Max 2600RPM???

[jimmmy]

I have a 3.0 L merc on a 17 ft 1989 Oliver that has given me endless low RPM problems at WOT. Had two seperate mechanic look at engine and lower drive over last 7 weeks -moocho$. Carb rebuilt, head gasket replaced and head machined, checked for exhaust restriction (as flappers were missing), checked engine alignment, new points, new plugs, new condensor, timing ok. The last mechanic seemed to think that the blow-by was a little excessive?? How much blow-by is OK? Compression check shows 150PSI in all 4 cylinders (after head redone). Engine in gear runs smooth to 2600RPM about half throttle, then there's no more between half and full (actually bogs slightly to full). Runs smoothly to 4500RPM in neutral. Exhaust not smoking but a lot of blow-by to spark arrestor (mainly air not much smoke).<br />Running out of $ and options. Could it be carb still? Could it be weak spark, that still shows as ok timing? Is engine pooched? Running 19 pitch prop.<br />Help anyone?

 

[Ribfan]

Re: Max 2600RPM???

I would personaly say, either the 2nd stage is not kicking in on the carb.<br /><br />Or too big prop, i have the same engine but a 2001 model, with a 19" prop i only get low 3200 WOT. I am yet to try a smaller prop, but everyone i speak too seems to think the prop is too big. try a 15" or 17"<br /><br />But i may be wrong

 

[jimmmy]

Re: Max 2600RPM???

We've tried a 17 pitch prop with little change! Actually when we bought the boat, it had a 4 blade 23 pitch prop and performed about the same. Both mechanics seem to think it's not a carb problem! The last one actually sprayed extra gas into carb at WOT and it made no difference.

 

[magster65]

Re: Max 2600RPM???

The mechanics and yourself have probably checked these things but;<br />- does the timing advance to where it should?<br />- has the exhaust been thoroughly checked for blockage?<br />I agree with ya', it sounds like too much prop but those things could do that too.

 

[McKenzie]

Re: Max 2600RPM???

Seems like a long shot, but maybe something is binding in your outdrive. Out of the water, does is still act weird if you try to give it the gas in forward (might wanna take your prop off )

 

[jimmmy]

Re: Max 2600RPM???

They checked the timing and it is advancing! They pulled the engine to thoroughly check for a blockage. Nothing..... <br />They checked for a blockage downstream of where the flappers should have been. Anyone ever hear of them being lodged upstream??

 

[Ralph 123]

Re: Max 2600RPM???

Jim,<br /><br />How fast is the boat going at WOT and how does that roughly compare with the mfgs spec/ expectations?<br /><br />Is it possible, just possible, that the tach is screwed up under load and reading wrong?

 

[jimmmy]

Re: Max 2600RPM???

At WOT (2600RPM) I get about 27 to 28 MPH if trimmed right. Im sure the tach is right. The throttle is about 1/2 way and you can hear that she's not running full out. When I give her more throttle she actually slows slightly and you can hear a slight change in engine pitch. If I rev her in neutral to 4000RPM you can hear the difference and she's smooth. Former owner claims 37 to 40 MPH was top speed. I'm stumped!

 

[Ralph 123]

Re: Max 2600RPM???

How is it possible that you can have such different props and get the same exact RPMs? Very different loads yet the same results. It just seems counter intuitive.<br /><br />I guess I'd be inclined to go with a radical prop change for a quick test - small diameter and small pitch and see what happens...

 

[jimmmy]

Re: Max 2600RPM???

Tomorrow we're trying new coil, new dist cap, new wires, new rotor. We'll see if we get any change. Hey what's another couple of hundred loonies $??

 

[jimmmy]

Re: Max 2600RPM???

Again I'll ask.... has anyone ever heard of the exhaust flappers getting stuck upstream of where they should be?? I know the last mechanic pulled the engine and looked downstream.

 

[magster65]

Re: Max 2600RPM???

I've never heard of it but who's to say it can't happen?

 

[Ralph 123]

Re: Max 2600RPM???

Jim,<br /><br />Just spit-balling here but has the outdrive been checked out? Gear ratio, etc.?

 

[oldboat1]

Re: Max 2600RPM???

could be a linkage issue or cable issue -- Throttle/shift control lever is not actually opening up the throttle past a certain point when in gear. You could verify at the carburetor linkage that you are at WOT (or not) when you are expecting it to be wide open.

 

[Bondo]

Re: Max 2600RPM???

Try a 15" prop...<br />I'm with Ralph Spencer, I think the outdrive was replaced with a 1.50/1(V8) ratio...<br />You Should have a 1.98/1 ratio......

 

[akriverrat]

Re: Max 2600RPM???

4500 rpm in neutral with throttle fully advanced? you're still in primaries at 4500 rpm in neutral. stuart has it right with no secondaries.

 

[jimmmy]

Re: Max 2600RPM???

A couple of people have said it sounds like the carb secondaries are not opening. My carb is a 2 barrel. How could I check this out? The last mechanic sprayed gas with a spray bottle, into the top of the carb at WOT to see if it reacted. It didn't!

 

[llerrad]

Re: Max 2600RPM???

Has the cam shaft timing been checked with all the work done might be one tooth out. Oliver Boats was in the 705 area you might check the tech guys at Hot Knots in Orillia good engine people.

 

[magster65]

Re: Max 2600RPM???

I would try the prop (less pitch) test next.

 

[akriverrat]

Re: Max 2600RPM???

jimmmy, i was one of those people and i am just stupid. still sounds as though it is carb or fuel restriction problem though.