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/23hon65kcs...ough1.m4a?dl=0
https://www.dropbox.com/s/b33ev7jgw4...ough2.m4a?dl=0
https://www.dropbox.com/s/r10y4oa6fr...ough3.m4a?dl=0
https://www.dropbox.com/s/sxgfhnovpp...ough4.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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