1972 1150 misses after it warms up...

[DVTO2]

I have a "new" 1972 Mercury, serial number 3362946. I have solved a number of problems, 1) new parts for lower unit obviously with a new impeller so it's working well, 2) new fuel pump diaphrams, 3) new filters for the fuel lines and making sure connections are tight and vents are clear, 4) cleaned the carbs. Each step in the process brought better performance. Cleaning the carbs was my last effort and the motor now starts well, runs well for about 10 minutes with a top end of about 5000 RPM, and then starts to lose power, sounding like it's not firing smoothly on all cylinders. I have two manuals but am not a mechanic and my next step is to drop it off at the marina, but I am trying to learn about the motor and thought I would see if anyone here had a suggestion.

 

[Moody Blue]

Re: Trouble with Mercury 1972 1150....

Re: Trouble with Mercury 1972 1150....

Two suggestions for you.

1) Try squeezing the primer bulb when the rpms drop off. If thing improve, chances are the fuel pump needs attention.

2) Is the vent on the fuel tank open? If not, a suction is created in the tank and the fuel pump can't draw enough fuel at higher RPM's.

One question. What size fuel lines and are there any in-line filters between the tank and the motor?

 

[DVTO2]

Re: Trouble with Mercury 1972 1150....

Re: Trouble with Mercury 1972 1150....

I have already dealt with the fuel line and vent issue and changed the fuel pump diaphrams. Squeezing the bulb does not help at this point. There are inline fuel filters which I changed. Again, things work pretty well and I get a consistent 5000 RPM for about 10 minutes and then the motor starts missing after it warms up.

 

[DVTO2]

Re: Trouble with Mercury 1972 1150....

Re: Trouble with Mercury 1972 1150....

bump.

 

[DVTO2]

Re: Trouble with Mercury 1972 1150....

Re: Trouble with Mercury 1972 1150....

The motor goes to the marina today but I'm still hoping sone might offer some suggestions.

 

[DVTO2]

Re: 1972 1150 misses after it warms up...

bump

 

[Moody Blue]

Re: 1972 1150 misses after it warms up...

When it starts to miss, what happens when you shut it off? Will it restart right away? Does it run any different when you restart? How long before it runs smooth again after shutting down? Does it have to completely cool down?

Try a high speed shut down after it begins to miss. Just shut it off without throttling back. Pull the plugs and see what they look like. Any signs of water? If any plugs look "steam cleaned" then you are getting water into the cyl(s). Would likely occur at higher RPMs when the pump is operating at a higher pressure.

 

[emckelvy]

Re: 1972 1150 misses after it warms up...

Pull the distributor cap and check the condition of the center spring-loaded carbon contact. It should be free to move in the cap, if not pull it out and spray the cap with carb cleaner and blow dry.

If the cap is full of a lot of "trash" inside that'll cause a misfire. Carb cleaner is a good way to get it clean fast. Have some paper towels under the cap to absorb the excess solvent as you flush debris out of the cap. Let the cap dry completely before reinstalling, you don't want to lite off any residual fumes!

Run the motor when it's dark outside and check the spark plug wires. If they're lighting up like a Christmas tree it's time to replace.

Check the spark plug boots, even a small hole in the end of one can lead to a misfire as the spark jumps to ground.

When the motor starts misfiring, check each plug wire with a timing lite to see if you can pinpoint the problem to one or more cylinders. If all cylinders are missing, use the timing lite around the coil wire. If spark is intermittent coming out of the coil, you've got other issues. Like maybe the coil is breaking down under load. Usually there's not much that goes wrong with these, but if an inspection reveals any cracks in the coil it's shot and should be replaced. One of the smaller coils found in later model distributor Inlines is a good upgrade.

Beyond that, anybody's guess. Maybe the switchbox?? Probably no good way to rule that out except by substituting a known good one.

Anyway, there's a few ideas to toss around. HTH & G'luck........ed

 

[DVTO2]

Re: 1972 1150 misses after it warms up...

Moody blue: The engine starts up OK after I shut it off but continues to miss. It will run max 4000 rpm when it would run 5000 rpm from a cold start. It has to completely cool down to run well again. I am not sure 5000 rpm is the top end for this motor - the manual does some tuning things at 5600.

wmckelvy: your suggestions sound like good ones for general misfire but what about the the fact that it only does this after it heats up.