RPM question 1972 Trojan

[BMac]

I purchased a 1972 30' Searaider last year and we just finished our vacation up in Georgian Bay. Lots of fun and mostly sunny days. During the trip the most I could get out of the Twin 318's was about 3100 RPM's. In smooth water the top end was about 25 mph and in rough water about 18 mph at top end. Both engines were rebuilt about 450 hours ago and have given me no problems other the the RPM concern.
The owners manual states the engines max recomended rpm should be 4000.

Anyone have any previous problems like this ?

[rossjo]

What RPMS in neutral? Full revs?

[JuiceClark]

Other than the obvious things like barnacles on the props or oversized props, I have one unlikely thing. One time I took a piece of junk Sea Ray I had out and could only get about 3000 rpm. I kept checking the engines because I knew the bottom was clean.
It turned out the trim tabs had clogged up and were frozen in the down position dragging my ass across the sea. The SeaRay design was so crummy, I couldn't even tell it was plowing a bit. You'll be able to feel it plowing in a Trojan.

Good luck,
Tony in Fort Myers, FL

[ronski]

I had a problem like that They replaced the filters tuned the engines replaced the carbs and finally they replaced the distributers and my power was back up to specks

[BMac]

The RPM's in neutral are ~ 750. I put on a snorkel and there is no buildup around the props. I will check the trim tabs next weekend (did not think of that) . The mechanic is suggesting carb rebuilding, not being a mechanic I just thought it odd both engines at full throttle are still the same. I thought if it was a mechanical problem the engines might perform differently.

Thanks for the suggestions.
Bill

[rossjo]

Can you get fll revs (4000+) in neutral? This is a quick 1st testt - then you know if your problem is very basic (alternator?) or a load issue.

[ronski]

My engines did have different top rpm's port ran to 2900 at the begining and the starboard ran at 3100. They were close. The most improvement came from carbs,and distrubuters

[rossjo]

You only got 2900 ad 3100 RPM in neutral @ full throttle?

If so - you've got ignition or fuel delivery issues!

[ronski]

NO!NO!

This was with the engines engaged. I was running the boat on a calm river.

[rossjo]

what RPMS do you get in NEUTRAL?

[ronski]

neutral 800 at idel to a clean 4400 redline

[BMac]

Engines went to 4700 in Neutral and the trim tabs were operating properly.
Bmac

[rossjo]

Ronski and BMAC - we have 2 threads going here - I think Bmac started this - so I'll conitnue with him ...

If both turn 4700 RPMS in neutral - you sound like you're getting fuel and spark (possibly weak). What kind of voltage do you get onthe primary side of the coil? You need 12-14 volts (10-11 won't work) to pull a load.

OIl pressures strong at this RPM (20PSI+). 30+PSI @ idle when cold?

Smoke (white, black)?

You need to measure your cynilnder pressures - they should all be within 5 PSI of each other. Any cylinder down 5PSI lower than others indicates a problem.

Do you have good plugs and wires?

When were the carbs rebuilt (what type are they)?

[BMac]

The engines and carbs were rebuilt ~ 450 hours ago. The oil pressure is good. As far as voltage, carbs, wires and the pressure in the cylinders go I will have to ask the marina to check that for me.
The exhaust was white on startup.

Thanks
Bill

[rossjo]

Is the exhaust clean or black (or different from side-to-side) under full load?

Plug wires don;t last long in the marine environment - I'd replace if they are over 5 years old just to be safe anyway. May not fix, but its cheap insurance. I also prefer the Champion SS plugs - they won't rust in place.

Compression check is imperative - if you've got a lot of hours, you may need a valve job ...