Coughing and wheezing at high rpm

"I've done a lot of work o

"I've done a lot of work on the engine and boat side this season, with help from some threads on the forum.

1977 Mercruiser 351 233hp, 2 bbl, elec ign, raw water.

I now have one remaining problem whenever it gets above 3,500rpms.

It should be able to WOT to 4,500 rpms, but when I throttle up and over 3,500, there is power loss, coughs, misses and the motor slows way down. At that point I have to pull back the throttle, wait a second, and then can bring it back up to speed. Same thing happens if I bring it above 3,500rpms again.

I've done the following work this season.

Compression tested:
- strong all cylinders

Cooling:
- replaced impeller
- replaced thermostat

Ignition:
- replaced the distributor, with new springs/weights
- new cap and rotor
- installed Ignitor II elec ign and flame thrower coil
- new plug wires
- new plugs, gapped at .40 for the elec ign

Fuel system:
- tested line for leaks, passed
- doped all fittings and connections
- changed water separater filter
- tested pump for correct vacuum, passed

Fuel tank:
- drained it and refilled

Carburetor:
- Had it rebuilt

Timing:
- Markings were wrong, probably b/c flywheel wasn't set when replaced way back when, so I had to set timing out in the water. Sequence: Made adjustment, ran, anchored, re-adjusted, ran, anchored, re-adjusted, etc., until it seemed right.

So the motor is now running great under 3,500 rpms, but fails above 3,500. So any ideas about what to look at? I was thinking issues could be-

1) timing problem still
2) vacuum problem somewhere
3) internal engine
4) ??? (problem probably here)

Any ideas that I can get started on?

Thanks,
Dave"

""...throttle up and over

""...throttle up and over 3,500, there is power loss, coughs, misses and the motor slows way down."

It sounds like a fuel issue; not enough at higher RPMs. Pump weak or fuel movement issue--filter dirty already. Carb float setting too low. Carb needs larger jets? Just my WAGs."

I haven't tried testing pu

I haven't tried testing pump pressure while out on the water and at high rpm. I'll do that next.

"I have exactly this problem.E

"I have exactly this problem.Engine just serviced (!!???). Problem started a few weeks ago when engine refused to start after refuelling. Mechanic diagnosed faulty king HT lead. Replaced, and started and ran ok.
" weeks later, started ok but could not get full power. Did a 10 mile run, and returned to base. Stalled many times during berting. Unfortuantely, boat is 1000 miles from my home.
My idea is to start with the basics, examine and replace all spark plugs, check fuel flow in carb, check spark and compression at each cylinder, etc, but difficult remotely. Any ideas?"

"Pau:

Please start a separa

"Pau:

Please start a separate post w/details about what year, engine model, type ignition, type fuel delivery. Thanks."