Need a place to start

[woodnknots]

I just got back from a 7 month deployment and would like to take my boat on the water soon, however... I had a nagging problem with one of the motors before I left (1978 Johnson 55hp), and was not able to sort it out. Soooo... I want to get going on it as soon as I can. Here's the problem as best as I can remember it.
-Engine seems to start and run fine for awhile (at all rpm's) but it eventually starts to surge. Not surge per se, but drop rpms dramatically, then jump back to wot, then drop, etc... So far, I have changed the fuel pump, de-carbed, and changed the engine wire harness. Usually, I can shut the motor off for a little while (a few minutes), and then she's okay for another 5-20 minutes, give or take.
My next course of action is to pull the carbs and soak them, but my friend that I bought the boat from has been insisting that I need new reed valves. I'm wondering if there's anything I should do either in place of pulling the carbs or while they are soaking to kill two birds. I've already done comp checks on both motors and they are A-OK. Any ideas.... Please???

 

[Xcusme]

Re: Need a place to start

The carb cleaning isn't a bad idea, but have you checked to be sure the tank vent is open? Does pumping the primer bulb fix the problem? I doubt very seriously if you have a reed problem. Defective reeds wouldn't cause your symptom to come and go. Another thought, you could be dropping a cylinder, possibly a coil heating up and killing the spark to one cylinder. When was the last time the water pump was services?

 

[iwombat]

Re: Need a place to start

Sounds like you've got a cylinder dropping out. Spark & compression check and tell us what you find.

Forget about the reeds.

 

[tashasdaddy]

Re: Need a place to start

agree, with iwombat. also is the vent open on the fuel tank?
a link N sinc problem, bad bushing.
http://forums.iboats.com/showthread.php?t=244013

 

[airframer]

Re: Need a place to start

take a timing light and run it up, when it starts surging, test it on each cylinder. worked for me once.

 

[woodnknots]

Re: Need a place to start

Thanks for all the replies! I am assuming that the tank vent is open, considering that I have no direct access to the fuel tank, and the other motor which sucks from the same tank has no problems. The compression for both cylinders is about 120. Pumping the primer bulb does NOT fix the problem. I tried that too many times to count. I like the coil heating up idea. I think I'm going to order a couple new coils and give that a try, along with cleaning the carbs. Water pump was replaced when I got the boat, last year.

 

[iwombat]

Re: Need a place to start

Before you go throwing parts at it heat up one coil at a time with a hair dryer while running and see if you can reproduce the problem.

 

[woodnknots]

Re: Need a place to start

I just checked my notes from last year. I had replaced the upper coil with a new one, and swapped the lower from the other motor. I still had the same problem. Now I'm wondering if it's maybe a stator? I'm getting a little nervous that I'll be able to get this problem licked. How hard is it to change a stator, and how much money am I talking here?