Mercruiser 140HP Overheating and low WOT

[tcooke]

I have a fresh rebuilt 1978 140HP Mercruiser running on a 1969 Merc 1 outdrive with a new water pickup pump installed (never turned dry other than by hand), 19p; 14.5" prop. The engine is pretty much brnad new, block and head came assembled from the machine shop, new manifold, riser, water pump, plugs etc..... Carb was rebuilt as well.

I set the timing as per the manual 6deg before top dead centre @ idle, but was just told today to check the timing at 4000RPM in nuetral with an advanced gun and see if it is at 30 deg. Is this right? Is the timing more important at the higher RPMs and is 30 degrees after tdc @ 4000rpm be correct?

I had ity out on the water for the first time and found that my Wide Open Throttle is 3300 rpm and the temp guage says its operating at around 185-190. The thermostat was just checked tonight, starts to open around 135-140 becomes fully open at 165. It only opens about 1/3rd the way at 150 and stays that way. Is this a defective thermostat? Shouldn't it open all the way at 140?

Thanks again fopr everybody's help!

 

[fat fanny]

Re: Mercruiser 140HP Overheating and low WOT

For what they cast buy the stainless type and not a cheapy butI would bet thats your issue unless your circulating pump is shot and or getting there.

 

[tcooke]

Re: Mercruiser 140HP Overheating and low WOT

Circulating pump is brand new, just got a new thewrmostat today, going to compare the two right now. What about the timing though? Is what I was told correct?

 

[T-Bone Coug]

Re: Mercruiser 140HP Overheating and low WOT

What about the timing though? Is what I was told correct?

Assyou-ME-ing your specs are the same as mine, your engine should reach max advance at 2200RPM. Any RPM's above 2200 you should have 31deg BTDC minimum, and 35deg BTDC maximum.

Check your Merc manual to be certain, but this should be correct.

 

[tcooke]

Re: Mercruiser 140HP Overheating and low WOT

Well checked the manual, says I should be at 24-28 degrees BTDC, but it also adds on the side to add it to the initial timing of 6 deg BTDC which puts the engine at 30-34 deg BTDC.

The new thermostat acted the same as the old one, starts to open at 135 fully open at 165f. The last trip out I payed close attention to the manifold and riser, both were cool. So it must be the sending unit or the tach, inspected the sending unit, it had a loose terminal as if busted inside. Bought a new one today and am going out to put it in.

When I was out though I still get a low WOT, 3500 rpm. Though the timing is set at 30 deg BTDC everything else seems fine, everybody I have talked to regarding the prop says it should be fine (19P; 14.5")

Any other ideas as to the low WOT culprit?

 

[Bondo]

Re: Mercruiser 140HP Overheating and low WOT

Any other ideas as to the low WOT culprit?

Ayuh,... I believe there's a listing in the Adults Only section dealing with That issue....

 

[tcooke]

Re: Mercruiser 140HP Overheating and low WOT

Ayuh,... I believe there's a listing in the Adults Only section dealing with That issue....

I know I read through that, but what I don't know is what is the most likely cause of a loss of 700 - 1100 RPM?

Everything keeps bringing me back to either re-pitch the prop OR some fuel issue. I have researched what I could regarding the prop pitch and everything is telling me the 19p is the right one. The carb runs great as far as I can tell......it was a complete rebuild.

Would the float adjustment cause this much of an RPM loss?

 

[havasuboatman]

Re: Mercruiser 140HP Overheating and low WOT

Your WOT problem is likely due to the amount of pitch you are running. How long does it take your boat to plane? Is it slow out of the hole? If it is, you should try a 17 in. pitch.

 

[tcooke]

Re: Mercruiser 140HP Overheating and low WOT

Your WOT problem is likely due to the amount of pitch you are running. How long does it take your boat to plane? Is it slow out of the hole? If it is, you should try a 17 in. pitch.

This is what makes me think it is not a prop issue. The holeshot is awesome, pops right out immediately, planes almost instantly. I have talked with a few fiberglass owners who run newer 3.0L mercruisers and they all run 19p props and get 4200 rpm. My boat is aluminum so it shouldn't have any problems with this prop.

I guess this brings me down to a fuel issue. I don't think it is on the "boat" side as I had someone pump the primer bulb while at WOT and no change whatsoever. I am running 92 octane gas, but still nothing more than 3500.

How would one test the fuel pump? is there a special guage to use? What about the float in the carb? I am pretty sure I had it set for the factory specs but would lifting it help?

Again I thank everyone for thier help as this is a very busy forum.

 

[Wakesilver]

Re: Mercruiser 140HP Overheating and low WOT

For me old 140+19p=4400+ WOT according to the tach.
Do you trust your tach? I wonder about mine sometimes.
Did the new temp sender work?

 

[ziggy]

Re: Mercruiser 140HP Overheating and low WOT

Quote:
Originally Posted by Bond-o
Ayuh,... I believe there's a listing in the Adults Only section dealing with That issue....


I know I read through that, but what I don't know is what is the most likely cause

any of the causes in that list can be the culprit. you need to check them off, one by one (really check them too, not just oh, surely i have the right ratio or whatever, go check the ratio to confirm, etc). there's also a list in the trouble shooting guide of the mercruiser service manual that may have some other ideas.

I guess this brings me down to a fuel issue. I don't think it is on the "boat" side as I had someone pump the primer bulb while at WOT and no change whatsoever.

there's someplace ya can start at. i/o's don't have a primer bulb. since it has one, i'd think checking to be sure the whole system prior to the fuel pump is'nt engineered by someone w/o you knowing about it.. there's also a good write up about testing a fuel system in the adults only section too. you may want to do some testing. i believe the fuel pump test is in that read... got new fuel filters i assume?

 

[tcooke]

Re: Mercruiser 140HP Overheating and low WOT

For me old 140+19p=4400+ WOT according to the tach.
Do you trust your tach? I wonder about mine sometimes.
Did the new temp sender work?

The tach is working properly, in nuetral it runs the whole range. Yes the new temp sender works! Thank god it's not overheating.

Quote:
Quote:
Originally Posted by Bond-o
Ayuh,... I believe there's a listing in the Adults Only section dealing with That issue....


I know I read through that, but what I don't know is what is the most likely cause

any of the causes in that list can be the culprit. you need to check them off, one by one (really check them too, not just oh, surely i have the right ratio or whatever, go check the ratio to confirm, etc). there's also a list in the trouble shooting guide of the mercruiser service manual that may have some other ideas.

Quote:
I guess this brings me down to a fuel issue. I don't think it is on the "boat" side as I had someone pump the primer bulb while at WOT and no change whatsoever.

there's someplace ya can start at. i/o's don't have a primer bulb. since it has one, i'd think checking to be sure the whole system prior to the fuel pump is'nt engineered by someone w/o you knowing about it.. there's also a good write up about testing a fuel system in the adults only section too. you may want to do some testing. i believe the fuel pump test is in that read... got new fuel filters i assume?

I have started to check off the list. I do think that my trim might be set improperly as I noticed last time out my bow puts out alot of splash. I am going to try the next setting out (old non-power trim). This winter I am going to switch over to power trim. I'll let you know what I find.

 

[Bronc Rider]

Re: Mercruiser 140HP Overheating and low WOT

I would look into that primer bulb as ziggy suggests. Then I would play with the trim. The trim will definitely rob some rpm's if you?re plowing thru the water. Unfortunately, if you don?t have power trim you will lose hole shot if you change the trim to a setting more suitable for higher speed.