Mercruiser 470 Rochester Carb flooding syphon

[Blufin 21]

Hi people.

I am running a 470 Mercruiser in a Savage Bluefin with built in fuel tanks.

Starting from the start,sorry if this is a bit long I will explain as much as possible to save a lot of questions and make thing easier for you people to answer. Thanking you in advance.

I had the boat in the water running perfect and it suddenly cut out during revs of around 4000 R.P.M with no warning at all,no misfire no surge nothing.
As it was the last time I was to have the boat out last season (Australian Season) I used my auxillary motor to get back to the ramp after no success of it starting after trying for a very short time. My guess was fuel problem after checking spark.

The boat sat on the trailer untouched for some months before I removed the carby.
Once I removed the carby I pulled it apart to find it was full of crap and decided to rekit it.

We have just entered a new season here in Australia.

I rekited the carby - Rochester 2BBl marine using genuine rochester kit.

Adjusted the float to specs - 11/16 Level & 1 9/32 Drop

I screwed both mixture screws all the way in,then backed them off two turns

I fitted the carby back to the engine put the muffs on the leg hit the key & bingo it started with no drama I let it idle for approx 5 minutes before turning it off thinking I would give the mixture screws a twink if needed when I know how it runs under load on the water.

The next day I filled both tanks with fuel ready to take it out on the water for the week end,checked all fluids etc as we do before going out and all was good being anxious as we do at the start of the season I fitted muffs again restarted engine to find no problem.

Boat sat on the trailer for three days untouched.

Ready to go out on the week end I wanted nothing to go wrong car hooked up I'm going to be fussy befor taking off towards the water and check fluids.

WOW... The engine was full off fuel in the sump,more fuel than oil in the sump.

I drained approx 33 litres out of the sump plug fuel obviously up into the block it had syphoned itself from the tanks while sitting.

Now notes to take into consideration

NO it does not have an electric fuel pump.

No the tank vents are not blocked

NO my carby DOES NOT have check balls it is the model carby released from factory WITHOUT CHECK BALLS NO CHECK BALLS REQUIRED . In doubt so was I & researched to find not all Rochester 2BBl had checkballs FROM NEW

This one has me completely lost suggestions please

Thanking you in advance

Mark

 

[Bondo]

Re: Mercruiser 470 Rochester Carb flooding syphon

Ayuh,... Welcome Aboard,.... My 1st guess is the fuel pump diaphragm ruptured,...

 

[Blufin 21]

Re: Mercruiser 470 Rochester Carb flooding syphon

Bondo thankyou.

Something for me to look at

 

[Eric 3.7LX Alpha 1]

Re: Mercruiser 470 Rochester Carb flooding syphon

Usually if the carb leaks fuel into the engine, while the engine is off, it will just empty the fuel bowl and be difficult to start until the bowl fills again. I have never heard of the carb being able to siphon fuel from the tank while off. You just rebuilt it and have a new needle and seat in there, just does not seem very likely.

So I am going with Bondo on this one, thinking fuel pump leaking into engine thru diaphragm.

 

[Blufin 21]

Re: Mercruiser 470 Rochester Carb flooding syphon

Eric I thankyou. It is 7.10am Sunday here in Australia. Checking diaphragm shortly today and will let you know the outcome. Thankyou

 

[NHGuy]

Re: Mercruiser 470 Rochester Carb flooding syphon

Hi, You didn't say whether there is a filter before the carb. If there's not, put one on there. Preferably a water separating filter. If the carb was dirty there could still be a quantity of junk in your tank. Check and change the filter seasonally or if there are problems.
Maybe you'll be lucky and find that the debris that was in the carb is bits of your old fuel pump diaphragm!

 

[Silvertip]

Re: Mercruiser 470 Rochester Carb flooding syphon

There is also a likelihood of a defective anti-siphon valve at the outlet of the tank. Yes -- a bad fuel pump can cause the problem but only if the anti-siphon valve is bad. The anti-siphon valve is required for below deck tanks to prevent the flow of fuel into the bilge during a leak, or anywhere else for that matter when the engine is not running. Contrary to popular belief, that valve is not to prevent fuel flowing back to the tank. It may be that the valve has been replaced with an open barb fitting. This would be the first thing to check.

 

[NHGuy]

Re: Mercruiser 470 Rochester Carb flooding syphon

You can see whether the anti siphon valve is functioning when you remove the fuel line from the tank side of the pump. Put a catch container under the open end of the fuel line. If the fuel stops after the contents of the hose have drained, the anti siphon valve is working. If not get one, they are cheap compared to the big boom that could take place!