1963 75hp Compression Issues

[yorab]

Motor in signature. The motor has been completely rebuilt. Some time ago, I tried to test compression while the block was on my bench using a Lisle gauge borrowed from Pepboys. The values were all over the place, but I chalked it up to error due to turning the flywheel very erratically by hand.

I now have the motor reassembled and I used the starter motor to perform a compression test today. My numbers were 80-70-65-62 dry and 65-65-65-62 with oil. This doesn't make much sense to me. I did hear a bit of hissing from one cylinder so the gauge must not have been sealing so well. I assume that this could be the case with the other cylinders. The Lisle gauge has a rubber o-ring for sealing. Does the crush washer on the spark plug typically seal better than a rubber o-ring?

My manual gives a torque spec of 20-20.5 ft-lbs for the spark plugs. I talked to my shop guy recently about a problem with a Helicoil installed in the spark plug hole and he was very surprised that the torque spec is so high. He was thinking more like 12-12.5 ft-lbs due to the limited thread depth at that hole. I'm worried that if I torque down those plugs to get a good seal with the crush washer, then I may end up boogering up the threads. Any opinions on this?

 

[Willyclay]

Re: 1963 75hp Compression Issues

My 2cents! Has the motor been run through the break-in cycle yet? If not, the rings are not seated and compression values would be expected to vary greatly from each cylinder and the desired range. Was the battery fully-charged so the starter is spinning at its rated speed? Were the carburetor choke plate and throttle plate open for unrestricted airflow? Is that gauge the "screw-in" type? Regardless, try a different gauge and see what you get. My Clymer manual shows 210-246 INCH-Pounds which is 17.5-20.5 Ft-Lbs. Also, my experience with torque wrenches is they are notorious for having significant error at values below half their max rating. Good luck!

 

[eavega]

Re: 1963 75hp Compression Issues

My 2cents! My Clymer manual shows 210-246 INCH-Pounds which is 17.5-20.5 Ft-Lbs.

The OEM manual for my '76 70 HP shows the spark plug torque 17.5-20.5 Ft-Lbs. I just put new plugs in mine, and it does feel like you are putting in a lot of pressure, but the threads held up fine.

Rgds

 

[AlTn]

Re: 1963 75hp Compression Issues

OEM 1978 9.9...same..17.5 - 20.5 ft.lb

 

[Chris1956]

Re: 1963 75hp Compression Issues

Those compression numbers are pretty low. I would expect about 90 lbs and all even, especially after a rebuild. I recommend a retest.

 

[yorab]

Re: 1963 75hp Compression Issues

Reason: Fat fingers

 

[yorab]

Re: 1963 75hp Compression Issues

Well, good advice. The motor has not been through it's break-in yet. I haven't even started it yet. I hope to do so by the end of the week. I guess that the rings need to seat as you noted Willyclay. Also the battery was fully charged, but the starter was not terribly strong toward the end of the testing. It is an old battery, but has had very little use. I took it to Advance Auto the other day to get tested. It passed at 939 CCA and 12.6 volts. It is rated at 878 CCA. However, I'm wondering if it is strong at first, but quickly drains due to age. It is 8 years old.

I will wait until after the break-in period to test compression. I believe that I will also torque the plugs to the lower end of the range. I have a 150 ft-lb wrench but I've been wanting to get a more accurate torque wrench for lower torque values. Now I have a reason to go get one.

Is there any way to test the "longevity" of a battery? If not, perhaps a new deep cycle battery is in order.

 

[Chris1956]

Re: 1963 75hp Compression Issues

I would look to the battery cables and connections. Make sure they are clean and tight. Any heat on the cable or other components signals resistance which makes the starter turn slow. I once had a motor where the starter turned at top speed for a few seconds and then slowed down. Turned out to be rotted out battery cables, under good looking insulation.

 

[boobie]

Re: 1963 75hp Compression Issues

Screw a new plug in until it seats in the cyl head and give it 1/4 turn more to tighten. Remember you're only dealing with a 3/8" reach spark plug here. As far as batteries, don't use a deep cycle for starting. They weren't built for it.

 

[Willyclay]

Re: 1963 75hp Compression Issues

I have a 150 ft-lb wrench but I've been wanting to get a more accurate torque wrench for lower torque values. Now I have a reason to go get one.

Boobie has the best advice for the plugs but you might want to invest in an Inch-Pounds torque wrench for other low foot-pound tightening requirements. I bought a Craftsman 250in-lbs. wrench recently and, coincidently, just used it today on the oil drain plug in my Blazer's cast aluminum oil pan or is it a sump. Good luck!

 

[boobie]

Re: 1963 75hp Compression Issues

Agree with Willyclay 100%. Use Inch-Pound torque wrenches when working on any outboards. The only time you need the FT-Pounds is for the flywheel nut or pinion gr nut in the g/c.

 

[Chris1956]

Re: 1963 75hp Compression Issues

There are 12 in-LBS in a Ft-LB. Conversion is easy....

BTW- no one ever uses a torque wrench on the spark plugs. Make 'em snug....life is too short

 

[yorab]

Re: 1963 75hp Compression Issues

I bought a new battery today. Same as the old one--Everstart Maxx Marine dual purpose. I don't want to get stuck away from the dock with a dead battery, even though I'l be close to shore on Pennsylvania lakes and rivers.

I'm gonna install those plugs tomorrow and hopefully fire her up. I'll keep you posted.

 

[Chris1956]

Re: 1963 75hp Compression Issues

Does that motor have a pull start? I used to pullstart my old Fat-fifty, w/o issue. That 75HP has more displacement, but should start at least as well as the old fatty.

 

[yorab]

Re: 1963 75hp Compression Issues

No pull start on this one. It was an available option, but wasn't on my motor.

So I installed the plugs today. I screwed in until seated and then went 1/4 turn more. Cranked her over and had a leak at the starboard plugs. I picked up a torque wrench that is a bit more accurate than the older wrench that I have. Turns out that the plugs were at 8 ft-lbs. I went to 9 ft-lbs and still leaked. Went to 10 and the same thing. Torqued them all to 12 ft-lbs and I think that the leaks have stopped. I still hear some rushing air, but I think that it is the sound of the air escaping the cylinder as the exhaust port is opened. I'd guess that I turned about 1/2 to 3/4 turns beyond seated in total to reach 12 ft-lbs. Is there a way to be certain that the plugs are seated and sealed without really cranking down on them? I know that if I crank down on them more, they will seal, but I've already had problems with the plug hole threads and I want to avoid more problems.

I also found a few other problems today that I will post in other threads. If anyboby knows this motor and is in a helpful mood, please chime in on those. Thanks.

 

[Willyclay]

Re: 1963 75hp Compression Issues

I still hear some rushing air, but I think that it is the sound of the air escaping the cylinder as the exhaust port is opened.

Most likely correct conclusion.

I also found a few other problems today that I will post in other threads. If anyboby knows this motor and is in a helpful mood, please chime in on those. Thanks.

If your plugs are leaking, you will find oil/gas residue around them. Hang in there, you can whip it!

 

[yorab]

Re: 1963 75hp Compression Issues

I believe that everything is in order and ready to start. However, the weather is still nasty here. We actually had snow yesterday in central PA. This is the ugliest spring season that I can ever remember here. I'll try to start it up in the next few days.

What I may do is sift just a bit of baby powder around the base of the plugs. If there is air leaking there, the powder should go airborne and it would be easy to see. I would think that it would be easy to clean up as well. If anybody can see an issue with that, please let me know.