mercruiser 120 cam to crank timing help....

[floridaboater1]

I have a 1975 mercruiser 120 2.5 153. I rebuilt the engine when i took the cam out it was hard to tell where the mark was on the gear. So i put the engine a tdc. and i made a new mark on the cam gear for reinstallation. Well i put everything back together and it ran good no real problems. I put 20 hours on the engine then i pulled the timing cover to replace the timing gears ( i couldnt get them at time of rebuild) When i got the new cam gear i found out the i had been running the engine 180 out. I just installed the cam with the new gear correctly but now the engine wont run... If i spin the cam 180 out again it will run. what could be the cause? Why will it only run 180 out?

 

[cr2k]

Re: mercruiser 120 cam to crank timing help....

My guess is that your distributor is 180 off. With out the distributor or cam there is no 180 off. just no. piston at TDC. With the cam and or distributor you now have a situation where 180 off means something.

 

[floridaboater1]

Re: mercruiser 120 cam to crank timing help....

if my dist was out 180 my rotor would not be pointing at the #1 cylinder at dtc.

 

[mariner1900]

Re: mercruiser 120 cam to crank timing help....

Did you rotate the engine before you dropped the distributor in? The cam gearing is 2:1 ratio. You can install the cam exactly 180 degrees out and 1 complete rotation of the crank will line the gear marks back up at #1 TDC. If you rotated the crank to get #1 TDC firing stroke before dropping in the dissy then this is probably what happened.

 

[floridaboater1]

Re: mercruiser 120 cam to crank timing help....

i can only get it to run if it is 180 out.(when the cam and timing marks are away from each outher with the dist pointing at the #1 it will run) If i put the motor at dtc with the cam and crank gear marks together and the distrubuter pointing at the #1 it wont run it tries to but it just backfires out the carb and exhaust. any ideas

 

[bruceb58]

Re: mercruiser 120 cam to crank timing help....

You then need to pull the ditributor and rotate it 180 as well because when you rebuilt your engine and put the cam at 180 out, you also put the distributor at 180 out. If you now want to put the cam back, you need to do the distributor at the same time.

 

[nyfireman]

Re: mercruiser 120 cam to crank timing help....

Im having the same problem with my 2.3 omc ford. Ran great when i put it away last yr and now it wont start and having same timing issues. hope someone can help

 

[floridaboater1]

Re: mercruiser 120 cam to crank timing help....

I did when i roatated the cam i had the distributer out then put it in pointing at the #1 cylinder

 

[Bt Doctur]

Re: mercruiser 120 cam to crank timing help....

The rotor DOES NOT FACE A PARTICULAR CYLINDER. Number 1 cylinder is set at TDC compression stroke.The rotor faces #1 position on the dist cap

 

[floridaboater1]

Re: mercruiser 120 cam to crank timing help....

yes that is where im setting it... I just pulled the dist again and turned the engine one turn back to 0 put the distrubiter back in and it will run but is shakeing real bad will not rev up and is sputtering

 

[bruceb58]

Re: mercruiser 120 cam to crank timing help....

yes that is where im setting it... I just pulled the dist again and turned the engine one turn back to 0 put the distrubiter back in and it will run but is shakeing real bad will not rev up and is sputtering

Did you use a timing light to set the timing?

 

[farmer9009]

Re: mercruiser 120 cam to crank timing help....

I suggest pulling the rotor button off and make sure that it has not moved on you. For some reason mine was not installed all the way and caught the inside of the cap and striped out the plastic key part on the rotor which rotated it on the shaft it sits on. It took me a little while to realize this one.

 

[mariner1900]

Re: mercruiser 120 cam to crank timing help....

Just had a look at the cam from a 120HP I have in the garage. When the cam timing marks are aligned the cams for #1 cylinder are in the valve overlap position (#4 is actually on TDC firing at this position), i.e #1 IS NOT at TDC on the firing stroke. The crank needs another full turn to bring up #1 TDC firing stroke. That is why it appears to be 180 degrees out.