What are the symptoms of dropping a cylinder?

I have a 1972 Evinrude 50 HP model 50273C which is starting easily (once warmed up) and running well at its mid-range. It runs very smoothly at WOT, although its not spinning at the correct RPMs (its running about 4400/4500 RPMs). I am having problems getting it to idle at under 1200 RPMs, and the standard carb adjustments are not getting it to idle lower. I am getting good spark using a spark tester, and my compression is 140 and even. The plugs don't look wet. However, I did a cylinder drop test and found that when I pulled the top boot, the motor kept running. A little rough to be sure, but still chugging along. When I pull the bottom boot the motor started running rough to the point that unless I throttled up some, it would conk out. Given that it didn't die immediately, I would assume that its working, but it definitely does not run as smooth as on the bottom cylinder. What should I be looking at here? Is this possibly one dirty carb, or should I be looking in some other direction? Other diagnostic tests I should run?

-E

Re: What are the symptoms of dropping a cylinder?

Well you HAVE done the compression test, and its good, right across the board. So given we need spark, compression, fuel/air mixture, and timing to run, I'd look at the other three. You have spark, and it seems fine. What is left, fuel and timing, seems like its it running ok, except for ???? certain rpm ranges. Low speed, is seldom affected by timing, since the timeing is adjust for the high end not low, yet you have a low speed issue. I'd be looking over the fuel delivery system, carbs fuel line, fuel pump, tank etc. Start in the middle, look over the line, primer bulb, and tank and filters. If they are ok, look at carbs fuel pump, etc.

Re: What are the symptoms of dropping a cylinder?

Hopefully, it is carburetor. Are you sure they are synchronized properly?

I could be something worse, but don't go looking for the worst till you have reason to.

Re: What are the symptoms of dropping a cylinder?

F_R said:

I could be something worse, but don't go looking for the worst till you have reason to.

Click to expand...

Okay, I'll bite. What is the "Worst" scenario? As it is, the motor runs, starts reliably, and moves my boat well enough for my needs. I intend to keep using it while I tweak it into submission.

-E

Re: What are the symptoms of dropping a cylinder?

Remove the spark plug on the cylinder your having trouble with, making sure to keep spark plug grounded so you don't burn up you ingnition box (Brain) If compression is above 110 your good sounds like a fuel mixture and delivery problem!!

Re: What are the symptoms of dropping a cylinder?

Did this motor ever run in the right RPM range (5500+)?