1990 OMC 4.3L HO Cobra

[PRiver44]

I recently bought a 1990 Reinell 181SR up in Medford, OR. First outing, it ran pretty rough, so took it to a mechanic for a service/tuneup. Next weekend, took it out and it idled very smooth, but at 2500 and above, it was surging, not really enough to get the tach needle to move, but I could hear ad feel it, and was only getting 4200 at WOT. took it back to the mechanic and they found that the points were at twice the gap as they should be. they re-gapped and re-timed it and didn't charge me for any of it. I'll find out how it runs on Saturday. It had a 3 blade 21P prop when I bought it. Last time out, taking it out of the water, I discovered a chunk missing on one of the blades (Near the hub). Not sure what I hit, but thought that was an odd place to have damage. I bought a 4 blade 20P and will try that out on Saturday as well.

Any guesses on what I'll get for speed and WOT??

 

[Scott Danforth]

with any new to you boat, do all the maintenance

that includes pulling the drive, doing the bellows, checking the gimbal and u-joints and checking alignment.

it also includes impeller, oil and lube changes, and in your case, points service, etc.

your WOT is what determines your propeller purchase, and the propeller and load determine speed. you did that backwards.

how do you know if your motor was hitting proper WOT with the prior prop?

the WOT range for your motor is 4600-4800 RPM. that is where you should be flat out, throttle mashed against the stops and with the right prop. if you are less that that, you are lugging the motor.

 

[Lou C]

I think that prop probably has too much pitch for the boat/weight that you have...might want to go down to a 19 pitch...
I used a 15x17 on mine but my '88 FW is a good bit bigger (20'9") and heavier no doubt.

The bellows and impeller are very easy to do on the Cobra. The gear oil change has to be done the way its laid out in the OMC manuals. Take a look at the Crowley Marine site, look in support, they have a bunch of OMC maintenance stuff right from the factory shop manual.

My boat will hit 4600 with the 17 pitch prop, but its not fast, probably maxes out in the high 30s on a good calm day. It really needs a 5.7 lol. I have a 15.5x15 I am going to try, just to see if it still under 5000 WOT. You really can't go by the dash tach, they can be off between 200-300 rpm at times. Its best to rig up a proper digital engine tach and get an accurate reading. Mine is 100 rpm off (high) even at idle.

 

[PITBoat]

Mine is 19 or 20' and heavy too. I've got a 14 1/2 x 19, and the most RPM I've seen is 4,300 on a new automotive tach. Speed is about RPM/100 from 3,000 or so RPM on up. I'd like to try another prop to see if I could get the rpms up just a little higher, but that's not real high on the list right now.

 

[PRiver44]

Ok, I achieved 4500 RPM with the 4 blade 20P Aluminum prop. Speed was 43 or so (non-GPS). Ran awesome up to 3300, then started to surge/miss again. When I was at WOT, I backed it sown to 3000 RPM and it was missing/surging. Started to pull back on the throttle, heading for a marina, and the engine started to speed up a little and smoothed out. Going to have to take the mechanic out on the boat to show him what it's doing, since I'm not able to convey verbally