1975 merc 500 idle issue

[oba97]

I have been working on my1975 merc 500. I have replaced the fuel lines, wiring harness, fuel pump gaskets,impeller (last year). When I start the boat it runs fine. I can get her going WOT with no issues and seems smooth. After it's warm it doesn't want to idle or go at low speeds very well and will eventually die if not given gas. I also noticed that when I shut the motor down the engine get very hot but when it's running I can keep my hand on it. Any thoughts on my issue?

 

[UKMike]

Does it have a poppet valve and have you looked at that?

 

[oba97]

I don't think so? How would I know? I'll take a look through the parts diagram.

 

[oba97]

Is it normal for a the cylinders to get very hot once shut off?

 

[UKMike]

With respect, that's like saying does water get more wet when it's been used?

Most engines will show a heat bloom after coolant flow shutdown, but that's not what your issue is (by your own description).

I don't know your engine but as far as I know you will have idle and main jets, if you engine is running well on WOT but bad on idle AFTER WOT, I would guess you have an issue with low chamber pressure, carburation or as I said before the poppet valve, which opens at a controlled time to relieve pressure/temperature. It's always a fine balance between WOT and Idle but there are lots of other factors that could be playing a part.

 

[oba97]

I couldn't find any reference to a poppet valve in the parts diagram so I don't think that's it. I know my heat questions was a little 'captain obvious' just wasn't sure if the heat bloom would be so hot that I can no longer hold my hand on the cylinders? Maybe I just need to monkey around with the idle jet a bit?

 

[racerone]

No poppit valve and no thermostat on these popular / built forever 4 cylinder motors !!-----Look for obstruction in waterjackets.----Possible pieces of a broken impeller.

 

[oba97]

I don't think I have a water pump issue because I get good flow when running.

 

[racerone]

Nope----The tell tale is to indicate that the waterpump is working.--------But that does NOT indicate good water flow through the motor and water jackets !!!

 

[oba97]

If I am able to hold my hand on the motor when running wouldn't that indicate I have good water flow through the motor?

 

[racerone]

Your motor will not idle you say.-----I guess it is time to do somme basic outboard trouble shoooting.