Cb750 carb idle screw. My question has to do with the bike being a "Frankenstein" build. Re-adjust the throttle stop screw to return the idle back to ~1500, or whatever you want it to be. Last weekend after rebuilding carbs (80 750c) started bike using auxiliary fuel tank with filter. Also, 50+ years later the carbs themselves have often receded where the air screws seat, so they must be opened less than the 1970s spec to get the same mixture. Then adjust the _sync_ screws on the other 3 carbs until the drill bit just fits them as well. Screwing them in makes the idle/pilot circuitry leaner or richer? Nearly all aftermarket carb kits come with newer idle screws that have a pre-bored metering hole in the end. The rpm hangs because you've got the idle speed set too high and the carbs are too far open. 1979 to 1983 DOHC CB750/900/1000/1100 Honda Custom: Forums View and Download Honda CB750F2 service manual online. Is there a hesitation off idle to 1500- 2000 rpm. 71-76 carbs run rich at this point, 77/78 carbs will lean out at this point. Author Topic: 1977 keihin carbs for cb750. Adjustments to the air/fuel mixture are made by turning the idle screw in or out, opening or closing the throttle plates, subsequentially adjusting the motorcycle's idle speed. I recently bought this bike knowing the carbs needed cleaning, so i took the carbs off, noting where the cables, hoses go etc. Fits 69-70 Honda CB750K (B750A code stamp). Adjust idle to 900-1000 Now throttle response off idle is the key. Turning counter clockwise (as if you're viewing it from above) increases idle speed, clockwise decreases. Enjoy the videos and music you love, upload original content, and share it all with friends, family, and the world on YouTube. Adjust to I understand the air screws, 1 3/4 (or so) turns out, off idle throttle response, etc. I'm nto sure if this is an EPA thing or not, but running free flowing air cleaners of any kind on the 71-76 style CB750 carbs often presents a bit of a problem wiht these pre metered idle screws. Tighten the adjuster screw lock nut without changing the adjustment. The bike started ran and adjusted idle and it idled on its own. so do I lower the fast idle speed or do I simply increase the idle? And in increasing the idle, do I adjust the pilot screws or just the black knob beneath the carbs? Thanks The one on the right is the throttle stop screw. But what is this other screw?? I know that on a lot of carbs, air screw is typically on intake side, then fuel screw is on the engine side, but it doesn’t look like these carbs are made like that. Loosen the lock nut and turn the adjustment screw until number 1 reads less than 2. Do not release the throttle, but just hit the kill switch, pull in the clutch and coast to a stop. The other three screws on the other three carbs are a unique screw style screw head. Number four was a pinch high (1/4"), but spec in the manual is all to be within 1". The screws on your carbs meter air from the air jet to be mixed with the fuel from the Slow jet, primarily for throttle settings of idle to 1/8 open. Pilot Air/Fuel Screw Adjustment Explained - Multiple Carbs - Part 2 How-To Motorcycle Repair 140K subscribers Subscribe THE IDLE: where is the idle adjust knob? This carb bank has a 'pulley' setup with push/pull throttle cables that actuate the pulley. Step 11 Locate the adjustment screw and lock nut between the number 1 and 2 carburetor. The Carb slide and attached jet needle set the mixture for 1/8 to 3/4 throttle positions. There is no individual throttle stop screw on each carb. HONDA CB750K PILOT SCREW ADJUSTMENT IDLE DROP PROCEDURE Nore pilot is and is the NOTE a with of 50 Turn pilot until it øats and back it to is a to pilot screw PILOT SCREWOPENING; 1-1/2 IVB42A) 1-3," IVB42cb C,' VT/O,v pilot CB750 Air Screws - Question On my bike, I have to compromise with an air screw setting that balances overrun "popping" and a off-idle stumble as I pick up the throttle - I accept a little lean popping to get a lean enough mixture to avoid the stumble. With a large inventory of vintage motorcycle parts and same day shipping in the continental US, we can assure that all your Honda CB750 SOHC motorcycle needs will be met here. Then ride for a few minutes at 1/8 to 1/4 throttle only and do a plug chop. 4 inches difference from the number 2 carburetor. 20 shipping 71-76 CB750 Idle Screw Modification for Free Flowing Air Cleaners Nearly all aftermarket carb kits come with newer idle screws that have a pre-bored metering hole in the end. If you haven't been through the carbs and cleaned them I'd suggest doing that. IMPO With the engine running at temperature, and with a fan or means of forced convection aimed onto the engine, the carbs fuel screws and idle are adjusted, then the synchronization is adjusted via adjustment screws on the carbs. If it's the large black screw between carbs 2 & 3 (dead centre) it's your idle adjust. 1969 1970 Honda CB750 K0 carburetor idle adjuster screw throttle stop screw $22. So screw them all the way in and out that much. I am familiar with the carb balancing procedure using vacuum gauges but I am curious to hear the best way to adjust the mixture screws on this vintage of carbs. Maybe it broke off? Where is the idle adjust screw on my all-stock '73 cb750 K3?. diameter screw driver held against head of idle screw, i can distinctly feel the slide strike the screw, when throttle is allowed to snap shut from full wide open. I'm not sure where to look first. Synchronizing the carbs using a set of vacuum gauges on the other hand will go a long way towards smoothing out the idle, improving throttle response, and quieting down the sewing machine noises at idle. « Last Edit: March 30, 2018, 02:17:21 PM by The service manual spec is an emission setting, you can go 2-2 1/2 max, needing more says your carbs have clogging in the idle circuits. And Idle mixture screw regulates flow of air and fuel after being mixed in the pilot jet emulsion tube. pod filters kerker 4 into 1 110 main jet 75 pilot jets 2. Couple questions: 1. The off idle hesitation is a switch from air mixture to slow jet. Idle screws may need to be adjusted or idle adjustment. I think the FSM says 1200. Bottom right is a mixture screw and they are usually set at one turn out for a baseline setting. With the bike warmed up, set the mixture screws to a baseline one and a half turns out from lightly seated and adjust the idle to around 12 to 1500 RPM for most bikes. I recently rebuilt my carbs, and now the bike won't idle. 00 + $4. Set the idle mixture screws to what the manual states and then ride the bike at normal operating temp. , etc. However, the idle has to be set highbecause it wont idle below 2000 rpm. Disengage the choke and the bike just cuts out. Does the bike have a modified exhaust or the airbox removed? You'll need to mess with the jetting if so. Mar 3, 2025 · Vacuum sync the carbs. Looking at the ignition coils and How to tune your carburetor for the best idle and throttle response. This is the real deal…if you've already got your carbs off for a rebuild, you might as well replaced all the stripped out factory hardware and restore the original look. It's not like a single, nor even a twin. I am in the process of synching the recently rebuilt carbs on my my stock CB750K2. Dime City Cycles is the leading destination for Honda CB750 SOHC Idle Adjustment Screws. Setting Air Screws on cb750. I must remove the choke to start it. In straight functional condition. This 71 piece kit for DOHC Honda CB750 models includes front stay plate screws (8), CV cap screws (8,) throttle cable bracket screw (1,) rear stay pla I don't care what make or model you have. 5 turns. What does this carburettor screw do? 2. i tried adjusting the timing knob in back of the cylinders and Here's how you do it: Adjust the idle screw (the main one) until the 1 carb outside of it is open enough to just let the 1/8" drill bit through. Make adjustments accordingly. One problem was that my floats where stuck, so i cleaned them and lubed. CB750F2 motorcycle pdf manual download. In your pic its very visible on the left side. If you've tried everything, but still lean at idle, post up a picture of your fuel level using clear tube method. As long as negative pressure exists at the pilot circuit exit, there will be a contribution from the pilot circuit. 3G Gross Weight: 15G Model Name: CG-DSLS Material Type: Copper Iron Plastic Iron Aluminum Report an issue with this product or seller Jan 26, 2026 · Carb idle adjuster screw with spring. Each one separately turned all the way to closed must kill that cylinder or again you have carb issues. Opening all 4 carbs too much because one is clogged does not work. Carbs like a Edelbrock 4 barrel only have 2 mixture screws in the front, so you adjust those like you would how I explained by screwing one in until the idle drops and then back out again until you get the highest possible idle, then go to the other screw and do the same thing. Using a long, sm. I have the air mixture screws backed out 1. The procedure is simple: Set the idle RPM to ~1500 using the throttle stop screw. Enjoy! Finally got the carbs synched! Airbleed screws set at 3/4 turns out. So I bought an 81 CB900f a while back as non runner and now getting to it. I just want to make sure I understand the actual causality and functionality. When cold and no choke, it fires the instant I touch the start button and revs nice under throttle. The idle screw adjustment stop doesn't seem to do anything. And, the Main jet sets mixture for WOT position. Pilot screw adjusts air or fuel? (Read 9755 times) 0 Members and 1 Guest are viewing this topic. my fuel pilot screws are on the bottom of the carbs next to the float bowls. Once the bike is up to temp set the RPMs to around 800-900 and one at a time turn the idle mixture screws in until the engine begins to stumble or until the screw gently seats (DON'T FORCE IT!!!). I'm assuming at idle, since you're referring to air screws, but not sure. The idle is set by a large knurled screw knob that sits up higher on the rack and to the left or right depending on the year. Essentially it sets the correct mixture for the hardware and fuel in use for you, along with the Using a tool, made from a small paper clip, to extract the rubber O-ring and metal washer after the idle-mixture (fuel) screw and its spring are removed. Only thing about that is when you lower the idle then it won't idle. I put everything back where it came from including the setting of the The carb boots weren't tight enough and it would idle around 4000, get lower when I touched the idle screw and then back to 4000 again. 5 turns on the a/f screw 4 washers on the needle the bike has good throttle response now when its warmed up but the idle is very Once the carburetors are installed on the bike, the idle screw (the one with the big black plastic head between carbs #2 and #3) needs to be adjusted and the carbs have to be synchronized. I need a bit of advice. You'll probably want to close the idle screw before you bolt them back on the bike, 1/8" openeing is Trying to set the idle mixture screws using the 50 or 100rpm drop method can be maddening since these don't always idle smoothly. It started running rough at idle a couple of weeks ago so I've stripped and cleaned the carbs including the pilot screws. CB750 Idle issues First, it doesn't start with the choke on. CB 750 Idle Air - Fuel Yes the top left is where you screw in a tube to synch. - Mark « Last Edit On alot of 4 barrel carbs, there only mixture screw on the primaries. what you see is what you get. I have a 1981 honda cb750 supersport that does not idle. CB750 carb air screws - which way to turn? There's a very good routine in the Honda manual about setting these though which I feel is advantageous to follow (we can get that to you if you've not got access) an innocuous looking instruction that has far reaching effects with these carburettor type. So 1980 CB750F Idle with choke engaged sits kinda high at around 3000rpm. tighten your boots and check for leaks and see if it idles again. The screws should be 2 1/2 turns out I believe. I've long used My carb is a VB42A. One set of throttle cables, all carbs are affected. At the very least the carbs need cleaning. Idle Adjustment Screws The idle mixture screw controls the air/fuel mixture at idle speed, and often is located near the throttle plate. Force will break the stems and designed to do so, be careful. The stock air cleaners Find many great new & used options and get the best deals for Honda CB750-F Supersport Engine Carbs Carburetor Bodys 1976 # ACP-7 at the best online prices at eBay! Free shipping for many products! If you hold carb with the front facing you then any teeny little holes that are just covered up with the throttle butterfly fully closed MUST be open and the small chamber that supplies them with fuel as well, it is blocked off with a plug under carb behind the mixture screw hole. Pretty sure the a/f mixture screw is on the bottom, then the idle adjustment is labeled in the pic. From lawn mowers to motorcycles this is the best way to dial in your carburetor for a fine tune adjustment for your Idle mixture screws. Setting Honda CB750 Carburetors [PD42 B K8 Carbs] | Adjusting Motorcycle Throttle Cables | Part 9 In this episode I clean and rebuild the PD42 carbs for my CB750 project, and adjust the throttle Author Topic: How do you adjust the CB750 '77-'78 idle mixture screws? (Read 13451 times) 0 Members and 1 Guest are viewing this topic. Idle speed also is in play. Good condition and low miles. The only mixture you can change is the idle mixture. I have a 78 cb750f (supersport). So I'm expecting to find one idle adjust screw. Other than Hi, First post here so be gentle:) I bought a 97 f2v around a month ago. Idle mixture screws are underneath each carb toward the front and easily broken having limiter caps on them makes it even easier as you cannot turn them more than maybe 3/4 turn without hitting the cap, an EPA thing for emissions. The fuel pilot screw my third carb is what looks like a brass cap screw. Know the problem is remembering how many turns the main jet and the idle jet screw in "they Author Topic: 77-78 CB750 PD42A Carb Rebuild Info (Read 84450 times) 0 Members and 1 Guest are viewing this topic. Author Topic: CB750 carb issues - rich at stock setting and poor idle (Read 9831 times) 0 Members and 1 Guest are viewing this topic. Now get a feel for the throttle and look for signs of an overly rich or lean air fuel mixture. How do I set the air fuel mixture on a 77 cb750? What are the settings that the screws are supposed to be at? A true air screw regulates flow of the air inlet from the air jet. The idle limiter caps if still in place may limit that and can be removed with a soldering iron and heat, they are loctited on. Whether that's inherent in the beast, just 40-yo carbs doing their thing, or a PO screwed up the jetting, I don't know. Carbs or engine condition and likely valve clearances or sealing. If I set it lower than that, it will idle about 30 secondswhile it just starts to drop revs and finally die. +1! It isn't possible to listen to the engine and adjust the air screws for 'best idle', for example, primarily because the cylinders interact. The 750 at normal operating temperature should idle at around 800-900 rpm. The stock air cleaners introduce some vacuum to the intake side of the carbs that Product Description Motorcycle Carburetor Idle Adjustment Screw Kit for CB550 CB550K CB650 CB650C CB750 CB750K CB750A CB750F CRF100 NSF100 Color: Red, Yellow, Blue, Green, Black Material: Plastic, Iron Net Weight: 3. Air idle should be at 1 1/4 as you stated. Thoughts? so i just finished putting together my 79 750f brat trying to get my carbs tuned right this is what ive got. The throttle stop screw is part of the carb mounting plate and sits between and in front of carbs 1 & 2 when they're installed. Pull ALL the plugs and check the colour. Adjust the air fuel mixture screw to achieve the greatest idle RPM possible. Honda CB750-F Supersport Engine Carbs Carburetor Idle Screw 1976 # ACP-8 MPN: 064A Retr0parts (24 Reviews) 100% positive feedback Contact Seller Honda CB750-K CUSTOM Carburetor Idle Jet Screw LOT 1979 1980 1981 1982 # AP-289 MPN: VB42A Retr0parts (24 Reviews) 100% positive feedback Contact Seller Putting things together. 77j5r, t5ls, muob0, trmw, z6ie, p4p4, tgr9n, n0uol, 7sayg, taoq,