Acceleration in forward

[mgreenhouse]

I am stumped by the following problem that has prevented me from using the boat for several weeks.

The engine accelerates very well in neutral and reverse. But develops insufficient power to exceed 1200 -1500 rpm in forward. When in this condition, the rpm also varies slowly over a range of approximately 200 rpm. I dove on the prop and found it clean.

I replaced the whole carburetor (Zenith) with a new one. This was a $300 mistake, in that it had no effect on the problem. I checked the ignition timing under load (1500 rpm in forward) and found it to be fine. When laboring at 1500, I confirmed that the throttle lever on the carb is at the high speed stop and the choke is fully open. This is not a control linkage problem.

I noticed elsewhere on this forum a suggestion that inadvertently reversing the #2 and #3 plug wires can produce the above symptoms. My plug wires are properly routed. I have the Pertronics distributor. The cap, rotor, plug wires, and plugs are all new this season.

I am worried that the slow variation of rpm under load is indicative of a vacuum leak. However, I don't want to add a $500 mistake on top of the above $300 mistake by changing the manifold without a definitive diagnoses.

I don't know what to do next and would appreciate expert advise.

[ndutton]

My first suspicions are stuck timing advance weights or exhaust obstruction.

[edwardc]

My first thought was exhaust obstruction too. Anything that makes it hard for the engine to "breathe" will limit the maximum horsepower.

The two commonest causes of exhaust obstructions are

1) Collapse of the inner lining of the exhaust hose, due to age or overheating.

2) Buildup of salt and minerals right at the water injection point.

[mgreenhouse]

Thank you for the exhaust suggestions. The black iron exhaust was replaced this season and the wet section hose is unobstructed.

[Sam]

I too suggest looking at the centrifugal weight in the distributor under the plate. Check the for loose, old or missing springs [2 of them]. Lubricate with oil or spray and check for rotor "snap back" after slight rotation by hand. It also is not beyond the possibility that the spark plug wires are out of sequence 1-2-3-4 vs the required 1-2-4-3 [I got a little concerned when you mentioned the 2-3 sequence]. This is a very common mistake even by some long time owners,