Getting my 1985 260 V8 past 4,000 rpm HELP

[pmjr0987]

Hey guys looking for some suggestions here

1985 fourwinns horizon 195 cuddy has factory mercruiser 260 5.7 V8 and alpha 1 (1.5 ratio)

Currently running a 14x19p stainless Propeller and wanting to go for a 14.5 or larger diameter or a 4 blade prop as I have alot of prop slip at slow speeds




Problem is the boat revs fast to 4,000 rpm but I can't get it to go over 4,000

Off the line the boat will rev up to about 3,000 rpm till it plains than make its way to 4,000 rpm and a top speed there of roughly 45mph

I can get there at 3/4 throttle but the last 1/4 throttle makes no change in rpm or speed but you can clearly hear the 4 barrels open up (bogs a bit too when it does than comes back) and the engine tone changes but no more rpm

So far I've done

-compression is all 145-150 psi
-new ngk plugs gapped .035"
-new Sierra coil
-new 10w40 Pennzoil maxlife oil
-new Sierra fuel filter (10 micron)

The boat is a little hard to start dead cold but the choke was removed and has a manual lever for it now so that could be because of that

Once warm the engine starts near immediately, repeadibly and idles completely perfect about 700rpm. Engine never stalls out even with alot of idle/forward/reverse work at the dock or extended idling

The motor runs with no hiccups besides the 4 barrel bog around 3/4 throttle which I read alot of these Rochester carbs do

The wires look newer and are in great shape no dry rot or tears

Moroso electronic distributor looks good with no abnormal wear

Boat also has an edelbrock performer manafold if that info is helpful

What should I look at next? Could there be an exhaust block? Maybe need a bigger flame arestor?

Could my 10 vs stock 20 micron fuel filter be restricting flow?

Idk where to start here as I really want to see what this boat does past 4,000 rpm I read my boat should be around 60mph at full proper wot rpm (46-4800 rpm)

 

[wahlejim]

Are you using the helm gauge or a shop tach for your RPMs? The gauge at the helm is unreliable. Get a shop tachometer and make sure you are getting the correct readings. You will probably need a buddy for this. If you are noticing a change in the sound of the engine, the gauge should be moving. Verify you are getting the correct readings.

For measuring speed, are you using a GPS or just the speedo at the helm? Use a GPS for a more accurate result.

 

[Watermann]

Your motor might be near max RPM or that's all she's got. A 30 year old boats tend to gain some weight. 45 MPH for a 30 year old glass cuddy with a MC 260 of the era would be incredible if you ask me.

On the prop, a little diameter means next to nothing as far as RPMs go, it's pitch that matters. There's no way on earth that 800 RPMs would ever give you 15 MPH either. If you're using an older prop switching to a new model is never a bad idea. They have increased cupping and an aggressive rake over the older style props.

When motors make the bog sound as throttle is applied it usually indicates a lack of fuel to the air ratio and if really bad is accompanied by a dangerous lean condition cough or even a back fire.

 

[alldodge]

Waterman has your answer, your boat is going as fast as it can with the prop you have. With your current setup you only have slightly more then 6% prop slip (sure wish I only had that little). Just need to go down to about 17 pitch, 15 might put you in the 4800 range

 

[pmjr0987]

I'm running a new quicksilver stainless 3 blade on it now idk if it has aggressive cupping but it was a newer prop and its in mint condition

That's sad I was hoping for more speed than that. It does feel fast compared to my old boat but I was thinking somewhere in the 50+mph was what it should do with the amount of fuel it drinks at that speed!

I'm going to try and get a new tach on it to see if its true

 

[Watermann]

The QS are good props alright, all the new features and advancements. I use the QS aluminum not stainless so I don't know much about them other than what the write ups are. I've read that it's advisable to drop an inch of pitch when going from AL to SS.

Yeah as AllDodge says that's an impressive prop slip percentage.

 

[airdvr1227]

If you get that old 5.7 to 4800 rpm take video...exploding motors always get good hits on YouTube.hoto:

 

[pmjr0987]

They sure do but why would it explode? Compression is basically at new levels, about 514 hours on the meter, no backfires or pings. 4800 is not an extreme number for a 350 but I just would really like to bring it at least to the beginning of the proper range (4600)

 

[Rick Stephens]

It is harder on your engine to have it propped where it won't go over 4k. You are absolutely correct to prop it correctly. A 350 that turns 4600-4800 is not going to blow up just because it is propped properly to turn 4600-4800.

Rick

 

[Bondo]

If you get that old 5.7 to 4800 rpm take video...exploding motors always get good hits on YouTube.

Ayuh,.... 4800 rpms is where ya want it to be able to run,....

'ell, prop it for 5,000 rpms, 'n it'll yank the arms off a skier,....

SBCs can turn to 5,000 rpms without issues,... 5500 is possible with the right cam, 'n induction,....
Higher than that with the right set up,'n a good balance job,....

 

[pmjr0987]

That's what I was thinking lol. But it just seems pretty efficient at light throttle... Some responses here pointed to a possibly faulty rpm gauge. I'm taking the boat out Sunday and I'm going to get some more accurate reading on the rpms with a different gauge and pay more attention to rpms and GPS speeds at different throttles and report back definitely want to get to the bottom of this before I go for a more dialed in prop for how I use the boat