Rough Idle, Fine above 1000 RPM

trailshredder

Petty Officer 1st Class
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Apr 25, 2011
Messages
283
My 97 Glastron with a 5.7L GL has developed a very rough idle, sometimes stalls. As soon as I hit 1000ish RPMs, runs like a top. Spark plugs were fouled out, so I replaced them. Was better for a few runs but doing it again. Cap, rotor, plug wires have all been changed in the past 2 years. Holey 2 barrel carb has been rebuilt a year ago. Fuel/Air mixture adjustment has been tweaked and didn’t help. The Choke plate is open and not stuck. Someone mentioned replacing the Idle Air Control Valve (IAC). Any thoughts?
 

alldodge

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Mar 8, 2009
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Carb doesn't have a IAC, you need the carb taken apart, cleaned real good. The idle circuits are clogged up
 

Saline Marina

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Sep 9, 2014
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162
Agree, IAC is a TBI fuel injection device.

Its also appropriate to look for vacuum leaks, there aren't many paths on the Penta setup but the gasket under the carb could possibly be a source that tampers with idle air/fuel balance.

If I clean the carb, I like to buy a "1 gallon paint can" of carb cleaner that's actually about half-full at the auto parts store where you fully dunk the carb in overnight and it has some special solvents which goto work on the gum very effectively. Then I like to use the wire from a paper wire-tag and poke thru all of the passages, very carefully, but to make sure they are all open. Finally blow clean with compressed air and reassemble + test.

Before you get thru that, some Sea Foam in the gas can potentially help but its more of a proactive maintenance item in my mind than a magic bullet which is going to solve all of your problems. But, for $6 (around here) its a fast thing to try with minimal effort.
 

jerryjerry05

Supreme Mariner
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May 7, 2008
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17,923
Test: can of starting fluid, start motor, spray around the intake side, carb area.
Make any difference? sucking air.
 

Bondo

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Apr 17, 2002
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Ayuh,..... Foulin' the plugs is too much fuel, rather than to much air,.....
 

Scott Danforth

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Jul 23, 2011
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47,297
first thing to check.

does the clear line from the fuel pump to the motor have fuel in it. if it does, replace your fuel pump

if not, go back to the carb pull it apart, and chase every passage. replace your power valve.

you have either fuel entering the motor externally, or you have an issue in the idle circuit.
 

Regal1973

Petty Officer 2nd Class
Joined
Feb 15, 2018
Messages
106
Doubt it would be the source for too much fuel like your speaking of, but I'd also check cap and rotor again, even if it's been replaced in last year. Once had a motor with similar symptoms. Corrosion on 1 yr old cap contacts was enough to cause ruff idle. Once rpm increased, charge was able to navigate through the corrosion and idle issue went away. A new cap and rotor fixed it. A 2 minute easy check for piece of mind.
 
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