Thumping in 1978 Merc 1400- VIDEO

[rustyscrew]

My 1978 merc 1400 has been running great for a year since I bought it. I was running for the 3rd day in a row at 2/3 throttle and it was sounding good- suddenly I heard a dull thumping sound (rapid)...maybe some loss of power. I shut it off immediately and when I turn it over with the electric start it has a repetetive dull thumping sound coming from within the engine. It sound like something knocking into something else in the powerhead...it doesn't seem to be coming from the lower unit at all.

Also the thump seems to happen maybe every other revolution.....

I suspected a broken rod or ring, but I just checked compression in all 6 cylinders (inline 6) and compression is good in all.

This leaves me COMPLETELY stumped...with salmon season rapidly approaching!

Thanks in Advance for any help.

 

[rustyscrew]

Re: Thumping in 1978 Merc 1400- HELP!

Re: Thumping in 1978 Merc 1400- HELP!

JUST IN- I am working on this engine right now. So I tried to fire it up since it didn't seem to be a compression problem. The thumping/knocking sounds pretty loud when it is cranked over and begins to catch. However when it catches, it immediately stops with a loud thunk!

Also, when we turn it over with the plugs detached it still knocks...therefore I am thinking it is not an electrical or timing issue.

Super stumped!

thanks

 

[rustyscrew]

Re: Thumping in 1978 Merc 1400- HELP!

Re: Thumping in 1978 Merc 1400- HELP!

JUST IN- I am working on this engine right now. So I tried to fire it up since it didn't seem to be a compression problem. The thumping/knocking sounds pretty loud when it is cranked over and begins to catch. However when it catches, it immediately stops with a loud thunk!

Also, when we turn it over with the plugs detached it still knocks...therefore I am thinking it is not an electrical or timing issue.

Super stumped!

thanks

 

[Scaaty]

Re: Thumping in 1978 Merc 1400- HELP!

Re: Thumping in 1978 Merc 1400- HELP!

Pull the plugs. One cylinder at a time, crank it by hand and when the piston is is going toward the crank, stick something in the hole and give the piston a good shove. It moves forward, ya got a piston pin problem. Bushing probably went south on one of them

 

[rustyscrew]

Re: Thumping in 1978 Merc 1400- HELP!

Re: Thumping in 1978 Merc 1400- HELP!

Thanks. I will try that. I guess that would be consistent with good compression? Piston is still working but is rattling...does this sound right?

Will that test work for an inline-6?

You guys rock...thank god for this site!

 

[Scaaty]

Re: Thumping in 1978 Merc 1400- HELP!

Re: Thumping in 1978 Merc 1400- HELP!

It will work on anything. What your trying to do is get the crank to pull the piston down. Then you push on the piston. You will feel it move and them make contact again if the pin bushing went south. You will still get good compression though

 

[rustyscrew]

Thumping in 1978 Merc 1400- VIDEO

Thanks scaaty. I will give that a shot. I am trying to post a video of the problem since it is difficult to describe. Maybe this will help the diagnosis. You can here the knocking as I turn it over and it stops suddenly as soon as it starts to catch.

Does the video change your thoughts at all?

thanks

 

[rustyscrew]

Re: Thumping in 1978 Merc 1400- VIDEO

Stand by for video...file is too big. I will try to put it on youtube a bit later. sorry.

 

[rustyscrew]

Re: Thumping in 1978 Merc 1400- VIDEO

Ok I put the video on you tube at:

http://www.youtube.com/watch?v=yGv2wrhgJkU

I don't know if this will be more helpful. Any thoughts?

thanks

 

[Scaaty]

Re: Thumping in 1978 Merc 1400- VIDEO

Wow, thats a tough one. Try getting a auto stethoscope and poke around and see if you can isolate the knocking

 

[jimmbo]

Re: Thumping in 1978 Merc 1400- VIDEO

I think it needs to come apart

 

[Chris1956]

Re: Thumping in 1978 Merc 1400- VIDEO

Rusty, Although I am not a professional, I have owned and repaired my '77 Merc 1500, which is likely identical to your motor. I listened to your video 3 times, and never heard anything unusual. I would have liked to have heard it run at idle, since the cranking sound can mask other sounds.

I recommend you remove the spark plugs and rotate the flywheel by hand. Feel for any tight spots. Next do the crankpin bearing test recommended by SCATTY and see if you find play in the wrist pin needle bearings. If both of those tests are negative, I would run it. If it has future problems, you can deal with them in the future.

 

[rustyscrew]

Re: Thumping in 1978 Merc 1400- VIDEO

The severity of the sound doesn't come across well, but you can hear the thump especially on the top view. Especially since it went from purring like a kitten to this rough sound in a second. I will try the pin/bushing test today or tomorrow and get back to you.

thanks fellas

 

[rustyscrew]

Re: Thumping in 1978 Merc 1400- VIDEO

I tried the piston check (5 of 6) and they all felt solid. I then fired it up again and idled it. It still has a knocking time sound to it. It does engage and spin the prop fine.

I took it to a local mercury mechanic to have him hear it. He agreed it sounds bad and he thought it was in the lower unit. So now I have the lower unit off and I fired it up briefly to see if I still hear the problem. I think it still is running rough...although it sounds different now that the drive is off.

I guess I am thinking the problem is in the powerhead again. Have I reached the end of tests before tearing it apart?! :%

thanks.

 

[Chris1956]

Re: Thumping in 1978 Merc 1400- VIDEO

Rusty, You can put a water hose on the copper water inlet pipe and run the motor w/o a LU for as long as necessary to isolate the noise to the powerhead or LU. I would hate to see you tear apart the wrong one. You might stick the midsection in a bucket of water to quiet it down. It them may sound more like it does with the LU installed.

 

[rustyscrew]

Re: Thumping in 1978 Merc 1400- VIDEO

THANKS. I was wondering about that. I just talked to the mechanic again and when he heard that it wasn't the lower unit, he immediately concluded it was toast. He says that often the crank-shaft shears beneath the fly-wheel. It shears at an angle, so it still spins..but roughly. He is super busy right now because salmon season is starting and everyone is bringing in their boats. So I can't get him to look at it seriously until the 9th....after the opener!:'(

I may dig into it this weekend and see what I can find. I will start with your suggestion Chris1956...thanks.

keep you posted8)