140 Shaking at Idle

Hello all,

It's been awhile. My wife and I have been busy, finally moved onto a lake in the Fox Valley of Wisconsin three years ago. I swear my boat tools and parts are still hiding from me in storage. I spent this summer getting a boat lift in place for the pontoon. It sits in deep water and has been quite a challenge.

The engine on my 1974 Mercruiser 140 seems to be shaking a lot at idle. It starts great and runs just a bit rough until the choke opens. I don't even try to idle until the temp rises to 130. It settles in around 140, which matches the thermostat. I had the carb rebuilt 10 or 15 years ago. The boat doesn't get a lot of use in terms of hours. It still idles at 550 rpm, which is about the spec, I believe. But it shakes pretty good at idle. I have thoughts of tuning the mixture (my gauge is in storage somewhere), maybe it's one cylinder missing (?), or maybe it's an engine mount (I sure hope not).

GM Engine 140 HP 4-cyl 181 cu.in. s/n 3888148

Mercruiser "Late Model I" Stern Drive s/n 3901959

I thought I'd run it by you folks with 100x the experience I have, so here's a video.

-- The video starts with the engine at 140 degF, in neutral at 1500 rpm.

-- At about 7 seconds, I throttled back to 1000 rpm (I went a little lower and came back up, as you can see by the shake).

-- At 15 seconds, I throttled back to the idle detent and the engine settled in at 550 rpm.

-- At 31 seconds I turned the ignition to off.

Before I go down the wrong path, I'd appreciate your thoughts.

Thanks.

Steve

edit: insert video didn't work

You'll even find mention of when you thought the carburetor was rebuilt back then. It might have been 2005

You have a Pertronix Ignitor, I wouldn't worry about that unless nothing else works.

Nothing to do with your running issues but, I'd also look at my fuel line to see if the rubber hose is USCG approved hose and I'd look at how you attached it. It looks like you cut the metal line and just slid the hose over it and hose clamped it. The hose is supposed to go over a flared or barbed fitting on both ends and be hose clamped, not over smooth tubing. It keeps the chances of a fire down 

I just realized I'm posting a Mercruiser issue on the CCFan forum... Good Lord, I'm really losing it. Sorry. Our Ski Nautique is running well (knocking on real wood).

Keno, thanks for the link. That search is a good idea. I can't believe how much I've forgotten over a decade or so. Use it or lose it, I guess.

Thanks for reminding me of regular maintenance -- I've been neglecting that. I'm getting cap, rotor, wires, plugs and both fuel filters (pump & inlet). I'll take a look at the fuel line.

I used a little carb & choke cleaner without any noticeable improvement. The carb was rebuilt in Spring of 2006. I may have to pull it and have it rebuilt. My fingers and eyesight don't do well with little springs and such.

The choke is staying wide open all the time. I can't remember if I disabled it, based on a post from Tim (TRBenj). I'll check the voltage and linkage. I switched to electric choke awhile back.

I thought I may have found the problem, but no. A long time ago, I replaced the pipe plug on top of the intake manifold behind the carburetor with a hose barb adapter. I had a rubber cap on the barb but it had split. I replaced it and even taped over it but no change. Maybe I should pull that hose barb and put a pipe plug back in.

I'll look for some tubing and listen around the base of the carburetor for any vacuum leaks. And if I can find my timing light, I'll check that, too. This stuff was a lot easier on my body 15 years ago...

Steve