Who all runs a q-jet? I currently have a big demon carb but fixing to get a q-jet.


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As big of a rottenchester fan as I am.

GOOD GOD WHY?

GT

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Well....

This is coming from likely the biggest Quad fan there is... I will tell you why in a second.

But first. Here is the deal as far as why I think you are out of your mind on a big inch jet boat... Jet boat engines get loaded like no other. By loaded, I mean that when a jet boat engine stops pulling to a higher RPM, there is literally NOTHING left. The vacuum in the intake is at close to atmospheric pressure as you can possibly get and still move mass quantities of air/fuel., the problem is that a Quadrajet uses "velocity and vacuum" to move the secondary dampers and the metering rods. It is because of this trait that causes the secondaries to be the weak link in a jet boat application, you need to leave quite a lot of potential CFM on the table to get it to work. So that stated, your 800 CFM vacuum secondary carburetor will act like a 700 CFM mechanical secondary Holley, but with no accelerator pumps for the exaggerated secondaries. Basically it will take away the instant acceleration you could experience with a mechanical secondary.

I don't remember the exact vacuum numbers, but want to say that once the vacuum drops below 1.8"H.G. on a Rochester, the gains virtually stop. A Holley mechanical secondary can work at virtually zero..... (Though "zero" is a myth IMHO). But if you were to measure the velocity pressure, it would be off the chart (The Holley on a jet). The Holley survives on velocity only, not vacuum. If you were to actually to a CFM calculation on an 850 DP or even a Dominator, the actual CFM is quite a lot more than the advertized CFM if the system is properly built.


With the advanced cam timing we normally set these monsters up with and the loads they will experience, the manifold vacuum levels will (or should say should) be so low, it does not allow the carburetor to work to it's full potential.

The reason they work so well on a dirt car is that the torque curve is reversed to that of the requirement of the jet. The jet boat engines torque curve is linear, whereas the dirt car is variable.

There will be a few on here that think I am being Mr. Smarty Pants about it, but I say go ahead and try it.... If you remove say an 850 CFM mechanical secondary and install the large bore (800 CFM) Rochester, you will likely experience a major drop in top end performance.


Now back to the reason as to why I am a fan... I worked for a Chrysler Dealer back in the early '90's and was the drive-ability technician and was responsible for approximately 22 Police cars that all had the 360 cubic inch MOPAR monsters and they all had the feedback version of the Rochester 800 CFM carbs. Let me just say that with some off the record tuning, these things could make in excess of 400 HP and still got close to 20 MPG.

When I built my Blazer (1972 CST 4X4) I built one of the secret recipe Rochesters and the thing was an animal, I had an opportunity to install a 1991 TPI injected 5.7 out of a wrecked Corvette with the "D" port heads and all and believe it or not, I pulled the EFI in favor of the Rochester after driving it for a month. With the Rochester, the HP was noticeably better and the mileage increased by almost 25 miles per tank, I was driving it approximately 270 miles every weekend.

I still own the carburetor, but not the Blazer... 

Hope I didn't get too deep there. 

GT

That all makes perfect sense to me. Let me tell u about my demon carb. Really had a bad spot about half throttle. When the secondaries open it would come alive. Primary jets was a 72 and a 74. I put in a pair of 72's and was worse. Put in 76's and was better. This summer I put in 78's and it's even better but still a slight sputter at 1/2 throttle.


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Does it have a constant sputter or just during acceleration?

With that jetting it had a power valve.  Any idea what size?

Unless you are saying it had two different jets in the primaries.  That would be incorrect.  If you are saying 72 primary and 74 secondaries, that would be closer, but still odd.

The secondaries were likely very lean.

I would have tried a 4.5" power valve and 76 primary jets and 78 secondary jets.  The rule of thumb on primary to secondary jet sizes is a difference of 4 normally.  But with them close together you will be able to tell which way you need to go on either side.

If it just happens upon acceleration, you may have too small of accelerator pumps or the wrong pump cam. Doubt that the pump nozzles were too small because Demon carburetors normally over sized the nozzles.

Also too,  what type of timing advance do you have? Meaning do you know what the max and base timing is?

The timing should be "all in" by around 2,500 rpm. It should also transition smoothly. If the weights are sticky, I guess it could jump and do some weird chit.

If you think a Holley type carburetor is difficult to tune, wait until you go to fart with a Quadrajet.

GT

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When my holley had problems I used my spare q-jet and cruising at 3000 rpms it would not hold a steady speed so then the secodaries would kick in and that was too much so you would let off the gas and try to cruise at 3000 rpm and same thing would happen over and over and over, secondaries come in and then you would take off. it was really annoying. I thought it might be the carb and I put it on my dads el comino and it ran fine.  The only thing I can think of is the primaries are too small and its a big difference when the secodaries kick in because there huge. I anit sure of this but its my 2 cents and just wanted to let you know.

Sounds like you have some dirt in the airbleeds or somewhere.  First try cleaning what you have really good.

On a side note, you should check to see if the distributor shaft bearings and gear have excessive play.  If the timing is bouncing around, it could seem like a carb problem.

Not too sure about the air bleed thing, but good call on the worn out distributor.

I am still stuck on the incorrect jetting.

If someone screwed that up, what else did they miss?

GT

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That's the way I bought it last summer.


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It's a torker II


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