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1978 6 HP Johnson Low Compression

D

DVF2001

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I recently purchased a 1978 6 hp Johnson Outboard. It ran fine for the first few times I had had it on my dinghy but then it started to cutout and die while running at high throttle. I assumed it was a problem with the gas or gas tank or fuel fittings so I used a new tank with new fittings. Same problem. I then rebuilt the carburetor, same problem. I then installed a new fuel pump, same problem.
The spark is very strong and when she is running , it seems great. I didn't think the compression could be bad because she starts easily and will idle smoothly for hours, but I tested the compression and only got 15-20 psi on each cylinder. Being that the motor runs so well most of the time, I have a hard time believing that the compression could be that low. Would bad reed valves cause low compression like this and then let the motor run great most of the time? Any help would be greatly appreciated.
 
The reed valves have nothing to do with compression in the cylinder !!----Pull the cylinder head off.-----Likely blown between the 2 cylinders !----The head is also warped.----They all are.----New Sierra head gasket is a better quality then original.----Resurface the head with emery paper on a piece of glass.
 
Racer is right on.....head gasket, very common problem. Could be low gauge too. I will post a gasket photo of what to expect....just changed this one in September...motor was still running but very hard to start.
 

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Boobie said:
Try a different compression gauge !!
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Boobie, I tested the compression gauge on another motor and got proper readings. I also checked the gauge with an air compressor and it read the same as the air compressor gauge. I ordered the head gasket yesterday and will replace it over the winter with all my other winter projects.
Timguy, thanks for the photo.
 
Well I changed the head gasket and I’m only getting 30 psi reading on both cylinders. The cylinder walls looked fine. I looked online to find piston rings but I’m unable to find them. Any help finding rings for a 1978 6R78B would be greatly appreciated.
 
Was the head gasket blown ?-----What kind of compression tester are you using ?---Running with 50:1 gas oil mix ?-------Any pictures of cylinder walls ?------Your location ?
 
Probably was "dry seized"....often little or no rust. Try a fresh mixture of ATF and acetone @ 50/50. Best loosener and buster ever. If the cylinders are not rusty or scored, head gasket was good....then there is hope for a simple fix here.
 
racerone said:
Was the head gasket blown ?-----What kind of compression tester are you using ?---Running with 50:1 gas oil mix ?-------Any pictures of cylinder walls ?------Your location ?
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racerone, the head gasket didn’t appear to be blown. I used 3 different compression testers and got the same reading. Yes, I was running with 50:1 mixture. I didn’t take photos of the cylinder walls but they were clean and rust free. I’ll try pouring the acetone/atf mixture into the cylinders and then crank the flywheel to hopefully free up the rings.
 
Looks like no problem here, but it was about ready to fail. Are you spinning with the starter cord to get your compression tests? After the rings break free, then be sure to flush through with gas/oil mix. This will rinse things out. Have motor laying down with carb facing up and run mixture through with plugs out. If my guess is right, the rings should free up in a day or two. Pull over every few hours if you can.
 
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timguy said:
Looks like no problem here, but it was about ready to fail. Are you spinning with the starter cord to get your compression tests? After the rings break free, then be sure to flush through with gas/oil mix. This will rinse things out. Have motor laying down with carb facing up and run mixture through with plugs out. If my guess is right, the rings should free up in a day or two. Pull over every few hours if you can.
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Timguy, are you saying to rum the Acetone/ATF mixture through the carburetor? I have never seen this done so my plan was to pour the mixture into the spark plug holes and let it sit for a while, is that the wrong way to do this?
 
That appears to be an original head gasket and replacements are better.-----If you could walk over I have new piston rings on hand.-----No location for you posted.----So sorry , no shipping on that item.
 
racerone, I live in Totowa NJ so if you are close I would gladly walk over to pick up the piston rings. I have also updated my profile. Thanks.
 
Right.....Canada closed. They locked us out....2 camps, one since 1964 and one since 1981.
No you probably have no rust in the crankcase, so just acetone/ATF the cylinders. Afterwards, flush from carb side as aforementioned. Why? To get any acetone off the crank seals....it will get there ......if left on for an extended period of time, it will soften the seals.
 
Well, I poured the Acetone / ATF mixture into the cylinders and let that sit for a few days. Then I put in more and worked the pistons back and forth many times. Everything looked great but I'm still only getting 30 psi compression. I decided to pull the pistons and check them. everything looks good inside the cylinders but 1 piston has some scoring marks on the side. The piston rings have the groves in the outer edges which I assume is the way they came from the factory so maybe they are not worn out. I'm trying to find new standard size rings but I'm not having any luck. I'm able to find 0.030 oversize rings but I don't know if I can use them. The bore in both cylinders measures 1.9350 with my digital caliper. Any ideas?

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Looks like a beauty. Put it together with new rings and should be fine..I really see a motor that should have had 90 psi.
 
I shoulda said that too, but I thought I'd get scolded. That solvent mix did a nice job, eh?.....reconditioned the rings too?????
So Racer....why 30 pounds compression and DVF tested the gauge on another motor and an air compressor?
 
Well, I'll clean up everything and put it back together. Should I hone the cylinders? Is 1.9350" diameter the correct bore size for this engine?
 
Well---Was this motor apart before ??-----1.9375" is standard bore ,but a digital caliper is NOT correct tool to measure a bore.---This motor has seen little use , unless new rings were installed.----Deglaze cylinders.----Resurface the cylinder head( it is warped ), install the improved head gasket. -----I say put it together.---Make sure there are 30 needles for each rod bearing.----It should run like new.
 
Right, there are inside micrometers that can give you accurate readings throughout the bore. I assume that caliper is all you have so you can only measure the end, which makes it difficult to gauge the wear at the ring travel area. There is an inexpensive tool that you can adjust to fit in various points of the bore, then you measure the tool with the caliper. As Racer mentioned, put rings into the bore and measure end gap with a feeler gauge, this will also let you see areas that may be worn. You can push the ring to various points in the bore, using the bottom of the piston to keep it square.
As a good general rule, a good tight fit for cylinder to piston difference/clearance, is .001" for each inch of cylinder bore. Find the fatest part of the piston, usually on the skirt, nearer the bottom and 90 degrees off from the pin. You also want to check size of any area with scoring. On tiny bores like chain saws and small outboards you should be meticulous.
 
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Thanks guys, I got the pistons back in but I'll take them out tomorrow and check the clearance of the rings before I put it back together. The clearance between the pistons and the cylinder walls was very close. I did place the head on a sheet of heavy glass and sanded the head using 220 then 500 then 1200 and finally 2000 grit paper and it appears to be flat. I did use a new Sierra head gasket and I will use another new Sierra head gasket when I put it back together again.
 
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