470 idles rough but runs good

[jabfd]

Hi guys my 470 started to shake after running on plane at 3500 rpm got back to the dock and at idle it shakes and moves a lot if I trottle up then back in to neutral it's fine for little bit then starts to shake and idle rough some times stalls
I changed plugs wires coil cap rotor all before it started to have rough idle

Also looks like a little foam or condensation in the valve cover cap and in the tube going to the carb oil was just change with 25w40 I did have the jug of oil in the boat all fall and winter spring until I use it to change the oil seal was open but it was caped is it possibly moister could get in antifreeze is full and oil is clean on dip stick

Thanks in advance this my first post happy boating to all

 

[nola mike]

Re: 470 idles rough but runs good

Did you have fuel in the clear line to the carb? Punctuation would make your post easier to read...

 

[jabfd]

Re: 470 idles rough but runs good

Did you have fuel in the clear line to the carb? Punctuation would make your post easier to read...

Hello thanks for the reply there is no fuel in the clear tube I noticed it was off the carb but put it back on and problem still there thanks for help

 

[Bamaman1]

Re: 470 idles rough but runs good

You've got a very simple motor, but it's a different animal. Your rough idle could be a number of things.

I would find a good auto mechanic that knows old generation motors (and works cheaper than a marine mechanic.) Put a set of muffs on the outdrive and turn on the water.

Warm the engine up, and have them check the timing. I'd have the 4 cylinders checked for compression--and make sure you've got good/new spark plugs. You'd also need to check the dwell on the distributer to make sure the points gap is right. If everything is okay so far, put the motor on an engine analyzer (like for cars), and go through the engine analysis.

It could be as simple as the carb needs a new kit. Last time I rebuilt my 470 carb (1980 model), I had to get a carb kit off a Rochester carb for a Chevy 307 V-8 for a 1967 Impala. (Mercury was never accused of being very modern.)

The 4 cylinder Mercruisers have never been known to be smooth idling motors. But they shouldn't be harsh idling either. Good luck!

 

[stonyloam]

Re: 470 idles rough but runs good

I would remove and replace the plug wires one at a time to see if you have a cylinder that is going dead (no effect when wire pulled). If so then do a compression check, could be that you have low compression in one cylinder, possible bad head gasket.

 

[Bamaman1]

Re: 470 idles rough but runs good

Note: I have a complete gasket kit, including a head gasket, that I'm not going to use.

 

[jabfd]

Re: 470 idles rough but runs good

Note: I have a complete gasket kit, including a head gasket, that I'm not going to use.

ok I am going to do a compression test i will let you know if i need it . i checked my oil this morning and it looks like it is a little higher on the dip stick but not milky . i checked the fuel tube going from the pump to the carb there was no fuel in it . However it was clogged up so i took the tube off the pump blow thru it now its clean . also noticed that the fitting at the pump is clogged up with stuff i tryed to stick a pieace of wire in there but don't know if i got it all.
Is it possible that the gas can be going in to the crank and not up thru tube becuase it is clogged. Btw it ran fine to day
at the dock and left it idle it stayed at 750 rpm and did not drop .thanks alot for all the help guys

 

[jabfd]

Re: 470 idles rough but runs good

Ok. I did a compression test. Had 150,155,155,145, this should be good right .

 

[stonyloam]

Re: 470 idles rough but runs good

Ok. I did a compression test. Had 150,155,155,145, this should be good right .

Right!

 

[NHGuy]

Re: 470 idles rough but runs good

If you had an issue with the gas getting in the crankcase you'd be able to smell fuel in the engine oil. It ran well today, well that's good, maybe your issue is gone. But if it comes back let us know.
If that clear tube was not connected correctly before you cleaned it you might have fixed a vacuum leak when you reinstalled it. Open vacuum ports will muck up yer idle and running for sure.