Mercruser 454 cid big block (340 hp) bogging problem

[comptiger5000]

Engine is 1986 model mercruiser MIE 340 big block with quadrajet carb. 1160 hours on engine. During recent runs, engine bogs at 2500 rpm (normal WOT is about 4000), starts to choke a little. I've tried adjusting timing, hasn't changed anything. My guess is it's getting too much gas, because if you try to throttle it up to wide open, it starts backfiring constantly until you back of to about 1500, and chokes, if you leave it at WOT for a few seconds, the engine stalls, still running at 2500-2800 rpm.

When running below 2500 rpm, it runs fine. Engine will not climb past 2500 rpm, and until backing off significantly and reapproaching 2500, rpm is somewhat inconsistant, varying 2100-2400 with tach needle wavering.

Any help would be greatly appreciated.

[stonebreaker]

Always start with the basics - fuel and spark. Make sure your ignition system is in good shape, from the plugs all the way back to the ignition module. Make sure your fuel system is in good shape, from a fresh fuel filter to the carb being in proper working order.

Just taking a WAG (Wild-Assed Guess), it sounds like it might be a timing issue. Maybe a broken advance spring in the distributor, or a vacuum line that's fallen off?

Have you checked the condition of the plugs to see if it's running too rich or too lean, or if it's detonating?

[comptiger5000]

I have not checked for vacuum lines or spark, however, I doubt it is detonation becuase the engine runs perfectly other than the bogging. I checked and adjusted timing several times, but it has not helped, however, on timing setup caused the engine to backfire sharply when it bogged. I tried carburetor cleaner to no avail.

At this point, i'm guessing that it's either richening the fuel mixture too much at higher throttle, or that the secondaries aren't opening properly. This engine had a similar problem a few years ago with the secondaries not opening, however, it was bogging at 2800 rpm.

Is there any easy way to check for proper opening of the secondaries without disassembling the carb?

We took a mechanic out for a run today, and we'll see what he comes up with after looking at it.

Also: when it bogs, it's not rough, it feels like the engine was throttled back intentionally.

[lee armistead]

I have an identical problem with a 302 V-8 Ford backed by a 12 YJ Jacuzzi jet drive. See thread under engine forum "Popping back through carb". I can't tell you for sure that we have the same problem but I have arrived at the following conclusion: Carbs deliver fuel, sufficient spark ignites it. If cylinders are loaded up with unburnt fuel you'll get a pop back through the carb when they do ignite, an evloving problem such as a flooded engine and it only occurs on acceleration (pouring more gas into the cylinders). If the dizzy (distributor) is rotated while in operation (accelerating) and it does not change the bogging down condition but the engine idles and starts as it should it seams to me that there is insufficient voltage being delivered by your coil to ignite the fuel in the cylinders. I do not know the type of ignition you have installed, mine is a Prestolite Electronic unit. Requires a high voltage coil that does not require an external resistor, the resistor reduces voltage to the plugs (not what you need). I have not gotten any feed back from the post that I made so it apparent to me that there are maybe only a very few people that know anything about the elecronic of an engine. I picked up on your thread while searching for an answer to my problem. I went out over the weekend and purchased a UC16T coil and will be replacing my coil today. I'll let you know what I get out of it! Lee Armistead, leearmistead1@netzero.net, Ruskin, Florida, 33570.

[lee armistead]

I have an identical problem with a 302 V-8 Ford backed by a 12 YJ Jacuzzi jet drive. See thread under engine forum "Popping back through carb". I can't tell you for sure that we have the same problem but I have arrived at the following conclusion: Carbs deliver fuel, sufficient spark ignites it. If cylinders are loaded up with unburnt fuel you'll get a pop back through the carb when they do ignite, an evloving problem such as a flooded engine and it only occurs on acceleration (pouring more gas into the cylinders). If the dizzy (distributor) is rotated while in operation (accelerating) and it does not change the bogging down condition but the engine idles and starts as it should it seems to me that there is insufficient voltage being delivered by your coil to ignite the fuel in the cylinders. I do not know the type of ignition you have installed, mine is a Prestolite Electronic unit. Requires a high voltage coil that does not require an external resistor, the resistor reduces voltage to the plugs (not what you need). I have not gotten any feed back from the post that I made so it apparent to me that there are maybe only a very few people that know anything about the elecronic of an engine. I picked up on your thread while searching for an answer to my problem. I went out over the weekend and purchased a UC16T coil and will be replacing my coil today. I'll let you know what I get out of it! Lee Armistead, leearmistead1@netzero.net, Ruskin, Florida, 33570.

[comptiger5000]

actually, I fixed the problem, it turned out that ethanol gasoline had degraded a rubber fuel hose, causing a clogged valve. When this was corrected, the bogging dissapeared. It runs well now.

[lee armistead]

No doubt you are pleased. I'm glad it was that simple and your back on the water. Have fun. Lee Armistead