What is the best fuel to run in a 68 Mercrusier 120 hp

[Spivy30]

I just bought a 68 starcraft holiday with a 120hp mercruiser and was wondering what would be the best fuel octane to run it. The boat sat for a few years but was fogged I had it running the other day but it would stay running under 3000 rpms and it was back firing every now and then. I put about 2 gallons of 87 oct pump gas in it before I can it and guy told me the fuel might not have enough octane. I was thinking it might be a timming problem too.

 

[Eric 3.7LX Alpha 1]

Re: What is the best fuel to run in a 68 Mercrusier 120 hp

The fact that it was backfiring has nothing to do with 87 octane. There is something else going on here.

Also if your are running on muffs 3000 is not a good idea.

 

[Spivy30]

Re: What is the best fuel to run in a 68 Mercrusier 120 hp

I'm thinking it has something to do with the timing and the points. I bought a tune up kit for the ignition with a cap rotor points ect. When you set the timing should it be at idle? Around 500-700 rpms? Thanks again you helped with me someother stuff too I apperciate it a lot.

 

[Fishermark]

Re: What is the best fuel to run in a 68 Mercrusier 120 hp

Yep - just good old regular unleaded works fine on that engine.

Yes - the timing should be set at idle. Set your point gap / dwell first, then the timing.

 

[bigskiohio]

Re: What is the best fuel to run in a 68 Mercrusier 120 hp

i run the highest octane i can get makes me feel better as my 120 sips the fuel it doesnt bother me to pay 20 cents more per gallon.
I just did my points and my boat ran great before and it ran better after the new points. dwell was a little tough to set. actually th timing should not change it only changes as points wear and dwell changes. so dwell is more important of the two but check anyways.

 

[dozerII]

Re: What is the best fuel to run in a 68 Mercrusier 120 hp

i run the highest octane i can get makes me feel better as my 120 sips the fuel it doesnt bother me to pay 20 cents more per gallon.
I just did my points and my boat ran great before and it ran better after the new points. dwell was a little tough to set. actually th timing should not change it only changes as points wear and dwell changes. so dwell is more important of the two but check anyways.

I`m with you Bigsky, I burn premium as well, in my 72 SS 120 that is what the manual calls for and with the little bit it uses what the heck.
Glen

 

[dubs283]

Re: What is the best fuel to run in a 68 Mercrusier 120 hp

premium, non-oxygenated fuel is what you want to run in your boat

no ethanol/alcohol at all

check out some of the how to's in the DIY section at the top of the threads page

a wealth of information there

 

[sethjon]

Re: What is the best fuel to run in a 68 Mercrusier 120 hp

i run the highest octane i can get makes me feel better as my 120 sips the fuel it doesnt bother me to pay 20 cents more per gallon.
I just did my points and my boat ran great before and it ran better after the new points. dwell was a little tough to set. actually th timing should not change it only changes as points wear and dwell changes. so dwell is more important of the two but check anyways.

The 20 cents isn't the issue. It doesn't make the engine run any better or faster or smoother or anything else you can think of. 87 is more than enough.

 

[bigskiohio]

Re: What is the best fuel to run in a 68 Mercrusier 120 hp

Like i said makes me feel better and i cant prove but i am sure it helps it from having any preigniton problems .

 

[dubs283]

Re: What is the best fuel to run in a 68 Mercrusier 120 hp

octane = resistance to burn, some say resistance to knock (AKI = anti-knock index)

the higher the octane, the cooler and slower the fuel/air mixture burns in the combustion chamber

less octane = greater chance for detonation (explosions that destroy pistons)

also, alcohol/ethanol ladened fuels will increase chances for vapor lock

use non-oxy, premiuim fuel in your boat, 92 octane

 

[Spivy30]

Re: What is the best fuel to run in a 68 Mercrusier 120 hp

I gave the boat a tune up and set the timing and it runs perfect and starts right up its amazing what new points and condensor will do. I was just curious about the fuel because it was so old I will try and run nonreformulated fuel but its hard to find in the Milwaukee area you have to go way west out in the county to find it.

 

[Fishermark]

Re: What is the best fuel to run in a 68 Mercrusier 120 hp

I will try and run nonreformulated fuel but its hard to find in the Milwaukee area you have to go way west out in the county to find it.

The older an engine gets, the more it loses compression. The more it loses compression, the less octane requirement it needs. If you engine runs good on regular, which I suspect it will, then that is all you need. Period. Higher octane will not give you more power or more cleaning agents, or anything else.

 

[HT32BSX115]

Re: What is the best fuel to run in a 68 Mercrusier 120 hp

The older an engine gets, the more it loses compression. The more it loses compression, the less octane requirement it needs. If you engine runs good on regular, which I suspect it will, then that is all you need. Period. Higher octane will not give you more power or more cleaning agents, or anything else.

Absolutely true!


But anything that makes you feel better is good for you I guess.... It's that placebo effect.......


I run the cheapest regular with what ever is inside. It doesn't make any difference.

No gasoline will fix your engine if it's missing, popping, farding, back or front-firing etc!!


With an engine that old, I would do a compression or leak-down check and carefully check the ignition, timing and carb......


regards,


Rick

 

[Brewman61]

Re: What is the best fuel to run in a 68 Mercrusier 120 hp

Another thing that can happen to older engines is carbon deposits in the cumbustion chambers. They can act as hot spots, prematurely igniting the fuel, and can also take up enough volume to change the compression, and in those instances, higher octane fuel might work a tinge better.
Essentially masking the problem. I've run many a can of GM top cleaner thru some of my older vehicles- stuff worked pretty well IIRC.
I've pulled the heads off some really nasty carbon fouled engines. Not marine, in this case, but I would think they could be subject to it as well.

 

[Spivy30]

Re: What is the best fuel to run in a 68 Mercrusier 120 hp

I did a compression check and it was 130 across the board. Once I did the tune up the engine started running perfect. I was really wondering about the best fuel because most marinas only sell 89 octane.