1985 Mercruiser 260 knocking sound and idles high after running on plane for a while

[Iceguy1]

Thanks to everyone for your help with my last issue with shifting. The boat shifts perfectly now. I have two more problems I wanted to ask about.

1) I have a little bit of a knocking sound coming from my valve covers. The sound seems to happen in a regular pattern, and every time it knocks, I can feel a vibration in the idle, and hear a difference in the exhaust. If anybody has any thoughts on what this might be, it would be greatly appreciated.

2) I set my idle to about 650 RPM is in forward gear, Obviously after the engine has warmed up and the choke on my Quadrajet is wide open. Then after running on plane for a while, when I pull the throttle back to idle in forward, or back to neutral , The RPMs jump up to about 1100. After killing the engine and restarting, the RPMs go right back to 650. Sometimes I have to leave the engine off for a few minutes, then restart and it will run back at 650. When this happened yesterday, I noticed kind of a wheezing or whistling sound when the RPMs jumped up like this. The wheezing sound was definitely associated with the engine and running at the higher RPM. Definitely wasn't my blower running. Anyone have any insight on this?

Thanks in advance.
Mark.
1985 Chris craft scorpion 210

 

[Bt Doctur]

knocking ,ticking,clanking is never good. Might be the lifters going bad,too hot an engine, too hot an oil, to far advanved
jumping rpm W/ hissing could be a manifold leak or carb base gasket leak allowing it to run lean.

 

[Iceguy1]

Thanks Bt Doctur. Just to clarify and give you a little bit more info... If anything, I feel like it might be running a little rich because of the strong smell of fuel in the exhaust. Engine temp is always on the cool side. Thermostat is supposed to be 140?, I never see it over 120?. I have a new thermostat I was going to put in, can't get the bolts off for the life of me and decided to leave it alone for now before I cause more damage and break the bolts off for the thermostat housing. Also, no noticeable hissing sound. It's more of a slight whistling sound, and it only happens during this condition of the RPMs running higher at idle then I set it to. It almost sounds like a component running, like a pump or motor. Didn't know if maybe it was something up with the fuel pump. But I have no idea. Bare with me, I'm just learning. This is all new to me. I'm handy and I fix stuff for a living, but engines are all new to me. Just minor things on my boats over the years is about all the experience I have

 

[Bt Doctur]

take a video and post it, might help to hear it

 

[Bondo]

1) I have a little bit of a knocking sound coming from my valve covers. The sound seems to happen in a regular pattern, and every time it knocks, I can feel a vibration in the idle, and hear a difference in the exhaust. If anybody has any thoughts on what this might be, it would be greatly appreciated.

2) I set my idle to about 650 RPM is in forward gear, Obviously after the engine has warmed up and the choke on my Quadrajet is wide open. Then after running on plane for a while, when I pull the throttle back to idle in forward, or back to neutral , The RPMs jump up to about 1100. After killing the engine and restarting, the RPMs go right back to 650. Sometimes I have to leave the engine off for a few minutes, then restart and it will run back at 650. When this happened yesterday, I noticed kind of a wheezing or whistling sound when the RPMs jumped up like this. The wheezing sound was definitely associated with the engine and running at the higher RPM. Definitely wasn't my blower running. Anyone have any insight on this?

Ayuh,.... Welcome Aboard,..... With #1, have ya pulled any spark plugs from the knockin' area,..??
What did they look like,..??

#2,... Does the throttle cable Push slightly against the idle adjustment screw,.....
There needs to be a tiny bit of pressure there,.....

What's the general history on the motor, 'n it's service record,..??

 

[wrench 3]

With the knocking sound under the rocker cover affecting the running I'd think more likely a sticking valve. Or if it comes and goes in a regular pattern, I once ran into one with a missing exhaust valve keeper. The remaining keeper was cocking the retainer and the valve rotator caused it to interfere with the rocker arm once every rotation. Then again that's a long shot.
Check the secondaries on the carburetor. Make sure the lower shaft is closing all the way when you come back to idle.

 

[NHGuy]

Try "assisting" the throttle to close. if you can get it to idle by helping the throttle close down there are adjustments you can do to get more pressure on the return spring..
If it's still running too fast and the throttle IS closed, check the items that were mentioned before I jumped in..
Hissing and running wrong does sure sound like vacuum leaking somewhere.

 

[Iceguy1]

Ayuh,.... Welcome Aboard,..... With #1, have ya pulled any spark plugs from the knockin' area,..??
What did they look like,..??

#2,... Does the throttle cable Push slightly against the idle adjustment screw,.....
There needs to be a tiny bit of pressure there,.....

What's the general history on the motor, 'n it's service record,..??

I took the boat out yesterday and adjusted the throttle cable to where there was just a slight pressure against the idle screw, thinking this would take care of it. Unfortunately, no dice. It still jumped up in idle just like before.

As for the spark plugs, I just replaced them. The old ones all looked pretty fouled with oil, though I did use quick silver's "power tune" before changing plugs and oil which could have fouled them up too, I suppose. I noticed an odd thing on the plugs though... The points on all 8 plugs had a groove in them. Don't know if they came that way, I've certainly never seen a groove in new spark plugs before. I'll dig an old plug out of the trash and post a pic a little later.

Not to sure of the history of the engine. I just bought the boat last fall. It's got 600 hours on it. And I know the boat hasn't been used for two years prior. Since I bought the boat, the engine ran strong, even with the knocking sound. I've had the heavy old girl up to 50mph gps on a windy day so I thought the engine was running pretty good.

Though, yesterday I noticed a loss of power while on and getting on plane. Almost felt like cavitation but clearly was a loss of power. And more than ever, the the exhaust smelled like fuel. Thought maybe misfiring after changing spark plugs like I did something to cause it. But at idle, the engine runs really smooth so I don't think it was misfiring. I certainly will take the advice from several people and take a close look at the carb, especially the secondaries. Figure if they were sticking closed, it could explain the loss of power I noticed. Also, I rigged up a vacuum gauge using a low side refrigeration gauge. Tomorrow when I take the boat out, I figured I would check and adjust the idle mixture screws if indeed it's running as rich as I think.

One question for you guys. After carefully listening to the engine and confirming the knocking is coming from a valve cover and which one, if I pull the valve cover, is there anything in particular I should be looking for? And temporarily would it be ok for me to re-use the old gasket just while troubleshooting?

 

[Iceguy1]

Here are pics of one of the spark plugs. They all look to be in identical condition. Upon closer inspection, it looks to be more of a carbon buildup and NOT fouled with oil as I previously stated. The oil was just on the threads. Just curious if the groove worn in the point is an indicator of anything, as well as the carbon deposits?

 

[flipbro]

Valve cover gaskets I wouldn't reuse unless there the rubber ones! Those plugs are suppose to be like that! And I would do a vacuum test along with compression test.. Google up vacuum test sbc..

 

[Iceguy1]

Valve cover gaskets I wouldn't reuse unless there the rubber ones! Those plugs are suppose to be like that! And I would do a vacuum test along with compression test.. Google up vacuum test sbc..

Wow. I looked up what a vacuum test is and how to do it and I certainly see how it is relevant. Thanks for the good advice. I have nothing but time on my hands this weekend, as I am very hesitant to take the family boating after the worsened problem I noticed last time out. I will certainly post my findings. Man, I hope it's nothing too major. She's a very cool old boat in otherwise really good condition but the point of getting an older boat was to keep it cheap. I've already put quite a bit in her in only 6 months of ownership. Bust Out Another Thousand... Huh

 

[flipbro]

I own a 84 wellcraft elite. I've completely restored . New stringers bulkheads transom and deakl. I built a 350 bored .40 over edlbrock 420 443 cam intake and carb. 193 heads I polished the Chambers. Rebuilt the leg. And currently it's getting a fresh coat or Acrylic urethane paint. White and grey. Old boats are great but can become an addiction and cost can add up quick. Hope you get her sorted out.

 

[NHGuy]

Sometimes combinations of things can happen too. Are your exhausts good? By that I am asking is there any chance that cooling water water is getting into the exhaust tract? That and/or too much timing can cause knock from too much compresssion (water) or too early firing (timing).
I can't think of a reason for the high RPM after taking it out of gear unless you have some kind of mechanical advance distributor that may be sticking. Could we get some pictures of your distributor, both exterior and down through the top with the cap off?