74' Mercruiser 120HP bogging down after prolonged idle....HELP

[Fordiesel69]

1974 Sea ray SRV190
Merc 120HP GM 4cyl

We can run this thing for hours at 3700RPM and it runs excellent. However if we slow down to idle (slow no wake) for 5-10 minutes it will not rev up back up to get on plane. It bogs down and kind of flutters up an down. It did backfire a few times as well. Pumping the throttle a little keeps it running at least. When acting up, it will still run nice at low rpms.

Distributor advance looks good, points look good, fuel filter cut open and looked good with just a tiny bit of dirt on the bottom, and the carb looks spotless inside with no water. We pumped the fuel tank out and found no debris or water.

Any ideas?

-New points in 2009
-New aftermarket manifold in May 2010
-New filter in April 2011

 

[Don S]

Re: 74' Mercruiser 120HP bogging down after prolonged idle....HELP

Probably "loading up" with excess fuel from idling. Tuneup and carb mixture adjust will usually fix it. Also check that your choke is fully open when the engine is warm.

PS: Check your private messages

 

[Fordiesel69]

Re: 74' Mercruiser 120HP bogging down after prolonged idle....HELP

From the sound I beleive the opposite. It seems to bog (lean) and backfire (timing) upon throttle up but when brought back to idle it runs nice and smooth. It also revs up with no load just fine.

Choke is fully open when warm. Engine always starts instantly hot or cold, choke has always seemed to work well.

 

[Don S]

Re: 74' Mercruiser 120HP bogging down after prolonged idle....HELP

Does the accelerator pump squirt fuel into the throttle bore when you operate the throttle?

 

[Fordiesel69]

Re: 74' Mercruiser 120HP bogging down after prolonged idle....HELP

Very much so. More than 2 pumps will make it very hard to start.

 

[jim372]

Re: 74' Mercruiser 120HP bogging down after prolonged idle....HELP

did you pull they spark plugs out and check them, are all fireing and color , wet from fuel not burned

 

[Fordiesel69]

Re: 74' Mercruiser 120HP bogging down after prolonged idle....HELP

Only checked them when we "unwinterized it" they looked good. I already ran it on the muffs so plug color will be no longer accurate.