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TroyNic

Need for speed

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TroyNic

I want my 88 sunsetter with 5.7 260 hp to go faster, I have a 13x13 4 blade prop on it and it tops out at 36mph and 3k rpms. Any ideas?

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jk13

Verify the tach is correct. 3k is really low. Typically the RPM:MPH ratio on the older boats is close to 100:1. 

If all is running correctly you could see low to mid 40s at best with that hull and a stock engine package. 

You may pick up a couple MPH with a lower pitch 3 blade but check engine condition and tach accuracy first. 

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ahopkinsVTX

Is your engine been tuned recently and running ok? 3k for rpms is very low. 

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TroyNic

I thought I was really close on the ratio until about 25 mph...it hits 34-35 strong but seems to hang at 3k rpms....I dont notice any screaming like the engine is at wot.....just replaced fuel pump and engine feels strong but I am not an expert by any means.

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barefootrocker

36 @ 3,000 RPM with 13 pitch doesn't add up, mathematically speaking. Verify top speed with GPS speedo (lots of free phone apps for this), verify prop information with stamped info on prop hub, and if possible verify engine RPM against tachometer. Harbor Freight sells a photo sensor tachometer for under $40. Slap some tape on the crank pulley on the front of the engine and the tach will tell you RPMs. You shouldn't rev the engine to top speed in neutral but getting it up to around 2,000 rpm (according to photo sensor techometer) should safely tell you whether the boat tachometer is correct or not.

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TroyNic

Provided the tach is wrong and am actually at 3600 rpms, what then wot is at 4000 4400 if not mistaken. Am I still missing something?

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Steve B.

You should be able to hit upwards of 5000 rpms (at least with a 3 blade 13x13 prop).

Good luck,

Steve B.

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Woodski

Ball park numbers - you should expect to see ~ 45 +/- 2 mph @ ~4500 +/- 200 RPM so as noted close to a 100 RPM : 1 mph ratio assuming the prop is of original specs or a 13 x 13 configuration (slight variations with the newer CNC stuff - less pitch).  Those are close to the as delivered performance numbers with the 265 hp engine packages.

In your case, it sounds like the engine is either not getting enough air (dirty or clogged spark arrestor, blocked air inlet vents, choke hanging closed), not getting full throttle or secondaries not opening, ignition timing significantly retarted or advance not working.  You mention replacing the fuel pump, did you issue crop up right after that?  If yes, focus on that first.  Perhaps the lines are not seated properly and there is air getting in the lines so carb is not filling with fuel.  Why did you replace the pump?  If it was due to poor pump performance and the new pump did not fix it a possibility would be the cam lobe for the fuel pump arm, assuming a mechanical pump.  Hopefully those are some helpful leads on your troubleshooting tree.

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