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1999 4.3 mercruiser tops out at 3200rpms

9

912Jonathon

New member
My 1999 Tahoe Q3 will only top out at 3200rpms at 30-35mph. It has a 4 blade 14” x 20p prop. I just purchased. I know it has a fresh tune up. What prop would I need to run?
 
That’s like a 18 ft boat with 190 hp? That should be able to swing that prop, maybe not to max rpm allowable but definetly should hit at least 4400. What brand and style of prop is it?

So a couple of thoughts. Verify tach and is speed gps?

would check some basic things like

fuel filter
look at plugs what do they look like
verify ignition timing and total advance
check ther is not exhaust shutter stuck in bottom of y pipe
 
Dieter said:
That’s like a 18 ft boat with 190 hp? That should be able to swing that prop, maybe not to max rpm allowable but definetly should hit at least 4400. What brand and style of prop is it?

So a couple of thoughts. Verify tach and is speed gps?

would check some basic things like

fuel filter
look at plugs what do they look like
verify ignition timing and total advance
check ther is not exhaust shutter stuck in bottom of y pipe
Click to expand...

it’s a aluminum prop. Not sure of brand. I will check the tach and mph as well as other things.

it seams to run good. With just me, it planes out quick. With a couple adults and kids on the tube, it takes a minute to plane out.
 
I'm thinking that's still way too much prop.

My 19 footer with a 5.0 V-8 runs a 3 blade 21 pitch (stainless) at 4,400 rpm, but I prefer a 19 pitch (aluminum) that turns (if I let it) over 4,800. Boat planes off much better with the 19, only cruises a mph slowed at 3,000 rpm, and still gets great gas mileage. It's especially useful towing the grand kids on a tube.

Jeff
 
Something doesn’t add up given info supplied, which is what lead to check the tach comment. If you were due to engine condition or incorrect prop selection only getting 3200 rpm at wot you wouldn’t be able to plan quickly even with one person.

if it were mine I’d verify the engine rpm and condition before playing with props. you say engine was tuned was this what the seller said? Who tuned it?

Most of my buddies with similar size boats and 4.3 power are running three blade 23” props. This assumes the drive hasn’t been changed out with an incorrect ratio one. You can check your drive ration on left side should be both stamped In There and on th mercruiser sticker. Should be 1.81 or 1.84 or something like that.
 
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