Firing problem 50 HP Merc

[tom2]

I have a 1984 4 cyl 50 hp merc. When I pull the wires off either #3 or 4 the motor will stall out. I seems to make no difference in the way the motor runs when I disconnect #1 or 2 cyl. compression is 120 in all 4 and I have good spark. Thanks for your help. I rebuilt the carbs but It looks like a lot of gas is coming out the throats of both carbs.:'(
Bobby

 

[Mike722]

Re: Firing problem 50 HP Merc

If you have good spark on all four cylinders using a spark tester, it sounds like the fuel is not being drawn into the number 1 and 2 cylinders even though you have good compression. I have not work with 2cycle marine engine enought to know what to check next.

Sorry
Mike

 

[twocycle]

Re: Firing problem 50 HP Merc

Mike's correct Bob. You need to know for sure if those cyls. are firing, however I can just about they are. Cylinders 1 & 2 are feed fuel by the top carb and cylinders 3 & 5 are feed fuel by the bottom carb. So the logical conclusion is that either the top carb is not supplying fuel or you have a reed valve problem. You should NOT have a lot of gas coming outta the carbs so you have a problem there. You should have spring loaded needle valves in the carbs and they can be a little tricky to set so that could be a problem, also do you recall what color the floats were? If they were a tan foam material then you need to get 'em outta there and replace them with new style floats. Oh yeah another thing if these carbs have fuel pumps on each carb then you have to watch how you install the flange gasket as they have a hole that you must line up with the hole on the intake as this is the pulse feed to operate the pumps. If it has a remotely located fuel pump then it's a non issue. Without a serial number I don't know which style pump(s) you have. Also it's possible for a perforated fuel pump diaphram to cause problems as it'll allow fuel to pass thru to the pulse side and then on into the crankcase. (Most likely not the case here)

Hope this helps.
Happy boating.
John
Owner/Operator
UB&M

 

[emckelvy]

Re: Firing problem 50 HP Merc

Have you checked for spark on #1 and #2 while the motor is running? Easiest way to do that is to clamp an inductive timing lite around each spark plug lead to see if it's got good fire while under compression.

Next pull the spark plugs on #1 and #2. See if they're dry or wet.

If the plugs are dry, try putting a few ounces of Pre-Mix into a spay-bottle and spray into the throat of the top carb. This should cause #1 & #2 to pick up momentarily. If you find this to be the case, don't run anymore until you find the problem, as if you have no fuel you have no lube.

If they're wet, you may be getting too much fuel instead of not enough. In that case, the float level could be incorrectly set, or perhaps idle mixture is too rich.

One thing you can do while it's running is to go full lean (clockwise) on the idle mixture adjust needle and see if cyl's 1 and 2 pick up while you're going lean. After that run the needle out approx 2 turns and see if that makes any difference.

BTW you say a lot of gas is coming out of the throats, do you mean spilling out the fronts of the carbs, or spilling into the throats of the carbs into the motor?

If it's pouring out the fronts of the carbs, you probably need to replace the float valve/seat assy in each carb. Check the floats for proper height adjustment, too.

HTH, let us know what you find.........ed