4.3 mercsuiers stalls, surges,

S

stang32

Member
Joined
Jul 17, 2004
RO Number
14425
Messages
56
my 96 4.3 I/O has been kicking me in the ass lately. I tuned it up last year, wires ,cap rotor, plugs. rebuilt the carb with new accelerator pump & yet I still have this problem, when i am in neutral & it doesnt matter if its cold or hot, i push on the throttle,it surges, i hear air being sucked into the carb, then the boat picks up the rpms, but this is after about 10-20 seconds.. sometimes it pops out the carb..
now when i am running the board will surge rms drop & falls very noticeably. then it will run fine. this is so inconsistent & it makes my passengers nervous.
an help is much appreciated
this engine was purchased 2 years ago and is a rebuilt unit from a popular rebuilding company.
 
Is the carb idle jet(s?) mix tuned for maximum vacuum reading at idle?

When you tuned it up you checked timing to spec in Timing mode, right? Then returned to Rum mode? (That is assuming your engine requires Timing & Run modes.

When you replaced the accelerator pump did you confirm then & now it provides a good squirt of gas when the throttle is advanced?

A vacuum gauge test may help identify the issue.
 
I expect you have a Q-jet( Rochester). Have someone run the boat as you check the carb. Remove the air cleaner and watch the action of the choke plate when cold and how it opens as it warms up. Then under way, watch the secondary throttle plate. This may be where the sucking comes from. If the plate opens too much and/or too soon it will do as you describe. When it starts chocking out or poping thru the carb try closing the secondary plate slowly with your finger. The boat may surge ahead so be aware. You're adjusting the fuel/air mixture..oh yeah, you have to be around 1/2 to 3/4 throttle so the secondaries are opening. The secondary never has to open more than about half way.
 
sandy, I did the timing, it is a thunderbolt IV ignition so it requires shirting the wire , i did that & had the timing set. the jets looked good to me, i will check them again. checking the vacuum may be tough, there are no vacuum ports on this carb.
 
The vacuum port may be a pipe plug or hose fitting between the carb and distributor on the intake manifold. It's useful to learn about vacuum readings, carb adjustments and diagnosing readings however your problem sounds more like fuel supply problem...but just check the distributor points and rotor/cap contacts are clean.
 
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