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Mercruiser 260 "knock" at idle

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racin84

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1982 Mercruiser 260. When I fired up the motor this spring it ran and sounded fine. Launched the boat, initially leaving the ramp and idling to channel it was still fine. 20-30 mins on a plane at 3000 rpm, then when I backed down to idle or anything less than about 1500 rpm I get a "knocking". Fearing the worst, I started pulling plug wires to isolate the bad cylinder. No changes in Knock when any of the plug wires were pulled. (I have successfully used this diagnostic trick on Chevy V8s before).
When I pulled #8 wire there was no change in engine rpm, as opposed to pulling #1 which causes dramatic change. Swapped #8 wire for #7 wire and there was no change. Like #8 cylinder dead. Changed plugs, cap and rotor. No change in knock. Ignition is a Pertronics point-less setup.
Here's what really has me puzzled...Today while troubleshooting, I pulled #2,3,5,and 8 plug wires all at the same time with very little change in engine rpm (maybe 50 rpm).
#2,3,5,8 are all on the same lower plane of the dual plane intake manifold. Is it possible I have a plugged idle circuit in the Quadrajet 4 barrel carb? The engine runs fine above 1500-1800 rpm with no knock or any lack of power.
Anyone ever seen or heard of this? Sure could use some input.
 
Last edited:
knocking is usually the lower end rattling is the upper end.

usually.

So on a warm motor with all the plugs out and the throttle in forward do a compression test.

Keep the plugs in order so you know what plug came out of what hole.

Read the plugs and get back to us.

Spark Plugs

there's no water in the oil, and the dip stick is not over filled ? Had to ask you seem knowledgeable and i don't want to insult you.
 
racin84 said:
1982 Mercruiser 260. When I fired up the motor this spring it ran and sounded fine. Launched the boat, initially leaving the ramp and idling to channel it was still fine. 20-30 mins on a plane at 3000 rpm, then when I backed down to idle or anything less than about 1500 rpm I get a "knocking". Fearing the worst, I started pulling plug wires to isolate the bad cylinder. No changes in Knock when any of the plug wires were pulled. (I have successfully used this diagnostic trick on Chevy V8s before).
When I pulled #8 wire there was no change in engine rpm, as opposed to pulling #1 which causes dramatic change. Swapped #8 wire for #7 wire and there was no change. Like #8 cylinder dead. Changed plugs, cap and rotor. No change in knock. Ignition is a Pertronics point-less setup.
Here's what really has me puzzled...Today while troubleshooting, I pulled #2,3,5,and 8 plug wires all at the same time with very little change in engine rpm (maybe 50 rpm).
#2,3,5,8 are all on the same lower plane of the dual plane intake manifold. Is it possible I have a plugged idle circuit in the Quadrajet 4 barrel carb? The engine runs fine above 1500-1800 rpm with no knock or any lack of power.
Anyone ever seen or heard of this? Sure could use some input.
Click to expand...


Have deffinately seen that on automotive engines. One side of carb feeds 4 holes and the other side feeds the other 4 holes, on the dual plane intake manifolds. Have you tried adjusting the idle mixture screws to see if anything happens? Have seen a leaking intake valve cause that on the dual planes, too. Completely scews up the mixture / air delivery on one side. Do the compression check to eliminate the intake valve scenerio. If compression is good, perhaps it's carb rebuild time. The knock makes me curious, though, but may be an indication of overly lean mixture on one side of carb. Another thing to consider is a vacuum leak.....

Just throwing some stuff out there.
 
Just finished carb rebuild. No smoking gun. Will install and try a run tomorrow.
No water in oil, oil level at full mark. Just did an oil change a couple weeks ago (after the noises started). Have yet to cut open filter to look for debris.
Will do compression check and vaccuum check tomorrow when she is running again. Curious as to why you say do the compression test "with the throttle in forward"? vaccuum test may be tough to get good results, only one vaccuum port on a 4MV Quadrajet.
The throttle shaft did seem to have a little play on the port side, which is the side that feeds the "dead" cylinders. Put an "O" ring on it for now to try to stop any vaccuum leak there. I'm thinking maybe that could cause a really lean mixture on that carb barrel.
I do greatly appreciate the feedback and help. Will let you know what happens next.
 
Why do I always start with the hard stuff...Rebuilt carb, no changes. Got engine running and warmed up. did compression test. #1,2,3,4,6,7 all 160-170#. No. 5 at 0#. No. 8 at 40# dry, 45# with some oil in it. Guess I'll be pulling the heads off. Will check valve travel first.
 
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