96 Boomerang w/PCM 351 Coughs

[Kevin Gould]

my 96 Boomerang has the PCM 351 and ran really well for the first 5 years I had it. Now its developed a "cough" at higher throttle levels. I've had the carb rebuilt and changed the fuel filter, but no improvement. I checked the spark and timing, and both are fine. Any ideas?

[Holdmybeer]

Vacuum leak.

[HFarr]

Can you record the "cough"

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[Kevin Gould]

How to test for vacuum leaks?

[Kevin Gould]

will do next week. thanks.

[Holdmybeer]

A little messy but.....

Start the engine (have to be in or on water). Rev it up without the prop spinning and spray the ends of the vacuum lines with WD-40. If it revs up or smooths out then the vac leak was temporarily sealed with the WD-40, which means the vac leak sucked in the fluid. Hence, you found the leak. Does the distributor have a vac line from it to the carb? If so, that is the vacuum advance when at high RPM's and would cause a delay in firing at TDC and it would "Cough" or almost act like a backfire.

[Kevin Gould]

I'll try that. THANKS!

[Kevin Gould]

Hey Holdmybeer - I looked for possible vacuum leaks tonight but could not find any sources. It has electronic ignition so no vacuum line for that or anything else. The intake manifold looks to have never been removed - all original paint is unbroken. I've replace the fuel filter and fuel pump. I checked the ignition timing at idle and it is fine there (did not check at higher RPM). I made 4 audio recordings of the "cough" (actually more like a backfire) - it happens at full speed 5,000 RPM - you'll hear it if you listen to the M4A files. Here are links to the files in my dropbox:
https://www.dropbox.com/s/23hon65kcst71ad/PCM351%20cough1.m4a?dl=0
https://www.dropbox.com/s/b33ev7jgw43g15j/PCM351%20cough2.m4a?dl=0
https://www.dropbox.com/s/r10y4oa6frufcr9/PCM351%20cough3.m4a?dl=0
https://www.dropbox.com/s/sxgfhnovppyhvzy/PCM351%20cough4.m4a?dl=0

Any ideas would be helpful.
Thx.
Kevin

[zabooda]

I replaced two electronic ignition modules in the 16 years I had my 98 Moomba. Also, the carburetor rebuild required a different power valve than what comes in the kit.

[HFarr]

Does it do it cold too, or only after running a while? Definitely sounds like a small misfire. Check plugs? Connections? Maybe something slightly loose or corroded just a bit.

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[Holdmybeer]

The motor sounds super strong.
It runs great and revs strong until the "miss". Does it continue on its way after the cough or does it lose power and slow down?

[Kevin Gould]

It does it at wide open throttle (5,000 rpm), which I only do when the motor is warm. Plugs and plug wires are new. Gas is new. Fuel filter and fuel pump are new.
Carb was rebuilt over the winter by a pro (not me) but made no difference. Pro said compression was fine. I'm stumped.

[Holdmybeer]

You have electronic ignition but you still have a distributor, correct?

How does the rotor, cap, and coil look? Check coil for cracks or corrosion. Next would be wires from engine to ignition box. Loose connector, loose ground.

Do you have a tach? (Hopefully electric and not mechanical)
What does it do when it coughs? Does it flicker?

[secondsupra2004]

my 96 Boomerang has the PCM 351 and ran really well for the first 5 years I had it. Now its developed a "cough" at higher throttle levels. I've had the carb rebuilt and changed the fuel filter, but no improvement. I checked the spark and timing, and both are fine. Any ideas?

I definitely agree with other members that it is ignition related. You might want to pull the cap and inspect the shaft play on the distributor. I'm not sure what setup you have for ignition, but make sure everything is tight and your grounds are clean.

[zabooda]

I had my carburetor rebuilt when I was having that problem only to find out afterwards the electronic ignition was bad for the second time in 8 years. They are cheap in price and it appears in quality. The difference is your boat has the Ford engine and mine had the Chevy.

[Kevin Gould]

It coughs, then goes on its way for a while, then coughs again. Also, it may not come through on the audios, but even when not coughing, it runs a bit rough at wide open throttle

[Kevin Gould]

I'll check the distributor and leads to ignition box next week. It does have an electronic tach - I'll check what it does when engine coughs. Thanks for your and everyone's inputs.

[Holdmybeer]

I'm convinced it's not Fuel related.

I would think timing issues which goes to cap rotor coil grounding.

When it coughs does the tach fludder or drop 500 os rpms? The gauge may move but the engine cannot drop rpms that fast without exploding. That would lead me to believe there is a drop in voltage signal at that time. Could be a weak battery. Is it new?

[Kevin Gould]

just tested - yes the tach does drop/flutter when it pops. The battery is not new but is on charger now - will see if that makes a diff. Thx for your ongoing help on this.

[Holdmybeer]

just tested - yes the tach does drop/flutter when it pops. The battery is not new but is on charger now - will see if that makes a diff. Thx for your ongoing help on this.

Well that helps a lot.
It is a voltage/ignition issue causing a engine miss at higher rpms. If the battery doesn't do anything to help, start checking ground cables, cap & rotor (maybe replace since cheap), loose spark plug or rubbed wire.

All those thing check out....ignition module time just like Zabooda has mentioned.

[Kevin Gould]

Will do, thanks. May be a couple weeks, but I'll let you all know what I find.