Another Look at the Carb

Posted: November 1, 2012  |  Tag: 

By: Grid Michal

I have a 1996 MerCruiser 4-cylinder 3.0L engine, with a two-barrel carburetor and electronic ignition, and Bravo One I/O. I have two problems. 1) When I accelerate from a standing start, throttle full forward, the engine sputters and dies, unless I back off the throttle for a second. 2.) When I do get to high speed, 3000 to 4000 rpm, it surges and then slows down to the original speed. Both problems occur only under load (i.e., when in the water). Also, the first problem doesn’t occur in the water and out of gear. I have replaced the carburetor with a new Rochester two barrel, the fuel filter (Merc) and also tried a rebuilt carb. I’ve replaced the ignition coil (OEM) and ignition module (OEM), but this has not cured the first problem. The second problem occurred after replacing the ignition coil and ignition module. I have checked the compression, and it is within spec +/- 10 percent. Spark plugs are new and gapped to 0.035. There is no oil or carbon deposits on the spark plugs. The spark plug wires are original. The choke is wide open when warmed up. In fact, it starts cold with no problem. I can see the carburetor inject gas when full throttle is applied. This leads to the biggest problem: Salmon season has started on the Sacramento River, and my boat doesn’t work.
There’s a big difference between fishin’ and catchin’. That’s why I go to the market for salmon. That doesn’t address your first problem, though, which I’m almost certain is carb related, despite the variety of carbs. The out-of-water tests using throttle are never a good idea, because of what happens to engine internals when not under load.

You have to deal with one problem at a time. It would be better to go back to the old module before checking timing and fuel. If the surging started after replacing the module, go back to the original module and start your diagnostics.

I didn’t see that you checked the timing for advance when the throttle is advanced. That would be a good place to start. You seem mechanically proficient, so I trust you’d perform this with a driver you trust, using all commonsense safety approaches. If the timing is spot-on, use your timing light to peer down the carb bore to see if fuel is being dumped into the body when the throttle is nailed (all still under load). If not, look to your accelerator pump adjustment.

If all else fails, use starting fluid or ether to check for air leaks around the carb base.

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