Holley Carburetor Tuning: Idle Circuit Mods for High Duration Camshafts
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- Опубліковано 15 жов 2015
- Part 1 of 2: In this video we take a in depth look at the Idle Circuit. We're going to explain how to properly check, adjust, and modify the idle circuit.
This is what a PhD in carburetors looks like.
Hey Destin
If anyone is looking for his other videos he switched channel names to "Holley Carbs: How To's & Advice ,40 yrs of exp." Just look over there and he has a whole bunch more videos.
This is the best darn video for explaining the idle circuit on a a Holley! Well done bro! More please!
Awesome video my friend!! I hope more people really tune in to this with big cams. I built a ford 429 with a big cam and had this exact problem. Fortunately I didn’t drill holes I just modded the secondary butterflies with an adjustment screw to open them up for more air and closed the primary butterflies closed. Worked perfectly. Thanks for sharing these tips. This is old tech that will fall to the wayside in the future. The youngsters will loose all this in the future.
I like the way you explain everything twice. Great video
I've been fighting with a 4150, 670 cfm, vac secondary for the last 3 months at least. I finally have idle fuel mix adjustment thanks to this video and others that I found on YT. I don't get setting the curb idle with the primary throttle plates when in fact you are exposing more of the transfer slots. This is where I was getting into trouble having the idle mix ineffective. Now I back that primary curb idle off the primary and set my idle with the secondary adjustment screw. What a difference.
There is nothing more genuine and authentic than a carburetor indtrucional video with a sweet Southern accent, its just how it ought to be, ha ha, thank you so much for teaching us in such great detail, can't wait to see the other videos
Agreed.
Did you know that the Southern accent originated from the British accent and probably Irish as well ?
This Gentleman has a Beautiful " Kentucky " accent !
@@chelseacurtis5498 But in reality it's well identified as REDNECK dialect, stopped formal education after 3rd grade
One of the most informative videos out there. Keep em coming.
Just found this as i'm having issues with my idle circuit with my QFT double pumper. After I double check my valve lash, i'll watch this video again and try the mod. Thank you so much!!
I have a friend just stepping into the holley world and I saved this as one of the tuning vides for him to watch and learn from...…. a good learning guide for a newbie
Excellent instructions. Just what I needed for my Anglia gasser with radical camshaft...
Just found this video, great job on taking the mystery out of the holley idle circuit
Great Video and explanation of the idle circuit. Ive been tell guys for years to drill the blades....they just don't get it, they try to compensate the bog by adding large boosters as a band-aid.
Great video! Looking forward to more!
Very helpful video and i really appreciate it that you took the time to share your knowledge.
Initial timing effects all of this. Great video!!!
Never had a Holley, but on my Weber's...they want zero exposure to the primary circuit.
It should idle on the idle circuit only.
I have heard to keeping the transfer slots "square-shaped" for Holley's...This is good info right here!
Best explanation I've heard! Great video!
Just found these videos - fantastic! Very much looking forward to seeing additional videos. Thank you so much!
Awesome thank you. The best explanation out there!!
great video randy my butterfly's are drilled on the primary side, 4777-4. i inherited this 81 z28 project pulled the carb off an noticed these mods. now it makes since with this car having a big cam. brian in ky
thanks Brian
Your video's and knowledge are outstanding! and have helped me more than I can convey, =)
Thanks for the great video!
Right on good job explaining it thank you. You learn something new everyday!
Thank you so much! It's nice to know what to do but even better to know WHY!
@Lydia Ilyina yes i should've known better but to quote the lamest imbecile in history "c'mon man"!
More videos like this. Very informative.
nicely done
Wow, Great info thank you!
Very Good! Hope to see more soon!
thanks Mike
Such a great video! Thank you!
Very well explained! 👍👍
Very helpful, thanks for posting!
Love your videos dude. I build carbs for people and u taught me some good stuff
Thank you! Thank you! sir for sharing your knowledge and time .
Thanks for sharing your Holley. Modifications . Will post the results tomorrow
Well done. Easy to understand now. Thanks.
mate this really helped me Thank u
More please. This is a dying art. My carb guy is getting old and I'm gonna have to be my next carb guy.
thanks for watching Shawn by the way my original channel got hacked , I have re-uploaded these videos and 8 others to my new channel (Holley Carbs: How To's & Advice ,40 yrs of exp.) please subscribe to this new channel, where we upload several new videos weekly thanks. -Randy
Shawn are you more interested in rebuilds or modifications .randy
Randall Cribb both? Mainly rebuilding and tuning.
This is a great video and the practice is actually been around for a long time. I was lucky enough to meet Grumpy Jenkins in his shop the year he died, and watched him work on my friends drag carb. These old timers will be the last of this carb pastime with the computers and injection taking over. Kids now push buttons on the computer instead of swapping jets and changing power valves etc. I still like carbs better because it will get you home without sensors burning out.
Been watching the folks over at the Cleetus McFarland channel. Reminds me of when we were kids but like you say they use lap tops to tune their cars. Just don't seem the same.
Hi Randy, I just read that you have another channel (to which I WILL subscribe) but I wanted to leave my thanks right here for this video. Had exactly the symptom you described on a 327 with a 4160 on it and the problem was that the transfer slots were too much exposed so I took off the butterflies and set them in again. Now the throttle shuts the transfer slot all way and I'll be able to set in the idle properly.
That video was awesome! I've got a 351 Cleveland, with a large cam, with lot of overlap. It stinks at idle so much, i don't wanna drive it. I've never tuned a 4 corner idle Holley before, so these tips are just fantastic. Thanks!
Jules from Melbourne, Australia
+Julian S , thanks for watching , if I can be some help on your holley let me know I have lots of tricks for the idle circuit, please share video. randy
Randall Cribb Hi Randall, that sounds great... if you let me know your contact details, i'll msg you or even call you. Cheers J
+Julian S Julian you can call me at 9046089447
If it stinks, you need to reduce jet size on the primary side to start with, pull the bowl off, and read the jet numbers. Order jets on ebay 2 numbers smaller and go smaller from there if you need to. And if you ever have to go larger on a motor, you can buy drill bits and drill the jets out in a pinch if you find the chart on jet sizes/drill sizes.
@@randallcribb7015 Where are you located? Can you send contact information?
great video I've have not seen one that explains like you have thanks
thanks Kevin hope these help
Update to my previous comment. This video taught me two things that nobody else did. 1) How the idle circuit works 2) Why you need to set the primaries differently, when you have a big cam. In the end, I didn’t need to drill the holes as my carb is a modern Quick Fuel HR 780. This has a (almost hidden) Secondary idle adjustment screw so you can crack open (in my case) the vacuum secondaries, to let the idle air in. But it’s the same idea. However, if I hadn’t watched this, I’d never understood it’s purpose. So now the all new FE, doesn’t crackle when you rev it from idle (because the exhaust isn’t full of unburnt fuel) and can be driven smoothly enough at low speeds. None of the other ‘experts’ got across to me any of this stuff. Thanks from the U.K., where nobody understands these carbs, except maybe those who watched this. Many thanks.
cheers for the tips an cheers to uncle google so we can get all this good info an videos easy to follow tips thanks
This is a great help. I have a new 427FE with a pretty big cam. Above 2000 it runs strong and smooth, but below it bucks and is generally horrible to drive. Also an unpleasant crackle when revved, which I think I sunburnt fuel. My carb is a Quick Fuel QFT 780, but I guess the principle is the same. My plugs are soot black. I thought I need to start messing with the jets, but now I see the butterfly hole mod is probably a better way forward.
Nice Job,
Very informative, Thankyou very much.
I did that to a 750 carb that was on a 302 chev motor that I built a long time ago . The motor was I think around 12.5 to one with a set of heads that came off a g gas car .The 600 isky roller had a lot of duration . Seems like I installed 2.5 power valves and added the holes to the throttle plate to lower the plate down the transition slot. I had read about this mod and a couple of there mods in a hot rod mag like 40 years ago or close to that . I had added reo pumps and a lot of accelerator pump nozzle . I appreciate your videos and will watch all of them Thanks
You sir are my hero! been fighting my pig rich idle for a while..With the info you provideed here, I finally understand what I have been doing wrong....Thank YOU!
What did you do to fix your Rich idle
Australia never got big block V8's, but, damn, it had some awesome cars, and some awesome engineers. What very few people realise is that the XY GTHO Ph III had a different camshaft to the Boss 351C in the Mustang. It was bigger, so it should have been making more power.
Would love to see a video on tuning air bleeds for vacuum secondary carbs
good video thanks
Thanks for the information. You said drill all 4 blades and I only saw 2 holes.
Thank you for sharing your knowledge. Your videos are a treasure. I have dual 750 demons on my 671 383 stroker. I have it running/driving but needed some advice where to begin on the tune. I have done some power valve referencing in house. What’s a good base line squirter? Accel Cam? Jets? Accel pump? Any help would be sweet.
Your videos are fantastic. More modification videos would be great. Have you ever worked with roots blower carbs. I have been trying to improve the holley blower carb I run. Any thoughts you would like to share would be great.
Great videos Randy, Thanks. I have a 850 double pumper with 4 corner idle circuits. It is on a 545 CI BOSS 429 street motor with a cam with pretty small duration numbers for the 545 CI. My question is should the secondary throttle plates be in a similar position with the idle transfer slot as the primary? Should the primary and secondary throttle plates be matched in there relationship with the slots? You mentioned the exposed slot should be around .040". I have read a lot of people say the slot with the plates closed should form a square which seems a lot smaller than .040". Your thoughts please. Thanks
hate to bump an old post ,but I have been pulling my hair out trying to get my carb to work, your videos are GREAT and have helped a TON, I have a double pumper 650 cfm 4150 and it likes the idle screws at half of a turn, that is where best vacuum and afr is at, but it wants to get real lean when I let off the throttle after a good hard pull, can't seem to tune this out,ANY help will be MUCH appreciated
I am curious as to what idle vacuum the high duration cam gives you that these mods are necessary.
thank you for some ideas on how to lean out the carb using some common sense
Carl Palmer Its actually running richer because it pulls more fuel from the idle circuit, richer not leaner.
Great video Randall, when will the next video on modifing the metering block be ready?
Thanks for the video. I need to know how to replace the booster Venturi on the 4150 flat nose carb. One has eaten away on the tube. Please reply
Great video, I'm having issues tuning up a 280 duration cam (224 at .05) in my 5.0 sbf and wondering how to know when I should resort to drilling the butterflies? I seem to need to adjust the Curb Idle screw to where the butterfly is cracked open to keep it from stalling. If so is it idling on the primary circuit? It's a Holley 4160 600cfm Vac Sec. Thanks for any advice.
can i ask you a question? i had a very strange problem with my dirt track race car a couple years ago, i say that because i put 2 different carbs on it and it ran almost the same. it would backfire out the exhaust ONLY, at all rpm like about 1 to 10 pops every 1000 to 5000 rpm. and also it would not idle down even with idle screw removed ! with throttle rod removed as well. this was a new engine valves checked and rechecked, with MSD box. any ideas ?
Would this modification help with a hard hot start? I have a sbc 350 with a long duration cam and a holley street avenger 670. Once the motor is hot and is turned off it sometimes starts at bump of a key and other times it doesn't want to start and acts like its flooding. There is still fuel in bowls and no fuel residue in bottom of intake manifold so not showing any signs of fuel percolation that I can tell. Timing is within range so dont think it's a timing issue. The transfer slot is definitely bigger than it needs to be when I get it tuned on car so going to do the mod just was curious if it could also be causing a over rich hot start
Thanks for making this video...I have a 390 cfm holley, I can adjust the idle mixture screws but when it get warm it won't idle, will drilling the plates work for me...Thanks
In theory could you screw the idle mix screws all the way in and if the engine still runs it would be running on the transfer a lot? Then if your engine is running off the transfer slot you could back the idle screw off until it stalls, then back the idle mix screws off 3 half turns and attempt to start and it should in theory be running off the idle circuit right?
I am having a problem I have a big cam 440 Chrysler with an 800 dbl pumper and holley blue pump I have changed the Quick fuel needle sets checked center hung floats.I can set the floats perfect drive the car out for a cruise everything is good but on the way back home all of a sudden the car starts flooding the carb is clean bowls are clean no debris but rear level is too high I adjust the float again car is fine. Question is what can make the levels change after you have set them many times before.
Nice video.
What about to create a controlled vac.leak connectting a petcock to a manifold port or something similar instead of drilling the blades?Thanks
Can the secondaries be opened up and help close the primaries instead of drilling holes in the butterflies?
I have problems "the other way" as it were. I can´t open my blades enough to expose the transfer slot!. What happens when I do set the bldes rigt is it idles around 1700 rpm andI have no vacuum leaks. I´m no where near exposing the slot. What that does is the when I´m just above idle on the throttle it leans out when I drive, not in neutral so much. I figure that it leans just befor the transfer slot is about to be exposed but is not "engaged". The fuel in the idle circut is not enough so that´s why it goes lean. If I press down just a tad more the fuel comes back but I´m nowhere near drawing from the main circuit. Therefore the fule must come from the idle cirut....At the moment I run the idle circuit a bit rich so it won't lean out so much.
Any help or ideas is greatly appreciated!
I thought mine was drawing too much from the transfer slot so I checked and it was good, just had a little showing.
Tore down a swap meet carb that I recently bought.... discovered someone drilled .035 holes straight through the idle feed restrictor holes straight into the bowl side of things. What is the reason someone did this? Thank you
Hey randy thanks for doind the videos my hot rod idles 900 rpm it never done that befor. But still is a little rich do i go down on jets now? Thank you from hawaii ALOHA.
Hi, My Holley double pumper doesn't have the lateral regulation screw. It have an lean idle.
Any advice? Should I change the plate?
Blow threw build !
Randy what about tuning a 1050 dominator for the Hot street application
I have a BB Chevy and I seem to be at 12-13 on my AF gauge at cruse until I give it throttle or up a slight grade then it runs @ 14.7 -15..
Is my transfer slot too rich or delivering the fuel ⛽ @ the wrong ratio?
before drilling, turn the idle up on the secondarys
Hi Randy, Thanks for the great video. Just a question. I have a Holley 600 (1850) that was idling very rich so I drilled the holes in the throttle plates like you did in the video. I drilled them to .070" My engine is not rich at idle anymore, but It has a surge like condition at slow cruise speeds. Any idea what might be happening? It didn't do this until I drilled the holes.
Thanks
Tony, when you drilled the holes, you allowed more air into the engine, so now the low speed surge means the engine is hunting for more fuel. it should not
be necessary to drill holes in an 1850 vacuum carb. your rich condition
could have been caused by other issues, such as float level too high, or
possibly the power valve had blown due to a backfire. Holley recommends
to never drill holes in throttle blades, because there are other methods to tune
the carb. Holley is the most tuneable carb on the market today. hope this
info helps.
what is the difference in a 650 meter block and a 750 meter block or are they the same
If your running vacuum secondaries would it be best to just open the secondaries a little bit rather that drilling holes on the primary side?
JMckeown6 Some people do it, but when you do, you have the same problem when vacuum pulls the secondaries, you get lean or even a lean bog. Thunderhead69 has a video on it.
Maybe you can answer this question I had 15 years ago.
I built a 381 stroker Chev. Ran first time on the dyno, set everything up perfectly (750 to 800 idle speed) with regular headers (some exhaust leaks).
I took it to my work for storage (worked at a marina where I built the engine) but set it up on a pallet with nice motor mounts where I could drag it out, hook up a hose and run it whenever I wanted too. I made a custom set of junk zoomie 10" headers for this purpose. I didn't have a real pair of headers yet. Thing is, I could only get the idle down to like 1100.
I rebuilt carbs at the time, at the marina, so I took it off and did some inspections. Nothing.
I finally installed the engine in the car with long tube headers and exhaust and the engine ran perfectly again!
Nobody knew what caused this.
Any clues guys?
Why does only 1 side of my carb idle mixture. screws adjust 850 holley .thank you if you. Can help
Friend, I have an original 302 from '75, put a 4-port intake and a Holley 4160 600cfm vacuum. For the engine to start I have to accelerate, I release the accelerator and it shuts off. There is no idle.I've been watching videos of similar carburetor rebuilds and I don't see that my carburetor is missing any parts. I already adjusted the floats, each hole in the joints matches, injects well, What can be?
we will have more videos coming soon , such as power valves circuits and mods , accelerator pumps and cams also how to adjust discharge nozzles and a video on the 4412 500cfm two barrel holleys, we will also do some videos on the holley #1850s and quadrajets and alcohol carbs.stay with me and I'll show you tricks learned over 40 years of experience. hope you like it. please subscribe if you like them, thanks, Randy & matt
+Randall Cribb Thanks for all of this. I just subscribed and cant wait for future videos. I've just built myself a 347 stroker with a pretty big cam and dialing in this thing just isn't as easy as I thought it would be.
I bought myself one of the relatively nice fully adjustable Ultra DP's and I'm trying to learn all I can about the emulsion circuits and bleed systems in the carburetor and how to adjust them, but cannot find ANYWHERE that teaches me what each do or how to go about adjusting them. This thing has little jet bleeds literally everywhere, and even though I've got it pretty close, I'm almost positive there's still more left on the table if I can just figure out how to fine tune everything properly. Is there anywhere you can direct me where I can learn more about these circuits and how to adjust them whether they be books or your videos?
I will do some videos and try to explain what all the bleeds are for and which control each system , and there effects on fuel.
Where should the air mix screw be set to for the baseline adjustment?
so once we set the slots right then we need to adjust the idle by mixture screws without disturbing the idle speed screw .did I get it right?
if you have a racing cam in your engine you might need to drill bypass holes in the throttle plates to bring the idle speed up after you set the transfer slots correctly, might help to watch the video again . hope this helps you randy
Before I would drill any holes is make certain the correct gasket is used between the throttle plate and carb. Also use a vacuum gage and install the correct size power valve. If you don't reduce a 6.5 front with high duration cams... fuel enrichment is too much.
Gordon the power valve will not cause an idle problem if its open or closed at idle, unless the power valve diaphragm is ruptured and gas is being drawed in pass the gasket or diaphragm , the power valve only flows fuel out of the boosters , now if power valve is wrong calibration and staying open at idle it will be rich above an idle , it took a long time for me to understand that, sounds like you been repairing holleys for a while, I have been repairing holleys and others for about 45 years and i'm still learning about fuel curves , any time you wont to talk send me a message. randy
Power valve formula : put a vacuum guage on the motor and see what the vacuum is. Divide the number by 2 and thats your power valve # you need. Example : vacuum reads 10 psi, you need # 5 power valves. But first you need to make sure there are no vacuum leaks by spraying carb cleaner on the motor where vacuum would leak while the engine is running, if the RPMs increase, theres your vacuum leak.
What size drill bit to start drilling holes in throttle plate
Some guy rebuild my carburetor and ever since I can't make it run or sound correctly. I messed with the idel screw and when I put the truck out in reverse or drive it hits hard like if the trans is going to break. So I moved the idel screw and the truck stopped doing that but I'm still having trouble
At the 11 minute mark you say all 4 butterflies were drilled and say "you can see this one has already been modified", but only the Primary butterflies are drilled... Can you explain to us why only 2 were drilled so we can understand better? Thanks
Having watched this video should the secondary match the primary when it comes to the transfer slot?
Not necessarily match but still be within the tolerance he stated. Most holleys come out of the box with too much slot exposed on the secondary blades. I made a tool to adjust them without taking the carb off but in most cases you will need to. A pocket screwdriver is all it takes once the carb is off.
Hello there Randy, I have been watching your video's and have gathered real good info from you, but I am having a real head scratcher with a lean mixture off idle? You have to slowly open the throttle, otherwise it flat spots and won't rev up instantly? I have just finished restoring a 1974 Pontiac Firebird with a 350ci pontiac block in it running with Hedman Headers and a Offenhauser intake Manifold with a 4brl double pumper 4776-5 Carb. It also won't start even when hot without the Choke and to get rid of the flat spot I have to set the choke at nearly fully closed with just about 3/16" opening on the flap to get it to run cleaner? It revs up ok then? Please could you tell me or anybody out there what I do to over come this part of leaness be much appreciated cheers Ty.
Tyrone Savill Lean bog is exactly what this fixes. Your 4776 is a 600cfm which will run, but you need to get more fuel in the engine. You can do this trick, but you can also try 2 sizes bigger jets on the primarys. Does it smell rich ? Or is it running lean?
I've found drilling ONLY the primary plates works well (as those are the only ones w/ idle mixture screws) for cams w/ 270+ duration @.050. Or using a 4 corner idle carb and not having to drill.
eric it does not matter if you drill the bypass holes in the primary side or secondary side ,or both sides all your doing is bypassing air past the throttle plates , it has nothing to do with it having a two corner idle circuit or four corner, your just bypassing air to close the throttle plates to correct transfer slot opening , I agree you can drill only the primary side, randy.
Full on race carbs have 4 corner idle mixture screws, if you notice on your carb there are blanks not drilled out where the screws would be.
So here is a dumb question: Why does holley not put a mark in the throttle bore or give a measurement from the top for the correct setting of the throttle plates relative to the transfer slots so the carb doesn't have to come on and off...
Not dumb at all. Anyone who has dealt with this has asked it. So has Holley and they sell a kit so you can adjust your sec without taking the carb off. My new 650hp on my 351w bracket car had great top end but idle sucked. Lean bog or dead rich. Idle screws were in all the way,out of the box. Opening them 1.5 turn got me stinking black smoke. So much for "out of the box". It is great to be able to find these videos from guys that know what they are talking about. Very little REAL tuning information is available. I am now confident that with proper sec adjustment and the proper air bleeds and maybe the holes in the blades, [if needed] I can improve my 60ft and light rt. I intend to replace the flathead screw with an allen screw or maybe just flip it over so I can dial the secondary from the top, engine running. Thanks for the video. Just what I needed.
It depends on the camshaft overlap. It may be different size holes drilled for different motors. You still have to remove it to check.
This guy is right about all this but you have to be a little bit Hands-On to mess with
Why is it important to have your idle port adjustment back? Apart from reducing base idling speed is there any other benefit? I think I need to do this but I don’t mind having an idle that’s at 1000rpm as apposed to 750rpm (6 cylinder)
Would like to know also mine runs great picks up perfect and I'm idling at 1000rpm I'm sure im in the transfer slot be runs great
Randy, what are your thoughts on cracking the secondaries as an alternative to drilling holes? Any pros or cons?
Old video but I think it depends if it has 4 corner idle or not. If it does it would be better to drill in my opinion.
Got a 4150 quick fuel and when I step on it it just floods it and take a second to go. Gonna try this tomorrow
Hi I like your videos I'm having problems with this brawler 750 double Pumper I did what you advise got to 70,000 of a hole in the butterflies now it's idling better but I still have no adjustment in the screws I'm running a high duration cam it's a 312 322 a run pretty hard runs good off at top RPMs but just won't idle all that good it idles a lot better having problems and this new brawler has the screw in the metering plate like you show to drill out so that's already been done in the new meet metering block if you have any idea appreciated your response thanks a lot Paul
Big cams having trouble idling... you don't say.
Did you notice the difference between the throttle plate angles?
if you're running a pcv valve with the hose to the base plate of the carburetor, that's enough of a vacuum leak. no need to drill holes in the throttle plates.