5.7L low idle vacuum

[Colsanders1955]

I have a 1988 Chapparal with a 5.7L Mercruiser 4 barrel that has an ongoing issue with stalling and intermittent hard starting. When I acquired the boat two years ago, it had an orig Rochester Quadrajet that was flooding and would not idle at all. I rebuilt it and it stopped flooding but hasn't idled well and sometimes is hard to start when hot. This Spring I opted to eliminate any fuel related issues by replacing the fuel pump and the carb with a new Marine Holley 650 spread bore (offered as a direct Quadrajet replacement). While it seems to have improved it still has that intermittent problem with starting and idling down smoothly. I initially thought of a vacuum leak but choking the carb while idling did nothing but stall it. It has an idle vacuum reading that floats between 8-10 inches. The idle speed screw is down to the point of the idle mixture screws doing little and won't even kill the engine if screwed in all the way. My initial thought is maybe the engine is just worn out (I don't know how many hours on it) but when I rev the engine it has a very solid 17 inches of vacuum and the boat performs flawlessly at cruising speeds. Just trying things I toyed with the initial timing setting which was right on spec at 8 degrees but I bumped it up to about 12 degrees and that seemed to help some but I cannot lower the idle speed to normal without it loping and finally stalling. I haven't pulled all the plugs out to examine them but did pull one (the easiest to reach) and it was clean as a pin. I'm now wondering if the timing weight on the harmonic balancer may have rotated slightly on the crank hub causing me to get an erroneous timing reading(?). Any advice would be appreciated.

 

[flipbro]

compression test is needed

 

[NHGuy]

You don't just bolt on a carb. It must be tuned to the engine unless you got one that was configured for the 5.7.
When the balancer is at tdc the crank notch will be around 12:30.
You may be on to something about timing. Your engine will run with barely any fuel if advanced too far. But it will knock at high rpms & loads so be careful that you don't hole a piston.
Do the carb spray test around the carb base and intake to heads interface, might be a leak.

 

[funk6294]

A couple of things come to mind. The thunderbolt iv ignition does not add any additional timing at idle like you would with a vacuum advance distributor so the idle vac will always be a little low. The 8-10 is pretty low on a motor with a stock cam. Check the timing first and make sure it's right. Second, based on your description of the carb mixture screws having little affect the idle speed screw may be too far open causing the the throttle blades to be open too far into the idle transfer slots. If you pull the carb and look at the primaries the blades should only be exposing .020 of the slot (should look like a square).

Also, does your motor have a PCV valve connected to the bottom of the carb? I have the Holley 600s on mine and when I got the boat I had the same issue with the mixture screws, I added a pcv set up and this allowed me to reduce the idle screw down and the mixture screws started to operate normally.