RPM loss on return trips

[megbert]

twin 350XL Crusaders on our 32' Voyager. Last two weekends on our return trip from our favorite anchorage, port engine dropped to 3600rpm from normal 3800-4000 for 30 minute trip. All gauges reading normal no CE lights ran at 3600 steady but would not go higher. Trip out in the morning normal, actually brought to 4100 to see if I could recreate and ran great. Starboard engine fine. Two separate fuel tanks.

Thoughts?

[mjk1040]

I'd start by checking fuel filters. Check inside distributor cap for corrosion of cap and rotor. Recheck timing. Spark arrestor clean and check choke working free. Check valve at tank? I would suspect fuel issue first.

[Cooler]

+1 on above. Are you burning gas with ethanol? If you are fuel injected, you may have some injector(s) getting dirty. Run some Star-Tron fuel treatment through that tank. Make sure it is gas treatment, not diesel. They package both similar. Make sure those flame arrestors are clean. Most people ignore them and they get covered with an oily film. er

[bud37]

Considering the trip out is fine....it is possible the fuel pickup in the tank is getting partially blocked as the boat bounces around, then after sitting calm for a long while whatever drops away and all is clear again......check your anti-siphon valve as well.
Another may be cumulative heat maybe a coil, try swapping sides.

[megbert]

not fuel at all, long story short, ends up compression on 1, 5 cylinders are "0". Head coming off tomorrow.

[bud37]

So if I read this right you have 0 compression in #1 and #5 cylinders....I will be very interested to see that cylinder head and gasket when you get it off.

I assume you had a look under the valve cover for spring/pushrod/rocker issues...?

[Viper]

Before you remove the head, have you confirmed it's an upper end problem and not the lower end? Should be testing for that before dismantling.

[km1125]

Wait, am I understanding this right? You had problems on two return trips, but the next morning the engine runs fine. You're measured "0" compression on two cylinders now? I'm wondering how those cylinders got their compression back for the morning run (or the next weekend). Something doesn't sound right.

[megbert]

Sorry for the delayed response. So next time out, boat ran lousy out and then back, fuel became less of logical explanation so compression check showed some definite issues. Head was removed, a crack in the head on an exhaust valve and the two plugs on the cylinders with 0 compression were melted on the bottom. No other visible damage to cylinder and related parts. Rebuilt head was reinstalled and engine runs fantastic. Only explanation on the initial theory of running good on outbound trip is as block heated up, crack in head would worsen?

[RGrew176]

Thanks for posting the end result. It may help someone else in the future.