1975 Mercury 150hp on the water Issues!

[Jhenry72]

Hello,
I have problems with my Merc throttling up in the water. It seems to run really good while on the muffs. And I also put it in a tank and it runs good. But when I go to the lake, just seems like its off. And I can't get it to rev past 3500 rpms. I've done the "lync and sync" while both on the muffs and at the lake. Have the timing at 4* BTDC at the primary pickup and 21* at Max advance. Also have reset the Idle mixture screws to 1 3/4 turns out from lightly seated. I have tried adjusting the screws out a little as I go but doesn't seem to make a difference. I have done various different tests to make sure that I have good spark including cleaning the carbs. I have brought it to a couple different mechanics, both with different diagnosis. 1 told me, bad switchbox and too old of a motor... other told me I had a bad trigger! The prop I have on is a factory 19 pitch. I should also say I have purchased this boat back in Feb. and the previous owner stated that the boat ran last year but didn't get on plane. Said it needed a good tune up. I have replaced the Internal wiring harness, Distributor cap, Spark plugs, Water pump and Impeller. Replaced the old fuel with fresh 93 octane fuel with seaform. Just getting really frustrated!

Thank you for any and All your Help! I have learned so much from this site!

 

[Chris1956]

Re: 1975 Mercury 150hp on the water Issues!

What kind of boat do you have it on? If the boat is too heavy, you will not get max RPM. Your tach is accurate?

Check for arc between plug wires and block. What do the spark plugs look like? Any of them look real clean?

 

[Jhenry72]

Re: 1975 Mercury 150hp on the water Issues!

Hi Chris, The boat is a 1971 Checkmate MX-18. It came stock with a 135 horse Mercury and the original owner replaced it with the 150 horse. I have all the original documentation and the boat is rated up to 160 horse. I did pull all the plugs out and 1-3 looked like they were firing good! 4 was a little watery and 5 and 6 looked really clean... But i did replace them new and they do look like have have fired just not as much as the first 3. I do have another distributor that I purchased when the one mechanic told me the trigger was bad, that I'm going to swap out and see if there's any difference. The plug wires, looked really good. No cracks or burns on any of the wires. I have also done a compression test and all cylinders were in the 135 to 125 psi range. The tach is an old Autometer tach that I've had lying around that has always worked in the past for me. I believe it's accurate I can get it to idle around 850 in neutral.
Thank you very much for your Advise! It is Greatly Appreciated!

 

[Jhenry72]

Re: 1975 Mercury 150hp on the water Issues!

Okay, a little update from today. I swapped out the distributor with another one that I purchased and also reset the floats on the carbs. Got it all back together and, Fired right up! checked the spark on all cylinders and have good spark! Then I hooked up the timing light and at idle it shows that I'm at 6* BTDC. So I think I might be a tooth off on my distributor?? I'm hoping that this is the solution to my problem!! Wish I didn't have to work today! Finally a Beautiful day, would like to head out to the lake to try it out!!