rebuilt chev 350 will not get over 3000 rpms

[ledsled50]

completely rebuilt chev 1980 truck 350(marinized) will not rev past 3000 rpms under a load. 24 ft seawind cuddy with completely overhauled omc electric shift 21/16 gear ratio and 14/13 stainless prop. will leap from idle to 3000 rpm nice and crisp , not breaking down at all .4 barrel is opening all the way. timing is 10 degrees btdc and 32 dgrees at full advance.engine will noload to 5700 rpms. i have gone from a 14/ 17 prop to a 14/15 to a 14/13 and have gained 200 rpms with each swtich.. this 350 with the 21/16 ratio and 14/13 prop runs good up to 3000 (at half throttle),going from half to full throttle does not make the boat go faster than 22 mph or gain any rpms..should there be a reduction in the outdrive ? for example engine rpms 4600 / prop rpms 4000? or engine rpms 4600 / prop rpms 5000 rpms ?with this 21/16 gear ratio the engine turns one revolution and the prop turns 1 and 1/4 turns.thanks dave

 

[rbezdon]

Re: rebuilt chev 350 will not get over 3000 rpms

Seems strange, my 88 5.0 OMC has a gear ratio of 1.5 to 1 (engine turns 1.5 for 1 prop turn. What kind of drive is giving you that gear ratio??

 

[ledsled50]

Re: rebuilt chev 350 will not get over 3000 rpms

ron, this outdrive is a 1972 omc electric shift 21/16 gear ratio( upper gear case)with a 14/13 prop. i have 2 other complete outdrives with ratios of 21/19 and using 14/17 prop (i havent tried these yet)and the other is 19/20 ratio without a prop. do all lower omc units have the same ratio? if there is different ratios, maybe thats why im getting 1.25 (eng) to 1 (prop).....respectfully ,dave

 

[ledsled50]

Re: rebuilt chev 350 will not get over 3000 rpms

ron after reading my last post i should clarify the 21/16 ratio when rebuilding the upper gear case i counted the teeth on the input shaft(21)and counted the teeth on the water pump shaft (16 teeth)...once together i mark the input shaft and turn that ball gear one revolution ,the prop will turn 1:25 turns. thank you

 

[rbezdon]

Re: rebuilt chev 350 will not get over 3000 rpms

That sounds better. Your first post had the ratio inverted and this would give some strange results. It seems you still have a much lower ration that I have seen on many of the postings and the one I have in my Cobra. I am running 1.5 to 1 and from what I have seenon this site, this is common for the small block V8 size engine/drives. 1.25 seems low to me. The net result is you would need a smaller prop. I run a 14.25 x 19 and my estimate is I would need a 15 pitch to get roughly the same RPM vs. MPH if my gear ratio were the same as yours. It seems the 14/13 prop is in the right range but you've got a hell of a boat for that poor little engine. It may be you have some other issues such as the fuel delivery issues mentioned in many of the postings (carb, sererator, antisiphon, pick up tube, dirty tank, fuel filters, fuel pump pressure, etc). Give the fuel system an thorough inspection, clean and test as well as a good tune up (dwell, timing, plugs etc)and see what happens.