tick over problems

[Medway & beyond]

Hi all,

I'm hoping for a bit of advice with my mercruiser 470 engine.
I recently purchased a remanufactured Rochester 2gc carb as the old one had problems with the idle mixture screws and seats, plus it was heavily flooding (most likely due to the float) on installing the new carb, my engine is still running extremely rich, even with the mixture screws turned in almost all the way, I'm reluctant to try to tighten them any further in case I damage the new seats, the spark plugs, after cleaning them are turning black and sooty within 10 minutes every time.
I have a new distributor, leads, plugs, and points.

Any advice would be most welcomed.

Thanks.

 

[spannerman]

First check the choke is going completely off when the engine has reached operating temp, also that the flametrap isn't clogged.
Your carb may have airbleed screws to adjust the mixture, so screwing them in makes it richer, screwing out lets more air bleed past the closed throttle plate giving a leaner mixture
You will have to play with the mixture and idle screw on the throttle linkage as they affect each other, and if the throttle is slightly open it won't be running on the idle jet, so get it warm set the mixture screws about 2 1/2 turns out, set the idle speed to around 700rpm, readjust the mixture and idle as req'd. The final idle should be 650rpm in gear, going back up to 700/750rpm in neutral.
Give it a quick rev in neutral, if it stumbles or bogs down its too weak.

 

[Medway & beyond]

Thanks for that spannerman
Choke and flametrap are both fine

I will try screwing them out a lot further today as suggested, and follow your procedure for idle rpm, I've currently got her idling at 600 rpm in neutral.

I'll let you know how I get on.
Thanks again.

 

[spannerman]

Just checked the manual, looks like your carb has normal mixture screws, in for lean, out for rich. I guess you have checked the float height is correct and the fuel shut off valve is working.

 

[Medway & beyond]

Thanks again

I have been working on her today, again with no joy, still running extremely rich with the mixture screws wound all the way in.
I haven't checked anything on the carb yet as I was hoping that it would be set up correctly being that it was remanufactured for my model engine.

Would I be getting these symptoms if my timing was out fractionally?

 

[spannerman]

If the timing was out it wouldn't run as effectively, but I can't see it making it run extremely rich, it would either be harder to start if it was too advanced and run unevenly, if it was too retarded it would be slow to pick up.
To turn the plugs black after 10 mins is excessive fuel which isn't burning as you have already used up all the available air in the cylinder, a bit like running with the choke on.
But check the timing, with problems like this take nothing for granted and make sure all basic settings are correct before diving in deeper.

 

[pampas]

is your air filter clean?

 

[Dave_Seager]

Are you certain that the new carburetor has the same jets as the old one? One carburetor can be equipped with a variety of jets to suit quite different engines.

 

[wipe_out]

I don't know the carb specifically but I would check main jets and if it has check the emulsion tubes as well..

Mixture screw, while effecting the full rev range, don't have a huge effect at larger throttle openings.. This is all down to main jets..

How are you reading your plugs?

Are you cruising for 10 minutes then immediately killing the engine and pulling the plug or just idling for 10 min or revving and then shutting off the engine?

 

[GrahamHR]

Ask the question on an American boat forum; a huge pool of experience there and they are rarely confrontational.

 

[wipe_out]

You could also grab yourself a "Gunson Colortune" (probably from Halfords or similar).. Its basically a glass spark plug that allows you to actually see the colour of the combustion and tune accordingly.. Used one for years on my old carbed cars to check and set air mixture screws.. Probably not good to rev over about 3k rpm though so not going to help much with WOT mixture testing..

 

[Medway & beyond]

Thanks again all

I've always had good success in tuning my cars and bikes over the year's, this problem has become baffling.
Doing further research last night, according to my manual, if I have a certain fuel pump installed that is faulty it can over pressurise the system (although I'd expect the float to still be able to shut the flow off?) So my next step will be to check the pump, and the float.

I'll update as to my success or failure.

Thanks again.

On my next visit I will go in armed with a vacuum gauage and a gunson colour tune also.

 

[gordmac]

The pump could be producing enough to flood it, is it mechanical or electric? Can you check the fuel pressure? I am not familiar with US carbs but if screwing the fuel screw in completely is making no difference I would suspect the fuel is getting in some other way, flooding due to too much fuel pressure (or a float/valve fault) would be my guess.

 

[Medway & beyond]

Ask the question on an American boat forum; a huge pool of experience there and they are rarely confrontational.

Hi all quick update, I dismantled the fuel pump, the diaphragms looked past there best.

I took Graham's advice and found pages of American forums all with the same problem, it seems once the pumps are old, the pressure valves give out, you need to use a spacer between the block and pump housing to reduce the pressure. I have opted for a new pump.

Will put an update on once she's running perfect again (fingers and toes crossed)