TH Posted 11 October – And keep in mind that rarely are all ports on a carb. Starting with the front: Choke pull-off and what you sayis the dist capI amassuming you mean the vacuum advance under the distributor cap? Looks kinda like a flying saucer. The metal line lookslike a vacuum line to the transmission modulator. You should be able to follow that and see where it goes. I personnaly have that line going to manifold vacuum and not ported vacuum like you ave but others can chime in on that. The back T coming off has 1 going to your brakes which is correct. This T-has manifold vacuum full time vacuum.
Question about vacuum lines
Carburetor Technical Information Rochester Quadrajet Quadrajets were used as original equipment on many vehicles from to Over million have been produced. QJets have a maximum airflow of from cfm. Because the secondary side of the Qjet opens according to the airflow requirements of the engine, the same basic carburetor size can be used on a large range of engine sizes. For example, on a mild Chevy the air valve will never open all the way – because the motor can’t use more than cfm or so.
M4MC Quadrajets have an integral choke and a different airhorn design; also, the throttle shafts are longer than in 4MV carbs.
These are competition carbs, and they are supposed to work this way.
In a squarebore, the bores are closer to the same size, though, in some squarebore carbs, like cfm Holleys, the primaries may be noticeably smaller than the secondaries. However, even then the bores share the same centerline. Many aftermarket intakes, such as mine, and the Edelbrock Performer and RPM, etc, are “dual pattern” intakes the area where the carb bolts on looks like a butterfly.
Either a squarebore OR spreadbore can be physically be mounted on them with no adaptors. Carter Thermoquads, Rochester Quadrajets, and Holley ‘s, ‘s, and ‘s are spreadbores. If the intakes are true squarebore intakes four individual holes or true spreadbore intakes again, four individual holes, though of different size , you cannot really bolt a squarebore carb on a spreadbore intake and vise versa. In the case of putting the squarebore on the spreadbore intake, the primary bores of a spreadbore, though small, are spread further out than the primaries of a squarebore.
The squarebore primaries of the carb would not line up well with the holes in the manifold. Also, most spreadbore primaries are smaller than most squarebore primaries, so even if they did line up, the manifold would present a sudden, constriction. In the case of putting the spreadbore on the squarebore intake, the secondaries of a squarebore intake are smaller than those of a spreadbore carb, so in that case, you’d have significant flow problems.
Tuning carburetors and setting timing
June 10, , It originally had the AIR system on it, but the entire system was removed shortly after delivery. It has the original distributor and the vacuum advance is hooked up to ported vacuum. The car has always seemed to run well.
This requires that lean mixtures have “the fire lit” earlier in the compression cycle spark timing advanced , allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature wasted combustion energy.
There is some variation depending on the model year and the country — emissions considerations and such. Alternately, of course, the vacuum port on the carburetor can be plugged, which accomplishes the same thing. You can’t set this distributor with the engine not running, as you need the idle front vacuum line working to pull the distributor plate back to the idle setting.
The port on the left provides vacuum advance, and the port on the rear provides vacuum retard. When using the single-vacuum distributor, the vacuum hose runs from the vacuum port on the left side of the carburetor to the vacuum canister on the distributor. In that case, the vacuum port on the rear of the carburetor must be plugged. There is also a vacuum port on the intake manifold just below the carburetor.
This port is for a vacuum line up to the stock air cleaner. See our Air Cleaner Discussion. If you are using a tricked-out chrome air cleaner which we do NOT recommend , this port on the intake manifold must be plugged to prevent entry of air into the system.
Distributor Vacuum Advance Hose
Be sure to save the illustrations separately. The opinions expressed here are my experiences, and your experiences may vary. I don’t put myself forward as the world’s leading expert on Holleys, but I do have some helpful ideas. In some cases, I may not be able to answer specific questions, but I’ll do my best.
Back to Top Post by hippy on Jun 16, 3:
And can even give a performance increase on carburetor engines. Here is the real explanation. For this example we will use say an 86 range 5. The stock marine unit has an iron gear, magnetic pick-up, external coil, No advance to speak of, and a seal to keep water out with a vent tube on one side of the housing. On this particular model carbed thunderbolt the ignition amplifier mounted on the riser acts as an electronic advance but is vague.
Basically you are set at 10 btdc at idle and the unit will increase the advance to 24 deg on top of that starting at rpm and becoming full circle 34 deg by rpm. Now in stock config in proper working order this is pretty good. However depending on engine modds and prop sizing load on the engine , that pre set config is not perfect. Vacuum advancing can improve every aspect of this. There is allot of speculation from tuns of different guys or gals on these forums on how the vacuum works.
Carburetor Tune Up Guide
Back to Top Post by hippy on Jun 16, 3: Jan 23, Can a Suzuki Gti engine fit into a Gl manual Justy? Jan 24, 8:
Next is the thin sheet steel plate.
Car Year, Make, Model: Tuning carb with a vacuum gauge Hook up your vacuum guage to the intake manifold port of the carb sucks at idle. Set you rpm about where you want it to be. Adjust your idle mixture screws to give you the highest manifold vacuum reading which should be the highest idle rpm reading with the throttle butterflies closed. Re-set your idle rpm. Run it a while and read your spark plugs. If they are sooty, or wet, then lean it out a bit with the jets and metering rods strip kit Be super careful to read your instructions with your strip kit because: I usually let my technician tweak the knobs.
She is that good! One small change at a time and you can crispen this carb up real nice.
Vacuum Lines Connection
There are a large number of modified vehicles that no longer use an ECM engine control module to oversee the emissions and performance parameters of their vehicles. Because of this, the pre computer-controlled type HEI distributor is often used; it is a relatively cheap, stand-alone unit with good to very good performance potential, and has a good track record for durability and reliability. A computer controlled HEI distributor work basically the same as a non-computer controlled HEI except for the lack of a mechanical or vacuum advance mechanism some early versions did use a vacuum advance ; the ECM determines the advance curve electronically.
The only other change you can make in the advance curve is to manually advance the base timing usually worth a little HP by itself. This is not a bad thing, it leaves funds available for parts that WILL make the car faster!
In order to set the floats on a Holley with externally adjustable floats, remove the sight plug with the engine off.
Before you start work, replace any cracked or missing vacuum hoses, and check vacuum hose routing against your service manual diagrams or, in most cars, a color sticker under the hood somewhere. Make sure you get the right grade of vacuum hose; anything carrying gasoline or emissions fumes require a special material. When in doubt, get the pricier but safer gasoline-safe hoses.
Vacuum hose, gauges, and Ts are sold by most auto parts stores. Vacuum gauges are usually large dials with a narrow rubber hose or metal hose grip leading out; most come with various plastic tips and fittings. Warm the car up by driving it around for a bit, then connect the vacuum gauge to intake vacuum; you can put in a T and share vacuum with another function, if you plan to use a vacuum hose inside the cabin, or use a port that supplies vacuum at all times.
Tuning carb with a vacuum gauge
Post by mx5tc on Sept 25, 0: I suspect what is happening on your scoot is that you now have the right mixture for best power at WOT but are too rich for midrange and less than WOT cruise. I’m wondering if a needle with more taper look at the various profiles that Keihin has would still give you the right WOT mixture while providing a leaner and more economical mixture for midrange and cruise. Has anyone figured which Mikuni or Keihin needle series can be swapped into our carbs??
The retard line will only retard a little of course, compared to the main vacuum’s advance.
Nearly 22 years ago now. Today, it has 95, miles. A large percentage of that 58, mile difference has been off-road in the Pacific Northwest. The rest has been getting to and from the trail-head to leave the pavement. I traced this rig’s history and spoke to it’s original owner not long after buying it. He had purchased it as a dealer “demo” with 4, miles on the odometer.
He used and abused it hauling a 55gallon drum, hunting dog cages, and pulling a 12′ trailer before the Repo-man mercifully rescued her from him, returning her to a Mike Salta dealership in Portland. That dealership was closing, and she was wholesaled to Beaverton Datsun, where she and I first we met. It’s been a love-hate relationship. Water leaks into the cab it’s been known to rain a bit in the PNW , the occasional ill-timed breakdown, and she even has a spot or two of skin cancer these days.
But, just about the time I’m finally convinced it’s time to say goodbye, some stranger will ring the doorbell, apologize for interrupting dinner, and darn-near beg me to sell her to him. That’s when I say again that I’ll probably be buried in her instead of a casket. I’ll get in and the Optima Red-Top fires her up on the first try even though that battery’s 72 months came and went long ago.
Mechanics I’ve talked to say they’ve seen these old AMC 6 cylinders go K miles before giving-up the ghost.