New 3.0 cobra sluggish

[cajun1957a]

So as the title implies, I had a very reputable marina/shop install a brand new chev 3.0 Cobra 4 banger in my 1991 16 foot glass Sunbird bow rider. The old motor developed a crack in the block when i had it out last month. Since i had just paid for a completely rebuilt outdrive i was kind in too far to let it go. Motor wasnt cheap either. So this is what ive observed. The old motor i was told was rated at 130 hp. It ran at 4700 rpm WOT at 35 mph by gps. I had myself and four grand kids in it and the water had about a 4 inch chop. Ran strong. The new motor im told is a 140 hp reman and it purrs and idles great at 700 rpm in gear and im told that is spec. The prop i had on the old motor was a high five 17 pitch stainless and although i perhaps think it was a tad big, it gave me those numbers on rpm and 35 mph. Same prop on the new motor with two adults and i could not pull it up on plane unless one of us got in the bow. Sounds like its bogging and WOT is 3700 rpm. The trim works fine and i know how to use it as i have had a boat for many years. When i got home i pulled the breather to look and see if the cab was wide open and it was on full throttle. (motor off of course). So im thinking of putting the original 15 three blade prop on to see what i get and if that bring the rmps up and back to what i had before in mph im happy. The previous owner said he could always get 35 mph out of it and that is what i was getting with the high five and the proper rpm, but i never used his prop only mine. Im wondering if perhaps the timeing is retarded back perhaps to far or needs to be advance. What say you.

Randy

PS Summer is over, weather was great today may have a chance to try the 15 and see.

 

[alldodge]

In short, old motor and with 17P-high five did 47 mph at 4700 rpm

New motor with the same drive (Ratio was not changed) will not even get up on plane without help

My first question to the drive rebuilder's, is did they change the gear ratio?
If drive is same ratio, then need to talk to the engine builders and ask why your having trouble

If you were starting from scratch I would say you don not want a 5 blade prop on the 3.0, and not even a 4 balde

 

[Scott Danforth]

verify the gear ratio of your outdrive.

a 16 foot boat with a 3.0 should spin a 3-blade 19p prop to 40mph at 4600 RPM

I ran a 15p, 17p and 19p in the boat depending on what I used it for

how was the motor rebuilt?

did you follow proper break-in procedure?

what is compression numbers on the motor? should be between 145 and 150 for a new motor

was the fuel system cleaned?

 

[cajun1957a]

The motor is brand new. the gear ration has not changed. I even opened the breather to see if the carb was opening fully. The breakin period was done according to the builder they ran it for 5 hours straight with no issues in some sort of a jig the boat was hooked too. I didnt see that but this builder is well respected. I got a good 47-4800 WOT with the previous motor and prop and outdrive. Im wondering if the timing is set to far back. I pay attention to engines how they sound, run and perform. Soon as i put the hammer down there was just nothing to speak of. Thanks for you input. Tomorrow ill try the
3 blade 15 that came with it and see. The boat is a shallow V weighing about 2000 lbs according to specs and its NOT water logged.

Randy

 

[cajun1957a]

Update : Upon closer inspection the original prop is a 17 Prop number is 391290 15 x 17. So this is not a prop issue and the lower end was rebuilt two weeks before the motor went. I got the above numbers with the high5 after the outdrive rebuild and the original owner got close to the same with the 3 blade 17 prior. What i can see is that the motor is not developing enough rmps at WTO. 3700 as apposed to 4750 rpms prior to the motor change.

randy

 

[alldodge]

A 17P 4blade is like a 3blade 19P. Adding another blade and the pitch goes up more, maybe 20P but thats just guessing because not many use them for a comparison

 

[cajun1957a]

I have rulled out this is NOT a prop issue given the previous motor with the SAME prop and outdrive performed well. I can NOT get the rpms past 3700 rpm WOT. One month ago with the old motor is was revving 4750 rpm WOT with the same prop and outdrive. It really acting like the timing is retarded back quite a bit however im calling the builder tomorrow and see what he says. Again, this is no fly by night back yard wrench. Every dealer, Marina, wrench and Marine shop say this guy is the best up where i am. Will keep you informed.

randy

 

[Randy stoner]

The brand new distributor was defective. Took it back to the wrench and after water testing and several days of investigation, it was determined the distributor was defective and wouldn't allow spark after a certain rpm. they replaced it and all is good. High 5 -17 prop and rpm at WOT is 4650. I never would have expected that.