Quadra jet bog 4.3 omc

[ciera]

1987 4.3 omc cobra quadra jet, rebuilt engine 50 hrs on it. Bogging at 3500 rpm, new points, cap and rotor, dwell at 38, timing at 6 deg, 18deg at 2500 rpm.. Electric fuel pump 4psi 35 gal per hour. Engine bogs right out and will die at 3500 rpm soon as second aries start to open. Carb was rebuilt twice. New plugs, wires, cap, rotor, fuel filters, removed anti syphon valve for testing. Also checked exhaust for blockage using inspection camera any ideas on this problem would be greatly appreciated thank u

 

[alldodge]

Still sounds like a carb issue, but what is the cylinder compression numbers?

 

[ciera]

compression 155 on all cylinders

 

[alldodge]

compression 155 on all cylinders

If that's actual, then that's the first 32 year old motor with more then perfect compression

 

[jimmbo]

Float set correctly? Is the Power Piston for the Metering Rods on the Primary side free and able to move? Is the choke pulloff functioning correctly(it dampens the opening of the Secondary Air Valve)?

 

[ciera]

float was set at specs, now it is set at 1/8 inch, power piston is free and able to move. Im currently looking into how to adjust carb secondaries

 

[jbetzelb]

Throwing this out there. I ran into this on my 89. I know you said new fuel filters but do you have a water seperator filter? After I replaced a bunch of other stuff I changed that and problem was solved. I had the best tuned 4.3 around. You will too once you find the solution.

 

[jimmbo]

The Secondaries are Mechanical on a Q-jet. However, they are on a spring loaded shaft, that is locked from opening while the choke is on the slightest. Dirt and Crud can gum up that mechanism, can prevent the Throttles from opening. The Secondary shaft does open in a couple of stages. There is an adjustable preload spring, that closes the Air Valve. There is also a proper spec for each particular carb. the info was usually provided in most Carb Kits. Unless it has been misadjusted by a previous Tinkerer, it doesn't need adjusting. You need to get the specs for your carb, so as to adjust it correctly. However before you go ripping it apart, eliminate all other possible causes. Clogged air bleeds and fuel tubes, leaky gaskets
Have you had someone else drive the boat, while you are observing the carb in action?

 

[ciera]

found the secondaries are opening slightly about 3/8 of an inch when the choke pull off is still pulled in
y understanding of the way they are setup is the secondaries shouldn't open till vacuum is lost. All fuel filters have been changed several times in the course of this season, water separator and the one in the carb
going out this weekend to test it again

 

[jimmbo]

found the secondaries are opening slightly about 3/8 of an inch when the choke pull off is still pulled in

Did you mean the Air Valve above the Secondaries, or the actual secondary throttle plates are opening 3/8"? Yes the choke pull off, on a lot of Q-jets is used to keep the Air Valve closed and to dampen the rate at which it opens. At exactly what level of Vacuum is needed for it to operate varies, depending on what the Engineers speced. With the engine running at idle, the choke pull off should pulled in. Can the Air valve be pushed open that 3/8". It shouldn't affect the engine speed as the Secondary Throttle valves are supposed to be closed. If the Air valve can be moved, then the linkage to the pull off is not adjusted properly, or their is something wrong with the pull off.

 

[PITBoat]

found the secondaries are opening slightly about 3/8 of an inch when the choke pull off is still pulled in
y understanding of the way they are setup is the secondaries shouldn't open till vacuum is lost. All fuel filters have been changed several times in the course of this season, water separator and the one in the carb
going out this weekend to test it again

Secondary air flap adjustment rod? Secondary air flap tension spring? A couple of adjustments Ken Styer addresses in his great rebuild tutorial.

Rod: should keep the secondary air flap from opening until vacuum drops with open throttle...

Spring: too little tension and you get bog; too much and the secondaries open too slowly...

 

[ciera]

Didn't get to take out on water yet did find the vacuum pot that keeps the secandaries closed wasn't pulling all the way in checked vacuum its staying at a steady 20 tried a different pot off an old carb and it seems to keep secandarys closed I guess I'm going to try a sorce a new vacuum pot going out in boat this weekend tieing secandarys closed for now thanks everyone for your help

 

[PITBoat]

going out in boat this weekend tieing secandarys closed for now thanks everyone for your help

That's one way to handle it...

My first car at 16 was a '70 Pontiac with a 370bhp 455; I lost the secondaries function at some point, and I suspected later my dad mighta disabled them somehow... I'll hafta ask him about that sometime.

 

[Silvertip]

At age 80 I have owned more quadrajet vehicles than most folks and the suggestions about the "air valves" opening too early is a classic symptom you are experiencing. When you pop the throttle open and engine bogs, begin pulling back on the throttle gradually. If the engine picks up and eventually gets past (or you feel it tries to get past) the bog, check that air valve adjustment.