4.3 OMC SLOW speeds

[mohogg]

Hey guys.... Need some help. I have recently rebuilt my rochester 2 bbl carb and set all the variances on my OMC 4.3L. The boat is a starcraft 18' bowrider and has a 15 x 17 prop on it. My question is what should I expect to see speed wise out of it. At WOT I am around 4700 - 4800 rpms can only get up to 30mph if I play with the trim. This is with just two people in the boat. I expected to see around 40 - 45 mph but was dissapointed. I tried a different prop (comprop) 13.5 X 22 and had even worse results. Around 23 mph and 3700 rpms. DO I need to look at the idle mixture on the carb? The motor runs great but the power doesn't seem right. Any help would be greatly appretiated.<br /><br />

 

[Lark40]

Re: 4.3 OMC SLOW speeds

Max rpm for that engine is 4400-4600. So, your under-pitched at 17".<br /><br />Try a 15 X 19 prop for your size boat.<br /><br />Use a GPS to determine speed, don't rely on an old water pressure meter to be accurate.<br /><br />I would agree that 45 is a reasonable top speed for an 18' boat.

 

[karayj]

Re: 4.3 OMC SLOW speeds

I too have a 18ft seasprite 205 hp 4.3 4 bbl and I have a 13 1/2 23 pitch thats what mercruiser says i should have

 

[karayj]

Re: 4.3 OMC SLOW speeds

check your timimg I played with mine last year and got more speed advancing the timeing..

 

[mohogg]

Re: 4.3 OMC SLOW speeds

When you talk about advancing the timing are you saying by the distributor rotation or how? Also is there any chance that I have the idle speed adjustment screws set wrong I heard that preliminary sttings should be around 1.25 turns? I'll try the 19 pitch prop to see what it does.

 

[Liquid_force]

Re: 4.3 OMC SLOW speeds

I have an '89 4.3 omc with a 2 barrel, but it's in a 19' sunbird. It has the recommended prop - 14.5x19 which I would say is in fair condition. I can get it to about 40 at 4200-4300 rpm's.

 

[akriverrat]

Re: 4.3 OMC SLOW speeds

what did it do befor the work was done? idle mixture setting has little to do with wot rpms.