1974 merc i/o 6 cyl fuel and compression questions

Hi everyone, I got a merc I/o running today. I think it is a 1974 or 1975 motor. the boat is from 1975. The motor is a 165 HP stright 6. I think this a gmc 250.

I noticed a sticker on the motor that said that said to use leaded fuel. Is this correct? That is the first question.

The second question is about the compression. After I got it running I took it to normal operating temp. Then compression tested it. The results were 90 psi in all of the valves. This seems a bit low but at least they are similar values. Any thoughts?

The reason i ask is because I am thinking or restoring this boat and want to be sure it will have a strong motor. I have spare parts for it, everything except a block and out drive. I will have to pull the motor for the restore. But want some info on how big a job it would be to get the compression back up, if need be.

Any info would be great.

Re: 1974 merc i/o 6 cyl fuel and compression questions

90 psi is very low. How did you perform the test? Engine warm? Carb open? How many revolutions did you crank per cylinder?

As far as leaded fuel, you can't buy that anymore except for aviaton fuel. You can run it with regular until it is time to get your valves done...with your compression measurements, that may be sooner than later.

Re: 1974 merc i/o 6 cyl fuel and compression questions

The motor was warm. I cranked it for a total of 3 seconds on each valve.

I have no idea if the carb was open. I am new to carb motors. I had the throtle in neutral position. When I think about it I should have had the throtle wide open during the. Test right?

Re: 1974 merc i/o 6 cyl fuel and compression questions

all yeah boy said:

The motor was warm. I cranked it for a total of 3 seconds on each valve.

I have no idea if the carb was open. I am new to carb motors. I hand the throtle in neutral position. When I think about it I should have had the throtle wide open during the. Test right?

Click to expand...

Compression test procedure:

1. Warm engine to normal temp.
2. Remove all spark plugs and disable ignition system.
3. Move throttle to full open.
4. With pressure gauge in spark plug hole, crank engine for a minimum of 5 compression cycles.
5. Repeat on all cylinders.

Chris........

Re: 1974 merc i/o 6 cyl fuel and compression questions

Throttle open allows the cylinders to suck in air through the carb.

Re: 1974 merc i/o 6 cyl fuel and compression questions

Awesome. Thank you. I will retest and repost tommrow. Any other insight about the leaded fuel?

Also if the motor does have low compression is it worth fixing given it age?

Re: 1974 merc i/o 6 cyl fuel and compression questions

The basic difference for unleaded is silicone-bronze valve guides (lead was used to lube the guides) and hardened valve seats. And of course the timing. You can advance the timing more on leaded gas.

You can buy additives to replace the lead, but when you spend $5-6 to treat 5 gallons of gas it raises the price per gallon of fuel quite a bit. Then it doesn't take too many tanks to warrant the proper valve guides and seats.

Re: 1974 merc i/o 6 cyl fuel and compression questions

cr2k said:

(lead was used to lube the guides)

Click to expand...

I think engine oil lubricates the valves guides.
Such as the engine with bad valve guide seals usually has a puff of smoke on engine startup due to oil entering the cylinder after the last engine shutdown.

Re: 1974 merc i/o 6 cyl fuel and compression questions

mkast said:

I think engine oil lubricates the valves guides.
Such as the engine with bad valve guide seals usually has a puff of smoke on engine startup due to oil entering the cylinder after the last engine shutdown.

Click to expand...

The lead in the fuel was to coat the valve seats and 'soften' the closing of the valve, hence leaded fuel engines run on unleaded fuel suffer from valve seat recession, and the need for hardened valve seats. Something I think is probably the problem with this engine...

Chris........

Re: 1974 merc i/o 6 cyl fuel and compression questions

all yeah boy said:

Awesome. Thank you. I will retest and repost tommrow. Any other insight about the leaded fuel?

Also if the motor does have low compression is it worth fixing given it age?

Click to expand...

By 1974 all the engine manufacturers were building engines with hardened valve seats.
A cylinder leakdown test will show you if the valves or piston rings (or both) are leaking.
If the valves are leaking, a valve job should seal the engine again.

Re: 1974 merc i/o 6 cyl fuel and compression questions

Can someone point I'm the direction for how to perform the cylinder leak down test.

If I do need to perform the vavle reseating, is this a task I can take on myself. I have decent mechanical skills, but would be doing the job in my backyard. Is the vavle seal job done with the motor in the boat or out of it?

Re: 1974 merc i/o 6 cyl fuel and compression questions

Any good engine machine shop can do the valve work.
I would want to pull the head to have a look at the cylinder walls anyway.
At least the numbers are even....just sounds like a tired old engine.