88 4.3 Mercruiser won't go over 2500 RPM under load

[Hondaman74]

Good Morning everyone. I recently purchased a 1988 Mark Twain super clean boat. Has a 4.3 Mercruiser in it. Got it on the water last weekend and would only go about 3200 RPM max barely planed out, running rough then eventually died running rough. Brought it home and did the following to it.

1. Spark Plugs
2. Cap and Rotor
3. Wires
4. Water Pump, it was making noise
5. Fuel Water Separator filter

I bought a carb rebuild kit but felt the running problem was ignition related. After installing all the parts I timed it to 8 deg. starts right up and idles good, revs in neutral well. So we got it back on the water yesterday and it will not go over 3,000 RPM sounds like its laboring, no more opting, spitting, or dying anymore, runs good but under load has no power won't plane out won't go over about 8 mph, super dog.

So we got it back home checked timing again, we even adjusted it on the water manually did not make a difference trying to manually advance it while running. So now we are left with three options

1. Carb problem but does not seem like a carb problem. It has a electric fuel pump installed
2. Exhaust plugged
3. Poor compression

We pulled the passengers manifolds yesterday looking at the flap and manifold didn't see anything still need to pull the driver side to see if flap is OK. I am hoping its in the exhaust.

The way it runs I have a hard time believing compression is a problem, but once I get both manifolds off I will check this maybe today.

Don't feel its carb problem, carb seems to be dumping fuel when running under load but just seems bogged down.

Anyone have any thoughts on this. Some other technical info. 1.84 outdrive gear ratio, 19 pitch prop.

Any ideas would be great, thanks.

Also having some weird electrical problems with gauges that I will post about after this but gauges flutter constantly when key on with fuel pump so I isolated pump with relay and its better but still flutter, gas gauges needled is all over the place doesn't to sit still if you think this could be related

Thanks everyone

 

[alldodge]

Do a compression test
What pitch prop?
With gauges bouncing around check your grounds, clean them

 

[Hondaman74]

Do a compression test
What pitch prop?
With gauges bouncing around check your grounds, clean them

I am going to do compresion test tomorrow. Pitch is a 14.5 prop by 19
I re did all the grounds that I can find and didn't change it at all. I am thinking possible Alternator seems more of a power fluctuation than a ground problem.

Thanks

 

[matt167]

check your plug wires. Sometimes they can be out of order but still run smooth without power

 

[Texasmark]

There is an I/O- Inboard Mercruiser forum where you may get better responses to your questions....see below.

 

[alldodge]

There is an I/O- Inboard Mercruiser forum where you may get better responses to your questions....see below.

Good point, GA_Boater can you move please

 

[GA_Boater]

Done. Tanks again, AD

 

[Hondaman74]

Gentlemen, thanks I thought I had posted it in the MerCruiser thread, sorry. So as an update did an compression test all 145-155, cold, both flaps open moving free, no exhaust restrictions. so its back to ignition not advancing or carb issues. Have a carb rebuild kit I am going to take it off this week and do that while I put manifolds back on and then I can test timing again for advance and total timing etc.

Thanks for all your help. Matt167, I have checked them several times and they are all good
Keith

 

[Rick Stephens]

I would support the need to do a carb rebuild. Pay special attention to float height. Make sure you got every passageway clean.

 

[Hondaman74]

So I rebuilt the carb and replaced the ignition module and I can get it to run a lot better now. I am still suspecting carb as an issue but was able to get it to rev up and plane out at 33 mph. I don't have a tach yet will be here Saturday to tell what its doing but its running pretty good but still not where I think it should be.

One question I noticed before the rebuild and after the rebuild the carb drips pretty bad when its running above idle. Fuel pressure is 5-6 PSI. Recheked float level and everything is good but I do not like the drip and think its running it rich. I can't guarantee this yet till I get tach running but never seen a carb drip like this in my 25 years of working on cars.

any thoughts out there. Its a Rochester 2bbl carb.
Thanks
keith

 

[Scott Danforth]

if your carb is dripping, you have either a poorly set float, the needle and seat are bad, or your welsh plugs are leaking