Weak cylinder on 73' 1500 inline

[luven2fish]

Cylinder 3 only has about 10-20 psi cranking compression. Bought this motor for my bass boat guy said it ran good. Is this possible? Tried squirting oil in cyl. then checked it again but little change. Would it run ok in the water (maybe down on h.p. a little cause off weak cylinder) , it seems to do ok on the hose. Any ideas? Thanks guys

 

[Chris1956]

Re: Weak cylinder on 73' 1500 inline

#3 cylinder on the inlines is usually the one to break. Pull off the center transfer cover and inspect the piston, rings and cylinder walls. If the cylinder is scratched, you should have it bored oversize and install a new OS piston and ring set. if the rings are stuck in the piston grooves, you might be able to loosen them up with some engine cleaner, however this would be an unusual situation.

In all cases you want to figure out what caused the damage. Usually it is caused by overheating or lean fuel mixture due to a dirty carb. It can also be caused by excessive ignition timing. Those motors should use a max spark advance of 21Degrees BTDC, versus the 23 degrees BTDC that they originally were spec'd for. You might also use mid-grade gasoline

 

[rpilote]

Re: Weak cylinder on 73' 1500 inline

I suspect you have a broken piston,,I just bought a 93 135 that supposedly was good..broken piston there also..It will not run right with bad compression. 4 stroke you can get away with a cylinder down..2 strokes rely on crankcase pressure to run right. Good luck, Rich

 

[luven2fish]

Re: Weak cylinder on 73' 1500 inline

Thanks guys, how hard is it to pull the center transfer cover? Can I see the damage through spark plug hole with a bore scope? Thanks again

 

[emckelvy]

Re: Weak cylinder on 73' 1500 inline

You'll be able to get a good look at the cylinder walls if you have access to a bore scope.

You'll want to pull the transfer port cover to get a look at the sides of the piston. You'll be able to see the rings, too, thru the intake ports.

Keep in mind that whatever you see on the intake side will be far worse on the exhaust side of the piston & cylinder as you've got so much more heat from exhaust gasses going out that side.

The transfer port cover is easy to pull, expecially the center one which will reveal cyl's #3 and #4. Just pull out the bolts and any fuel pump interference and the cover should pry right off.

All this will give you an 'inkling' of what's in store, but of course from there you're looking at complete teardown and inspection to determine the extent of the damage and whether a rebuild is feasible.

HTH & G'luck.........ed

 

[luven2fish]

Re: Weak cylinder on 73' 1500 inline

I pulled the cover and here's what I saw. The top ring on the #3 piston is broken. The side of the piston I can see (intake) looks fine. The piston seems loose in the bore but so does # 4, is this normal? The top of the piston that you can see from the side looks ok, not burnt. Also noticed the rod side of the bore looks like a die grinder was used at one time to radius the outside edge. Is that normal? Just trying to figure out if it's worth fixing. I'm open to suggestions. Thanks

 

[emckelvy]

Re: Weak cylinder on 73' 1500 inline

Sounds normal. Once you get it apart, you can compare the pistons in question to other pistons and even insert a "good" piston into the "bad" hole. Of course if you have access to a bore gage or micrometer and a ball gage you can measure out the cyl.

Failing that, a comparison of ring gaps on a new piston ring inserted into various cylinders, measured at the top, middle, and bottom of the bore, will tell you a lot about the wear and taper on the cylinders.

And yes, you'll have a bit of side clearance in the pistons, of course if it's way beyond tolerance it can cause problems.

Does this motor looks like it's ever been apart before? Someone could have easily broken a ring during the overhaul. If the wire and lead seal is intact and attached to the (1) crankcase bolt with a hole thru the end, it's never been apart.

May as well tear it down, it's nothing but a Big Lump until you do!!

 

[luven2fish]

Re: Weak cylinder on 73' 1500 inline

Yea I think it's been apart. How does the power head come off? I can;t see some of the nuts. Does the middle housing come off first?

 

[emckelvy]

Re: Weak cylinder on 73' 1500 inline

The smaller, horseshoe-shaped one-piece metal wrap underneath the main cowling comes off. If you'll tilt the motor up, it's easier to see the (2) screws per side that fasten this cover to the lower cowling.

Once this cover is removed, all the powerhead bolts are accessible. The (2) front nuts, one on either side, can't be fully removed until the powerhead is busted loose. Run them down flush with the end of the stud, and you can use a prybar between the nut and the structure of the center section to pry on and break the powerhead loose.

Of course there's plenty of interference on the powerhead to unbolt including the throttle & shift cables; the shifter arm; telltale hose; distributor wiring; fuel line (from the main connection to the fuel pump); various cowling support structure, etc. There are also some ground straps, Fore and Aft, that need to be undone. When you remove the horseshoe lower cover you'll see them.

If any nut or bolt won't come loose with normal hand-tool force, don't "push" it any harder 'cause it's probably corroded in place. Use a combination of heat and penetrant to bust 'em loose. PB Blaster is Good Stuff for that.

The powerhead is quite stout so best have a hoist or an extra pair of strong arms to help lift it off the motor.

Lay it on its back (with the spark plugs removed). That'll make it easier to work on. Use wood blocks as needed to keep it from rocking on the bench. Then you can strip off all the parts in the way.

You won't need any special tools at all except for a flywheel puller, and if you don't have the Mercury-style puller that screws directly into the flywheel, just pull (2) opposite bolts in the top of the flywheel and use an automotive harmonic balancer puller. Use Grade 8 or better 5/16"-NF puller bolts.

Just remember to put a drop of Red Loctite on each flywheel bolt and torque to approx 25 ft-lbs upon reassembly.

The flywheel nut takes a 15/16" socket and it's a heck of a lot easier if you have an air impact wrench to use on both flywheel nut and the puller.

Once you get the flywheel off there's (4) bolts on the main bearing cap to pull. There's (3) bolts on the lower crank bearing cap to remove. If any bolts are gonna be frozen, these are the most likely. They break Real Good, too. Just don't ask how I know! 0:

Once you get those, it's just all the bolts holding the flange of the front cover to the block, and bolts that go thru the front cover into the main bearing carriers. Use caution where you pry on the cover/block, as this flanged joint is a critical-fit sealing surface.

Well, have fun, your First Inline Teardown is always an exciting experience!....ed