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How to correct wrong gear ratio omc cobra

L

Lassepelle

New member
Hi :) I have the omc 88 mod 4.3 litre 205 hp engine in my boat, but unfortunately a outdrive for the 5.0 litre. The problem is that i cannot reach more than approx 4000 rpm @ wot.
My prop is a 15 3/4 x 15. Will a different prop correct the wrong gear ratio? I am not looking for a very fast top speed, but atleast reach wot and be able to cruise at a nice speed :)
 
Hi. No i bought the boat with the 5.0 drive, but i do have a drive for the 3.0. Will this drive be able to handle the torque from the 4.3?
 
I would have taken the prop to a shop, asked them to re-pitch to 14" and at the same time reduce dia to max 15".
Probably the cheapest.
 
That engine should be be between 4400-4800 WOT so your close. I would go the prop route as well being the easiest and cheapest. Either resize yours or look around for a smaller pitch prop (used, ebay???) Rule of thumb you will pick up close to 200 rpm for every inch of pitch you drop.
 
I wonder if you might just have a power issue with the 4.3

205HP is very close to what the 5.0 put out, and with a 15P prop, unless you have a heavy 21 footer, you should be making rpm ok...

You could try a lower pitch prop, but that's why I was a king about previous prop/ratio

I hate switching props without verifying engine is making good power (kinda like fixing a power probelm in a car by gearing down)

full tune up - new filters, clean flame arrester, timing light to check advance curve...etc etc.
 
Thanks for the replies :) So if i understand correctly a smaller prop would compensate for the wrong ratio? I was thinking of something like 14 1/2 x 19 or 14 3/4 x 19.
 
hystat said:
I wonder if you might just have a power issue with the 4.3

205HP is very close to what the 5.0 put out, and with a 15P prop, unless you have a heavy 21 footer, you should be making rpm ok...

You could try a lower pitch prop, but that's why I was a king about previous prop/ratio

I hate switching props without verifying engine is making good power (kinda like fixing a power probelm in a car by gearing down)

full tune up - new filters, clean flame arrester, timing light to check advance curve...etc etc.
Click to expand...


Hi. The engine was rebuildt 2 years ago and runs perfect at idle and accelerates very nice. I am pretty shure the engine is ok. BTW i have the 4 barrel carb. the engine is from -88.
 
Lassepelle said:
accelerates very nice.
Click to expand...

not past 4000 rpm it doesn't, but that's good- sound and feel are pretty telling.

I'd still be looking for stuck advance weights in the distributor, and looking for fuel delivery issues at WOT.

And I'd clean the flame arrester.

A prop change will get your WOT rpm up, and that is important for engine health, but you won't go any faster..

rebuilt engine.. hmmm - who knows what power you have... any idea what camshaft is in there?
Maybe you have an automotive cam.... that's can rob you of as much as 50HP

Are the secondaries opening wide on that 4 BBL?

Is your 22 year old remote cable and throttle linkage even pushing the carb linkage all the way?

There's a lot of simple 3 or 4 minute things I'd check before swapping props...
 
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Compression test first. lets see what it is. Then if you want a vacuum test heres how:

On Fire vacuum gage

How to Use and Interpret a Vacuum Gauge

So say timing is good plugs are good you will know cause to do a proper compression test you need to pull all the plugs and have a warm motor with the throttle fully down, you can read the plugs heres how:

Spark Plugs

Then it's time for a prop pitch change lower.

Point of information on my part i thought wot on that motor for that year was 4200-4600 rpm, i could be wrong.

So in theory if you go down 2 degrees in pitch you will be in the proper range of wot.

I didn't see if anyone asked if your boat is overweight, bottom dirty, or the stringers are rotten or you have hidden water in the hull, all of them would weigh you down and not allow wot.

Too long a reply to you but one last thing less pitch on a prop will hurt your top end speed just be aware is all.
 
hystat said:
not past 4000 rpm it doesn't, but that's good- sound and feel are pretty telling.

I'd still be looking for stuck advance weights in the distributor, and looking for fuel delivery issues at WOT.

And I'd clean the flame arrester.

A prop change will get your WOT rpm up, and that is important for engine health, but you won't go any faster..

rebuilt engine.. hmmm - who knows what power you have... any idea what camshaft is in there?
Maybe you have an automotive cam.... that's can rob you of as much as 50HP

Are the secondaries opening wide on that 4 BBL?

Is your 22 year old remote cable and throttle linkage even pushing the carb linkage all the way?

There's a lot of simple 3 or 4 minute things I'd check before swapping props...
Click to expand...

From what i know the engine should be equipped with all the right parts. The last owner said that a professional mech had done the job so i trust that this is right.
The secondaries are opening as they should and both the gear and throttle cable is new. even the gear cable from the engine down to the outdrive is new and adjusted after the "Hastings method" ;)

The boat weight is 1900 kg and is 21 foot long. It is a boat produced by the norwegian company "Fjord"

How it is with the fuel pump i cannot tell other than i guess it also is working since the engine works very smooth. At 4000 rpm the engine sounds fine, but i miss the last rpm`s hehe :)
 
Lassepelle said:
At 4000 rpm the engine sounds fine, but i miss the last rpm`s
Click to expand...
The exact symptom of a fuel filter that is getting plugged.

Also the exact symptom of a dirty flame arrester.

Also the exact symptom of stuck timing advance weights in the distributor.


and I've yet to see a 22 year old boat that didn't have all three of those issues at some point.
 
hystat said:
The exact symptom of a fuel filter that is getting plugged.

Also the exact symptom of a dirty flame arrester.

Also the exact symptom of stuck timing advance weights in the distributor.


and I've yet to see a 22 year old boat that didn't have all three of those issues at some point.
Click to expand...

Sounds like i must take a look at theese things asap :) I will keep u updated as soon as i have checked the timing, fuel filter and flame arrestor :)

I tried theese questions at iboats forum, but all i got as an answer was change outdrive since cobra sucks :(
 
Lassepelle said:
I tried theese questions at iboats forum, but all i got as an answer was change outdrive since cobra sucks :(
Click to expand...

you need to have a thick skin if you ask questions about OMC on iboats, most of them would rather give you their opinion about OMC than help you.

1900kg....that's a pretty heavy 21 boat almost 4200 lbs for that 4.3 to push, good thing you got the HO model w/4 bbl carb.
 
boatin_bob said:
you need to have a thick skin if you ask questions about OMC on iboats, most of them would rather give you their opinion about OMC than help you.

1900kg....that's a pretty heavy 21 boat almost 4200 lbs for that 4.3 to push, good thing you got the HO model w/4 bbl carb.
Click to expand...

True hehe. But the came originally with a penta aq 140 :) 4 cylinders and 120- 140 hp. Later the came with either the 6 cylinder penta 170 hp and aq200d 220 hp v8.

Regarding iboats i dont care what they say. Here in norway i have atleast 5 places to buy parts for my drive and the prices isn`t that bad :)
 
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