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View Full Version : 79 S&s Please Help!!


michaelh_26
06-28-2009, 04:24 PM
Hey everyone, my boat will run fine in my front yard but as soon as its under a load it will barely accelerate from idle to around 2000 rpm, i have to VERY slowly push the throttle forward and wait for the rpm to catch up. if i push it too fast it will almost backkfire from the carb and die. Once its built up rpm i can fly down the lake going 25-35 mph and the throttle seems to respond great. It is impossible for me to pull a skier and ruining my summer. heres some things ive done. last year when this problem came up i found broken advance weights in the distributor and a broken baffle clogging my right hand muffler, i replaced these things thinking all would be fine and the boat performed the same. This boat has been in my family since new and any help would be greatly appreciated.

compression test-all around 125 cold
new coil,wires,plugs, cap and rotor look ok, pertonix ignitor
checked voltage 9 volts to coil, 12 volts to ignitor per instuctions
new fuel filter/water seperator whatever it is
new fuel in tank with clean vents, maybe a clogged screen im unaware of?
new carb edelbrock 1406 marine
throttle cable check, seems the new carb isnt wide open when at full throttle, cable needs adjustment?
vacuum leak test around carb base and manifold(sprayed with carb cleaner)
checked tranny fluid
new impeller an d belts engine temp is 150

michaelh_26
06-28-2009, 06:44 PM
uhh, jimn where are you???

JimN
06-28-2009, 07:25 PM
Hey everyone, my boat will run fine in my front yard but as soon as its under a load it will barely accelerate from idle to around 2000 rpm, i have to VERY slowly push the throttle forward and wait for the rpm to catch up. if i push it too fast it will almost backkfire from the carb and die. Once its built up rpm i can fly down the lake going 25-35 mph and the throttle seems to respond great. It is impossible for me to pull a skier and ruining my summer. heres some things ive done. last year when this problem came up i found broken advance weights in the distributor and a broken baffle clogging my right hand muffler, i replaced these things thinking all would be fine and the boat performed the same. This boat has been in my family since new and any help would be greatly appreciated.

compression test-all around 125 cold
new coil,wires,plugs, cap and rotor look ok, pertonix ignitor
checked voltage 9 volts to coil, 12 volts to ignitor per instuctions
new fuel filter/water seperator whatever it is
new fuel in tank with clean vents, maybe a clogged screen im unaware of?
new carb edelbrock 1406 marine
throttle cable check, seems the new carb isnt wide open when at full throttle, cable needs adjustment?
vacuum leak test around carb base and manifold(sprayed with carb cleaner)
checked tranny fluid
new impeller an d belts engine temp is 150

Cold compression test is pretty meaningless. You need to check it at normal operating temperature, which means that you need to have a way to remove the plugs when they're hot without burning your hands. I use an old plug wire with the boot left on, cut off about 6" long and a box wrench to loosen the plugs. Once they're loose, the wire can be slipped on and you can spin it out. Leave the other plugs out when you do the test. If you have a bad gasket and the other plugs are in, the compression from a neighboring cylinder can affect the one you're testing.

If you can, do a vacuum test, too. A compression and vacuum test will give you a pretty good idea of the motor's general health.

How old are the plug wires? If they're more than a few years old, replace them. The moisture they absorb causes cross-fire, which can seem like what you're experiencing. Not only can they arc to adjacent plug wires but they can also arc to the motor itself. If you want to see if it is cross-firing, take it out in the dark with the motor box open/removed and run it fairly hard. If you see lightning, you need new wires. If the cap is old, it can develop traces of carbon, from arcing. These conduct and will take the path of least resistance to a ground.

Bellinghamster
06-29-2009, 12:06 AM
Sounds like a classic accelerator-pump-not-working symptom.

The accelerator pump's purpose is to squirt extra fuel into the carb throat under... wait for it... acceleration! This is needed because when the throttle butterflies are opened quickly the air going into the engine speeds up faster than the fuel can get moving in the venturis.

Without accel. pumps (or when they're not working) the engine will lean out and often backfire or die when accelerating, but will run just fine once the engine is running faster (if you can get there).

Pull off the carb flame arrestor, open the choke butterfly and actuate the throttle butterfly while looking down the carb throat. You shoud see a little stream of fuel squirting in from the side while the throttle is being opened. If it's not squirting, the linkage could be not assembled correctly, the pump diaphram could be torn or assembled wrong, the float level could be way too low (unlikely since it did run), or the passage could be clogged.

michaelh_26
06-29-2009, 12:35 AM
the plug wires are brand new and probably too nice for this old motor, ive had problems with wires in the past so i bought the best MSD wires i could find, the cap is old however but looks ok, i will replace it this week and pray that fixes it, as for the accel pump, it is working and i even stepped it up to deliver the most fuel, too much maybe??the carb is pretty much brand new, i replaced it a couple years back because of misdiagnosis, i had a bad coil. thanks for your input and ill do a hot compression test tomarrow.