1978 Mercruiser 140 hard to idle

[RandyJ]

I've got a couple of old boats with the 4 cylinder chevy engines, Mercruiser 140's around 1978 vintage. Love them but don't know why... I've been through these engines time and again but still have problems getting them to run at idle using factory adjustments. I can't get them to idle with the idle screw adjustment so have to take out the slack on the throttle cable which keeps the idle screw very far away from the cam it is supposed to rest against. I've tried to adjust valves, timing, etc. just to get it to run better at idle. I assume the big change is the fuel we use today. Any tips on what changes I might need to make or what to look for?

 

[Bifflefan]

Re: 1978 Mercruiser 140 hard to idle

You did not mention a carb rebuild, or replacement.
If you put your hand over the top, will it idle? If yes, then you have some big vacuum leaks that need attention. If no, the idle passages may be plugged.
When you say "been through", what does that mean?
There is something that is not right or it would idle fine.
The gas is not the issue or everyone would be having the same problem.

 

[EddiePetty]

Re: 1978 Mercruiser 140 hard to idle

........ I can't get them to idle with the idle screw adjustment so have to take out the slack on the throttle cable which keeps the idle screw very far away from the cam it is supposed to rest against.......

...if you have the idle screw all the way to the end of its travel (in), the 'slack' out of the throttle cable and the idle screw is completely off the cam......well my friend, by that time the butterflies are well past the idle ports and you are trying to idle the engine(s) either on the transistion ports or main jets!!!

You need to go back to step #1 and conduct a COMPLETE tune up to permit the engine to idle on the cam with the mixture screw(s) correctly set.

 

[BF]

Re: 1978 Mercruiser 140 hard to idle

my Dad had similar prob's with his '76 mercruiser 120, which I think is what you have (2.5L), I think (?) that they weren't 140's (3.0 L) until later. The engines look the same..

Anyway, he'd been chasing rough idle/hesitation probs for the whole season. He was on his second set of cap/rotor wires etc... I had gone through adjusting the idle mixture etc, and couldn't get it much better. It ran well from mid throttle up, but had a rough idle and a bog/hesitation when getting into the throttle.

I showed up while he was checking the points or something (again) and flipped thru the manual as we chatted. The pict of the firing order got me thinking, so I said, you probably flipped 2 wires around. He gave me the "don't call me an idiot" look, but then I looked at the wiring, and sure enough 2 were firing in the wrong order. Despite that he was careful when putting the new cap on, he made a mistake, probably during a tune up at the end of the previous season. He actually had replaced the cap again since then to try and resolve the idle prob, but carried the mistake forward by making sure he put the wires on the same spot.

After flipping the wires, I had to redo the carb settings again, but got it idling great, and the bog was gone. He was happy, but what surprised us most was how well it ran with the wrong firing order!

So, there's something for you to check, since your story sounds a bit like our experience.

 

[Bondo]

Re: 1978 Mercruiser 140 hard to idle

You did not mention a carb rebuild, or replacement.

Ayuh,.... Sounds like a Carb Issue to me too...

 

[RandyJ]

Re: 1978 Mercruiser 140 hard to idle

The carb has been gone through, cleaned and put back together a couple of times since the boat was brought out of the cob webs. It had set up for a few years. I'm running it on a jerry can (out board tank). Before I put the tank on it had lots of issues due to trash in the fuel line. I'm not sure where the idle screws should be set but I know that normally idle screws are out 1-1/2 to 2 1/2 turns.
As for the plug wires... I pulled the book when I put a cam in since the wiring got really confused. Points set on .020, timing is/was set to specs... I agree that it is carb issues. Guess I just need to give it one more cleaning and check for vacuum leaks.

 

[86 century]

Re: 1978 Mercruiser 140 hard to idle

I just rebuilt a rodchester 2 jet that had the wrong throttle plate gasket. The fellow I got it from said it worked good above idle. After a good cleaning and some new gaskets she works like a new one.