Merc 500 carb/ignition bugs.. questions

[hotrod53]

For those of you who have not been helping me with this already... '78 Sylvan Sea Master with a '77 Merc 500 50hp 4 cylinder, just launched after 13 years and the boat was recently rebuilt.

I had a little bit of a rough idle in the driveway, but when I launched it, it cleared up after about 3 minutes. The idle is now smooth as can be, and it sounds great.

Issues....

* it seems to have a miss every now and then eventhough I have all new plugs, wires, rebuilt carbs, rebuilt fuel pumps... I have already ohmed the coils and they are all identical.
* if I nail the throttle I get a slight momentary bog
* it was tough to get it on plane with two 200+ people inside, maybe due to not being full throttle. Once it planes out the rpms come up a little and the speed comes up.
* I don't think that I'm hitting my 5200 RPMs, it seems as though my throttle cable may need to be adjusted. There seems to be a deadband in the middle of the throttle, it rpms come up right off of idle, the middle really doesn't do much, and I have to go a little more than 90 degrees to be full throttle which may not even be full throttle.
* I also have bubbles in my fuel line.. may be normal???
* this lower unit is not the original and the prop is now a 13 pitch, but I still have the 12 pitch.

I ran about 1/2 tank of fuel thru it today, minor issues but finally on my way!

 

[PHE]

Re: Merc 500 carb/ignition bugs.. questions

You should first buy a manual and do the full link and sync. This will get your cables, timing and carb linkages set right. I use the cheap Clymer's, but I'm sure the real manual is better.

The manual will also say what your WOT RPM's should be. When you have it running right, you want to be at the top end of the WOT range with your typical load. Its much easier on the motor. If not, drop down a prop pitch. This will also pull you up on plane quicker.

If I was you, I would put the 12 pitch on and see if it corrects the problem.

 

[hotrod53]

Re: Merc 500 carb/ignition bugs.. questions

I pulled my plugs and found that cylinder #2 is not firing. These are brand new centerfire plugs and 1-3&4 show a flamefront mark, #2 looks like it just came out of the box. Further investigation revealed that coil #2 is blown, there is a hole burnt thru the side. Now I'm on a quest for a coil. Oddly, this is the only coil that is not original to the motor. I ohmed all of these out last week and they were all good, it must have blown today. There is no continuity from the secondary to ground, all others are about 1100 ohms.

 

[hotrod53]

Re: Merc 500 carb/ignition bugs.. questions

I found a new coil and also ordered replacement carb inlets. When I got home I realized that the carbs were still leaking!

I'm going to wait till I get this thing firing on all 4 before changing the prop. I'm sure that I was way down on power dragging a dead cylinder.