to hole 3.
Interesting , sounds like the same issue I'm having - but engine refuses to idle below 1000rpm and have to really rev to take off from stop. I have a lopy cam with .230/.234 dur on a 110 LSA. I was reading that changing nozzles wont' work. At the point of drilling the holes. A Holley performance book written but Mike Urich describes this problem to a tee.
I was having this problem. You make it very easy to understand what you are explaining. I want to pick up a EDELBROCK AVS 2. for a stock 1999 gen 6 454 (c10 swap) what do you think would be best for this stock engine, 600 or 650 CFM. It has the stock hydraulic roller cam still in it. All i did was remove all the fuel injection stuff for carb swap. Thanks
Thanks boss! That stock engine is never going to see more than 4500 rpm. The valvetrain and the heads on that thing suck. You’ll be just fine with the 650 AVS2.
Hello,
I have a Ford 302 with a set of aluminum heads, trick flow stage 1 cam and a edelbrock 650 Ava carb. The engine is running rich around 12 to 12.2 a/f ratio, my transfer slot is exposed about half away I have it idling about 900rpm timing is about 13 degrees before tdc. The issue is that when I try to close the throttle blades down the engine is stalling out, the plugs are getting black of the richness of the mixture, what can I do in this scenario? Thanks in advance
If an engine was dieseling , would that effect the transfer slot position on a 4160?
My zz502 has had a history of dieseling and I got it to strop by reducing the timing- but then started again after playing with the idle mixture and speed screws - which is one of the reasons you probably said to not mess which them a bunch!
Any other suggestions for dieseling / run on?
Explained very well, Brian, the transfer slots make a lot of sense once you know what they are for, i also know about drilling the holes in the throttle plates, i always thought if they were supposed to be there the factory would have put them there, lol, i realize sometimes there is a need for certain measures, thank you for the video really enjoyed it, later buddy
Thank you sir. Appreciate you stopping by.
@@MuscleCarSolutions My pleasure buddy
The factory never installed big Mutha thumpr comp cams into their engine tho 😉😉😉😉
@@jakefriesenjake Why you have to know what you are doing when changing things! Note: Comp Cams Thumpr' Cams want LOCKED TIMING..... yup, it's true. Confirmed with Comp Cams and running one in my engine. Locked at 35° and fires right up no prob.
@@joevalicenti2722 cool. I didn't know that. I got the big mutha thumper in my 381 Chev.
My distributor timing is 22 initial and 36 total.
With my Ignition, I'm able to put in a "start timing retard", to make it easy on my starter.
I think I set it to pull out 12 or 14 degrees. Sometimes it pops out the exhaust while trying to start. Not good, and hard to start.
I adjusted it to only pull out 2 or 4, and now starts better.
So yeah, I can see that being true!
The cam is a trick flow stage 1 with 221/225 degrees of duration and .499/.510 lift with 112 lobe separation, about how much initial timing should I be looking for with that cam profile ? Thanks again
Hey I have a Holley 4150 blow through carb. I'm confused I see some vids say set all for tranfer slots some say dont expose the secondarys just right above the square. Some say dont even touch the secondarys at all. I have a 351w big cam w 11 or 12 merc of vacuum. Motor wont idle unless I expose the primary slots more then a square.. My question is do i leave the primary's tranfer slots at a square and adjus the rear for the curb idle to get my idle were I need it? Or do I set all 4 slots at a square an start there's? Anything you can tell me will be greatly appreciated thx!🙂
As noted, once the holes are drilled in the primary throttle blades, the hole cannot be reduced at least in theory. The only way that hole can eliminated would be to use a pop rivet in the hole. I know it sounds strange but it might work. It will cause a slight change in airflow.
You discussed timing, idle screws, vacuum gages, but not about the role of a fuel-air ratio meter.
You can get better results with these three gages than just one or two of these gages.
Great video. Thank! Lots of guys talk about setting up a "square" transition slot or about a 0.040" opening. However, it is basically impossible to determine the exact size/position of the slot once the carb is installed. I noticed that the timed port (at least on my Holley 600) opens at about the same time as the transfer slot. Since the timed port should be zero inches Hg at idle (throttle blades closed), does it make sense to use your vacuum gauge to "set the slot" to where you just start to see some vac on the timed port? I did this on my BBC with a mild cam and it seems to run great now. It will idle smoothly at 800 with the idle mix screws out about 3/4 turn. BTW, my base timing is 24*
Squaring the transfer slot is to help the Holley get ahead of moving fuel from off idle. It can be set before the carb is installed and then tuned from there to see what the engine wants. The timed port will see vacuum when the throttle plates are open. When you crack open the plates to expose the transfer slots, you for sure can see vacuum at the timed port. Tuning really just depends on what the engine wants. Sometimes it follows a pretty predictable path and other times not. Mild cam with 24° initial is pretty wild unless you’re at a very high elevation. But if the engine is asking for it, give it.
Thanks for the explanation. I have a new holley brawler and it runs great, but I was wondering why the idle mix don't really do anything. Also, it leaks fuel long after I shut it off through the primaries.
Couple things. If you’re leaking fuel after shut off (you’re probably also while running) you Helier check your fuel pressure. Needle and seats to make sure there’s no trash in it. Might as well change both your fuel filters at that time as well. As far as mixture screws doing nothing, that’s a sure sign you’re into the transfer slots, or you’ve got garbage in the ports and it’s not metering any fuel.
@@MuscleCarSolutions when it's running the butterflies are totally dry. I think I fixed it though today. I had changed out the primary metering block gasket from a holley trickit and that gasket quality was poor compared to the original. I put the original back in and it's not leaking anymore. It's actually holding well enough that the fuel pressure rises to like 9.5 psi age I shut it down when it's really hot. The pressure slowly comes back down as it cools.
Another issue i hace is that I have fuel coming out the pcv port, I set my fuel pressure a 5.5 psi with a regulator, but when I connect the pcv port it doesn’t get fuel, only when the port is capped of with a vacuum plug then I see fuel in the port
Generally if you're drilling holes in the throttle blades you have the wrong carb or the wrong tune or some of both.
I feel like you’re speaking directly to me lol
I have a 1991 318 fuel injection d150 engine I put a weiland 8007 intake and a avs 500 elderbrock carb in order to get truck to pull good I need to increase the timing engine will work good but idles a bit to fast and when timing is decreased exhaust sounds nice and throaty but not when timing is set for engine to pull good any thoughts would be appreciated engine still has factory cam for fuel injected
Sure. Add the timing and turn down the idle speed to where the engine wants it.
I've read a lot about exposing just a little bit of the transfer slot (I believe so that you can just see a square area of it). Where did that come from? Is that what Edelbrock recommends? You're saying absolutely no transfer slot should be exposed? Is that possible when setting the idle?
Same question. Are we talking zero transfer slot showing or is there some wiggle room and adjusting for best performance? My SBC 350 is stock, old and a little tired.
Running into the high speed idle issue after being told to square my transfer slots on primary blades, but not knowing if I need to keep the secondaries closed all the way or open them to show squares also. Can't find the answer regarding that adjustment. Holley 750DP / mechanical sec.
I understand the common practice of always setting a Holley to a “square” box on the transfer slot, but what that does is cause issues at times that weren’t necessary. I’ll always start with them not exposed when it’s a mild cam, street application. If I need to open them as I tune, that’s an easier adjustment than trying to correct an issue that is assumed to be correct. It’s not wrong, but it’s not right for every application.
is it possible to richen up the air/fuel that is released from the transition slot on an AVS2 / 650? I 'm trying to fix a lean condition off idle on a 302 with a moderate cam. At first tip in the air/fuel jumps up to ~17 for a short period of time. Fuel pressure is 5.5 lbs, have tried base timing from 10-24 degrees. The condition has been improved a little bit by putting the accelerator pump arm in the top hole and using the strongest step up springs. Enrichening the idle mixture to 12.5 helps reduce that lean bog a little bit.
What step up springs are you using?
using the silver springs which are rated at 16" of vacuum although the engine has only around 12" at idle.
@@joedirt9397 sounds like you’ve found your answer!
On my edlebrock AVS2 only one screw is adjusting the air fuel mixture the other mixture screw doesn't do anything. Do you please have any advice for me? Thank you
Could be a number of things. First thing I’d consider is taking the airhorn assembly off and checking for trash in the float bowls. Blow through the idle mixture screws with some carb cleaner. Blow back through the idle port in the venturi. You’ve either got an obstruction or you’re into the transfer slot too much.
@@MuscleCarSolutions Thank you very much for your advice it is truly appreciated.
I have a sbc 350 with a summit 1102 cam and avs2 carb.I could not get it to idle in idle circuit.idle would be erattic when I come to a stop.tried different timing settings but nothing worked.decided to read Edelbrock installation manual and it said that if you have a performance cam and having trouble with idle switch vacuum advance to manifold vacuum (drivers side port).I did that and it idle consistently at 850 rpm and ran great.hope this helps someone.
That’s a really small, mild cam. Should do better with timed vacuum. You’ve got a separate issue.
my idle has to be at around 1000 - 1100 RPM , otherwise it just chugs like an idling harley davidson . its a 4 speed manual . but i cant stop it " dieseling " . it wont do it if i idle it right down to 900 or so , but it just chugs slowly then . i tend to think its pulling fuel from the transfer slots at that higher idle , so when i shut it down it runs on . its not electrical , i can yank the pos wire from the coil and it still does it . it runs 15-19 BTDC
Your issue is initial advance. Your timing at idle if far too retarded for your cam. Lumpy idling cam have low dynamic cylinder pressures and need much more initial advance. Cams like the Thumpr' for example require LOCKED timing. They run great at 35 degrees INITIAL advance!
So if I'm having this issue with transition idle and have done your suggestion with no luck . Should I go from 650 cfm to 750cfm carb? Or maybe try the drill hole trick in the primary flaps?
I would need to know what you’re working on.
@@MuscleCarSolutions chevy 383 stroker with fairly good size cam . Currently has a quickfuel sq650. Runs rich.
@@tommiller2101 hate to ask more questions but I need to know a lot more info. What’s the cam specs? What intake? What’s your base timing set at? What PV did you put in it? What is your idle rpm? What is your pressure at the carb? Checked for anything in the needle and seats?
Love the video. I am having a hard time getting idle below 1000rpms without stalling. Running a 427 Ford crate engine with Holley 870 cfm, vacuum secondaries.
Cam Specs: .608 intake .608 Exhaust 248 intake/ 258 exhaust @ .050 114 degree lobe separation. Any suggestions greatly appreciated.
What is your initial timing set at?
@@MuscleCarSolutions 18 BTDC
@@MuscleCarSolutions I feel it is a little advanced as I am having some hard starts after the engine is at running temp.
Yup. That’s a lot, but it’s one way to get the rpm down. You’ve tried starting over and backing the idle speed screw back out and resetting the idle mixture screws? The trouble you’re having in not enough air. That’s why you’re having to set the speed so high is to get more air into the system. The cam is your worst enemy here. I’m sure it sounds good but the long duration like that makes me wonder what the cam was spec’ed for. It’s got high rpm and race written all over it. Race cars don’t generally care about idle. The wide LSA tells me it’s probably not a comp thumper series cam and it was spec’ed for only race type use. There are a number of things you can try but I’m not sure how we’ll it’s going to play with that set up. Can I ask what you’re using the engine/car for?
@@MuscleCarSolutions It is a Factory Five Cobra, built it as a project with my son. Take it to car shows and such, do not really race it. Have fun with it.
My problem is a timing's all right the tunes all right but it idles at 9.1 AFR it's a 400 small block with a 650 AVS Edelbrock basically a very mild RV cam all stock 20 HG vacuum but when I screw in the idle mixture screws it leans it out and runs at a higher RPM and smoother way smoother but doesn't die the fuel floats and everything are okay and fine it just idles excessively rich runs great. outside of drilling holes in my butterflies the transfer slots look as they should be I've never had this problem on any of my vehicles I thought maybe my engine was too big and causing the engine to run overly rich not enough air I also considered vacuum leak in my intake gasket no vacuum leaks anywhere else
What is your timing set at? What is your pressure regulator set at? What level did you set your floats to?
@@MuscleCarSolutions I'm running 14° before top dead center my float Heights are set 7/16 to 1 in drop my fuel regulator is set at 5.5 I've tried lesser settings it's a stock 400 with new Springs and cam and valve job a very tame .420 lift my vacuum canister is adjustable set at 8° if I add more it gets slower I assume it's such a large bore and stroke small block that the intake is very small and it just needs a little more air the transfer slots are nice and square it idles but the pipes reek no black smoke but it seems to be kind of Happy but the gas mileage sucks in town actually good on interstate highway I thought about drilling the smallest holes in the butterfly but when I turn in my idle mixture screws it doesn't kill the engine maybe my intake gaskets are screwed up and I got an internal vacuum leak
So completely hide the transfer slots then adjust from there? The secondary adjustment is on the underside base plate. Does this mean you have to keep taking the carb off and on just to get it to idle??😮
On an Edelbrock carb yes. Cover the transfer slots. As you’re adjusting, the engine will let you know if it needs more fuel/rpm on a more aggressive cam profile.
Good video...I have a question about the transfer slots on an edelbrock AVS 2 800 cfm...chevy 454, mild street cam...it idles pig rich, fouls the plugs very quickly at idle, i think the transfer circuit is the issue...my question is how much of the transfer slots can be exposed below the throttle before that circuit engages? Do they need to be completely covered to stay on the idle circuit?
Completely covered. Any amount shown will cause an increase in idle speed, air flow and enough pressure drop to meter fuel. Depending on what cam you have, it shouldn’t be difficult to get the idle down to where no exposure is necessary.
Thanks for the prompt reply! I'm running a lunati 262 voodoo cam so it's not real aggressive...I'm at 5300 ft above sea level but still pull 19lbs of vacuum at idle. Fast idle wasn't really the issue, actually the opposite, had a hard time getting the idle much above 650 rpm with just the idle screws...pulled the carb and set the throttle blades covering the slots and I'll start over again tomorrow
Elevation tuning is different due to the reduced oxygen. You’ll need to start much leaner to get to the right calibration. Timing is super critical also. I have a separate video on tuning at higher elevation. ua-cam.com/video/l7fG6eMxv5A/v-deo.html
I set the carb up for this altitude right out of the box, I'm running 8% lean primary power, 9% lean primary cruise and 8% lean on secondary jets from factory setup. Car runs and drives fine except for the pesky plug fouling at idle issue. Timing is set at 15° btdc @ 900 rpm, floats set at 7/16" up, 1" dropped, and fuel psi is 5 lbs.
@@mikeschilling9713 typically at higher elevation you can push the timing a bit more. Try up to 18 for initial and see if it helps.
How do you inrichen the t slot?
99.99999999% of the time, it’s not necessary to modify the transfer slots.
Raising the base timing will also raise the full timing. How do you compensate?
You can start by limiting your vacuum advance to maybe 8 or 10° Max it's a big game of give and take you could try a heavier spring in your centrifugal if it advances too early
You shorten the mechanical curve by installing limiters. Depending on your distributor these may be levers (Pertronix) or bushings (MSD) or you may have to modify your particular OEM distributor. The springs should not change. You want the springs that allow your centrifugal advance to start coming in just above your idle RPM and be all-in around 3000-3500 rpm.
I am trying to close off some of that transfer slot , but I cannot because the engine will cutoff, my idle circuit hardly respond to adjustment
What’s your timing set at?
More initial advance is required!
I have a question and maybe some back ground would help . I purchased a 65 Mustang for a really good price. The car came with no paperwork or information. Little old lady's husband's car. She was ready to move on from it. So the car came with a Holley 650 double pumper and form the sound a large cam. I pulled the Holley 650 double pumper for Edelbrock 1406. May I need to retard the timing some to get it to idle? I could put a bunch more about what I can see the car has but I'm sure your really busy. Thanks for any information again.
Typically with a larger cam, more timing is needed. What is your initial set at?
@@MuscleCarSolutions my old timing light has seem to have disappeared I'm actually at harbor freight as I post this buying one. Of course they didn't have the $30 light so I had to spend $109 on a light that does all kinds of extra stuff which isn't bad. I'll comment back and let you know thanks for responding
@@MuscleCarSolutions Last night I took the 1406 off just to check the transfer slots. The plate was so far down I couldn't see the transfer slots at all. So I brought the plate up until the slots resemble the square put the carb back on the car and it started right up and idle that about 800. So my original question was if I go with a smaller carburetor (I removed the Holly 650 double pumper and replaced it with a Edelbrock 1406) should I have to retard the timing a little to compensate for the smaller carburetor? I'm at work today. 4th of July 😐 at least it's double time. I'll try and get the timing one night after work this week. Thanks for all your input I really appreciate it.
@@BPattB kind of depends where you timing is set right now. I wouldn’t be in a hurry to retard it. Leave it until you have a base calibration. At what elevation do you live? Less than 3000 ft, then 12-14 degrees. Above 3000 ft? Then you’ll bump the timing up.
@@MuscleCarSolutions I'm at 40 feet so I'll check the base line. Is there a way to donate to you, super chat or something? I'm sure I'm not the only one but I'm very grateful for you responding and sharing your knowledge.
13 degrees
Add more timing and try again.
Sometimes an owner just has the timing too far advanced and it idles so high the idle speed screw is out all the way and has no effect.
What if the transfer slots are uneven?
What do you mean by uneven?
@@MuscleCarSolutions They are not cut the same depth in the base plate. One side is lower than the other.
@@Preacherman5051 Brand and Part Number for your carb?
So where are the adjustments screws to set the transfer slots
Transfer slots are a fixed orifice.
If you’re asking how to adjust how much of the transfer slot shows, that is done with your idle speed screw.
Mine are opened way to much thanks
So I have the idle screw screwed out it's not touching the linkage and they are still opened to much. Do I need to adjust maybe the I think it's the accelerator pump down to the bottom hole to close the transfer slots up some. Sorry I'm not familiar with. Carbs at all. It's a new Edelbrock carb and the air mixture screws was doing absolutely nothing is why I'm adjusting and trying to figure it out.appreciate the video and the answers thanks again
@@nobodynothing1717 Just the idle speed screw adjusts it, don’t go messing with anything else. One other possibility is the linkage and return springs for your gas pedal is not getting the throttle plates to fully close. If you can pull on it and it goes all the way closed, that’s your cause.
The throttle plates should be able close all the way to the carb body walls when it’s off the car. If it’s a brand new carb, this is not likely the problem but I’ve seen rebuilds that were not put together correctly and the throttle plate hits too high. To fix that (don’t mess with this either until you check the vehicle’s linkage and return springs) you would loosen the screws that hold the plate to the arm and get it sitting correctly. Lock-title the screws when you put them back in, they’re not something you want coming loose and finding their way into the engine.
You can fill holes in your butterflies with lead
I’m almost every instance I’ve seen this tried, it works itself loose and guess where it ends up? Tig is the only correct way and no one will spend the time to even do that.
What was the carb that has corrected that ? 11:55