Simple motorboat question...

[WJorgensen]

I recently acquired a 1995 Chris Craft 187 with a 305 Mercruiser. Previously, it had not been started or run in 3-4 years. I recently replaced the fuel pump and both fuel filters. It had a quarter of a tank full of old gas, which I added some octane booster and sea foam. I started it up in my driveway (connected to the water hose) and it seemed to run fine. I then filled the gas tank up with new gas (87 octane) and took it to the lake.
My problem: As I pin the throttle to take off, the boat seems to have little power until it reaches ~2500 RPM, at which point it takes off. It takes a while for the engine to get up to speed, making it extremely hard for a slalom skier to get up. It has a 3-blade prop with 14” diameter and 19” pitch.
Do you have any suggestions?

Thank you

 

[Medskipper]

Is this a two stroke engine? If it is, it sounds as if the jets are blocked in the carb by the old fuel mix. The oil goes hard in the petrol and then blocks the jets.

Barry

 

[WJorgensen]

The engine is a 4-stroke 8 cylinder (305 ci, 5.0 L) with a Rochester Quadrajet 4 barell carb.

 

[WJorgensen]

Does anyone have an idea as to what the problem is? The engine runs smooth, which leads me to believe either the prop is too aggressive, or there is too much friction somewhere in the out drive?

 

[SoulFireMage]

Old unleaded fuel may be one particular issue. Referring to barry's post I'd put new fuel in, though I'd have the carbs off and clean them through if possible. You don't mention the oil so I'd change that and the filter, basically a full and complete service include the plugs. Especially these as again, you could do with checking the state of the plugs. If these are gunked up you might be in a bit of bother.

 

[WJorgensen]

Thanks SoulFireMage.

 

[SoulFireMage]

Not a problem WRJ. Let me know how you get on .

 

[Lakesailor]

Does the carb have an accelerator pump and jets as well as main jets. Could it be these at fault that are slowing down the pickup?

Ah, just found this description."The QJet is a large 4 barrel carburetor that has a small primary side for fuel economy and good emissions and a large secondary side for good performance. QJets have a maximum airflow of from 750-800 cfm. Because the secondary side of the Qjet opens according to the airflow requirements of the engine, the same basic carburetor size can be used on a large range of engine sizes. The basic 750 cfm QJet casting is used from 231 CID to over 400 CID - the big secondary air valves open only as the engine breathes. For example, on a mild 350 Chevy the air valve will never open all the way - because the motor can't use more than 600 cfm or so. "

Could it be the secondary chokes are not opening?

Even better a repair kit I found includes
"Air valve cam
This nylon cam lifts the secondary air valve and metering rods; a worn cam won't lift the metering rods all the way, hurting performance"

The Carburetor Doctor

 

[WJorgensen]

That very well could be it!! The link from the vacuum break to the choke worked itself loose. I replaced a pin to keep the link in place.
Thank you for directing me to this page!