1972 mercury 1150 low rpm?

[74sidewinder]

Looking for some opinions on my rpm issues. Took the boat out today for the first time this season, finished the link and sync and the boat runs great no issues other than only being able to reach 4000 rpm and about 35 mph. I’ve read they should spin at least 5000 rpm. I redid the compression test once I got home all 6 were at 120 except cylinder 3 and 4 which were 118 nothing to be concerned about. All the plugs look good but I noticed a few water droplets on the base of the plugs ( where the electrode is ) not sure if it was just condensation or something I should be concerned about? No external leak on the water jacket around the plugs. My prop is a 13.25 17p and the engine is on a 16 foot sidewinder with 2 adults. Should I just reprop? Or is there any other suggestion

 

[Faztbullet]

Make sure carbs are fully opening as that motor should spin a 17p easy on that hull

 

[Chris1956]

water droplets on spark plugs are bad. Which plugs have water?

 

[racerone]

Sounds like it is running on 4 / 5 cylinders.------Stop running it until you determine what is wrong.-----Lost spark on one lead.----Blockage in a carburetor.----Cleaned the distributor cap ?=====Pin hole in fuel pump ?

 

[74sidewinder]

Cylinders 3,4,5 had what looks like water on the plugs, clear liquid. Wasn’t soaked but had maybe 10 small drops. Fuel pumps are both rebuilt, new plug wires ( I’ll check for spark tonight ) caps clean and carbs were just rebuilt.

 

[racerone]

Plug wires going to the correct cylinders ?-----Sounds like 2 of them may be mixed up.

 

[Chris1956]

Well, if it is water on cylinders 3,4,5 and not cylinder 6, it is likely exhaust baffle or inner exhaust water jacket cover is leaking, putting out the fire in one or more cylinders. Not uncommon if that motor has seen salt water.

If water on #6 cylinder as well, it could be leaking lower crankshaft seal. Much better problem to fix.

 

[74sidewinder]

Did some testing tonight, all 6 cylinders have spark I believe I routed them correctly, I cannot find a wiring diagram of the distributor cap of the routing for the wires. As for the water on the plugs is there anyway to test the leak? I was thinking leaving all 6 plugs out, disconnect fuel, turning on the muffs and cranking the engine to see if water comes out of any cylinders? Is this a reliable test or is there another way. I cannot remember if cylinder 6 had water on it, the motor has never seen salt water. I’ve attached photos of the plugs after about 2 hours total run time on the engine first picture is cylinder 1 on the far left then 2,3 etc. I’ve also read the cylinders can be washed clean or steam cleaned if there’s a water leak but all 6 pistons are equally carboned

 

[jimmbo]

Well, you will spray gas and oil out of the plug holes until the carbs and crankcase are dry. I doubt you will see any water. A leak from the exhaust baffle might only enter while the engine is running, and at only certain RPMs, because of how Exhaust tuning works.
You don't have to worry about a leaky Head Gasket, as there is none on that engine, I doubt the cylinder itself is cracked.
Wipe the plugs dry/clean and go for a High Speed Run, then check the plugs. I mean check them before any low speed running after the run

 

[74sidewinder]

Thanks I won’t be able to go to the lake for another 2 weeks, I’m probably gonna order the power head gasket set and change them as well as the lower crank seal and see how it goes the next time I’m out

 

[seraphter]

The distributor cap has the cylinder numbers embossed on it at each plug wire connection. I had the same search issue - couldn't find it. Eventually I figured it must be somewhere obvious. Use a scope, mirror, or phone camera to look underneath.

 

[Faztbullet]

Those plugs don't show any steam cleaning and if it was water intrusion in that many cylinders it wouldnt idle...

Took the boat out today for the first time this season, finished the link and sync and the boat runs great no issues other than only being able to reach 4000 rpm and about 35 mph.​

 

[74sidewinder]

Update. Took off the exhaust baffle and cover today, broke only 1 bolt I attached some pics, don’t see any cracks but gaskets are falling apart and corrosion is built up, anyone chime in if you see anything strange. As for the spark plug wiring I have it correct and the throttle plates are fully opening, the motor ran 2 hours tops on brand new plugs for that reading. I did notice small amount of rust forming on the cylinder walls of 2 cylinders.

 

[jimmbo]

The pics show the cover off, but the baffle is still bolted on. Exhaust tuning wise, Cylinders 1, 3, and 5, make up one 3 cylinder engine, while 2. 4, and 6, make up another.

#3 seems to have a cleaner exhaust cover...

Only 1 broken bolt, You really should be buying a Lottery Ticket tonight

 

[74sidewinder]

The baffle is now removed and unfortunately I found a large crack right through in between cylinders 3 and 4. Is this repairable by welding? Or should I source a new one?

 

[Chris1956]

The crack is in just the inner water jacket cover, and not the block, right? If so, replace it.

 

[74sidewinder]

Update: I had the baffle plate fixed by a machine shop, welded the crack and resurfaced. Couldn’t find a new or used one that would ship up to Canada. Got everything back together with new oem gaskets and took the boat out to the lake for a test. Starts great but still have very low top end couldn’t get past 3200 rpm now. Seems worse than before, brought the boat home and now I’m gonna redo the link and sync and double check spark on all cylinder. Maybe I messed something up during the exhaust work? Also I notice with the motor in neutral in the water, it would only rev up to 2500 rpm is this a normal number? I thought I could get above 3k before.

 

[jimmbo]

Is the spark advancing properly to the proper max advance? Are all the carbs opening fully?

 

[74sidewinder]

I believe so the distributor is turning with the throttle and yes they are opening 100%. Is it possible the switchbox is failing? Anyway to test it. It’s the only component I haven’t changed other than the distributor and coil, everything else is new. I have never had this engine working properly and would like to see what kind of power it has

 

[Chris1956]

Gee, switchbox, coil and distributor are common to all cylinders. If it works for 5 cylinders, why not 6?

Do check spark plug wires for resistance and arc to ground.