Problems setting up a Quadrajet on Mercruiser 7.4 Bravo 2

[Mischief007]

Hello everyone,

I recently rebuilt my quadrajet carb on the 7.4L Bravo. I had to do this due to a dirty fuel issue. Since the rebuild I am struggling to set the idle air screws, she starts and runs with the screws 3 turns out but if I knock off the engine the hot starts are horrible and take minutes of cranking.

I attached a Vacuum Gauge to the manifold (via the square plug in the manifold behind the carb) and can’t see much difference wherever the screws are set. The best Vac I can achieve is 10-12 hg… any ideas what I have missed here?

The engine is only 95 hrs after a full rebuild and runs great… once started…the hot starting is the problem I really want to crack.

 

[kenny nunez]

Was that happening before the contaminated fuel issue? Next time you shut off the engine open the engine cover, remove the flame arrestor and look down the air horn to see if gas is dribbling into the Venturi.

 

[jhande]

I can only relate to automotive applications as I was an auto tech for 36+ years. Most of my career involved engine rebuilding and tuning.

If the 454 cu. in. was rebuilt to factory specs and does not have a racing cam, then...

A vacuum reading from a direct intake manifold should be 18-22 Hg. A steady 10-12 Hg reading would concern me a bit.

First let's address the good cold start, difficult hot start.
Is the choke operating as it should? Closed cold, open wide hot?
Once the engine is warmed up is the timing properly set (probably is)?

Adjusting the Q-jet...
Turn one idle/air mixture screw at a time. Turn it in 1/4 turn and watch the vacuum gauge. Continue with 1/4 turn increments until you see the vacuum start to drop, turn it back out a 1/4 turn. Repeat procedure for the other screw. Now set the idle screw for around a 750 - 800 RPM.

If the choke is functioning properly, it's idling fine, the timing is good but the vacuum is still to low I would look for either a vacuum leak or an oop's in rebuilding the carb. You can use a spray bottle filled with water for testing vacuum leaks. With it warmed up and idling start by squirting water along the intake and heads. Along the bottom parts of the carb and all vacuum hoses. When a leak is detected you will hear the engine slightly raise in RPM's and smooth out a bit.

I would run thru those things before digging into the carb.

OOP's
I would first do as Kenny suggested.

 

[kenny nunez]

The vacuum readings, are they with the boat in the water or on a trailer?
The last 454 I had would idle @ 14-15 inches on the trailer, it was a Holley, when the boat was in the water it was only around 8-9 because of the back pressure of the exhaust underwater. It was a bone stock 330 hp marine engine.

 

[Mischief007]

Was that happening before the contaminated fuel issue? Next time you shut off the engine open the engine cover, remove the flame arrestor and look down the air horn to see if gas is dribbling into the Venturi.

The engine was running fine after its rebuild but the yard that did the work neglected to refit the stock water separator and fed the fuel line straight to the pick up pump. (This was just before I bought the boat). The boat had spent its life on a river. Not being familiar with the Merc’ 7.4 i didn’t notice the filter was missing I took the boat out to sea which stirred up the sediment from the bottom of the tank (rookie error I know). So I stripped and rebuilt the carb and fitted an external water separator unit. The starting issues have only shown since the rebuild.

Can’t see any fuel leaking down the Venturi.

 

[Mischief007]

I can only relate to automotive applications as I was an auto tech for 36+ years. Most of my career involved engine rebuilding and tuning.

If the 454 cu. in. was rebuilt to factory specs and does not have a racing cam, then...

A vacuum reading from a direct intake manifold should be 18-22 Hg. A steady 10-12 Hg reading would concern me a bit.

First let's address the good cold start, difficult hot start.
Is the choke operating as it should? Closed cold, open wide hot?
Once the engine is warmed up is the timing properly set (probably is)?

Adjusting the Q-jet...
Turn one idle/air mixture screw at a time. Turn it in 1/4 turn and watch the vacuum gauge. Continue with 1/4 turn increments until you see the vacuum start to drop, turn it back out a 1/4 turn. Repeat procedure for the other screw. Now set the idle screw for around a 750 - 800 RPM.

If the choke is functioning properly, it's idling fine, the timing is good but the vacuum is still to low I would look for either a vacuum leak or an oop's in rebuilding the carb. You can use a spray bottle filled with water for testing vacuum leaks. With it warmed up and idling start by squirting water along the intake and heads. Along the bottom parts of the carb and all vacuum hoses. When a leak is detected you will hear the engine slightly raise in RPM's and smooth out a bit.

I would run thru those things before digging into the carb.

OOP's
I would first do as Kenny suggested.

Hi, thanks for the reply… the choke is coming off as it should and both blades opening right up, it a simple type with a bi metal coil spring which opens up with the heat from the head.
I’m not sure what level of tune Mercruiser put on these marine conversions but the vacuum gauge needle jumps around a lot.

I can screw the needle screws all the way in and way out again and little much changes… the engine still idles pretty smooth with the screws all the way in at 700rpm. Could the needles be worn? The look ok to the eye.

I will try the trick with the water spray thanks.

 

[Mischief007]

The vacuum readings, are they with the boat in the water or on a trailer?
The last 454 I had would idle @ 14-15 inches on the trailer, it was a Holley, when the boat was in the water it was only around 8-9 because of the back pressure of the exhaust underwater. It was a bone stock 330 hp marine engine.

Hi Kenny,

The boat is in the water, she is a 2655 Ciera Sunbridge. I take her out every winter but otherwise she is on a pontoon berth.

 

[Mischief007]

Hi, thanks for the reply… the choke is coming off as it should and both blades opening right up, it a simple type with a bi metal coil spring which opens up with the heat from the head.
I’m not sure what level of tune Mercruiser put on these marine conversions but the vacuum gauge needle jumps around a lot.

I can screw the needle screws all the way in and way out again and little much changes… the engine still idles pretty smooth with the screws all the way in at 700rpm. Could the needles be worn? The look ok to the eye.

I will try the trick with the water spray thanks.

The vacuum readings, are they with the boat in the water or on a trailer?
The last 454 I had would idle @ 14-15 inches on the trailer, it was a Holley, when the boat was in the water it was only around 8-9 because of the back pressure of the exhaust underwater. It was a bone stock 330 hp marine engine.

That makes a lot of sense Kenny, hopefully it’s as simple as that.. I’ll raise the drive leg all the way up to see if the reading changes..

 

[nola mike]

That makes a lot of sense Kenny, hopefully it’s as simple as that.. I’ll raise the drive leg all the way up to see if the reading changes..

No, that's not what he meant. "Out of the water" means running on muffs, not with the outdrive raised up.

 

[Mischief007]

No, that's not what he meant. "Out of the water" means running on muffs, not with the outdrive raised up.

Yeah I realise that but if I raise the leg closer to the surface of the water there will be less back pressure so i should see some change on the Vac gauge…

 

[kenny nunez]

Make sure the flush out is secure on the lower unit.

 

[Mischief007]

Was that happening before the contaminated fuel issue? Next time you shut off the engine open the engine cover, remove the flame arrestor and look down the air horn to see if gas is dribbling into the Venturi.

Hi again Kenny, I had another look today after giving the boat a good work out, although I cannot see any fuel dripping I did get white fuel vapour coming up past the choke flap on the Starboard side barrel… so I am guessing there is fuel going down there… what do you think? What would cause that?

 

[kenny nunez]

It may be possible that one of the plugs in the bottom of the float chamber is leaking. That has always been a problem with Qjets. Flip the carburetor over, remove the throttle plate base. Clean the area around the plugs and coat over and around them with JB weld.