Prop size question

[roger-UK]

Hi all,

Just a quick question if anyone can advise please.

My 2004 185 Sport with 4.3 has a Mercury Vengance stainless 21p prop (added by the previous onwer I guess). The Sea Ray parts list I have states the boat should have a 23p on it. The reason I ask is the boat wont rev out at WOT and seems to run out of steam at 4200rpm. Speed also seems to be down running just over 40mph when all the specs I've read suggest the boat should do over 50.

Is the prop causing this? Boat is clean underneath before anyone asks and engine seems fine otherwise. :huh:

 

[springer3]

First things first: is the tachometer reading accurately? You should be able to over-speed the engine when the prop is lower pitch than the standard prop. Failing to reach max power on a smaller-than-spec prop tells me the engine is not producing spec power. Perhaps that explains why the previous owner moved from a 23 to a 21.

Engines lose power for a number of reasons. These can be checked in any order, but the order given will stop wasting time and money replacing parts that are marginal but not going to change the real problem:

Compression loss: Run a leak-down test on the engine. Valve job or rebuild are needed is this item is off. Too many people waste time and money on the fuel system and ignition system when the engine needs the attention.

Ignition system: Check the distributor advance and make sure that is working OK. Does the engine have fresh spark plugs? How old are the plug wires and distributor cap? Is the coil working at spec? The engine will run with a weak coil, but not up to spec. An ignition scope is the best way to check the health of the ignition system. Ignition parts are cheap enough to use the shotgun approach, especially if these items have a lot of age on them. Ignition parts replacement is standard maintenance, but it is cheaper to troubleshoot and replace only the parts causing the problem.

Fuel delivery system: Check this last, as parts are expensive and items above need to be at spec first. Is the engine carbureted or FI? Either system can cause power loss. Is the fuel in the tank clean and free of water and dirt? Has the fuel filter been changed recently? Is the fuel pressure at spec? Engines will run with dirty or partially-clogged carburetor jets. Dirty FI injectors also lower power. Try a couple of tanks of Techron fuel treatment (no affiliation). If that does not work, parts need to come out for a cleaning.

A lumpy idle is a pretty good indicator of compression loss, ignition issues, or a fuel system that needs attention. How is the idle smoothness?

 

[roger-UK]

Thanks for the reply,

Its a carbed 190hp engine. As I say, it appears to run fine, smooth, near silent idle and doesn't sound laboured or poor running through the rev range. The motor is immaculately clean and I have service records so it doesn't appear to have been left to rot so to speak.

I guess I'll have to get my hands dirty and look at the ignition parts... Can someone tell me how to get the carpeted panel that sits over the distributor off to allow easier access?

 

[springer3]

....I guess I'll have to get my hands dirty and look at the ignition parts...

Check the leak-down first. You have to know the engine is sound before chasing other problems.

 

[roger-UK]

I dont have a compression tester so it will have to wait until the boat has its end of season service (which isn't long)...

 

[Knotty Buoy]

Just a quick check WITHOUT THE ENGINE RUNNING !!! push the throttle wide open and check down the carb to make sure that you don't have a linkage adjustment issue.....ei is the carb opening fully? Also if you've got one, check the speed with a GPS. The pito speedos on boats are notoriously inaccurate and they often "under-read"

 

[coolings]

Did your boat ever go faster than 4200? If not, I would think you are overpropped. I went through six props trying to find one that would run properly with my boat. There are going to be a lot of differences going from difference size props (not just pitch, but diameter) and from aluminum to stainless. Stainless props can be repitched some (I think maybe two down or up. Repitching to a 19 with the same prop should increase your RPMs by 400.