High compression in 1 cylinder - 7.4 carb

[snowbrd84]

1997 Sea Ray 270 with a single 7.4L carbed engine and Bravo 2 sterndrive

Boat runs great at low speed but will not accelerate past 3800ish rpms. Gets up on a plane and will cruise at 27-29 mph with no bogging. Just won't go any faster. Bottom is clean, new wires, cap and plugs this year. Motor rebuilt 2 years ago. New 21p prop (spec is 23p).

Timing verified at 8-10 deg.

Ran compression test totesting warmed engine with all plugs pulled at wot and got 135-145 psi on cylinders 1-7. 8 came up 180. Tested again at 180. Tried 6 cylinder again and got 140. Came back to 8 and 180 again. Not a guage issue.

8 plug has blackened insulator and sounds like a knock or ticking from 8 cylinder. See pic attached. Otherwise engine sounds great and fires right up and will idle forever without issue.

My only thought is carbon buildup causing a knocking which is then triggering knock sensor to retard timing and limit WOT? Thoughts?

 

[jimmbo]

You might have carbon buildup, likely from oil being burnt in Quantity. You could run a Borescope in to take a look, easier than pulling the Head. I suspect that knock is a Connecting Rod Bearing has gone bad and is what is throwing up the great quantities of Oil, which is overloading the Oil Control Rings

 

[tpenfield]

Salt water? or Fresh?

If it was rebuilt 2 years ago, excess carbon should not be an issue at this point. #8 does not look like it is firing as well as the other cylinders. A borescope is a good idea so you can see if there is fluid (oil, water?) in the cylinder.

 

[snowbrd84]

Upper Potomac river - will have to order a borescope to take a look inside cylinder.

 

[tpenfield]

Make sure to get one with a 7mm (or less) camera probe, as 10mm is too big to get a good look into the cylinder.

 

[flashback]

While you're waiting on the scope pull the valve cover and give the rockers a look..

 

[kenny nunez]

You may have a camshaft issue. As Flashback suggested remove the valve covers and watch the rocker arms on #8 compared to the other rockers. It might be a pulled rocker arm stud. Sometimes the press fit in the head is not tight enough. It is easy to drill and pin the stud in place.

 

[nola mike]

Upper Potomac river - will have to order a borescope to take a look inside cylinder.

Hey neighbor! Just took a 35 mile cruise up to Tim's in Fairview Beach today.

 

[snowbrd84]

Piston looks clean with bore scope. Didn't have time to pull the valve cover so will have to try to open it up Sunday. I would say it's definitely more of a tapping than a knocking.

 

[snowbrd84]

Here is a video of it running. Also when I rev it up and it seems to stumble at 4000 rpm in neutral, the tach jump all over the place.

Side note - previous owner converted to a 1 wire alternator and I found out this morning that the 12v supply to the choke was never reestablished. So choke has not had any power supply.

 

[Kola16]

Like others have stated, it sounds like you have a flat lobe/lifter on the camshaft. Pull the valve cover and inspect the exhaust and intake pushrods for cylinder 8 to see if there is any play in them. Is it a flat tappet cam? Flat tappet cams require oil with high zinc/ZDDP. I found that out the hard way too once.

 

[jimmbo]

I don't think it is Cam Lobe Issue. Lightening the Pics does show some Goop on the Head, Exhaust Valve area is drier, likely cause it is hotter. Pull the Head, and pull the Intake Valve out, likely a bad/missing Valve Seal

 

[Kola16]

I don't think it is Cam Lobe Issue. Lightening the Pics does show some Goop on the Head, Exhaust Valve area is drier, likely cause it is hotter. Pull the Head, and pull the Intake Valve out, likely a bad/missing Valve Seal

That wouldn't cause it to tick like it is though would it? If that cylinder is not firing properly, stuff is going to build up in that cylinder wouldn't it? Check the pushrods first before pulling the heads off. Roll the engine over with the valve cover off to make sure your intake and exhaust valves are opening and closing properly.

 

[snowbrd84]

Baby's first bday today so stuck at home but hoping to get out to marina tomorrow morning to pull valve cover. Are all the bolts accessible without removing exhaust parts?

I've read other similar cases in auto engines searching online where the problem ended up being backed out nut on rocker arm.

 

[brodmann]

I'm not an inboard/outboard guy, but that knocking is definitely a valve knock. Either like someone suggested above the rocker stud has worked loose, lifter has gone bad, or for some reason a valve is not opening on that cylinder. I'd certainly remove the valve cover to look for issues before removing the head.

 

[Lou C]

If you have been running with the choke like that (nearly closed) it’s been running way too rich & may have a lot of carbon build up in the combustion chambers. One issue with electric chokes is that they are actually an electric un-choke. Unless they have 12v continuously it will stay closed. The old fashioned manifold heated chokes like what I have on my pre Vortec 4.3 do not do this; they open slower but stay open as long as the engine comes up to normal temp & you keep the choke plate & linkage clean.
I’m not hearing a knock or tic in that vid maybe the idle is a bit too high to hear it on the phone I’ll try it on the desktop.
It’s possible that when the engine was rebuilt the hydraulic lifter preload wasn’t set right or one of those self locking nuts backed off. It could be that, a pulled rocker stud or a bent pushrod. I’d run some marine Techron thru it after you fix the choke.

 

[snowbrd84]

If you have been running with the choke like that (nearly closed) it’s been running way too rich & may have a lot of carbon build up in the combustion chambers. One issue with electric chokes is that they are actually an electric un-choke. Unless they have 12v continuously it will stay closed. The old fashioned manifold heated chokes like what I have on my pre Vortec 4.3 do not do this; they open slower but stay open as long as the engine comes up to normal temp & you keep the choke plate & linkage clean.
I’m not hearing a knock or tic in that vid maybe the idle is a bit too high to hear it on the phone I’ll try it on the desktop.
It’s possible that when the engine was rebuilt the hydraulic lifter preload wasn’t set right or one of those self locking nuts backed off. It could be that, a pulled rocker stud or a bent pushrod. I’d run some marine Techron thru it after you fix the choke.

Appreciate the feedback. Going to connect the choke supply to purple 12v+ to close that loop and pull the valve cover tomorrow. Will report back here with whatever I find.

 

[snowbrd84]

Hey neighbor! Just took a 35 mile cruise up to Tim's in Fairview Beach today.

Heard they reopened. Good to hear. Once I get everything straightened out would love to make it down there.

 

[04fxdwgi25]

Check the exhuast valve components for that cylinder. Lifter, pushrod, rocker arm and even cam lobe. A stuck closed exhaust valve will build excess compression in that hole.

 

[snowbrd84]

I don't know much about rocker arms, so not sure how much movement is normal. See video.