Montego 534 rpm at WOT

[jfast3]

Have a 1992 with twin Mercruiser MIE 7.4i. Just purchased and had engines gone over - carbs rebuilt, new fuel filters,new distributor components, plugs, wires. 9 Run fine and will go to over 5000 rpm in neutral but only reach 3200 in gear. I think they should get to 4400. Any ideas? Wrong props? Issue with v-drives? What else can I do to diagnose?

[AaHubb]

Hey there dock mate! Welcome to the forum. Other than props one thing you could check is the vent to the fuel tank.
..Aaron

[Lyndon670]

If you are getting WOT of 5000 in neutral, then you have eliminated cables etc. it sounds like you are under propped. Also check the condition of the hull as well - not sure where you boat, but barnacles and growth are capable of knocking some serious RPMs off. How was the boat loaded? How old is the fuel - what about fuel filters? Are the secondaries in the carb actually opening WIDE? I doubt you need to worry about the V-Drives - if you can get 3K + in gear then they at working. With both engines doing the same thing, look to shared issues (hull, fuel, props, etc).

[jfast3]

Thanks. Hull is clean - just had it out of the water and went over it. Have topped tanks with fresh fuel, but some of it was certainly old. I'll check the fuel filters. I believe the carbs are opening wide, but worth checking again.

[g36]

x2 fuel filters and also check the advance on the distributors are operating properly ie not stuck or not moving freely and the advance is where it should be for engine specs

[agskinner1]

Hi There,
I had a 36 ft. Uniflite before purchasing our 1989 4207 Carver. I had a similar problem with it and it took a little investigating to get just the right diameter and pitch props. First ,I did a complete tune up, checked the bottom condition, etc. I made up a chart with rpm, vacuum and speed, and then ran the boat out, loaded as I would when I usually cruise ( full water, fuel,food etc.). Every 300 rpm, I wrote down the speed and vacuum, up to WOT. I could only get the revs up to about 3600 rpm. Then I called a good prop repair shop and talked to the owner for a while. He told me that 1" of pitch equals roughly 200 rpm, add 1" and you get about 200 rpm less, and take out an inch of pitch and you gain roughly 200 rpm. With diameter, you get about 400 rpm difference for every inch up or down.
I had 19" X 18" 3 bladed props when I started this experiment, and wound up with 17" X 16" four blade props. I went from a top speed of 18 1/2 knots at 3600 rpm, to a top speed of 26 knots at 4400 rpm (which is the manufacturers rpm at WOT) and doubled the vacuum at 3300 rpm. To me, that means less stress on the engines and better fuel economy.
Also, if the boat has spent it's life in the chuck, check you exhaust manifolds for restrictions.
Good Luck