Q. I have a problem with my boat engine backfiring upon acceleration. A broker bought the ’97 Sea Ray from England, and it appears the engine had been swapped at some point, and is now a ’98 Mercruiser 250hp 5.7L (type 4311025LS). It was a pleasant surprise, as the ’98 got Vortec heads, Thunderbolt V ignition with a knock sensor, and hydraulic roller cam. The boat only has 240 hours on the odometer, but the engine might have more hours on tap. It sometimes backfires at cold start, or when accelerating from idle, again usually cold. I suspected a vacuum leak but couldn’t find any. I overhauled the two-barrel Merc carb, but there wasn’t anything wrong with it, and the accelerator pump worked fine. Ignition timing is spot-on, 10 degrees with the timing lead grounded. I browsed through the documents that followed the boat, and found a service report stating the intake manifold gaskets had been replaced because of a suspected vacuum leak. Obviously, that didn’t cure the backfire. What else should I look for that would be causing the backfire? Also, the idle quality is not as good as I’m use to. Could this be spark related? Kind regards.
A. GM built some very special engines for Mercury Marine over the years, offering some very cool small- and big-blocks. The engine you have in your boat is a mix of truck parts, special Marine hydraulic roller, and carbureted technology. We believe you have a lean condition causing your backfiring at cold start and accelerating from low engine speeds while cold. What type of choke control does your carb employ? Is it electrically controlled by heating a coil that unwinds as it heats up and opens the choke blade or does it use some other type of external heat to open the choke? As you know, the Vortec heads do not have an exhaust crossover, so we suspect it’s an electric choke arrangement. Your backfire could be related to the choke and how quickly it’s opening after cold start. Also, if you have the ignition on for a period before starting the engine, it could be preheating the choke coil, causing the choke to be out of time for the temperature of the engine. Also, we’re sure Mercury uses some type of vacuum-assisted choke pull-off. This could be out of adjustment, causing the choke to be pulled open too far when the engine starts. This will also give you lean bog, or backfire when pushing into the throttle.
You also mentioned that the idle quality isn’t up to snuff. Yes, this could be a vacuum leak, or that the carburetor idle circuit is adjusted too lean. Most of the later-model carburetors had either plastic caps or stainless covers placed over the idle fuel screws to prevent adjustment. Remove whatever is preventing you from adjustment and set the idle fuel of the engine hot with a vacuum gauge. Connect a vacuum gauge to a manifold vacuum source and adjust the idle fuel screws with the engine at operating temperature until you can achieve the highest idle vacuum. These adjustments are identical to what we’d recommend for both the choke and idle circuit on any carburetor.
Finally, you could have a vacuum leak at the intake manifold gaskets. We’d re-torque the intake manifold bolts and try adjusting the choke and idle fuel screws. If you cannot bring the idle quality in line with the idle fuel screws, you’ll need to dig deeper to find your vacuum leak. Good luck, and happy boating!