350 Mercruiser backfires at 2500 RPM.

Just put in a rebuilt 350 mercruiser. New distributor and plugs/wires. Starts and idles beautifully. Runs smooth up to 2500 rpm and then it backfires. Rechecked the timing, OK, adjusted the distributor while running, but still backfires. Filled up with new fuel, still backfires. The carburetor was rebuilt, Still backfires. Anyone have any ideas or suggestions? What am I missing? Appreciate any input. The boat has twin 350's The port one runs fine. Thanks. Good and safe cruising.

Maybe the secondary throttle (if carbureted) isn't opening. Easy to check. Pull off both spark arrestors and watch the carbs as both engines slowly accelerate. When they reache 2,500 is the starboard opening at the same time as the port.

Also could be ignition. Is it electronic or points/condenser- I know, it can't be THAT old.

David

Joe- what's WOT ROM?
Can you install a vacuum gauge in an intake port?
Possible mechanical/ vacuum advance issues with the distributor?
Does it have a "pattern" or erratic?

Sometimes to narrow down a miss I'll use the Lisle spark tester, install it in line on any accessible spark plug, and then watch it CLOSELY as it begins to miss and watch for the ignition fire to deteriorate. If not the case, possibly low fuel pressure, plugged spark arrester or mane a few worn can lobes...
http://m.summitracing.com/parts/lil...BI6tE_MipS8w6_NW8mex9FO1eQcwYAAnDYxoCHqDw_wcB
Good luck!!


1983 Present 42 Sundeck
Twin Lehman 135's

Had this sort of think happen on an OMC I/O and it was sticky valves.

Ken

kchace said:

Had this sort of think happen on an OMC I/O and it was sticky valves.

Ken

Click to expand...

Yea, does kinda sound like intake valve issues. Cylinder firing with intake valve not fully seated. Bad seat, bent stem, broken spring etc etc.

Go aboard at night and watch the ignition wires in the dark.

If its a light show , change the wiring.

Lean mixture, late timing, leaking valves, plug wires mixed up, ignition crossover in cap or wires, etc, can cause this.

First thing is check that firing order is absolutely correct. A guy on the dock was having this trouble and I asked him to double check wires. He said it was right. Hired me to help. First thing I did was go through firing order. Found two wires mixed up. Amazing the thing ran pretty well on six.

How was valve lash set? If you set with dry lifters you can get valves too tight. Need to set with lifters pumped up, which is a mistake many make.

Usual suspect after that work is plug wires mixed up or distributor one tooth off.

Ski in NC said:

Lean mixture, late timing, leaking valves, plug wires mixed up, ignition crossover in cap or wires, etc, can cause this.

First thing is check that firing order is absolutely correct. A guy on the dock was having this trouble and I asked him to double check wires. He said it was right. Hired me to help. First thing I did was go through firing order. Found two wires mixed up. Amazing the thing ran pretty well on six.

How was valve lash set? If you set with dry lifters you can get valves too tight. Need to set with lifters pumped up, which is a mistake many make.

Click to expand...

True True. And usually if you find two cylinders "not contributing" as you pull ignition wires one at a time- those will be the ones switched .


1983 Present 42 Sundeck
Twin Lehman 135's

Sounds like you have an older distributor ignition system engine.

I used to get that. Pull the distributor cap off and spray the inside with WD40 or similar. Even a tiny bit of moisture collecting in there will bleed off the spark. Wipe out the excess carefully so you don't leave debris behind.

At idle the spark actually doesn't need much voltage [comparatively] behind it to jump but when a load is on , then the ign. system requires higher voltage to fire meaning it may find an easier path than to the plugs.

Be sure all wires are good and clean. I used to silicone grease the towers and the plugs, lightly, not gobs.

Air leak. Check that the carb base and all access to the intake manifold are sealed.

Joew2604 said:

Just put in a rebuilt 350 mercruiser. New distributor and plugs/wires. Starts and idles beautifully. Runs smooth up to 2500 rpm and then it backfires. Rechecked the timing, OK, adjusted the distributor while running, but still backfires. Filled up with new fuel, still backfires. The carburetor was rebuilt, Still backfires. Anyone have any ideas or suggestions? What am I missing? Appreciate any input. The boat has twin 350's The port one runs fine. Thanks. Good and safe cruising.

Click to expand...

Joe, how did the engine run BEFORE all of this work was done, was it backfiring? Is this the problem you were trying to solve with the new distributor, plugs/wires? Is it backfiring through the carburetor or exhaust?