Mercruiser wont run under 1500 RPM

[deep tempest]

After some help, tearing my hair out....

Have a Merc 5.7L carb with about 14 hrs on the clock, new long block.
It has a new delco electronic distributer( timming perfect ) and is fitted with a Rochester Quadrajet.
Had heaps of issues with flooding, replaced fuel pump twice, Carb rebuilt by best in city? and still had flooding problems.
Purchased a rebuilt carb from " the biggerst carb supplier in the USA", looks great, but wont idle under 1000rpm. Had a mercruiser guy look at it and he couldnt do any better.
Even at 1000rpm, once in gear, slowly dies off and stalls. So I put the old carb back on, wont run under 1500 rpm.?

The port motor, with the same setup, same fuel tank, is perfect, what am I missing here, maybe 2 MPI motors..

Any ideas out there.

 

[wellcraft-classic210]

Re: Mercruiser wont run under 1500 RPM

It could be many things given the recent work and its hard to say over the internet..

Have your tried monitoring vacuum , compression -- Are you 100% sure cam timing is OK.

Is spark being delivered at lower rpms-- A timing light or spark sensor might help verify that.

 

[deep tempest]

Re: Mercruiser wont run under 1500 RPM

It could be many things given the recent work and its hard to say over the internet..

Have your tried monitoring vacuum , compression -- Are you 100% sure cam timing is OK.

Is spark being delivered at lower rpms-- A timing light or spark sensor might help verify that.

Checked cam timming OK, I have a new Delco electronic distributer kit which was my next step. A mercruiser mechanic check the compression ok, the new long block only installed in April.
Once the engine is above 1500rpm, runs fine, temp and oil all in the green.

 

[Bondo]

Re: Mercruiser wont run under 1500 RPM

and is fitted with a Rochester Quadrajet.
Had heaps of issues with flooding, replaced fuel pump twice, Carb rebuilt by best in city? and still had flooding problems.
Purchased a rebuilt carb from " the biggerst carb supplier in the USA", looks great, but wont idle under 1000rpm. Had a mercruiser guy look at it and he couldnt do any better.
Even at 1000rpm, once in gear, slowly dies off and stalls. So I put the old carb back on, wont run under 1500 rpm.?

Ayuh,.... Welcome Aboard,... Sure sounds like a fuel delivery issue to Me....

What do the Spark plugs say about it,..??
Are they Wet when it stalls,..?? Dry,..??
Showin' signs of over-fuelin',..?? Under-fuelin',..??

 

[grewvin1]

Re: Mercruiser wont run under 1500 RPM

Same engine and carbs, had the carbs rebuilt and one engine refused to run well could not find problem was it carb or other engine issues so i swaped the carbs to see if the problem followed to eliminate the carb. Maybe a last resort.

 

[deep tempest]

Re: Mercruiser wont run under 1500 RPM

Same engine and carbs, had the carbs rebuilt and one engine refused to run well could not find problem was it carb or other engine issues so i swaped the carbs to see if the problem followed to eliminate the carb. Maybe a last resort.

Some good sugestions,
I;ll go back to the basics and check plugs for a fuel symptom, cant be spark, have had two complete ignition systems on it, electronic, and went back to points setup just to confirm. Out on a fishing charter for a week from Friday, I'll udate the thread with what I find later next week.

Thanks

 

[NHGuy]

Re: Mercruiser wont run under 1500 RPM

I'd try the known good carb on the trouble motor, If that solves it, fire your carb guy and get in touch with cliffshighperformance.com.
He is the man on Quadrajets. I am halfway through doing mine right now with his very detailed help.
My main concern though is why the motor won't idle with 2 different carbs. I hope the known good carb is the solution, if it is you have every right to ask for the rebuilder to do a refund or swap.

 

[fishrdan]

Re: Mercruiser wont run under 1500 RPM

Had heaps of issues with flooding, replaced fuel pump twice, Carb rebuilt by best in city? and still had flooding problems.

Have you checked the fuel pressure? Even with 2 new pumps I'd verify it's putting out the correct pressure. The procedure is in the stickies at the top of the forum.

I'm SWAG'ing the fuel pressure might be slightly too high and pushing past the needle>seat, enough that it won't idle, but at slightly higher RPM's can handle the additional fuel. That's just an internet guess though... :facepalm:

 

[Don S]

Re: Mercruiser wont run under 1500 RPM

Sounds a lot like a leaky base gasket between the carb and the intake. Did you use a new gasket? Was it the proper one?
Start it up and spray some carb cleaner, or WD40 around the base of the carb and see if it changes how the engine runs.

 

[deep tempest]

Re: Mercruiser wont run under 1500 RPM

problem solved , finally.
Carb was sent back and rebuilt, main needle snaped off in jet??.
So reinstalled with new gasket , started and idled straight up. so could have been a combination of jet blockage and bad gasket

thanks all for your input

Now punching over 36 kts at WOT out of a bertie 28

 

[NHGuy]

Re: Mercruiser wont run under 1500 RPM

Good that you found it out, damn carb guy. I hope mine is right in the spring. Can't wait to try it out.
That guy I posted about claims in his book that many rebuilders rarely do it completely right. He had me pull out the idle tubes, and clean out passages that Mercruiser doesn't even mention.