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"Hello. I have a 1980 vintage
"Hello. I have a 1980 vintage OMC 800 Stern Drive (Stringer), Chevy 350 engine, 260 hp, Rochester Quadrajet carb. My problem is that my engine develops a rough idle after it is warmed up, which eventually causes it to stall at idle rpm which is set to 600. (Not too friendly when trying to dock the boat.) When first started cold, and allowed to idle to warm-up, the engine idle is OK. Then it runs at speed OK (no hesitation or misses). But then when I throttle down after running at speed for a while, the problem occurs. It is not a definite miss, but rather a "stumble" in the engine idle that eventually gets worse until it dies. I suspect the carb, but I'm not sure. My understanding is that the Rochester Quad carb has a separate idle circuit apart from the main carb operation.
The reason I'm not sure about the carb, is that the problem first occurred after I replaced the spark plugs!? Altho the low speed operation of the engine has never been completely smooth (some roughness), at least it didn't stall. I replaced the spark plugs because I developed a hard engine miss - one cylinder out - that was due to a fouled plug, which in turn I suspect was due to oil leaking from the valve covers - another problem. The new plugs cured the engine miss, but now I have the hot idle stall. I replaced the old spark plugs, which were NGK-BR6FS installed by a service shop, with AC-MR43T which were the original plugs called for in my GM engine. The old plugs were gapped to approx .032. I set the gap on the new AC plugs to .035 per the owners manual. But the original OMC factory service manual, which I have, calls for the gap in the 260 engine to be .030. Which is correct? And could the larger spark plug gap in the new plugs possibly cause this problem? In general, what is the effect of the gap difference in spark plugs?
Any thoughts on what could possibly be causing my problem, or how I could further trouble-shoot it, would be greatly appreciated."
"Hello. I have a 1980 vintage OMC 800 Stern Drive (Stringer), Chevy 350 engine, 260 hp, Rochester Quadrajet carb. My problem is that my engine develops a rough idle after it is warmed up, which eventually causes it to stall at idle rpm which is set to 600. (Not too friendly when trying to dock the boat.) When first started cold, and allowed to idle to warm-up, the engine idle is OK. Then it runs at speed OK (no hesitation or misses). But then when I throttle down after running at speed for a while, the problem occurs. It is not a definite miss, but rather a "stumble" in the engine idle that eventually gets worse until it dies. I suspect the carb, but I'm not sure. My understanding is that the Rochester Quad carb has a separate idle circuit apart from the main carb operation.
The reason I'm not sure about the carb, is that the problem first occurred after I replaced the spark plugs!? Altho the low speed operation of the engine has never been completely smooth (some roughness), at least it didn't stall. I replaced the spark plugs because I developed a hard engine miss - one cylinder out - that was due to a fouled plug, which in turn I suspect was due to oil leaking from the valve covers - another problem. The new plugs cured the engine miss, but now I have the hot idle stall. I replaced the old spark plugs, which were NGK-BR6FS installed by a service shop, with AC-MR43T which were the original plugs called for in my GM engine. The old plugs were gapped to approx .032. I set the gap on the new AC plugs to .035 per the owners manual. But the original OMC factory service manual, which I have, calls for the gap in the 260 engine to be .030. Which is correct? And could the larger spark plug gap in the new plugs possibly cause this problem? In general, what is the effect of the gap difference in spark plugs?
Any thoughts on what could possibly be causing my problem, or how I could further trouble-shoot it, would be greatly appreciated."