compression for QD-17

[opposedtwin]

what should compression numbers be for a 1956 Johnson QD-17 10 hp? I’m thinking mine are mighty low at 75 lbs. it is the same in both top and bottom cylinders, but the paint on the head, exhaust plate, etc is quite brownish compared to the recoil and other under the cowl parts.

the pistons look a bit scuffed through the inspection windows (although I realize that tells me only about 25% of the story), but I have seen much better compression on motors with worse scratches on the pistons.

no spark in top cyl. I have a nice prufex coil and a black one–which I suspect is the culprit.

I will drop the lower and see if the impeller is roached.

scott

[chris-p]

That is a touch low for a QD. Most are around 80 to 105 or so.

[opposedtwin]

chris would you happen to have a head gasket for this motor? #301862

[fleetwin]

A little low, but maybe just from sitting, these 10hps don’t have a lot of compression.
Like you say, the normal scuffs/bruises for an engine of this age, but nothing really bad. Maybe it was overheated and left dry. Go ahead and pull the gearcase to make sure the impeller didn’t break apart leaving pieces throughout the cooling system. Go ahead and pull the head for a better look, then give it the 50 cent hone job with light emery cloth and replace the head gasket, I’m betting the readings will come up to an acceptable level 80-85PSI.

[opposedtwin]

I dropped the lower and the impeller is fine. removed the exhaust cover and the pistons actually look pretty good. finally, I removed the head. no broken bolts thank goodness! the cylinder walls I could see looked to be in good shape. I believe I’m at the point where the previous owner turned back/gave up or whatever because the head bolts seemed to come loose pretty easy like they were not re-torqued.

so I’m left with a couple more questions.

I have to assume the motor was run hot at some point due to the discoloration around the head and block. so without any visual damage to the inside, what effect does being run hot have on an outboard? what part(s) in particular bear the brunt of having been run hot? in other words what should I continue to look for?

also, I’m going to assume the rings may be a bit snug in their lands. what can I do to coax them out short of cracking the block and mechanically loosening them up? I have heard of using sea foam in the fuel. should I try to soak the rings in the cylinder? if so, with sea foam? something else?

this motor is cosmetically very nice so I would really like to do what’s possible to get it running well. as soon as I get it started and back in one piece, i’ll post some pictures.

thanks guys!

scott

[wiscoboater]

Setting the motor on the floor or bench with the pistons facing up, head off and both pistons at about half way up the hole fill the cylinder about 1/2" over the rings with Marvel mystery oil. Let it sit a few hours, moving the piston up a little bit, then down a bit every 20 minutes or so. Dont move it enough to loose the oil down the ports, but just enough to work the rings a little up and down. This works well for loosening up a tight carboned up set of rings. It works for stuck pistons as well. I got a very stuck Champion 1J loose using this method.

[fleetwin]

Overheating affects the pistons and cylinder walls mostly, bearings/bushings are usually not affected much. The high heat can cause cylinders to distort/become out of round. The high heat usually causes the pistons to smear/scuff some material onto the cylinder walls which may also partially trap the rings in their grooves. The high heat often causes the cylinder head to distort and burns the head/exhaust gaskets. You have the head off, so be sure to check it carefully for flatness (along with the cylinder deck) before replacing the head gasket. Pulling the exhaust cover is a "judgement call", mostly determined by whether or not the engine has been used in salt water, sounds like your engine is a fresh water engine. I guess I would try to loosen the exhaust cover screws, but surely wouldn’t force them if they seem stuck/corroded in place, those little screws break off easily and are a bear to drill out/helicoil. Removing the exhaust cover will allow you to see a more of the piston ring/cylinder wall situation. You might consider removing the intake bypass covers which will allow a peak at the intake side of the pistons, these screws are easily removed. Again, don’t force those exhaust cover screws, leave it alone if the screws are stuck. Be sure to clean all cylinder head threads and use sealer on the threads/gasket during reassembly.
You don’t report seeing any aluminum transfer onto the cylinder walls, and only the usually minor scratches/scores that are found on most of these engines. So, I really think you will be OK and have a nice running QD once reassembled. Again, don’t be afraid to "rough up" those cylinder walls with some fine emery cloth, simply blow out any dust with compressed air/carb cleaner spray.
Be sure to post pictures if you have more questions or want more opinions on internal condition.
One final note, these engines are known for premature wrist pin/bushing wear, so be sure to rock the crank back and forth slightly looking for "lost motion"/listening for the clunk which may indicate problems.

[1946zephyr]

Yep, thats what I was thinking too, Fleetwin. A motor with a discolored head got pretty hot in the past.
A de-carb may be needed too.

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