motor hesitation

[jclever]

Hey everyone. I have a 1971 Merc 500 50hp. Whenever i try to go full-throttle it hesitates a lot. It will reach top speed but it takes a while. It has brand new gas and all the fuel lines look good. Brand new fuel pump also. Both hot and cold compression tests are good.
The interesting thing I found was the condition of the spark plugs. Its a 4 cylinder. The bottom#4 cylinder's plug was clean, almost looked brand new. I took the dirtiest plug out of one of the other cylinders and put it in the number 4 cylinder. I ran the motor for a few minutes and then pulled the plugs again. The new #4 plug came out very clean.
I told this problem to someone else and he was very convinced that it is because of a bad lower crank shaft seal. he said that water was getting into the number 4 cylinder and steam cleaning the plugs. I have no reason to doubt him I just want to get a second opinion before I do a power head tare down. Thanks

 

[emckelvy]

Re: motor hesitation

It's certainly possible. Is there any water showing on the plug when you pull it out? Have you done a high-speed shutdown (run at a good clip in the water then shut the key off) to see if any visible water remains? Note if you do this the motor will kick up some so you might want to tie a rope thru the transom 'eyes', across the back of the motor, to limit its travel.

Also possible to have a perforated exhaust baffle throwing water in the cyl but usually you see more than just one getting water in it.

In cases of 'washed plugs', besides water I'd also be suspicious of excessive fuel washing things clean. I'd be especially suspicious of this if you have no indication of water on the plugs. Perhaps a fuel pump diaphragm causing lots of fuel to be sucked into #4 cyl. Of course this would only be applicable if the fuel pump mounts down low; I can't recall on this one.

Have you checked to make sure the spark plug is firing under compression pressure? Not good enough to just lay the spark against the block, you need to see if the spark is strong enough under pressure. Best way to do this is with a firing indicator or timing lite.

Does #4 kick in at all at higher speeds? Just diagnosed an old 50hp where one reed had a small chunk broken at the very tip, it wouldn't idle on that cyl but when you revved it up, it would kick in. A real hard one to troubleshoot 'cause it didn't spit back thru the carb. Only teardown was able to confirm the diagnosis.

Note that you don't have to disassemble the powerhead completely to check the bottom seals. The lower bearing cap will pull out after removing the 3 bolts that hold it in place. You'll need some kind of puller to grab hold of the cap; don't pry between the bolt 'tabs' and block 'cause you'll break the tabs right off!

If you find nothing wrong with the lower crank seals, I'd split the crankcase while you have the powerhead off. At least then you can get a good look at the internals and make sure everything's OK.

HTH.........ed