'73 Merc 1500 stalling after long low speed running.

[14.4volts]

Since I have acquired this boat and the 1973 mercury thunderbolt 1500 (3628325) I have read a lot of very useful threads here that have answered my questions without having to join, so thank you all that contribute here!

I have a 1977 19' reinell, I got the boat in poor shape and went through it top to bottom, I replaced the 850 with a 1500 that gave me more horsepower and power trim, the guy I bought the 1500 from said the power head had been rebuilt 3 years ago and the lower half was replaced last year.

I felt confident with this motor as it had recently been on the water 8mos before I bought it.

I added an inboard tank to my boat, I didn't feel like having a bunch of gas cans taking up space, I used all new fuel lines (fill,vent and send) and if I had to guess I used about 5 to 6' of line from tank to motor with the priming bulb about 10 to 12" from the fuel port plug, I think the tank is anti-siphon as outlet line comes out pointing down at 60 degree angle.
(I completely rewired the boat, all components are new on the boat side from boat side engine harness back)

it idles great, seems to start right up (once I read here trim out full, squeeze ball till rock hard then a little more, choke and 2/3rd advance) and take off from the dock just fine.

My problem comes from prolonged low speed cruising, I can run WOT or half all day with no issue, I seem to reach speeds of about 37mph at WOT.
Once I get into an area that is a long "NO WAKE" leaving seems to be a chore, when I try to throttle up (slow or fast) it hesitates and dies if I don't immediately drop the throttle, the first few times it was baffling. It would die but fire right back up and hold idle and as soon as I tried throttle it would die again.

I've tried various methods of taking off, increase advance to idle the motor up for a bit and it will increase RPM then engaging, feather throttle out slowly and neither works.
Going to the back and squeezing the ball a hand full of times seems to be the trick, soon as a do that I can full throttle out or even gradually throttle up.

It seems that as long as I stay around 15mph or higher I have no issue slowing for a bit and speeding back up, unfortunately anything below that starts to create some sort of running/power issue.
one thing I wanted to add was a tach but ran out of time to get one before boating and I am not sure what tach I can use for this particular motor so I have no RPM to report with this problem and I know that's more important then MPH.

My thought is fuel pumps, or floats or both. I haven't tried running my emergency portable tank yet and since I have only one I am leery to drain it testing the possibility of it being something to do with the inboard tank I installed

Looking at the carbs I cant see any exterior issues of cracked lines or worn looking parts, all seems to be fairly newer looking.
The carbs are a little wet though on their exterior, also the linkage rod for the carbs seems to have a light coating also. I don't see fuel dripping or any major pooling of any kind, looks more like it was fogged with WD40 or something, I assume the carbs will need a rebuild soon and my plan is to go through them over the winter.

Looking at the fuel pumps they seem to look easy to replace and if that is my quick fix to get my last couple weeks of boating in I would be pretty happy, if not I guess its a good thing I have a few weeks off depending on my ability to get carb rebuild kits quick enough.

Thanks in advance for any replies, I've been a mechanic for 20years and can do any conventional engines easily I know how two stroke outboards work and have no issue working on them, I just lack the experience on these old gems and know its easier to ask questions then dig a hole!

 

[SkiDad]

have you set your air/fuel mixture crews to the correct turns out from lightly seated ? maybe you are too rich/lean here ? Have you put in fresh spark plugs and are they the recommend plugs for your engine ? have you rebuilt the fuel pump ? Sounds like fuel issue to me

 

[14.4volts]

I am confident its a fuel issue, I just ordered two new diaphragm's. haven't messed with air/fuel as it operates normally under most circumstances and I will when I rebuild the carbs.
I am think that maybe one diaphragm isn't keeping up with the other and its creating a lower float bowl on one carb... not entirely sure. once I prime up the bulb more and the floats are even then it takes off and then the one diaphragm does the job for both.
Just a guess though, I am in no way an expert on outboards but by looking at the motor and diaphragm set up it seems plausible.

 

[Mi duckdown]

quite normal for 1973. they hate trolling speeds. An OEM manual will be your best friend for that motor. Sorry iboats

 

[14.4volts]

Mi Duckdown, it was a little erratic below 15mph. I was just camping on a lake that had a 10mph speed limit, keeping 10mph constantly was almost impossible and required a lot of playing with the throttle no matter how the engine was trimmed. 15mph seemed much much easier to maintain, if that was all I had to deal with it wouldn't bother me so much, the stalling/dying trying to get out from a lower speed just reeks of fuel starvation, especially sine pumping the ball seems to fix this problem.
I have read things about timing going to 21 over the spec 23 and different methods of adjusting the air/fuel screw for performance, is there any issue with those diaphragms sucking fuel what is the reason for having two fuel pumps?
I will be getting a shop manual!

 

[Chris1956]

Max spark advance should be 21*BTDC. Idle pickup timing should be 4-6* BTDC. After you replace your fuel pump diaghrams, if you still have the same issue, you may have the idle mixtures slightly lean. You can open your top carb 1/8 turn and retest acceleration. Repeat with mid-carb and then lower carb, testing after each adjustment. Count the adjustments so you can go back to the original settings, as they are pretty close, and adjustment is a bit tricky.

 

[wired247]

Mi Duckdown, it was a little erratic below 15mph. I was just camping on a lake that had a 10mph speed limit, keeping 10mph constantly was almost impossible and required a lot of playing with the throttle no matter how the engine was trimmed. 15mph seemed much much easier to maintain, if that was all I had to deal with it wouldn't bother me so much, the stalling/dying trying to get out from a lower speed just reeks of fuel starvation, especially sine pumping the ball seems to fix this problem.
I have read things about timing going to 21 over the spec 23 and different methods of adjusting the air/fuel screw for performance, is there any issue with those diaphragms sucking fuel what is the reason for having two fuel pumps?
I will be getting a shop manual!

1973 was a weird year for those. You have whats called a "circle C" block . The ports are the same as the later "Circle J" race motors. As noted above they dont like low speeds much. Ports are too high for the motors intended use. They lowered the ports in 1974 to make it trolling friendly but that doesnt do you much good. As Chris noted its a delicate balance adjusting the off idle transition onto the mains. A little more timing helps but it also helps melt the top of those pistons so 21 is about where you should be. You are getting on plane when you pump the bulb because its pushing fuel past the needle and seat and richening the mixture same as if the idle screws were opened up a bit more. Anyones guess why they used two fuel pumps or why they chose to cool the fuel pump diaphragms with the water jacket ( another 1973 only feature ) .

 

[14.4volts]

I appreciate all the replies so far! thanks everyone, wired247 I didn't know any of what you said (except timing) so thank you!
I am not having trouble getting on plane, I am having trouble leaving at all!
Basically once I start the boat cold at the dock I don't have any issue powering up to speed or keeping light speed away from the dock, it seems to only happen if I travel at trolling speeds, basically anything below 10mph for extended periods (5 to 10 min)
I will adjust the air/fuel appropriately, my thought process on adjusting that (if it were the problem) was I should still be able to feather the throttle up.
Strangely I am a little more happy I have a one year kooky motor!

I think I also read something about fuel boiling, maybe that's why the pumps get cooled? or maybe the rubber brakes down at higher heat?

 

[SkiDad]

we had a 1973 115 when I was a kid - it was fast - loved that sound - i think we got it in 1974 when the 73's were still available, my dad got the 73 for 1350.00 brand new (no power trim) - those were the days

 

[14.4volts]

wouldn't it be nice to buy a brand new outboard for $1350! today!

 

[wired247]

wouldn't it be nice to buy a brand new outboard for $1350! today!

If you bought $1350 worth of gold in 1973 you'd get 31 ounces of the stuff. Yes, Over two pounds. That would be about $39,000 in 2014 dollars. $1350 invested in real estate in 1973 would net $13000-$50000 depending on the market. My dad made around $20K a year in 1973 as an experienced engineer. Figure an 8 fold increase in those wages today.
Considering you can buy a 150 opti for around $10K today I'd say you are getting a better deal financially on a motor that will start whenever you turn the key.

 

[14.4volts]

I paid $750 for this one with the power trim pump and switch box and control box, lower half is new and the power head was redone 3 years ago. I felt like I got a good deal.
far as the plugs go it has BUHW-2 and the factory ones are supposed to be BUHW-1 with the only difference ive been able to find is the BUHW-1 has more copper.
I have noticed a little oil on the outside threads of plugs 3 and 4, when I got the motor #3 was loose and if I touched near the distributor it would give me a shock.
I cleaned the cap and rotor and tightened the plug, seemed to solve the errant shock I had gotten.
retail people are proud of that distributor cap, need an once of 1973 gold to buy one!
break out another thousand.
is there anything else I should look out for on these motors in particular?

 

[wired247]

I went through a couple of those early '73 blocks before I figured out what was going on. Tough buggers to get to run right even more so than "normal" inlines .

 

[14.4volts]

double posted, don't know why?

 

[14.4volts]

Haven't has a chance to get a manual yet, been busy.
Just replaced the fuel pump diaphragms, they didn't look all the bad and the check valves seemed to be ok.
IF ANYONE has the torq spec for the fuel pump bolts I would appreciate it!

just pulled the float tops, floats look ok, screens aren't too buggered, I did notice there is no gap between the inlet needle and secondary lever.
is that expectable? the instructions I have say .040" gap between secondary and inlet when held up right.

 

[14.4volts]

clarify, there is some gap but seems to be around .025, that would be restrictive to how fast the bowl fills up.
also, after playing with them a bit I could get almost all the inlet needles to stick.
I think that helps my theory, basically forcing fuel into the bowl and dislodging the needle once the float actually hits it and vibration from the motor at higher RPM helps keep the needles from sticking.
Just a theory but seems plausible from what I was thinking originally.
I am gonna just gap these @ .040 anyways and hope that if I am doing it wrong some one will reply before I get it all put back together!
Fingers crossed!

 

[CVX20SPRINT]

quite normal for 1973. they hate trolling speeds. An OEM manual will be your best friend for that motor. Sorry iboats

I don't know about that.Mine will idle for a long long time and then still get up on plane in a couple of lengths of itself(78 Carlson CVX20,not a light boat)and pretty sure this motor has never been built.When I bought it the OMC guy where I picked it up said he figured the reeds were toast.I just started from scratch and went though the carbs again,cleaned the dist cap and then did a complete Link & Sync.Made sure the exhaust/waterjacket plates weren't leaking and made sure the bleed down restictors were in place.If these restictors are gone it will just load up at trolling speeds.Mine does almost 50 mph with 2 adults and 10 gals of fuel and an alum prop.Motor sitting on the transom.

 

[14.4volts]

as I suspect too, I can understand throttle play at low RPM, not absolute failure because of it.
I remember people having these motors my whole life, all the boats I was around were double or single V8 or diesel double 6's , only outboards I saw torn apart were dingy engines.

I've torn plenty of dirt bikes apart, these out boards crack me up! I am sure there are a couple mercury engineers in hell right now for playing such a dirty joke on us!

I was convinced that automobile engineers road bicycles to work, figured it explained the absolute lack of thought for service, then I grew up and realized most engineers don't work on anything!

 

[14.4volts]

well, it runs, I have no idea if it is any better. Ill find out tomorrow when I put her in the water.
The carbs were adjusted way lean (from what I have read) and the floats were way out.
I'd prefer a conventional I/O, there is something magical about a thin stick of motor sticking up in the air behind you though.

 

[14.4volts]

Ran like a champ absolutely no issues, I adjusted the air/fuel by sound in the driveway.
I had two miles of no wake and it ran better then ever and went right to WOT, I had no issues going to idle or back and forth from high RPM to low.

I replace both fuel pump diaphragms, all three floats, strainers, and bowl gaskets and plugs.

prolonged no wake was smooth and consistent.

With 6 people, a fully loaded boat and a broken 3 blade aluminum prop I was able to get 35mph.

Thanks for the help.