Quadrajet flat response berfore secondaries open

Hi
Struggling with my rebuilt qudrajet, and it is improving.
But something is littlebit wrong; just before the seconaries are opening the reponse are very flat. I dont know which way to adjust, is it a lean condition on primaries or ? Its not the windup spring thats holding back its the vacuum valve, and probably it takes a little bit to long time to reach rpm to let the low vacuum realse the valves. When the secondaries open its all right.

Also if I push the throttle to max from standstill it will accelerate fine until i reach the flat spot, and then i have to pull back a little.

OMC Cobra 1988 5.7l, 260 hp
prim 71, rod 45, second rod CL, Hanger E

Re: Quadrajet flat response berfore secondaries open

Sounds like the secondary air valve is opening too late. There should be a smooth overlap of secondarys opening as the primarys are reaching wide open; Q-jets not designed to run WOT on primarys alone, hence the flat spot.

Re: Quadrajet flat response berfore secondaries open

yup, probably to late, but its steerd by the vacuum, and the actual vaccum is only dependent on the valve opening and rpms. So i think my primaries is to small to rev up to correct rpm for vacuumbreak to open butterflies. Or Not??
The sync between primary and sec valve seems to be OK.
Still confused...

Re: Quadrajet flat response berfore secondaries open

concentric said:

yup, probably to late, but its steerd by the vacuum, and the actual vaccum is only dependent on the valve opening and rpms. So i think my primaries is to small to rev up to correct rpm for vacuumbreak to open butterflies. Or Not??
The sync between primary and sec valve seems to be OK.
Still confused...

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Maybe its because I have been away from the q-jet for a little while or your use of terms, or both, but it is not clear to me what you are saying.

Some baseline questions first:

- Is this the original carb off your boat and you performed a rebuild and now trying readjust? If so, have you changed any of the jets or rods from stock?

- Or is this a generic marine q-jet rebuild supplied by someone that you are trying to tune to your engine?


The q-jet has mechanically linked primary and secondary butterflys. When the throttle has been moved passed the point where the 2ndarys begin to open, then when enough vacuum is produced from the engine to overcome the spring force, the air valve plate will begin to open.

Is the flat spot coming before, during or after the throttle position where the 2ndarys begin to open? I assume after, but not sure from your description.

My 1st thought was the same as zbnutcase, but maybe the linkage between primary/secondarys has been messed with. ie arm bent from original stock.

Re: Quadrajet flat response berfore secondaries open

The seconday butterflies -only- open when there is sufficent vacuum to overcome the spring that holds them shut. It is independant of RPM, you will notice that the secondaries never open when you just rev it up in neutral.

If you have a flat spot before the secondaries open up, then you need to look at the primaries and figure out why. The primaries should be able to deliver the proper air/fuel mix from idle to WOT even if the secondaries -never- open. You won't get full power from the boat without the secondaries but it should run well and not hesitate or bog.

On an older boat, I'd say you have crud in there somewhere restricting the fuel flow in the primary circuit causing a lean condition at higher load and RPM. But you said you have already been through the carb. In any case, get the primaries sorted out and you should be golden.

Is there tech support for the Q-Jets? I don't suppose Edelbrock will answer questions about a carb they didn't build or sell. :-(

Re: Quadrajet flat response berfore secondaries open

Its marine and probably the original carb, but someone before me have "tuned" it with BIG jets and small rods, very crappy then, but now better.
And yes the flat spot comes when the mechanical valves have begin to open.
Still the problem is as I understand that when the flat spot comes the vacuum break have not yet released the butterflie valve. Waiting it out the vacuum break releases and the butterflie gently opens, no smooth and quick transition.
Seems to be a problem with the primaries you can think, but there is also some amount of air/fuel delviered by the secondarys by the little gap in the valve and the holes in the butterflievalve. So maybe it could be a question of another higher hanger to get moore fuel from start.
Sorry for rambling on with this... It should not be starvation on primary side cause i?m 10 % richer then mercruiser spec for 350 260 hp, and 2 % richer then OMC spec found on iboats forum.

Re: Quadrajet flat response berfore secondaries open

SeanMcl said:

The seconday butterflies -only- open when there is sufficent vacuum to overcome the spring that holds them shut. It is independant of RPM, you will notice that the secondaries never open when you just rev it up in neutral.

Click to expand...

You are confusing the air valve which sit over the butterflies.

Re: Quadrajet flat response berfore secondaries open

Maybe I've been away from mechanics too long but I've rebuilt a ton of those carbs in my automotive days. The secondary throttle plates as I remember are mechanically linked and open in correlation to the primarys at a certain throttle angle. Vacuum never comes into play. If you look down the carb at the secondary throttle plates and open the throttle to full with the engine off you willl see that they are wide open. (You'll have to push open the "air door" plates above them to get a look) The only thing that gets opened by vacuum or suction off the engine is the "air doors" located above the secondary throttle plates.
Saying that, there are numerous critical adjustments that need to be done when rebuilding that carb. The slightest error in being "off spec" will lead to a performance problem. They are a fairly straighftorward carb but at the same time easy to screw up if you don't get it perfect.

Re: Quadrajet flat response berfore secondaries open

fossill said:

Maybe I've been away from mechanics too long but I've rebuilt a ton of those carbs in my automotive days. The secondary throttle plates as I remember are mechanically linked and open in correlation to the primarys at a certain throttle angle. Vacuum never comes into play. If you look down the carb at the secondary throttle plates and open the throttle to full with the engine off you willl see that they are wide open. (You'll have to push open the "air door" plates above them to get a look) The only thing that gets opened by vacuum or suction off the engine is the "air doors" located above the secondary throttle plates.
Saying that, there are numerous critical adjustments that need to be done when rebuilding that carb. The slightest error in being "off spec" will lead to a performance problem. They are a fairly straighftorward carb but at the same time easy to screw up if you don't get it perfect.

Click to expand...

x2- also make msure the choke linkage is open all the way.the secondary throttles will bypass when the choke is on any amount.at least all i have seen.

Re: Quadrajet flat response berfore secondaries open

Yup, the secondary plates are mechanically locked out unless the choke is open.

Re: Quadrajet flat response berfore secondaries open

concentric said:

It should not be starvation on primary side cause i?m 10 % richer then mercruiser spec for 350 260 hp, and 2 % richer then OMC spec found on iboats forum.

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I am not familiar with the OMC configuration, but the Merc is known for being on the rich side. Have you tried to run with a Merc jet/rod or the OMC stock config? A flat spot does not mean it is too lean.

I have some q-jet tuning info I will look for this evening and pass the info on. I believe you are correct abot the vacuum break, but will check some info tonight. I don't believe there is any adjustment to it, either.

Re: Quadrajet flat response berfore secondaries open

I thought that the problem was in the primaries because you said in your first post "But something is littlebit wrong; just before the seconaries are opening the reponse are very flat."

If there is an issue before the secondaries open, then the issue is with the primaries.

Now you say "And yes the flat spot comes when the mechanical valves have begin to open." Which sounds like the trouble is in the secondaries.

As soon as you can narrow down what the issue is you will get better help.

In any case, I was taught to jam the secondaries closed and get the carb running well from idle to WOT, then allow the secondaries to open and fix any lean/rich condition by adjusting only the secondary springs, jets, rods, what have you. That is what I would do.

Good luck, and keep us posted.

Re: Quadrajet flat response berfore secondaries open

imported_John_S said:

You are confusing the air valve which sit over the butterflies.

Click to expand...

I meant the secondary throttle plates, but I was trying to use his terms. He talked of the butterflies opening at a certain RPM, and I followed his lead. My bad.

Re: Quadrajet flat response berfore secondaries open

sounds like the secondaries are opening too early to me.
too early causes lean condition, not late.

Need to go the other way on the adjustment and tighten up the spring.

Re: Quadrajet flat response berfore secondaries open

Ok, a lot of missunderstanding due to my sloppy language.
I meant flat spot when upper valves not yet are open, but second. throttle valves (guided by linkage) are opend.
On a hard acceleration for sure the secondary throttle valves are open to fully open, guided by the linkage. The linkage is fine cause att fully open primarys (90?) the secondarey plates are about 85? (as they should)

Re: Quadrajet flat response berfore secondaries open

concentric said:

the secondarey plates are about 85? (as they should)

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You mean the throttle plates on the bottom? or the air flaps on top?

I'm no Qjet expert, but as I understand the adjustable spring for the secondaries is to prevent the air flaps from allowing full air.
i.e. they should not just open on hard acceleration.
There is a tuneable delay by adjusting the secondary air flap spring.

If all 4 barrels are wide open on accel, that would lean the mixture out and cause a wicked bog

Re: Quadrajet flat response berfore secondaries open

imported_John_S said:

I am not familiar with the OMC configuration, but the Merc is known for being on the rich side. Have you tried to run with a Merc jet/rod or the OMC stock config? A flat spot does not mean it is too lean.

I have some q-jet tuning info I will look for this evening and pass the info on. I believe you are correct abot the vacuum break, but will check some info tonight. I don't believe there is any adjustment to it, either.

Click to expand...

Here is some tuning tips that I found useful in understanding the q-jet: http://www.lbfun.com/warehouse/tech_info/fuel & intake/Q-Jet Tuning Paper.pdf

The same info can be found on a number of different sites.


Unless you have made other engine mods from stock, would find, beg, borrow or steal, the omc service manual for your engine. Set it back to stock config. Make sure your engine is in good mech condition, initial and advance timing working correctly. Then go back to tuning the carb, if needed.

Tuning to an excessively rich primary, doesn't make sense. I think you will end up redoing it all over again. I don't understand why the omc would be that much richer than a Merc. Maybe wrong info, and why a service manual needs to be had. The 350 Mag does have richer primaries, but that is to help compensate for the intake.

I did take a few minutes to look at my carb while I was doing some other work. Mine is a latter model with electric choke. It was a marinized carb from a rebuild facility. Not the original on my engine. While the Merc service manual shows a measurement gap for the air valve dashpot, that is adjusted by bending the rod, mine has an adjustment screw. The windup spring is adjusted using a gram scale. I used a stack of nichols and paper clip to check what mine was set to.

Re: Quadrajet flat response berfore secondaries open - solved!

Re: Quadrajet flat response berfore secondaries open - solved!

Just letting you know.
Stepped down to original merc 5.7 spec; 68/71/CL and that did some improvement but not good, same hestitation and flat spot.
Still had to look into the power valve spring. measured vacuum at idle to 13" Hg and choosed a spring from the Edelbrock 4-spring kit 6" Hg.
This spring is longer and stiffer then the swaped out spring.
Out on the lake i pushed the throttle to max and get a nice smooth acceleration all the way to air valves fully open.
Conclusion: a overly soft power piston spring can give a hesitation and lean condition in mid throttle.
I suppose the previous owner had the original carb swapped for a rebuild. Maybe the rebuild was of an carguy used to speed tuned american muscle cars and therfor used to light springs and big jets.