Not Reaching Max. RPM's....

[powbmps]

So, after fiddling around with my motor (5.7 Vortec, re manufactured) all summer, I have one last issue/concern. I cannot get more than 43 mph/4,300 rpm's out of my setup. Company I bought it from says it should make maximum hp at 4,800 rpm's. It will rev right up to 4,800 and then cut out when the boat is in neutral.

Engine Won't Reach Operating RPM. Check

1. Fuel condition. Type and Octane possibly old fuel Only run 93 octane. 5 gallons at a time from one of several local gas stations.
2. Propeller pitch or diameter, damaged blades ACME 515. Definitely the recommended prop for my boat/motor/use combo. They say it isn't the prop selection.
3. Restricted fuel pickup tube or anti siphon valve Fuel System Test Performed all fuel system tests. It all looks good.
4. Crankcase oil volume, high oil level can cause aerated oil and lifter collapse Oil is right at full mark on dipstick.
5. Marine growth on hull and outdrive Freshwater boat, stored in driveway. Hull is clean, but not waxed.
6. Wrong gear ratio in outdrive Original 1:1 Velvet drive transmission.
7. Restricted carburetor air intake (clogged flame arrestor) Flame arrestor is clean.
8. Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive Exhaust is clear.
9. Poor cylinder compression Compression Test I have done multiple compression tests. 155 to 175 psi.
10. Carburetor defective, or wrong type. Problem existed with original 750 cfm Weber. Now have a new 600 cfm Edelbrock. Adjusted properly.
11. Fuel pump pressure and vacuum New Carter 6-9 psi, with FPR set at Edelbrock recommended 5.5 psi. Does not go below 3 to 4 at WOT.
12. Boat overloaded, improperly loaded, or improperly trimmed. One to three people in the boat when out on the water.
13. Engine Overheating Running new 160 thermostat. Temp gauge reads 150. Laser thermometer reads 150 at thermostat. Reads 180-190 right at heads.
14. Engine timing and ignition system operation Timing is right on. New plugs, wires, cap, rotor, coil, distributor sensor.
15. Remote control cables and linkage for proper travel to open throttle plates fully. No issues here.

The next thing I was thinking of replacing is the original Thunderbolt V distributor itself (probably with something simpler). Not sure how to check if this could be an issue. Boat is running great right now (just in time to put it away for the winter).

Thanks for listening!

 

[kenk63]

might be the prop pitch

 

[cwburkeva]

Are you getting any backfire through carb?

I had similar issue just the last few weeks. Turned out it was plug wires crossed on distributor cap - meaning I had wire 4 & 7 on wrong post. Put it in the right order after banging my head - and all was well. Went from max 4200 RPM to over 4800 RPM at WOT.

 

[Starcraft5834]

what pitch you running on your current prop? look on the collar of the prop, there's usually numbers stamped there, ####-## last two.. ie- 19,20,21 will be pitch.

 

[harleyman1975]

I agree check the firing order then play with prop pitch. How big is boat, how heavy, how old?

 

[Rick Stephens]

Sounds like too much prop to me. Anyone can list the suggested prop for a given model boat - that's just a starting place. Reality is what it is. If you have all the motor bases covered, prop down a couple inches.

RIck

 

[powbmps]

The boat is a 1995 Response, 20' long, 2,450 pounds. With the stock 265 hp motor and stock prop it should go 46 (per original manual). I would assume with the Vortec motor and 300+ hp it should go a bit faster. The ACME 515 (13 x 12 with .080 pitch) has come standard on the Response for quite a few years now. They only recommend a different prop if you are loading up the boat with ballast for wake boarding (we only ski).

I did actually get a back fire through the carb last week at WOT. However, it only happened once. I have marked both ends of the plug wires to be sure I get them hooked up correctly, so they should be good.

Thank you for the responses!

 

[Starcraft5834]

what Rick said.. prop pitch down (2), ideally would be best to test this theory first.. original specs dont account for the ease it is to get few hundred pounds heavier with age.... 1995 is certainly not ancient... but surely heavier than original, water soaked foam.... at least is likely

 

[harleyman1975]

Not an I/O?

 

[powbmps]

Not an I/O?

Inboard. Direct drive.

 

[Rick Stephens]

Inboard. Direct drive.

Ah... that's why that prop "The ACME 515 (13 x 12 with .080 pitch)". I missed the Velvet drive part. And I see why you would stick with that prop....


What did you mean by "It will rev right up to 4,800 and then cut out when the boat is in neutral." ?

I'd be hard pressed to rev a motor to redline without any load. But I would not expect it to cut out without a load either.

 

[powbmps]

What did you mean by "It will rev right up to 4,800 and then cut out when the boat is in neutral." ?

I'd be hard pressed to rev a motor to redline without any load. But I would not expect it to cut out without a load either.

The Thunderbolt V has a built in rev limiter that cuts the spark when you hit max. rpm's, which I believe was 4,400 to 4,800 for the motor that was originally in the boat. The motor I have now makes max. power at 4,800 rpm's, but the TB V could be cutting it a little short. I wanted to see if it would rev above the 4,300 mark. Just did it once.

 

[Scott Danforth]

the rev limiters are well above 5k RPM's check your tach

 

[Rick Stephens]

the rev limiters are well above 5k RPM's check your tach

Great catch.

 

[Bondo]

With the stock 265 hp motor and stock prop it should go 46 (per original manual). I would assume with the Vortec motor and 300+ hp it should go a bit faster.

Ayuh,..... Have You ever had this boat runnin' to 4800 rpms in the water,..??
Or did ya buy it as a dead boat,..??

What color are the spark plugs, Right after ya run it up to Wot,..??

If it backfired, I suspect it might be runnin' lean,....

Even a brand new carb outa the box, usually needs tunin',....