Choke Adjustment

[plrgpr]

Once again I am confused by the service manual. The 66 supplement says, in one place, to adj the choke by holding choke closed, actuate accelerator cross shaft to set fast idle cam and screw out choke rod until it mates with the choke lever hole. Couple pages over under “adjustment procedures” it says to place the fast idle tang on the second step of the fast idle cam, close the choke valve trip lever till it just contacts the choke tang. At that point you should be able to get the appropriate gauge between the body of the air horn and choke plate. These are two different settings. One has the choke completely closed and one has the choke open by the amount of the “appropriate guage”?????? My 66 140hp 4-sp is hard to start cold. My chokes are open, maybe 1/8” when cold.

[64powerglide]

The choke rod is at the back of the right carb & the left one is in front of the carb as you are looking at them. They are held on by a clip, take the clip off & make sure the hole lines up with the rod, if they don't screw them up or down until they go in the hole. The rod comes up through the engine tin. That's my story & i'm sticking to it. I will add do this when the engine is cold.

[bbodie52]

The mechanical automatic choke linkage functions to fully close the choke butterfly valve when the engine is cold and the choke thermostats are applying full spring tension on the choke mechanism on each carburetor. Moving the upper mechanism to the closed position also makes it possible for the fast-idle cam to drop in to the set position, once the throttle mechanism has been opened and released by the vehicle operator one time to move the throttle linkage out of the way to allow the fast idle cam to move into position for cold engine operation.

Successfully starting the cold engine produces a high intake manifold vacuum below the throttle butterfly valve on each carburetor, which in-turn applies a vacuum to each vacuum break, which pulls the closed choke butterfly valve on each carburetor open slightly to give the engine more air to breath when it is running cold. As the engine warms to normal operating temperatures the spring tension on the choke mechanisms is gradually reduced as the choke thermostats respond to engine heat. When the engine warms enough to open the choke butterfly valves the vacuum break no longer has a function on an engine that has reached full operating temperature.

The whole mechanical choke system is analogous to a modern computer-controlled electronic fuel injection system that responds to engine electronic sensors that tell the computer what the engine is doing and what its operating status is. The computer makes changes to the throttle body to modify its operating parameters to help start and operate a cold engine. As the engine warms, the sensors tells the computer of the changes and the computer makes pre-programmed changes to reset the throttle bodies to normal operating settings.

In the 1960s and earlier they did not have electronic sensors, automotive computers and electronic fuel injection, so all of these same cold engine processes had to be accomplished with mechanical devices that responded to engine temperature and speed, engine vacuum and throttle position changes. The carburetor, distributor (with its vacuum and centrifugal advance mechanism), and the Powerglide automatic transmission (which amounts to a mechanical, vacuum-controlled, and hydraulic fluid-controlled computer) have gradually been upgraded to take advantage of modern electronic and computer technologies that did not exist decades ago.

The first eight pages of the attached DELCO ROCHESTER - Models H, HV Carburetor Service Manual does a pretty-good job of explaining the carburetor subsystems and how they function. It also covers the differences in model years as they carburetors evolved. The explanation of the choke systems begins on page 7. The remaining pages cover maintenance and repair of the Corvair Model H and HV carburetor. This manual is a good supplement for the Corvair Shop Manual.

More tune-up info...

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[terribleted]

I think you are looking at the choke adjustment settings and the choke lockout setting for the secondary carbs. The chokes them selves are adjusted as follows. With the engine stone cold remove the choke rod from the choke lever (choke rods go down into the head by each carb simply remove the clips). Actuate the throttle linkage and while holding the throttle linkage partly open grab the choke shaft lever and rotate it so that the choke plate is fully closed. If you can not get it fully closed make sure the throttle is applied enough so that the carb linkages are clear allowing the choke plate to close (if it still can not close you have carb adjustment issues or some other issue with choke plate function). Holding the coke plates fully closed twist the choke rod up or down until the rod enters the hole in the choke shaft lever cleanly. then adjust the choke rod out 2 full turns, insert it and affix to the lever again with the clip. done. manual says something about holding the rod up while doing this adjustment, I would not, since some chokes never expand to hit the top of their slots anymore. This is why stone cold is the key. The choke will be holding the rod about as high as it ever will and you will get as close an adjustment as possible (this would perhaps be different if you were doing this at an ambient temperature of over 100 degrees).


The other choke adjustment is to adjust the choke lock-out levers on the secondary carbs of the 66 and later 140 linkage if still equipped. These instructions are on page 6M-4 of the 66 shop manual supplement. I have found most 66 linkages are bent and worn and do not function very well. I would suggest either 65 style non lock-out linkage for simplicity and more consistent function. Roger Parent makes a super nice new replacement setup for either. they are really nice altho not inexpensive.

[plrgpr]

Ted, I made the adj EXACTLY as you and the manual instructed (both carbs), stone cold engine. However, when i started the engine, it ran rough. The left carb choke would not pull off, remained closed after running a few minutes. I reset that choke back to where I started and it opened up and ran fine. Check out the 66 supplement page 6M-3 under “Choke Rod Adjustment (Fig. 9)”. As I said, that seems to indicate a totally different procedure for adjustment?

[terribleted]

6M-3 is a basic carb adjustment to set the choke opening at fast idle and has nothing to do with the basic adjustment of the external to the carb bi-metallic choke coil and rods. These carb adjustments need to be correct (all of them) before exact choke (or even carb) function can be attained. Is your choke pull-off (vacuum break) functioning and adjusted properly? Are they plugged into the proper bottom straight out vacuum port? If the pull-off is functioning and the carbs are properly adjusted, the choke rods into the head are properly adjusted and nothing is binding it is impossible for the choke plate to stay fully closed with the engine running unless there is almost no vacuum on that side of the engine (hard to with pistons moving air). The choke plates should open to the dimension set in Vacuum Break adjustment as soon as the engine starts....it should never be fully closed in operation. The left choke will open more slowly (from this minimum opening set by the pull off) than the right choke as the right front of the engine heats faster than the left rear.


Look at the more complete instructions in the 1965 Main book for complete carb rebuilding and adjustment procedures (covers what you saw on 6M-3 as well). Perhaps this will help things to be more clear. The adjustment steps there must be followed in order to be fully successful as some later steps build on earlier steps. The only need for the 66 supplement concerning carbs and adjustments in the 66 secondary cold lock-out adjustments that were new for 66.

[plrgpr]

I agree Ted, but after I adjusted both chokes, the left one did not pull off. Set it back to the approx originall adj and all was well again. The right one works fine after adjustment, and the left one works well once again, so all systems are functioning correctly. The car runs superbly once it’s running......couldn’t ask for a nicer running car, hot or cold. Just have to coax it too much to start when the engine is cold, and I can’t explain the failed pull off on the left carb after the adjustment.

[bbodie52]

...couldn’t ask for a nicer running car, hot or cold. Just have to coax it too much to start when the engine is cold, and I can’t explain the failed pull off on the left carb after the adjustment.

When the engine is started, manifold vacuum is applied to the spring-loaded vacuum break diaphragm. This moves the diaphragm plunger into the assembly until it strikes the cover, which in turn, opens the choke valve to the point where the engine runs without loading or stalling. The length of the connecting link determines the amount of choke valve opening.

As the engine is warmed up the choke thermostatic coil is heated. It relaxes its tension, thereby allowing air velocity to open the choke valve. This continues until the engine is warm. At this point the choke coil tension is completely relaxed and the choke valve is wide open.

With the choke mechanisms adjusted properly and equally on both sides of the engine, the tension on the choke rods will fully close the choke butterfly valves on both carburetors. Opening and closing the throttle one time will allow the fast idle tang to move into position to hold the throttle open at the fast idle setting. Fully closed choke butterfly valves would be needed for starting the engine. Once the engine is running at idle, the intake manifold vacuum is high enough to cause the vacuum break diaphragms to fully open, which slightly opens both choke butterfly valves to the proper position for a smooth idle on a cold engine. The rest of the opening process occurs as the engine warms and the tension is reduced on each choke mechanism.

If your vacuum break diaphragm on the left carburetor is not functioning, the choke butterfly would remain fully closed even after the engine starts. This would cause a rough idle until the engine begins to warm and the tension is reduced on the choke butterfly as a result. By reducing the tension on the choke butterfly manually by improperly adjusting the choke rod, the left butterfly remains partially opened, which is optimal for a running cold engine, but not for starting a cold engine. You are in fact compensating for a failed vacuum break on the left side by adjusting the choke mechanism so that it never fully closes. While this allows for a relatively smooth idle once the engine starts, it causes the engine to be difficult to start because the choke mechanism on the left side is never fully closed. The vacuum break mechanism normally covers this transition period between the choke setting for starting a cold engine and the choke setting for allowing the running engine to idle smoothly while it is still cold.

If the vacuum break on the left carburetor fails to open the choke butterfly partially in response to the presence of intake manifold vacuum on a running engine, and if the pull-off rod has been adjusted properly, the vacuum break internal diaphragm or plastic housing may have developed a vacuum leak. The vacuum break unit may need to be replaced. (You could confirm this by swapping the functioning vacuum break from the other carburetor, if desired). A fully closed butterfly on an idling engine would likely cause the engine to run rough at idle if the vacuum break fails to open the valve to allow some air to enter before the engine reaches normal operating temperature.

http://www.corvair.com/user-cgi/catalog ... IN&page=55


Part number C560: VACUUM CHOKE CONTROL- CHOKE "PULL-OFF" BRACKET IS PLASTIC **62-69 PRIMARY CARBS**

Weight: 0 lbs 4 oz
Catalog Pages(s): 55(20),U-8(20)
Price:
1 - 2 $ 15.95
3 - 4 $ 14.85
5 - 9 $ 14.70

[plrgpr]

Thanks Brad, but the left side is working OK now after resetting the adj back to where it was before.

[bbodie52]

But with the current setting you are using, didn't you state that you "have to coax it too much to start when the engine is cold"? If that problem still exists, replacing a faulty vacuum break would correct the hard starting problem as well.

[plrgpr]

Welp, started the stone cold engine this morning with the chokes adj as per Ted, Brad and the service manual. Started right up first time, no coaxing needed! BUT......it was instantaneously running rough. Jumped out and looked at the choke pull offs, right side was fully actuated. Left side was not at all actuated. Manually pulled the pull off rod on the left and the engine smoothed out. Verified that a vacuum was present by unplugging the hose. Ya’all were right.....bad pull off. Funny, though, that it didn’t exhibit any symptoms, other than hard to start, until I adj the chokes. It always ran smooth immediately after it started, once it started.

[Wagon Master]

You can always run the right side choke thermostat rod down (in) to where at full up it leaves the choke blade cracked about 3/16", or whatever it would be if the pulloff worked, until you get a replacement.

[bbodie52]

A faulty choke pull-off could also create a minor vacuum leak on that side, if the failure is due to a torn or perforated vacuum diaphragm internally, or a cracked plastic housing that is permitting an air leak into the diaphragm area.

[plrgpr]

True that Brad. Good suggestion Wagon Master.

[MtnVairMike]

Use a vacuum pump like a Mitty vac and pull 15 inches on the pull off. It should move the lever In fully and hold it until you hit the release button on the pump. With this method I found I had a pull off that was leaky. Took about three minutes for the vacuum to go to zero. The other held it for until I released it. If you have a failed pull off it will not move the lever or hold vacuum.

A lot easier than swapping them.

Try this and report back.

Mike


Sent from my iPad using Corvair Forum mobile app

[plrgpr]

Thanks Mike, but I’ve ordered the pull offs and they will be here tomorrow. In the meantime, I’ve adj the chokes as per the manual and as soon as the engine starts I run back and hold the pull off open for a minute or so and then the engine runs fine. With the chokes adjusted properly, the cold engine starts right up now.