Secondaries not opening quickly

[lushka16]

I have an 88 4.3L Mercruiser with Rochester carbs, and the throttle response is very poor. For a little while, full throttle would put me at 3200RPMs, and it would not exceed that unless I opened the secondaries myself. Now, they seem to be opening, albeit slowly. This problem appeared after I rebuilt the carb.

The vacuum actuator seems to be working well, it would appear that the mechanical linkage is in the wrong position. Here is a picture of it with the motor off.


Yes, I know my backfire arrestor is dirty, it's from a blown belt

Any ideas?

 

[Silvertip]

Re: Secondaries not opening quickly

I think you misunderstand how the Rochester Q-jet secondaries operate but we will get to that later. First -- with the engine off, push the throttle lever wide open. Manually open the choke plate on the primary and see if the throttle plates are standing vertical. If they are that's good. If not, you either have a linkage adjustment problem or the linkage was assembled wrong. Now open the secondary air valve and see if the secondary throttle plates are standing vertical. If they are that's good. If not, you need some pictures to get the secondary linkage connected properly. Now for the tricky part. The secondary air valve (that's the plate at the top of the carb over the secondaries, does not work through a linkage. It's called an air valve secondary for a reason. With the throttle wide open and the engine under load, the secondary throttle plates (not the air valve) will obviously be open. Vacuum below the air valve and air pressure above the plate causes the air valve to open in varying amounts and it now begins to add fuel. They dont's snap open and snap closed. This secondary system is load dependent, not mechanically connected to the primaries. The vacuum diaphram is called a choke pull-off. It opens the primary choke plate slightly after the engine starts. The choke stove pulls the choke open fully as the engine warms up. And you have one carb -- you don't have carbs. If you don't have a service manual I suggest you at least visit the local library and check the carburetor section of any GM V8 4BBL auto service manual for any car in the 70's - 80's. Full theory and pictures are usually provided. The secondary tension spring controls how easily or how difficult the air valve opens. If that spring is weak, missing or misadjusted you will have very erratic secondary operation.

 

[xltier]

Re: Secondaries not opening quickly

in addition to silvertips great explanation.if any part of the linkage is in any way choke mode,the secondaries wont open.the ckoke has to be totally relaxed.

 

[Coors]

Re: Secondaries not opening quickly

He He, and here we go. The Q jet is forgiving, never letting you open the secondaries until the engine needs it's flow. People that install Hollies, or any other carb, and you can feel the exceleration, all of the sudden, or the sudden noise of the secondaries opening, is due to a mis-adjusted carb- it was bogging before the engine caught up with the un-needed air. And that actually makes you slower (et, and mid-range).
Boom can tell you

 

[Coors]

Re: Secondaries not opening quickly

Just like Silvertip said; I type slow.

 

[smithrock]

Re: Secondaries not opening quickly

Not to hijack but, When my secondaries open (edelbrock carb) under full throttle I notice hesitation then it will go away and then come back. I took off the flame arrestor to look at it and that's when I noticed on the rear of the secondaries, one of the jets it kinda sputtering out fuel sporatically. It's not spraying like the other 3 are. Is this a simple fix?

 

[lushka16]

Re: Secondaries not opening quickly

Thank you for the great explanation. I actually do have the service manual, but to be honest, I have a hard time understanding carb theory unless it is said plainly (like you said) or physically shown to me. Also excuse my nomenclature, my motorcycle has 4 carbs, so I'm used to saying carbs, even if I mean just one.

I checked the throttle plates, both sets are standing completely vertical. The secondary air valve moves very freely, so I don't believe I've assembled it wrong.

Where might I find this spring that you mentioned?

 

[Coors]

Re: Secondaries not opening quickly

It's hard to explain; google it, it will be there; try chevrolet hot rod.

 

[Coors]

Re: Secondaries not opening quickly

Not to hijack but, When my secondaries open (edelbrock carb) under full throttle I notice hesitation then it will go away and then come back. I took off the flame arrestor to look at it and that's when I noticed on the rear of the secondaries, one of the jets it kinda sputtering out fuel sporatically. It's not spraying like the other 3 are. Is this a simple fix?

Rebuild of carb, and seafoam or berryman in tank; including changing filters regularly

 

[smithrock]

Re: Secondaries not opening quickly

I actually don't have a fuel filter hooked up right now. I will install one, does it go before the fuel pump or after (hose to carb)?

 

[lushka16]

Re: Secondaries not opening quickly

I see the secondary air valve adjustment spring on the manual. It's raining right now, so I can't check to see if there's anything wrong with it, but at least I know where to look. Would playing with the secondary air valve adjustment shaft do anything worthwhile?

 

[Silvertip]

Re: Secondaries not opening quickly

I just know it was going to be asked. To find the air valve spring, look at the end of the air valve shaft, sort of tucked up under the casting. If the air valve opens with little pressure and moves freely it is probably ok.

 

[Coors]

Re: Secondaries not opening quickly

Lol silvertip, there is a 2 1/2 turn involved for starters, but the info for tweaking is on line.
guys, read it, and go from there. That big choke-looking plate above the secondaries, controls your HP an engine running good.
Read up, and life will be good. It is on-line, probably in a chevy room; as soon as you understand it, it is a piece of cake to tune.

 

[Coors]

Re: Secondaries not opening quickly

Not to come out wrong, Silvertip- that was directed at the newbies with q jet; It is hard to explain with out pics, and hands-on.