Tyler, thanks for getting back to me so fast on the idle mixture tool needed, I will try to find one. Since watching your video I have rebuilt my carb and it runs like new, my son (who is a fuel injection loving mechanic) is even impressed. The guys at the local carb shop also were amazed when I showed them how to tell the 800 from the 750! Thanks again.
I always find the fastest speed that the engine idles with each mixture screw and then turn them out 1/2 turn to allow for any contamination that can get into the circuit later on which will lean them out. This will cut down the chances of idle problems sooner and postpones further idle problems. A little too lean is worse than a little to rich so it's better to give yourself a little "cushion" with that extra 1/2 turn out. I've done this for 43 years and it works better for less maintenance.
I know this is an old video, but damn if it hasnt helped me out tremendously. Just bought an 86 motorhome with a rochester 4bbl, and even though I've worked on my own cars my whole life, I've never had the "privilege" of working on a carbureted motor. My poor old Bertha was running rich, but when I patched up the fuel pressure regulator (it was literally just pissing out fuel), she was running so rich that she barely ran at all. I should be able to get her dialed in just right now, and its thanks to this video!
I forgot to add that my old QJet in my car is a 750cfm. Reason why I bought a new one is that current carb is horrible on cold start and bogs on second gear along with I saw fuel leak spot on front of the carb. When the engine is warmed up, carb runs flawlessly in normal and WOT driving. Your video has been added to Corvette Forum in C3 corvete tech section.
Awesome, I got a 78 dodge b200 with a 360 motor and it doesn't like to start, once running fine. I have to question the automotive center I just paid 630 dollars for oil change and tune up. I'm doing this myself as per your video, excellent explanation of what I'm doing. Thank you!
$630?? Wow, they saw you coming, sorry to say. Those old Mopars are super easy to learn to work on. Feel free to ask anything you want. Those V8's like a lot of choke, your hard starting issue is probably related to that.
Hey,want to chime in and say thanks from the everyday guy who is not a mechanic,but watched your video and was able to get my truck to idle again by adjusting the three screws until I achieved that sweet spot. It also helped when I plugged that vacuum hose back in that got disconnected!
Tyler - Thanks for this video about adjusting mixture screws. This is the 2nd of your videos I've viewed and find your style of teaching very helpful. I appreciate your thorough and methodical approach to covering a topic. I'm prepared now to tackle adjusting the mixture screws on my CJ-7's Carter BBD. I feel like I know what to expect and how to preform this adjustment correctly. Great job! Keep 'em coming.
ran across the plug over the adjustment screw on a toyota 22r i used a dremel and small chisel didnt feel like taking it off helped the truck a ton. Good video i was trying to explain this to my cousin how you just have to feel the sweet spot but he said i sounded crazy.
Thanks for all the help! the engine is currently not in a vehicle, i am either building a break-in stand or borrowing one, so i will need to throttle up to 2000 rpm directly on the carb, i will install a longer idle screw to make sure i can get it high enough
great video man, I'm 18 and just bought a 65 mustang coupe and I'm trying to learn how to work on everything and this video explained it perfectly! thanks man!
Wanted to tell you, by far this is the best training method I have seen regarding carbs for vehicles, especially for 2 bbls. Thanks for the info, gonna get a vacuum tester today and do what you instructed....and yes, shorter videos result in more watches....have a good day...
Great Video and easy to follow instructions. After months of trying to fix hot hard starts and cutting out when adding throttle on my Mercruiser 4.3L TKS I rebuilt the carb and she work great (for now). I couldn't figure out however why Mercury or GM had a tamper proof thing around the mixture screw until this video - and now it makes sense. Anyway I cut it off so I can adjust the mixture and can now finish the job with this guide then get on the water - thanks heaps from Australia.
I don't know what I like better the info or Tyler! Would love to have you talk about the 1985 K5 Blazer Rochester with all that crap emission stuff on it. I have a bog in the mid range performance. When sitting the engine performs right, but under a load (driving) the engine stumbles. There was some crap in the tank, but I don't think anything is in the float bowl. I love your way of putting things, thanks so much.
Great video. Just inherited a 1987 Monte Carlo SS with the 305 High Output Engine. I was having a problem with what I thought was the OEM Rodchester Carb. The car would start fine but within 3 to 5 miles will start to miss, skip, and backup not out the tailpipe but within the exhaust manifold or perhaps the intake manifold, not really sure. I was not sure it was the carb so I pulled the distributor cap to look at it and the rotor button. On a G Body, what a pain. First, it is at the back of the engine and secondly to get it reinstalled you really have to pull a few of the sparkplug wires from the cap because it will not mount properly because of the tension of the attached wires. As it turned out, I discovered to hot wire for the choke was not correctly fitted because someone had used an undersized spade connector to connect to the blade connector of the automatic chock. I refitted the choke hotwire with a proper size spade connector. I believe the hot wire for the choke was loose, so every time I hit a bump in the road, the choke hotwire would jiggle loose and then the choke would activate thus send excess fuel into the carb and thus the skip, miss, and backup. Problem solved, Car runs fine but I have to adjust the fuel/air mixture that requires a special tool which I ordered from Amazon and will have in a couple of days. Again, thanks!
Dood! you just saved me from loosing my mind. I have a 92 Dodge ram d150 and its running extra rich, and so I have been needing to find where those mixture adjustment screws were. So thank you so much man!
Good job, Gm was forced to build the feed back carb so they could maintain a 14.7 fuel ratio, this is the fuel ratio the Catalytic converter needed to operate efficiently. Keep up the great videos.
Thanks for the explanation - very helpful. I just rebuilt a carb for the first time for my truck (Rochester 2GV); I believe the accelerator pump was bad, but the rest of the carb was due for an overhaul, also. The truck runs very well now, but I didn't know exactly how to tune the carb. I bought a vacuum pressure tester, and I'm looking at buying the RPM80 adapter for my Fluke meter to fine-tune it. This project turned out to be more fun and less of a chore than I first thought :)
very helpful !!!!, thank you so much i broke apart and rebuilt my carburetor (rochester 2v), but didnt now how to ajusted , as engine running rough and smelly, now i feel confident and tomorrow ill give it a shot armed of course with a vacum gauge from harbor,,ill see how it plays out,,thanks again!!!
the small vacuum Port you pointed to on the right front of the carburetor is for the vacuum advance in the distributor. this is the most commonly hooked up wrong part of any carbureted car it only gives vacuum while the throttle is opened don't try to set idle air with that port
If you look around at the parts store you can usually still find a long, flexible adjusting tool for electronic carburetors. They come with 4 different tips that are stored in the handle.
Man thanks a lot for the helpful video.Its good to see people out there that don't want money to help the fellow man out, awesome video to, easy for the average person to follow.
Sometimes on quadrajet carburetors they have factory stainless steel plugs you have to notch them out in a v section on the bottom of the carburetor then you can go ahead knock out the stainless steel plugs then adjust your carburetor.
Excellent details! I have 2 issues here. I have a 78 lincoln in excellent original condition with 65000 miles on it. The car never was used in the rain or winters. with a 460 and a motorcraft 4150 carb on it. The car has all the original smog stuff on it and it all works using a vacuum pump and leak tester. Both catalytic converters have been emptied some time in its life and the exhaust has no restrictions. The engine vacuum indicates a strong fault free engine, plugs are perfect with a little soot and are new. Air filter is new. Gas filter new. The problem is that it bogs from idle when its sitting or if I'm cruising and decide to hit the gas to pass a farm vehicle. When I'm cruising, I have noticed it seems to sputter here and there at cruising speed. With the car cold the choke system is working ok. There does not appear to have any vacuum leaks as I tested that with carb cleaner. When I try to do the vacuum idle air/fuel adjustment, there is no problem doing the right side but the left side makes no difference. I have not pulled the bad side idle mixture screw out to see if the taper is broken or if some clown screwed it in so tight that it broke through the horn. Next issue, I cannot get a slow idle adjustment when it's normally running at 190 degrees. I cannot seem to adjust the main idle screw down enough to be able to properly adjust the idle RPM. It always seems to run a little faster in RPM than it normally should at idle even if the screw is all the way backed out so it doesn't touch the plate. The choke fast idle is OK so that's not holding the idle open. When I force the throttle manually from the carb to close it then dips to what seems as a normal idle speed. There is no bind in the carb to pedal adjustment holding it open either. I'm thinking that whatever is causing the higher than normal idle, the hesitation from acceleration, the sputtering at cruise speed and the inability to adjust the left mixture screw may be the problem area. The accelerator spits a nice stream on both sides so I know it's not that. Any input would be appreciated on these 2 issues. Your videos are the best I've ever seen because you go into great details.
Thank you for the nice comments. Well, you have several pieces of evidence that at first glance appear to point to a vacuum leak. If the throttle plate is closed tight with the screw backed out and away, and the engine still idles, then air must be getting "around" the throttle plate somehow. I suspect that you may have either a worn throttle shaft bore or possibly a leaking baseplate gasket under the carb. When I was about 17 I had an Impala with a common old 350 engine and a Qjet. It always had a tip-in stumble and also idled high and rather uneven. One day I decided to take the carb off and work on it, and when I got the carb off I found the baseplate gasket was cracked and had caused a vacuum leak. I replaced that and it solved both issues immediately. I'd also recommend following each and every vacuum line to it''s destination including the PCV valve and look closely for cracks. I think you are going to find your issue in one of the areas I mentioned. Btw, i had a '78 Marquis myself and I believe that carb is a Motorcraft 4350, which is a spreadbore somewhat similar to a Qjet.
I think mine is a 4350. The marquis is pretty well the same car in many aspects. Heres what Im going to do. Im going to replace a few valves. The PCV, the heat valve in the lid of the breather, Definitely the EGR cause when I spray carb cleaner around that area, there was a dip in RPM. Once I've done all that, Im going to change out all the vacuum lines with new ones and color code them as original. When I bought the car, The owner said they had the carb rebuilt and it looks like there was some work done to it because the welch plugs are new. Im going to strip the carb down and rebuild it if nothing is broken. I will rebush the throttle plate shafts also. I have all winter to get er done cause I dont use this car in the winter. I have some vacumme guages and a vacuum gun to run some tests but for sure everything will be replaced and then we will see whats up after that. The old smog systems were a real spider web of hoses. I know that the mixture screws might be an issue but if they look OK then there has to be some sort of leak bypassing that side of the system. Hey, thanks for getting back to me. I need all the help I can get.
A replacement comes in a carb rebuild kit. Ethanol eats away at the older cups that were not the updated material. You can pull the breather off with the engine OFF and cold, and see whether the carb squirts gas into the venturi when you work the throttle.
helped me out a lot. i been doin it the opposite way. i just bought a 1974 gmc c1500 with a small block 350 in it and couldnt figure out what i was doin wrong until now
Cool video, thanks! Can't wait to start building my 72 Nova 350 coupe. Planning headers, exhaust, intake, carb, and a better camshaft over the stock peanut cam.
Thank you. You explained air fuel better than anyone if ever talked to about. Wish I would have watched this couple years ago, when I was working on 77 Ironhead. I sold that daily maintenance piece of shit and bought suzuki. Anyways thanks a lot. My rochester is now getting way better gas milage.
tyler ur videos are by far the best...i have a 86 caprice 5.0 not sure if its 307 or 305 but anyway someonecstarted ripping off emissions stuff...which ee dont have to worry so much about in ky but i cannot find a video on how to correctly remove what's left and get vaccum lines correct dont know what to block or what not to block off. its my daily drivervand i depend on it to get my kids to appointments etc. now it is dieseling an wont shut off. also someone has turned the big screw on top of carb way down in and i cannot find a video on how to adjust this. it also will not idle down even with idle screw not even touching any help would b greatly appreciated. I am sure u r very busy guy so if u cant reply i completely understand.bthank u in advance in love your videos very informative.
On those you have to find a special tool which is essentially a long, flexible driver about 1-1/2 foot long, with a screwdriver handle on it. It comes with 4 special tips that fit over various types of mixture screw heads. It's sort of obsolete now but I'm sure you can find one online if need be. I think it's called "GM carburetor adjusting tool".
haha..I think it's basically finding another route to the same destination. Your end goal, in ANY method of adjusting idle mixture, is to achieve the highest idle and highest vacuum reading. I guess people have different methods but you cannot do it correctly just by saying turn out or in the screws X amount of turns and that's it.
That sounds very much like an ignition problem, because it's under load, but there are a few other things that you should check: Have you checked the timing? That's the absolute baseline you start with dealing with potential ignition issues. Also make sure the ignition advance is working. I would also do a compression test to rule out a valve problem. I think with an unknown engine I would pop the valve covers and watch all the rockers with the engine running-con't
Damn good video very informative. I'm trying to learn this dying art cause its hard to find anyone that know anything remotely about carbs. Thank you very much!
Here's maybe a strange carb question: how can you adjust them for best fuel economy and cleanest emissions ? I have a 1972 Volvo. The previous owner installed a Weber 32 / 36 but I think it's running rich. The car came from the factory with a Zenith Stromberg dual barrel carb which I still have. Any preference on which is better? Thanks for the easy-to-understand video.
Awesome video! I'm about to swap two Rochester 4bbl on Mercruiser 260s tomorrow (350 small block). One engine idled faster than the other and we thought it might be the throttle cable. Thanks for the idea to check the idle screws!
@danbirchjr My guess is that the float is sticking or the inlet needle has trash in it and is noy closing off the fuel supply completely. When you have the mixture screws that loose it is introducing air into the mixture and band-aiding the rich running. You'll need to pull the airhorn off so you can inspect the needle valve and the float. Good luck!
Thanks! This is not what I have read from other people but the more research I do the more it points to this direction. I also have a 96 boat with a qjet on it. I'm assuming a boats sweets pot will be the same??
hey i have learned alot from your videos. i have a boat with a 305/w a quadra jet. i bought a new carb for it. im trying to adjust the idle mixture screws. as much as i turn them it doesnt change the rpms of the motor. im trying to get this adjusted so i can use it for the 4th of july.
Very helpful video, but I still have a question I have a 84 GMC with the 454 Big Block, and it idles really high when I first start it for almost 4 solid minutes then I have to pump the throttle at least twice pretty hard before it idles down to where I feel it should be, temperature doesn’t make a difference weather it’s 95 or 55 out side it does it no matter what...where do I go if this doesn’t make a difference? Thank you!
Great Video lots of info.. quick question I have an fe 360 ford engine with 2bbl carb. I got the engine to start but every once and a while i have a fireball shoot out of the carb. Will the mixture screws fix this problem.. Iam assuming that the engine is burning to rich???
If the mixture screws are out of whack can that caused the carburetor to flood at a higher RPM like getting on the freeway ?? Thanks for all the cool videos 👌🏼
sounds like the accelerator pump leather is going away. On those carbs it's inside a small cover on the front face of the carb. 4 screws hold it on. Make sure it isn't leaking at any time.
@89blackcamaro It has to be a source that delivers a full 12v that goes on and off with the key. I don't know if it's a camaro as your name says, but GM makes that easy because there are extra terminals provided on the fuse block. Look for one labeled IGN. Don't use BAT, those stay hot all the time. If it's another kind of car or truck you might take the 12v source off the power feed for the wiper motor. Be sure to ground the black wire if the choke has one.
Back in the day they either ran three 2bbls, or two 4bbls. For a street engine I think dealing with the linkage negates any real performance boost. When I say a GOOD 750 Qjet, I mean one no later than about a 1971 model, because later than that the calibration built into the carb got progressively leaner(less fuel) because engines got more suckier.
Great video! Thanks for explaining everything. I have a Valiant Charger RT with 3 double 45 Webers. Engine just rebuilt and carbs redone but running rough and a bit rich I think. Is there a particular way I should attack the tuning?
How come I haven't found this video before?! Very informative, but my Rochester Quadrajet doesn't react in the way you describe to adjusting the idle mixture screws. It runs perfectly when reved higher, but in idle it works very rough, occasionally stalls when hot. When I adjust the screws the idle just gets more rough. It produces loads of HCs when idling 2000-3000 (norm 50).
I am pretty new to messing with carbs. I bought a 79 jimmy with a stock 350 a few months ago and has pretty much ran like crap. Major bogg when under throttle. I watched this vid a few days ago and this morning went out to see how off the mixture was. On both mixture screws, either way I turned the screws the RPM never changed. What do you sugest. Thanks for the vids.
Great video, thanks! I do have a couple questions though. The highest vacuum reading that I can get is 15" and that is about 5-6 revolutions open from starting at all the way closed on both idle mixture screws, but when I do this it seems to run poor. If I take it down to 13" which is 4 revolutions open it runs a lot better. Why is it running better on 13" as opposed to 15" and secondly, does 4-6 revolutuions from open seem too rich, or would that seem reasonable?
@escatel123 That's more commonly from a lean accelerator pump shot. That's probably an autolite carb, and you can replace the accel. pump leather easily. It's on the front of the carb, a small cover held on by 4 screws. You need to adjust the mixture anyway, that's just basic maintenance.
@KawiRider20 I haven't worked on a jet ski, but most carbs of any sort have mixture adjustment provisions. They may have 2 screws, one for low speed mixture and one for high speed, esp if it's a 2-stroke.
@manlymud No. The idle mixture screws only control the mixture at idle, has nothing to do with mpg or power. What it means is turn the screw one direction or the other until the idle speed is the fastest. No set spot, just experiment. The engine slows down when you go too far either direction.
@britishav8tor75 Yes, black pipes is too rich of a mixture. I would check the choke operation first as well as adjusting the mixture screws correctly. If by not kicking down you mean not dropping a gear, TH400's use an electrical switch to activate the kickdown. Not enough room to explain it here but you can google it and learn a lot about it.
Great videos, I replaced an old carter fuel pump on a 72 400ci with a quadrajet simply because it was hard to start after being parked a week although it was running good, now it starts right up but I hear popping sounds out of the left exhaust tail pipe at idle, do you think this is a carb fuel mixture issue or a fouled plug. Thank you.
@mikeinsanediego well you want the idle to be close to normal before you begin so that you aren't running on the main circuit. So, the first thing is to back the base idle down to a normal range, then begin your mixture screw adjustments.
Excessive timing advance along with manifold vacuum being used for the vacuum advance can do that or it's running too LEAN. Back the mixture screws out and see if that helps. If it's not that make sure the vacuum advance is connected to PORTED vacuum (no vacuum at idle until about 1000 rpm or so).
Yes, adjust the mixture screws first. You might be able to screw the idle screw in far enough to get 2000. Otherwise just have a helper keep the foot on the gas. You may have to stop and start it a few times to let the cooling system burp, but don't let it idle in that break-in period. Start it, rev it to 2K as soon as it has oil pressure. Good luck.
Tyler I have a Quadrajet on a 403 Olds in a 57 Buick 2 door I built. I put an electric choke kit on it. Was having trouble with the hot air one. Now hot starts are tricky. The idle is too low to kick off on its own and if I touch the gas it floods. Is it the choke’s fault? Idle mixture screws? Lots of questions with our seeing it in person. Any input would be appreciated. Thanks.
@ChubbyLoveDumpster It shouldn't matter. I would adjust the mixture first(there's only one screw because there's only one barrel) and try that. If it still lopes then check the choke. If neither of those resolves it then it probably needs a cleaning and overhaul.
Very nice information guide. Sir I would like to ask, on my '95 Toyota Tercel, previously when i start the engine in the very morning, i heard of the high idling engine sound, but after an overhauling on the oil leakage by the mechanic, that high engine sound gone which is abnormal because when I activate the A/C the engine vibrate and shutting down. What do you advise sir?
Thanks so much for the video! I'm trying to help an old disabled lady remove and replace her carb in a 85 e-150. I'm a car enthusiast at heart, and I've got a rebuild underneath me. But I'm new school- so adjusting the carb is something I don't have much experience with. She plans on getting the carb from autozone- will everything already be set? Should i check or set anything before I install it? Any guidance you can give me would greatly be appreciated! Thanks again!
What I do is find the two idle mixture screws, run them in till they lightly seat, then back each out 2-1/2 turns. That's a good ballpark to get it running, then after it warms up you can fine tune them as outlined here. You can also tweak your idle speed adjustment at that time but do that AFTER you have the mixture set.
WhoSaidTyler Thank you so much for the quick reply! After hooking all the vacuum lines up, and the fuel line- is there anything other I need to do before I start the car? I know after I rebuilt the fuel injected engine, I primed the fuel rail by cycling the fuel pump on and off a couple of times. Obviously, this isn't the case here, but are there any other similar steps? The more I do research on the carb, the easier it seems like a straight and forward install. This lady's van kept spitting black sut on start up, and would die a few minutes after idle or when she rolled to a stop or a light. However, if she just drove it on the highway- it'd drive perfectly fine. She wants the carb replaced- so I'm just doing my research.
only1balto No, you just mostly have to crank a while until the carb fills up with gas. What I do is disable the ignition system temporarily and then crank enough to ensure that there are no gas leaks. Once you check that, reconnect the ignition and it should start right away. You may need to do some minor choke adjustment also..what kind of carb is that, an autolite 2100?
WhoSaidTyler To disable the ignition system, just remove the the plug wire from the distributor to to the coil correct? Some videos I've seen, people open up the float bowl and let fuel trickle out- will this be something that I will need to do as well? In other videos, or how to install- this is never mentioned, so I'm a little confused. I'm not 100% on the type of carb. Its a replacement for a 1985 E-150 302, 5.0L. The part will arrive this Saturday from autozone. They normally come with a instruction pamphlet do they not? Thanks so much Tyler- you've been a huge help.
SVPrelude I'm actually very surprised at how little information is floating around about this vehicle or carburetors in general. I understand the vans 28 years old- but normally the interweb is full of useful and not so useful information. There's barely any- and when I do stumble onto something, the descriptions are very vague.
Thank you for the incredibly informative vids. I rebuilt my Quadrajet while watching your rebuild series. Car runs great on 2bbl only. secondary air doors don't open hardly at all when kicked in. The secondary is getting fuel, but with little air it floods out when trying to kick in the 4 bbl. Any ideas? (I did notice it seems like the dampener, (choke pull of) keeps a high vacuum even when warmed up and throttled.
Thanks for the info, just a little question, i have a Bocar carburetor on a vw 1.8l motor, it consumes tons of fuel, ralenti at 800 rpm, no black smoke, a little gas odour on exhaust, could you tell me which screws do i have to adjust and how? i'd thank you.
It is a quadrajet. I did better with it today with setting mixture. I have much less dieseling now. I still have a bit of hesitation upon accelerating from a stand-still. Also, the idle adjustment is all the way out (disengaged from the throttle linkage). That must mean something is out or worn.
So i am in the process of building my first engine, and i will be using a quadrajet, is there is a good rule of thumb to get the idle mix settings in the ball park for the initial start up? i want to be sure it fires on the first crank, to avoid damaging the brand new cam/lifters and new crank/rod bearings
Hey did you have any luck figuring this out? I have been trying to adjust my carb to idle lower too, I adjusted the screw all the way out and it is still super high. Let me know
