vacuum secondary

[79datsun620]

hey guys,i've got a 79 620 with a stock carb on it and i think my secondary is only opening sometimes i geuss maybe because of the high lift cam i've got poor vacuum.do you think i can remove the spring in the vacuum diaphram or maybe cut a little off to make it open with less vacuum.have any of you guys had this prob. and have a way to fix it.

[Mandingo]

I have a 79 also, my carb seems a bit wimpy, not sure even if my secondary activates, how do you test it?

[DanielC]

The reason for a vacuum secondary is that the engine does not move enough air through the carb. venturi to correctly meter the fuel, until the rpm is high enough to need both barrels of the carb open. Making the secondary spring weaker will cause it to open sooner, and driveability may suffer. The secondary should be opening when the engine rpm is high enough. Are you sure it is not opening now? Once you are off idle, your vacuum should be good, if the rest of your engine is in good shape.

An old, but very good piece of advise:

Do not over cam, or over carb your Datsun

[banzai510(hainz)]

if you make the spring weaker you might have low end proplems. bogging out mainly.

 

Does it feel like it running out of gas.? fuel filter?

Adjus the timming, mixture in the carb.

 

One way to ck the secondar is push on it to make sure its not jammed up.

then maybe rev the motor high then put your hand over the main barrel and for a second or so the seconday will open due to all the vacuum.

 

what is a high lift cam size?

 

Big cam stock carb is a Oxymoron

 

you still running a stock aircleaner assembly? You might get more volume running a stock set up. But what made you determine its a vacuum propelm?

Edited December 17, 2008 by banzai510(hainz)

[datzenmike]

The vacuum for the secondary opening is a the combined vacuum across both venturies and not the manifold vacuum at all. In other words, the if secondary opening were dependent on manifold vacuum, it would try to open on deceleration or idle when it is high and not want to open when vacuum is low like full throttle. Venturi vacuum is dependent on how much air is passing through it and can be high even if the manifold vacuum is low.

 

It is possible that the diaphragm is leaking or torn.

Edited December 17, 2008 by datzenmike

[Mandingo]

So if you rev the engine and look down the carb will you see the secondary squirting gas?

[Llittle_Llama]

i would think only under load, i am most likely wrong though.

[DanielC]

You probably cannot get the secondary to open without a load on the engine. The secondary has no accel pump, so the only way gas flows in the secondary, is if there is enough vacuum in the venturis. Unless you have an engine or a chassis dyno, no way will you be able to load the engine enough to open the secondary.

[datsunfish]

I once made a mechanical secondary linkage for a stocker. I have also cut and weakened the spring before. Both worked out just fine. If the secondary doesnt open than you would definitely know by driving it around. Your rpm range would be very low and you would run really weak under a load.

Like Hainz said though,A big cam and a stocker dont make much sense. It would make about as much sense as running points and resistor with twin sidedrafts.

[79datsun620]

allright i know its kinda silly runnin the stock carb with a big cam (maybe 280

460 i can't remember?) i've got a set of su's to go on it but i'm kinda intimidated by the linkage,manuel choke,syncronizing and all that mess.the reason i think its not opening is because i'll be in the throttle and it accelerates kinda slugishly and then boom! it peps rightup and go's like it should.everthing else should be working fine its all new i've just recently re-built the motor.thanks for all you guys imput i'm sorry i did'nt reply sooner but my "encheranet" is down at my house.

[banzai510(hainz)]

i've got a set of su's to go on it but i'm kinda intimidated by the linkage,manuel choke,syncronizing and all that mess.

 

I dont blame you brother!!!!!!!!!!!!!!!

 

I just need to know the cam size. The latter l20stock carbs ,people say they almost flow as much as a Weber DGV.

I had a 460/270 Scheider and it ran on my Weber DGV carb just fine(L16). Matter of fact they should have put that cam size in there stock!!!!!!!!!!!!

matter of fact a L20 and a cam with stock carb that 2ndry should be OPEN when your motoring down the road

 

If this was a propolem changed all of a sudden or was already there? Ask your self this first.

 

try the rev the motor up then put your hand over the main barel of the carb,The secondary will should open up as this is the only opening.,How much i dont know.?

 

Maybe the jet has dirt floating around. Maybe the chain is off a link or just alot of strectch?

 

I would go thru and adjust the valves,do a tune up.ck the cam timming and use a timming light. maybe adjust the dizzy and run it hard then do it again.

[Datsun720]

Sounds more like an issue with the primaries, you say it feels sluggish at first and then it picks it, the point where it picks up sounds like the point where the secondaries are opening, alleviating the primary issue. try going a little richer on the primary side, if that doesnt help or u see soot coming out the tailpipes,try going smaller a little at a time.

[datsunfish]

Cant believe I forgot to ask this. Did you check for vacuum leaks. They cause all sort of problems similar to this. Once your opened up the air is moving fast enough to fly past the leak. Check it.

[Jester]

This is GOOD STUFF for me. My Hustler does the exact same thing. I just wrote it off as a bad carb throttle shaft causing a vacume leak. Curious if you checked that yet. If yours is good, I'll just sit back and listen.

[79datsun620]

actually,after driving it the other day i shut it off and i heard a sizzling noise coming from under the hood,i looked and there was stuff bubbling out from around the throttle shaft.so maybe that is my problem too.