78 merc 150hp compression???

[Natalie P]

What should the compression be on my 78 150hp ... I get 120psi across all 6... is this good or bad? I also have a 81 merc 90 that gets 150psi across and a 78 chrysler charger that gets 150 across all 4. I am thinking that 150 is good and 120 is low???

 

[hoytvectrix]

Re: 78 merc 150hp compression???

They all sound good to me. 120 psi is just fine

 

[8hygro]

Re: 78 merc 150hp compression???

I also would consider 120 psi. to be near the lower end of exceptable compression. If possible, I like to make 2-stroke compression checks after a short warm-up.

8hygro

 

[Natalie P]

Re: 78 merc 150hp compression???

Thanks... yeah i was thinking 120 was okay but not great ... thinking maybe its time for a newer one tho... just a little confused about what the 150 should be... have read older mercs had compression around 180psi and i couldn't find what the compression when new for the 150hp I-6 was

 

[Chris1956]

Re: 78 merc 150hp compression???

An inline six 150HP motor is no newer than a 1977. In 1978 merc introduced the 150HPV6, and changed the name of the Merc 1500(150HP) to a Merc 1400(140HP). Just a note so you will know.

120PSI is excellent for an inline six...

 

[Natalie P]

Re: 78 merc 150hp compression???

Thanks must be a 77 then ...its a 1500 I-6

 

[Dukedog]

Re: 78 merc 150hp compression???

uh, first V6 was '76. But yeah, I agree 120 is just fine. Even if it's a V6 or inline...........

 

[Faztbullet]

Re: 78 merc 150hp compression???

Unless it been rebuilt with low dome pistons 120 psi is Ok but toward the low end as 90% of motors has high dome pistons of this age. Most 150hp you will see 140-150 psi range but since all 6 are pretty even it should run ok.....

 

[Natalie P]

Re: 78 merc 150hp compression???

yeah she seems to run fine....get 45mph out of her ....I cant find the exact yr tho...any ideas its a canadian serial number 7049088

 

[Faztbullet]

Re: 78 merc 150hp compression???

1975..

 

[starcraft1982]

Re: 78 merc 150hp compression???

120s on all holes on a 150 i-6?these posters are right.Thats not good.You should send me that motor so i can dispose of it.I wont even charge you to do it.I just want to see it get a proper burial.By burial,I mean proper burial of the tach on the boat I would quickly mount it on!Those numbers are fine.If they arent,there are THOUSANDS of people running the dog poo out of "not so good",30+ yr old inlines.

 

[PrinceValium]

Re: 78 merc 150hp compression???

You have the same motor that I do then! It sure is a finicky beast.

 

[Chris1956]

Re: 78 merc 150hp compression???

Duke, Merc did not make a 150HP V6 in 1976 or 1977. I am trying to help Natalie id her motor, and accuracy is important.

 

[Natalie P]

Re: 78 merc 150hp compression???

LOL @ Starcraft ...yeah i thought the V6 was introduced in 1978 too... i think they did make a 1500 I-6 in 78 but renamed it the 1400 so as not to confuse with the V-6 ... I had just presumed 78 as its on a 78 checkmate but most likely predates the hull ... i just wish i knew where to find the serial numbers ...could be 75 too...who knows ... I did pick up a 78 1400 for parts ... the lower leg bolted right on to the 1500... but that still doesn't really help in identifying the exact yr ... thanks for all your input guys and if anyone comes across the numbers let me know

 

[Chris1956]

Re: 78 merc 150hp compression???

Natalie, The original serial number was on the transom bracket, between the thumb screws. There is a part number for the block on the transfer cover side (starboard side), about 1/2 up, and very near the seam between crankcase and crankcase cover. You can match the partnumber at the crowleymarine.com website parts catalog.

The way I understand it the '78 1400 will be identical to a '77 Merc 1500.

 

[Faztbullet]

Re: 78 merc 150hp compression???

uh, first V6 was '76

Duke didnt say it was a 150hp, the first V-6 was a 1976 175 (1750)..

I had just presumed 78

If its a 1978 it will have ADI igniton, if it has distributer its 1977 or earlier..

The way I understand it the '78 1400 will be identical to a '77 Merc 1500

1978 block and ignition system different than a 1977

 

[Chris1956]

Re: 78 merc 150hp compression???

Fazt, You are correct, however, Duke's posting about the 175HPV6, when we were discussing an IL 150HP motor, was likely to confuse Natalie.

The '78 Merc 1400 was a distributor-based ignition. The change over to ADI occcurred on the 140HP and 115HP motor in the middle of 1979.

Are you sure about the block differences between the '77 1500 and the '78 1400?

 

[Natalie P]

Re: 78 merc 150hp compression???

Natalie, The original serial number was on the transom bracket, between the thumb screws. There is a part number for the block on the transfer cover side (starboard side), about 1/2 up, and very near the seam between crankcase and crankcase cover. You can match the partnumber at the crowleymarine.com website parts catalog.

The way I understand it the '78 1400 will be identical to a '77 Merc 1500.

I will have a look...thanks