Secondary Carb Question

[Gasman63]

I know Ive read or asked this before, probably Steve G. told me, but...

The secondary carbs only open up when you have your foot to the floor. But isnt a certain amount of RPM needed too? Without having an excelerator pump the fuel wont enter the engine without the air flow to pull it out of the carb. I know Im not using all the proper terminology but I hope Im making myself clear

So would rpm be what is needed to get the secondary to work and how much, or speed, or am I just not understanding the function correctly?

I had plans to have Steve G. fix up a set of four primary carbs for me as he has done many times but at this time Im just wanting to understand more. BTW, is anyone running this 4 primary carb setup? Opinions?

Thanks.

[pete]

I have a 65 so the secondary’s do have accelerator pumps installed. The linkage is progressive and the secondary’s start opening at about 50% throttle and quickly go full open as throttle is positioned full open. No specific RPM or speed will change that. Plenty of vacuum at all operating conditions to pull fuel out of bowl. The engineers designed these to function at all operating limits/conditions and they work just fine if properly tuned and maintained. No different than many other carbs with mechanical secondary’s such as Quadrajet and Carter AFB. Good day all.

[Dennis66]

Think of it as a big 4 barrel carb. "Certain RPM" would be more like a vacuum operated secondary 4 BBL.

[66vairguy]

Good question and answers. On some big engines the secondaries will NOT open if you go full throttle at low RPM, usually a vacuum device is used. This was done to prevent hesitation or a drop in power at lower RPM using full throttle. THIS IS NOT THE CASE FOR THE CORVAIR SECONDARIES. In 1966 there was a secondary "lockout", but it was related to cold operation when the primary chokes were on.

NOTE: The Corvair secondaries were revised often to deal with "operator problems". According to the books, Chevrolet did extensive testing of the "4 carburetor" design and the setup worked well and was reliable, BUT the testing was done under "demanding" test track conditions that did not take into account "daily driving" use which resulted in issues!!!

Here is an article from one of the local club newsletters that is a summary of information from different Corvair books that has a year by year explanation of changes towards the end of the article. It also discusses the problem of not being able to get the secondaries fully open due to linkage slop between the gas pedal and engine.

See

[66vairguy]

Oh!!! I just remembered years ago Steve Goodman started adding idle mixture circuits to 65 and 66 secondaries to the engines to pass emissions testing (since then Colorado has dropped "high altitude" area emissions testing for old cars - so I've been told). Per the article this was a feature on the 68 and 69 secondaries, although they are not common.

Steve would replace the secondary bases with 62-63 bases and adjust the secondary idle mixture to eliminate an idle misfire common to the 140HP when the primary idle mixture was adjusted to PASS inspection. I'll add that to get the idle speed down I kept the secondary "solid" throttle plates and did NOT use the "notched" 62-63 throttle plates.

[gbullman]

@Gasman,

You can open them at pretty much engine speed (assuming the engine is warmed up). From my experience you want to be at at least 3000 RPM, 3500 better before you put your foot in it. It sill winds up pretty good at lower RPMs but you definitely feel it kick in more 3500 and above. I think the coolest time to put your foot in it is 4th gear at highway speeds, it pulls really well for a 59 year old car.

And I do open my secondaries often!

[Gasman63]

I see you do put your foot in it Gary. Lol

Thanks for the attachment 66. I will give ita read tonight.

I was wrong. Its been a while since i read to carb book i have. Its the idle circuit that is missing not the pump.
Im not into going fast so i normally shift between 2800 and 3200. I guess I was looking for more in town/ off the line, between gears speed.

I brought this up because I havent noticed much gain when I put my foot in it. At one time I blocked off my secondary carbs and ran with just primaries . After unblocking I dont think I gained much if any. I guess I will try it at higher RPMs. Im just not sure how accurate my factory tach is.

I think bring a truck driver and running much lower rpms all the time Im afraid to push it up as it sounds like its winding too high to my ear. Or Im just a sissy. Ha.

[gbullman]

I see you do put your foot in it Gary. Lol

Thanks for the attachment 66. I will give ita read tonight.

I was wrong. Its been a while since i read to carb book i have. Its the idle circuit that is missing not the pump.
Im not into going fast so i normally shift between 2800 and 3200. I guess I was looking for more in town/ off the line, between gears speed.

I brought this up because I havent noticed much gain when I put my foot in it. At one time I blocked off my secondary carbs and ran with just primaries . After unblocking I dont think I gained much if any. I guess I will try it at higher RPMs. Im just not sure how accurate my factory tach is.

I think bring a truck driver and running much lower rpms all the time Im afraid to push it up as it sounds like its winding too high to my ear. Or Im just a sissy. Ha.

I get my off the line secondary action as well. Merging with traffic I’m usually rolling in first, basically time it takes to get my foot in it in 2nd I’m in the RPM range, wind it up to 5200-6000 in 2nd, shit into 3rd and foot right back in it. The difference isn’t like a 4 BBL Small Block but it gets moving pretty good and is a lot of fun banging through the gears.

[66vairguy]

I was wrong. Its been a while since i read to carb book i have. Its the idle circuit that is missing not the pump.
Im not into going fast so i normally shift between 2800 and 3200. I guess I was looking for more in town/ off the line, between gears speed.

I brought this up because I havent noticed much gain when I put my foot in it. At one time I blocked off my secondary carbs and ran with just primaries . After unblocking I dont think I gained much if any. I guess I will try it at higher RPMs. Im just not sure how accurate my factory tach is.

The secondaries are more noticeable around 3,000 and up RPM. Nothing wrong with shifting around 3,000 around town, and why I tell folks for most driving conditions the two carburetor 110 HP is a great engine.

Of course I like showing off those four carburetors at shows, but don't use the secondaries much due to all the traffic in my area. I DO find the opportunity open the secondaries at least once on each drive to run fresh fuel into the bowls (although the old gas dries up sitting).

Just a thought, check all the linkage and carburetor opening action. The 140 HP linkage take some "tinkering" to make it work well. Also make sure the distributor is working (and the correct number). A timing light and revving in neutral to about 2,5000 will tell you if the distributor is advancing (Clark's sells the distributor timing vs. RPM booklet).

While the EM under car throttle linkage bushing are "stiffer" compared to LM, they do wear out and you loose linkage travel.

Good luck.

[66vairguy]

I was wrong. Its been a while since i read to carb book i have. Its the idle circuit that is missing not the pump.
Im not into going fast so i normally shift between 2800 and 3200. I guess I was looking for more in town/ off the line, between gears speed.

I brought this up because I havent noticed much gain when I put my foot in it. At one time I blocked off my secondary carbs and ran with just primaries . After unblocking I dont think I gained much if any. I guess I will try it at higher RPMs. Im just not sure how accurate my factory tach is.

The secondaries are more noticeable around 3,000 and up RPM. Nothing wrong with shifting around 3,000 around town, and why I tell folks for most driving conditions the two carburetor 110 HP is a great engine.

Of course I like showing off those four carburetors at shows, but don't use the secondaries much due to all the traffic in my area. I DO find the opportunity open the secondaries at least once on each drive to run fresh fuel into the bowls (although the old gas dries up sitting).

Just a thought, check all the linkage and carburetor opening action. The 140 HP linkage take some "tinkering" to make it work well. Also make sure the distributor is working (and the correct number). A timing light and revving in neutral to about 2,5000 will tell you if the distributor is advancing (Clark's sells the distributor timing vs. RPM booklet).

While the EM under car throttle linkage bushing are "stiffer" compared to LM, they do wear out and you loose linkage travel.

Good luck.

[Gasman63]

Thanks for the reply Bob and everyone.
I will go though the linkages. I did replace a couple bushings but i know i do have some worn connections on top.
I will do more testing running the engine up a bit more for the secondary carbs to open better and get more response from them. Just need to get home and find the time to tinker.

[jimbrandberg]

Gary does what going into 3rd...
I've printed off the Bob Nichols article for posterity, looks like good reading later.

[Gasman63]

[Torskdoc423]

Take off your shoes before you go in the house, Gary.

[Gasman63]

As usual, you guys were right. If I quit shifting at 3,000 RPMs then I find secondaries actually do make a big difference. At 3500 I put my foot to the floor and she really takes off.

It's just hard for me to run the RPMs up that high but I guess I will have to learn to put my foot in a little more often if I want to go quicker.

Im thinking of a deeper toned exhaust system might make me feel better about higher RPMs. Maybe.

[66vairguy]

As usual, you guys were right. If I quit shifting at 3,000 RPMs then I find secondaries actually do make a big difference. At 3500 I put my foot to the floor and she really takes off.

It's just hard for me to run the RPMs up that high but I guess I will have to learn to put my foot in a little more often if I want to go quicker.

Im thinking of a deeper toned exhaust system might make me feel better about higher RPMs. Maybe.

In a convertible you hear more engine noise. Air cooled and aluminum engines make more noise. The Corvair engine can be run up to 4,000 without any issue if in good shape. Shifting maximum of 5,000 RPM is good for stock valve springs - more than that and you are getting into valve bounce issues. Racers use higher tension valve springs.

Yes at least once per trip you should OPEN the secondaries for a second or more - other wise they get "gummed up". NEVER open and immediately close the secondaries SUDDENLY. This causes the throttle plate to get covered in gummed up gasoline - an issue when these cars were new that lead to deleting the accelerator pump cup in the secondary and the INFAMOUS secondary lock out with choke on mechanism. I'll add Roger Parents lock out linkage works better than the factory set up!
Have fun!