Mercruiser 120 Backfire After Rebuild

[Leedanger]

Greetings,

I'll try to be succinct. Working on a Merc 120 Hp (3849449) thought to be a 1975. When I try to start (it's out of the boat on a pallet) she cranks and wants to fire and then I get a *POP*. The popping noise seems to come from the carb and I even got traces of smoke, guess it could have been steam.

Here's a little history:
Had the head off as well as the distributor so timing could be equated, but I feel confident that I located TDC

I am also reusing the head gasket, it calls for the bi-metal gasket and it looked good, figured no harm to reuse. (I realize that buying a new one is the best practice)

The most likely problem, although many told me this would work, I am using (trying to use) the exhaust manifold off a Merc 140 (2800770) also thought to be a 1975 but unless I am mistaken that is impossible by that model #. There is obvious variances between the two manifolds, but from visual inspection it did appear to be fundamentally the same.

Thanks for your help, it's appreciated. If I can add any info to help you remotely diagnose my issue let me know and I'd be happy to update.

Cheers

 

[Bondo]

Re: Mercruiser 120 Backfire After Rebuild

I am also reusing the head gasket, it calls for the bi-metal gasket and it looked good,figured no harm to reuse.

Ayuh,... Headgaskets are NOT reusable, Period....

 

[Leedanger]

Re: Mercruiser 120 Backfire After Rebuild

Huya,...but I don't like that answer... punctuation

Ok how about this one...I is the heads off a Merc 140 (2770070) synonymous with the head of the Merc 120 (3849449)? Why you might ask? Oh well thanks for asking. The head gasket for the older 140 is $20, for the 120 it's $80. The coolest part about that is I have bills that are exactly $60 so it should fit perfect.

Excuse my crass tone, it's not directed at you or anyone specific, thanks for you help.

 

[mkast]

Re: Mercruiser 120 Backfire After Rebuild

Had the head off as well as the distributor so timing could be equated

While cranking the starter, rotate the distributor a tad.

 

[fossill]

Re: Mercruiser 120 Backfire After Rebuild

I am also reusing the head gasket, it calls for the bi-metal gasket and it looked good, figured no harm to reuse.

Good luck with that rebuild

 

[Pete104]

Re: Mercruiser 120 Backfire After Rebuild

The "used gasket" will prove your point!

AYU, ???? It will atleast get this guy running!

What's with BOND-O & POMPOUS attitude? IS he A GOD?

 

[fishrdan]

Re: Mercruiser 120 Backfire After Rebuild

What's with BOND-O & POMPOUS attitude? IS he A GOD?

Chill...... Was Bond-o wrong? No! Matter of fact, he is exactly correct and I agree with his answer and how it was stated.

A used head gasket could induce a new problem since there is no way to ensure that the gasket will land exactly where the old gasket was, and , there is no way to re-crush a crushed gasket.

 

[Pete104]

Re: Mercruiser 120 Backfire After Rebuild

If you don't have to stand behind it. And this guy gets his boat on the water. What's Difference what he does? When he developes his next problem, it probably won't be a head gasket!!!

There, I Chilled!!!!!!!!~

 

[Bondo]

Re: Mercruiser 120 Backfire After Rebuild

If you don't have to stand behind it. And this guy gets his boat on the water. What's Difference what he does? When he developes his next problem, it probably won't be a head gasket!!!

There, I Chilled!!!!!!!!~

Ayuh,... Yer obviously No Mechanic....

A USED Headgasket won't get Anybody back on the water....
And,...
I'm ALWAYS standing here behind MY advice....

 

[Leedanger]

Re: Mercruiser 120 Backfire After Rebuild

Ayuh,... Yer obviously No Mechanic....

A USED Headgasket won't get Anybody back on the water....
And,...
I'm ALWAYS standing here behind MY advice....

I don't know about anyone elses qualifications, but as per myself I am certainly not a mechanic. Just a guy that bought a lemon of a boat and wants to get it going so maybe I can get the family out on the lake once. I appreciate all 3 view points actually, it's good to know what I SHOULD do, what I CAN do, and WHY I should do it. I didn't know that a gasket actually is crushed, therefor when I see a gasket that looks new I assume incorrectly it is reusable.

To put a cap on this thread...none and all of the above worked in a sense. The timing was slightly off, thank you sir, and it's true gaskets are not reusable. However, the head gasket was not the culprit (for at least today anyhow). A brand new shiny exhaust manifold gasket seemed to do the trick.

Thanks gentlemen, I appreciate your time and patients.