Mercruiser 120 help

I have a Mercruiser 120 on a Trihull Galaxy...while out skiing with son it seemed to tighten up.... oil pressure was good...45 psi .....never dropped...There was a definite knock in either drive or engine...also seemed as though engine was missing.....when getting home the compression was #1- 135#, #2-120#, #3-110#, #4-140#.....going to pull the drive and see if its tight.....but that will not cause engine miss....if lost a bearing wouldn't oil pressure drop?....anyone have any ideas before I jerk this engine out and tear it down....

Re: Mercruiser 120 help

Pull the lower part of the drive first. If the engine is still stuck then pull the top plate off the upper drive and look at the gears. If they look ok then pull off the upper drive and see if the engine is still stuck. You must find out if its the lower, upper (or both).

Re: Mercruiser 120 help

Think about it this way, the motor is trying to attain a certain rpm. if the drive is binding it. it will start to cough and sputter. hold your foot on the brake pedal of your car and stomp on the gas. What happens?????? good luck

Re: Mercruiser 120 help

pulled outdrive it was free.....pulled engine and found #2 rod bearing spun.....may get by with just basic rebuild all other looked good....<br />head gasket was copper....can it be reused?...used to run copper head gaskets on racecar n was able to reuse 10 times or so....think just go with another but was just wondering......any other tricks that might give a little more go to this little motor...<br />thinking of using 1.6:1 rockers from a ford in place of stockers have done this on cars and had good luck....this motor has the yellow stripe springs think it may make too much pressure on cam and waste it?.....just wondering if these things from hot rods might apply to boat....or would the restrictive exhaust make them a mull point

Re: Mercruiser 120 help

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Re: Mercruiser 120 help

crazyray, Most all marine engines are considered to be high performance. Any souping up of course will help you get out of the hole and on plane, but however the outdrive is only going to handle its specified RPM. It will also limit the amount of torque that the outdrive gears can handle. If you do soup it up, Go with a higher pitch prop to handle the extra horses to bring your RPM down so you don't over rev your outdrive. aAso limit how fast your hole shots are since the extra torque you're getting can over stress your outdrive gears. Good luck with your adventures.

Re: Mercruiser 120 help

what has anyone on here tried...with sucess or not.....I will be porting head somewhat mostly cleaning it up with some minor pocket porting so as not to affect durability....will much work on exhaust side of head help or is this counter producive.....will gasket match intake and head...think my same outdrive was used for bigger motors so feel that this little motor won't be hurting much.....rpms will stay in 4500 range....whats the difference between the ealy 120's with their 4300 redline and the later models that redline at 4800<br />is the stock balancing sufficent or should this be done also.....isn't this the same block as the iron duke motors?.....if so there is a big list of hipo stuff out there....this may sound as wishful thinking but have been around block in autos for yrs just wanted to try see what may apply to this engine<br />

Re: Mercruiser 120 help

Needs a blower

Re: Mercruiser 120 help

My 67 Stingray has a 120 in it. It is a 66 but I just did the major overhaul on it. Everything new except crank and had it turned. I have it mounted to an 89 merc outdrive. Exhaust didn't line up and couldn't find any adapter, so I had a stainless pipe and flange made up and it now has through hull exhaust. I mounted the exhaust flapper on the end of the pipe so water can't back up into the head/valves. It sounds real sweet when your being towed on a tube.<br /><br />As for your question on porting, I think that just about anything that can add to your flow threw the heads is beneficial. As far as the outdrive goes, you need to make sure that you have the right ratio. Just because it may have been for a big six or eight it may handle the extra torque, but without the right ratio, your prop speed my be slower due to the difference in ratio between the 8's and 4's. I personally don't exactly know what the ratio is. I don't work on them everyday to have the numbers imprinted in my brain. Trent will probably have the answers that you need to know. He is a Wizard of Marine Mechanics. Good Luck

Re: Mercruiser 120 help

I have done this same project. I had my head opened up from 1.70 intake valves to 1.94's .The exaust was 1.50 now is 1.65's. I put in a comp cam. Bored the block 30 over and went with 10 to 1 pistons. I found a carb from a merc 165. with this config the boat did great. Gained about 30hp <br />or so. RPM's increased to 5300. But I found that I had to put a carb spacer under the carb for better atomization. what a differance. Email me and I will send some pics. paintboytroyAhotmail.com