5.7merc,carbed, surges and wont idle

[dead boat]

Hello all,

5.7 1996, thunderbolt V merc cruiser, 2 barrel carb:

Rebuilt the carb in May, ran great for 2 tanks of gas. The boat sat for 6 weeks and now it surges at 1200 thru 2600 rpm, I see gas dripping from right jet looking down at the carb at mid range rpm. It won't idle or holds at 500 rpm. Electric choke is OK. Stalls when i give it gas. seems to run good at 3300 + rpm.

New plugs, cap, rotor and timing last fall.

I found a little whitish-greenish crud in the filter at the carb and I emptied the water separator, no water but 6 small red flakes.

Is their a filter in the tank and in the fuel pump?

 

[John_S]

Re: 5.7merc,carbed, surges and wont idle

If the crud is in the carb, it will need a good cleaning and kit. If you see dripping at idle, could be needle and seat, or fuel pump going (putting out too much presure).

On a '96 I'd expect you'd have thunderbolt V, which would require base mode for idle speed, idle mixture, and timing. No knock module, does not mean you have T4.

 

[dead boat]

Re: 5.7merc,carbed, surges and wont idle

I rebuilt the carb in may, the crud I found was on the input side of the filter.

 

[D_J_Winn]

Re: 5.7merc,carbed, surges and wont idle

Just an idea its a $5.00 part
I had a similar problem on my 1997 merc 5.7lx, i replaced the An anti-siphon valve. The seat of the ball was gummy/ broken and worn out, the ball stuck partially to the seat choking the fuel passage so the pump could not get the volume of gas the engine required. (Buy a copper one, hopefully your never have to replace it again)


-symptoms of a bad anti-syphon valve are, hard starting, surging RPM's while under way, hesitation or backfiring upon acceleration and lack of wide open throttle (WOT) RPM's."

 

[dead boat]

Re: 5.7merc,carbed, surges and wont idle

I check the anti siphon valve, pickup tube and screen, looked good.

I replaced the separator in may, at time time of the carb rebuild. I cut this one open to look for crud, found nothing and was able to transfer fuel through the filter membrane.

I replaced the water separator and it idles and revs great. Looking down both venturi jets:they are dropping fuel at the same rate. I will have to put it the lake to see if that was the problem. I doubt it, this boat like to mess with me.

 

[John_S]

Re: 5.7merc,carbed, surges and wont idle

Find the idle problem before trying to find mid-range problem.

Unless someone put in an older motor or changed over module and wiring, you should have T5, which has to be in base mode. Do you have a purple/white wire with a terminating bullet plug running down near the starboard valve cover?

When you adjust the idle mixture screw, how is it behaving? Are you able to easily determine the middle spot. Go back and forth with idle mixture and idle speed. T5 wants to be set at exactly 650 rpms.

If you have any dribbling or puddling of fuel on butterflies, you do have a fuel problem. Float level adj, needle/seat, or too much fuel pressure.

A number of people have had too much fuel pressure problems. When the mechanical pump is on it way out, it actually puts out more pressure. I don't understand why, but it does.