Occasional backfire through carb - MerCruiser 7.4L

B

Bob St John

New Member
Nov 30, 2022
29
Boat Info
1993 27' Sea Ray Weekender (7.4L with Bravo 3)
Engines
7.4L with Bravo 3
Folks,

I've got a 1993 27' Sea Ray Weekender with a 7.4L MerCruiser engine. The current MerCruiser 7.4L is an OEM long block that was installed about 10 years ago that has about 250 hours on it.

The boat has not been used in 2 or 3 years, but it was occasionally started and ran for 10 - 15 minutes.

I want to use it this coming season. My mechanic has recently:
  • Installed 2 new batteries (Group 27, 800 CCA), and put new connector terminals on all connections to the batteries.
  • Installed new fuel pump and raw water pump.
  • Installed new alternator.
  • Installed new spark plugs, distributor cap, and rotor.
  • Oil change including oil filter and fuel/water filter.
  • Probably several other things that don't immediately come to mind.
The gas tank held about 1/4 tank of two or so years old gas that had been stabilized. On top of that I added 3/4 of a tank of fresh 89 octane gas.

The engine now starts better than ever, purrs like a kitten when idling, never gets above 160 - 170 degrees when running, easily gets up to near WOT (4200 rpm), etc.

The "sweet spot" for this engine is 3100 - 3200 rpm, and I suspect it could run forever at this sweet spot.

Earlier this week I took it out for about a 90-minute run, ran it up to 4200 rpm several times (for 1 or 2 minutes maximum), then back to sweet spot, and basically ran at sweet spot with zero problems.

Yesterday I took it out again and it occasionally backfired through the carb while I was in the sweet spot. It would backfire once or twice in 30 seconds or so, then run smoothly for several minutes, then backfire again once or twice in 30 seconds or so.

Note that I think the sweet spot (3100 - 3200 rpm) is just below where the 2nd set of barrels on the carb cut in.

Does anyone have any thoughts on what may be causing the occasional backfires through the carb?

Thanks,

Bob St John
 
Backfiring through the carb under load is usually a sign of running lean. I would run it for a couple of minutes at cruise, chop the throttle, and take a plug reading. That way, you could eliminate a weak carb. Have you checked under the distributor cap and rotor for carbon tracks? They usually manifest as a worsening miss as the load/rpm increases, but it's still something to verify.
If, as you surmise, it's in the primary/secondary transaction, my money would be on the carb needing rebuilt. Quadrajets are great carbs but don't take kindly to sitting for long periods of time.
 
It's common for 454s to have sticking valve guides that cab cause the valves to not fully seat. Try adding a pint of Marvel Mystery oil to the fuel as a top cylinder lubricant. Run it for several hours and do a compression test with engine after it has been run for at least a half hour and still pretty warm.
 
Folks,

An update with possible (probable?) problem discovery and resolution. I suspect this is a somewhat unusual problem, and that some of the contributors to this thread may find it interesting.

Having not yet resolved the occasional backfire through the carb problem I describe above, my marine mechanic was replacing the throttle linkage cable when he noticed the Mercathode block (which is held in place by the bolts in the back of the carb) was loose.

He started to tighten the front carb bolts (no problem there) and moved to tighten the back carb bolts (big problem there). The back left bolt was loose enough to be turned by hand (which he did as much as possible), and the back right bolt (which is the primary attachment for the Mercathode) also was loose enough to be turned by hand, but this bolt kept turning ... it was stripped!

These loose bolts apparently allowed un-metered air to enter the carb and that upset the fuel/air mixture and caused the engine to run lean. That, in turn, probably caused the occasional backfire through the carb. I think that Nater's response above (where he mentions that backfires through the carb are often caused by a lean fuel/air mixture) was right on ... thanks Nater!

The resolution was to remove the carb, drill the stripped bolt hole, and put a HeliCoil in. Note that when the carb was removed, the gasket was "stuck" at the front of the carb, but not at the back, which is evidence of air entering the carb there.

So, the carb was removed, a HeliCoil was put in the stripped hole, and the carb was reinstalled with a new gasket and studs (rather than the bolts which were removed.)

We took one abbreviated run with the boat (due to bad weather) and it performed flawlessly.

Additional runs are necessary to confirm resolution, but we are quite confident the carb backfire problem has been resolved.

I believe I know which of my previous mechanics (in a different state from where I am now) may have stripped the threads, and I was somewhat leery of him at the time, so he wasn't my mechanic for long.

Thanks,

Bob St John
 
This thread reminded me of a Boat Trader recall add for Bayliner boats. The recall said “ engine may backfire, causing muffler to break , causing water to enter, causing boat to sink”
 
Bob St John said:
Folks,

I've got a 1993 27' Sea Ray Weekender with a 7.4L MerCruiser engine. The current MerCruiser 7.4L is an OEM long block that was installed about 10 years ago that has about 250 hours on it.

The boat has not been used in 2 or 3 years, but it was occasionally started and ran for 10 - 15 minutes.

I want to use it this coming season. My mechanic has recently:
  • Installed 2 new batteries (Group 27, 800 CCA), and put new connector terminals on all connections to the batteries.
  • Installed new fuel pump and raw water pump.
  • Installed new alternator.
  • Installed new spark plugs, distributor cap, and rotor.
  • Oil change including oil filter and fuel/water filter.
  • Probably several other things that don't immediately come to mind.
The gas tank held about 1/4 tank of two or so years old gas that had been stabilized. On top of that I added 3/4 of a tank of fresh 89 octane gas.

The engine now starts better than ever, purrs like a kitten when idling, never gets above 160 - 170 degrees when running, easily gets up to near WOT (4200 rpm), etc.

The "sweet spot" for this engine is 3100 - 3200 rpm, and I suspect it could run forever at this sweet spot.

Earlier this week I took it out for about a 90-minute run, ran it up to 4200 rpm several times (for 1 or 2 minutes maximum), then back to sweet spot, and basically ran at sweet spot with zero problems.

Yesterday I took it out again and it occasionally backfired through the carb while I was in the sweet spot. It would backfire once or twice in 30 seconds or so, then run smoothly for several minutes, then backfire again once or twice in 30 seconds or so.

Note that I think the sweet spot (3100 - 3200 rpm) is just below where the 2nd set of barrels on the carb cut in.

Does anyone have any thoughts on what may be causing the occasional backfires through the carb?

Thanks,

Bob St John
Click to expand...
I just had this problem sort of and it was the choke had had a new carb installed with electric choke and the chock slipped i guess after installation so finally the mechanic was able to get back to fix this choke and it ran fine last week may at least check since your boat is about the same age as mine
 
While we hot rod motor heads worked on the engines, we used to find vacuum leaks on the intake and carb, like that vacuum leak, by spraying starter fluid around the outside of the carb and intake manifold and listen for the RPM to change. Very effective, but not sure it is a good idea in a boat's engine compartment.
 
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