Volvo penta aq120 only hitting 3000rpm

[Canadianbuds]

New to the forum, hey all!! So I just picked up what I believe to be a 1967ish hydrodyne.. it has the aq 120/200 with twin zenith stromberg 175 CD carbs.. boat starts, runs and shifts perfectly.. But I'm only getting 3000-3100 rpm on what looks to be the original penta tach, and reaching a top speed of 26mph on GPS.. I have fresh fuel, no temp issues and with flame arrestors off it sounds like the carbs are perfectly in sync.. thanks for any help you may be able to provide.. cheers!!

 

[Bondo]

Ayuh,..... Welcome Aboard,..... My 1st Guess is,.... To much prop, try a lower pitch,....

2nd thought is,..... Do a Compression Test to see it's general health,.....

 

[Canadianbuds]

I'm at camp right now so I'll need to wait to get back to town for a compression test.. but the plugs are a nice paper bag brown, but I'm not even getting a hint of smoke, so it could be burning a touch rich, but I dont suspect its burning any oil. And I dont know what prop is on it, (10308) but I do know that props for the penta 200 o/d are almost impossible to find even used.. I've traced this boat through a few sales from about 8 years ago from the states all the way up to Canada, I really dont think it would have been molested since the restoration.. if that means anything.. but the guy I bought it from never had it in the water so he has no idea about what the performance of the boat should be.

 

[kenny nunez]

The first thing I learned in marine service school on a problem like that is to be sure the throttle is opening the carburetors all the way. That may not be the problem but worth checking. If your engine does not have a front mount the transom might be flexing and the bow is not coming out of the water enough.

 

[Canadianbuds]

Than you for the responses,, the throttle is opening to wot, that was the first thing I checked ;-).. I'm starting to think bondo was right, I have the wrong prop. When this was restored it was by a gentleman who ran a ski club.. so I'm thinking this was setup to pull skiers.. now my issue is figuring out what prop I have #1 and what i need #2.. i know its 17 spline, and i think it's a short "standard " hub, but the stamped numbers might as well be in Chinese for me... on the outside of the hub it reads 10308, and under the cone it says, 592 in one spot and 11 in another, also reads Michigan and smc.. if that means anything? I consider myself very mechanically inclined, but when it comes to the outdrive and prop pitch this is all new to me.. so I really appreciate all the help and I love learning.. anyhoo measuring the prop it has a diameter of 14" so would the prop pitch be 14×11?

 

[kenny nunez]

Some of the #s on the prop may be from a propeller shop’s job #. Your drive should be more than 2;1 gear ratio. However you may want to check the ratio by turning the engine by hand and watching propeller revolution. Someone could have swapped the drive from a larger engine which would be 1;61 .

 

[Maclin]

If it was setup for skiing then it would lean more towards revving to the higher range of the recommended RPM's, in my opinion.