Popping and not getting on plane

HYDBits

Recruit
Joined
Jul 9, 2020
Messages
2
I've used this forum to solve problems before but this is the first time I've joined/posted. I have a 1979 Cobalt with the 260 hp engine I think that is the 5.7. (350 chevy engine). I have done the following... checked compression (all around 115-120), new starter, new alternator, and new carb.

Boat cranks right up and will run all day smoothly in the parking lot. Put it in the water and it bogs down. I'd say around 2000-3000 rpms. Starts popping and I've seen a little too closely down the carb when it does because there is a small explosion in there. If I lay the throttle all the way down, it will maintain that 3000-3500 rpm without the popping but still no more power than that.

Tach, speedo, and oil pressure gauge do not work. I replaced temp gauge and it works shows boat at a constant 140. All gauges needles jump to one side and then the other when I turn on the key, but the temp, gas gauge and trim are the only ones working after the boat is running. The tach needle stays maxed out and the speedo goes back to zero. Oil pressure switch goes back to zero as well. I only bring this up for insight, dont care if they work unless that could be a reason electronically the engine won't push harder.

I've read most likely culprit is timing but then I also read something about grounding tach wire when checking compression and I didn't do that. What would that have damaged by not doing? My next move is distributor cap, wires, plugs and coil. I'd think if it's a timing issue it wouldn't even run right on the hose ???? Bad coil be the reason my tach doesnt work and loss of power after 3k rpms?
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
43,318
If your compression gauge is accurate then the motor is getting tired (150 is new) but this is your issue

The popping is a backfire from the motor not getting enough fuel. Need to check fuel pressure (3-7 psi) and also might need a rebuild of the carb. Do check the float level because if its to low it can also cause the issue
 

Rick Stephens

Admiral
Joined
Aug 13, 2013
Messages
6,118
Fix your gauges. They are necessary.

Rebuild your carb. Replace fuel filters - dump them into a clean container to see how much crud is in there..

Check timing. Most common reason for popping in the carb is lean, but timing or sticky valve can also cause the same pop.
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
51,351
Welcome aboard....

AD and Rick have you covered
 

Oldnbold

Seaman
Joined
Jun 27, 2017
Messages
60
Also double check your spark plug wire firing order. They’re easy to cross and engine will start but also can give symptoms like yours
 
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