Corsa Tune Up

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Corsa Tune Up

Unread post by lalkie » Fri Jun 07, 2013 8:40 pm

I have watched videos for syncing the carbs on a corvair. I have been unable to find on for the 4-carb model As I understand it the two back carbs are like a 4 barrel carb on other cars and the back barrels are not used until hard acceleration. Is this true with the Corvair Corsa. Do the rear carbs need sycned at idle or are the only use on hard acceleration. I am new to corvairs but have experience with SU carbs on MG's and british cars. Any help would be appreciated. Thanks Larry

Scott V
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Re: Corsa Tune Up

Unread post by Scott V » Fri Jun 07, 2013 8:44 pm

the secondary carbs are the 1 in the back on the passenger side & the 1 in the front on the driver side.

sync the primary carbs like the vids you watched. the secondary carb are adjusted - with the linkage - to open up @ the same time.

-Scott V.

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Re: Corsa Tune Up

Unread post by lalkie » Sat Jun 08, 2013 10:16 am

Thanks. I thought both on the back would be the secondary. It should be eaiser now. Thanks again.. Larry

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Re: Corsa Tune Up

Unread post by bbodie52 » Sat Jun 08, 2013 1:12 pm

:wrench: Corvair carburetor maintenance, tune-up and adjustment procedures are covered in detail in the shop manual. I have attached downloadable shop manual sections in Adobe Reader format for you to refer to. I have also attached the Rochester H and HV Carburetor Repair Manual to supplement the information in the shop manual. This well-written guide is about 50% carburetor theory and explanation, and about 50% actual procedural instruction. Specific explanation of the 140 hp secondary carburetors and linkage are addressed starting on Page 12.

Having the proper reference materials – including the factory shop manual – is important. You can download all of these references at no cost onto your computer in Adobe Reader format from the links below. You can then read through the procedures and print the appropriate pages that you will need to refer to as you go through any procedure on your Corvair. Of course, you can also purchase hard copies of the shop manual from Clark's Corvair Parts or other sources. I have also included a link for downloading and installing Adobe Reader, if you don't already have this free application installed on your PC:

Downloadable Corvair Shop Manuals, Parts Manuals, Service Manuals, etc.
:link: ... Index.html

Early Model Corvair Shop Manual & Other Useful Manuals

Adobe Reader Free Download

The Uni-Syn is a carburetor balancing tool that visually displays the velocity of airflow through the carburetor. It can make adjusting the balance between the primary carburetors easier, and help you to get your Corvair carburetors under control. I thought I would add these comments for your info...
bbodie52 wrote:
flat6_musik wrote:...I bought a Uni-Syn and only used it once. It seemed to me that at idle, there was barely enough air flow to raise the ball. It pissed me off and I put it away...
The Uni-Syn Carburetor Balancing Instrument is a tool that I remember as far back as the 1950s and 1960s. It was designed for use in multi-carburetor automobiles, motorcycles, etc. that were initially common in European vehicles, but were later found in American vehicles like the Corvair.
Image :confused:
:idea: The Uni-Syn is an airflow gauge that must be calibrated to match the specific engine airflow moving through the carburetor throat. The flow control in the center of the Uni-Syn is mounted on a threaded pin. Rotating the flow control raises or lowers it, which increases/decreases the air gap, which impacts the air velocity passing the oriface that allows some airflow to pass through the gauge. With the Uni-Syn held firmly on the carburetor intake with the engine idling, the flow control is adjusted so that the plastic float is approximately centered in the glass sight tube. Once calibrated, the Uni-Syn can be moved back and forth between the two primary carburetors to compare airflow at idle. The goal is to play with the idle speed screws to get an even airflow level that matches between both carburetors, and at the same time produces the desired idle speed. Before fine-tuning this idle balance with a Uni-Syn, the ignition system, idle speed and idle mixture should have been adjusted following the standard procedures as described in the factory shop manual. The balancing step with the Uni-Syn gauge is added at the end of the tuning procedure to measure actual airflow though both carburetors at idle, and balancing that airflow using the airflow meter as a measuring tool, rather than simply relying on a physical balanced "calibration" using a strip of paper to detect initial contact between the idle speed adjustment screws and the carburetor linkage, followed by counting the screw turns needed to obtain the desired idle speed (and ensuring that the same number of screw turns are applied to each screw). That procedure achieves a fairly close initial setup. The use of the Uni-Syn airflow gauge as a final step ensures balance between the two carburetors by measuring actual airflow.

:wrench: Once the balanced idle airflow and desired idle speed have been achieved, a similar procedure can be used to synchronize the airflow when the carburetors are held open at a faster engine speed by the throttle linkage. Again, the shop manual procedure can be used to set the initial mechanical balance between the two sides of the throttle linkage. The linkage segment connecting the accelerator pedal to the cross-linkage between the two carburetors is temporarily disconnected, and a turnbuckle is temporarily attached to hold the throttle opened against the pull of the throttle return spring.
The turnbuckle can be adjusted to hold the throttle open at approximately 1500 RPM. The Uni-Syn center flow control is readjusted to re-calibrate the float to a position somewhere in the center travel of the sight tube, based on the increased airflow through the carburetors at the higher steady RPM maintained by the turnbuckle arrangement. As with the idle synchronization procedure, the re-calibrated Uni-Syn is used to measure the airflow moving through the two carburetors. The goal is to fine-tune the threaded portion of the carburetor actuation linkage so that the same airflow reading is attained on each carburetor, but this time with the throttles held open by a pull on the throttle linkage, instead of by the setting of the idle speed screws. When the airflow has been balanced using the throttle linkage adjustment, the balancing procedure has been completed and the normal accelerator pedal throttle linkage can be reconnected.
Uni-Syn.jpg (47.27 KiB) Viewed 617 times

With the engine at idle, you want to open the Uni-Syn flow control as much as possible, but still keep the float in the sight glass at about mid-level. Then check the other carburetor, which you want to read the same flow rate. You may have to adjust the flow control a few times as you adjust the carburetor settings. Just remember to check each carburetor with the flow control set at the same point, and to keep the sight glass in the vertical to prevent the float from hanging up in the tube.

The wheel in the Uni-Syn venturi controls the flow through the tube, or in other words how high the bead is in the cylinder for a given engine speed. You need enough flow to not strangle the engine, and the bead works just as well in the lower third (which still lets good air flow through). Make sure the idle speed doesn't drop when you place the tool on the carb: if it does, open it up.

Before setting the carbs, make sure that the ignition is right: points and timing set, good wires and plugs. Rich running is often blamed on carbs when in fact it's a weak, retarded spark. Check the throttle shafts: loose ones let in air and lean the mixture, raising idle speed, as well as throw off the linkage action. Check the linkage that connects the carburetors. if it's loose, one will open before the other. Make sure there are no vacuum leaks.

With a twin carburetor set up, disconnect the linkage that connects one carburetor to the other. Set the idle speed with the linkage screws first. Use the Uni-Syn to check that each carburetor is drawing equally at idle. This may take a few tries until you get both drawing equally at the speed you want. Blip the throttle to see if they come back to those settings (worn throttle shafts can fool you). Set the idle mixture screws in accordance with the shop manual instructions. Check the balance again. Hook the linkage back up. If one carburetor now draws more, adjust the linkage until it's back to roughly equal.

Once the carburetors are drawing equally at idle, hold an engine speed: at about 1500 RPM. This checks that the mechanical linkage is pulling equally. You'll need to open up the Uni-Syn center wheel to draw more air and bring the bead down in the tube. If both carburetors are within a bead's thickness of each other, that's good. If one is definitely off from the other, the higher flow carburetor's linkage is being pulled more than the other. You'll need to figure out why that's happening mechanically to rectify it. :chevy:
:link: ... IN&page=58
1965 Corvair Chassis Shop Manual - SECTION 6 - ENGINE TUNE-UP.pdf
1965 Corvair Chassis Shop Manual - SECTION 6 - ENGINE TUNE-UP
(2.92 MiB) Downloaded 21 times
1965 Corvair Chassis Shop Manual - SECTION 6M - ENGINE FUEL.pdf
1965 Corvair Chassis Shop Manual - SECTION 6M - ENGINE FUEL
(8.87 MiB) Downloaded 14 times
Rochester H and HV Carburetor Repair Manual.pdf
Rochester H and HV Carburetor Repair Manual
(1.79 MiB) Downloaded 18 times
Brad Bodie
Lake Chatuge, North Carolina
Image 1966 Corvair Corsa Convertible

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