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brass monkey

4k Carb Queries

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just a newbie and would like to know, how will i know if my 4k carb second barrel is opening? and could i adjust some bolts or linkages so that it would open sooner? thanks for the insights...

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Not all 4k secondarys are Vac operated some have Mech sec with the flapper valve

 

Weren't they a 3k carb? They didn't have any anti-polution stuff on them.

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It is actually spring controlled, vacuum activated, and mechanically locked out by the first choke on mine.

 

As the first choke lever rotates past about 2/3 throttle it picks up the mechanical lockout arm and moves it down out of the way.

 

This allows the diaphram to open the secondary throttle plate, which is does via a spring on the throttle shaft. So the diaphram is not directly connected to the throttle shaft.

 

You could try removing the lockout plate and see what that does, and if it is detrimental then put it back and change its shape to alter its release point.

 

You could play with the spring strength, which I don't think will do anything, but really you will need a bigger diaphram to make it open at a lower vacuum.

 

I converted my Datsun from a simialr system to a mechanical linake years ago, but I can't remember the details.

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We don't run a spring on the secondarys on our vac sec, But the accelerator pump has a few mods to squirt more fuel so it may be compensating for the sec opening early.

The other factor is my 11 year old races the car and he may not know what a bad carbys with a flat spot is like.

He does say the mech sec are better.

I haven't run this carby on the daily driver, If you want I can chuck it on and give you a real report.

For the street I like altezzaclub idear of a lighter spring, This may do the trick for you.

 

Note

The 4k-c came out with mech sec and the 4k-j had the vacuum sec, I think the 4k-c had the throttle postioner diaphragm and 21mm pri and 25mm sec, The vacuum sec had bigger sec again about 26mm???.

The 4kc carbys are hard to get, The basic body is much the same as the 3k, just a few extra holes to plug up, and remove that hot idle thingey it will give you trouble

 

Here's a pix of our setup on the vac sec carby.

Beware we don't run the standard throtle return spring as that got removed along with all the choke stuff ages ago

Cheers

Andy

post-8902-1279876140_thumb.jpg

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lol about looking down carburetor and revving engine....... what happens if it backfires? no eyebrows.

 

use a mirror!

 

better still most carburetors on a corolla are mechanical secondaries, leave the engine off, open the weighted butterfly and open the throttle and have a look.

 

hmmmm... I think I have or have had at some point one of those vac sec bodies lying around, I thought it was off a 5k.

 

 

Robert.

Edited by rob83ke70

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A simple test is to connect the diaphragm of the secondary lockout to vacuum port and see the action of the secondary, as you open up the main throttle.

 

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