1963 40hp Big Twin Low Compression

[Bollie]

Hi, I would like to know what you guys think. This old motor runs nice after I have put some time into it. It just did not have the power it should have. I did run some Sea Foam through it, but the compression on both cylinders is 60 psi. I test it with a second tester and get the same reading. Will installing new sets of rings help to get the psi up and will it produce more power. My son have a lot of fun on the tube but the boat just can not get me on plain on the tube.

 

[Bollie]

Re: 1963 40hp Big Twin Low Compression

"o" I forgot to a that I squirt some engine oil into the cylinders but it made no difference on the reading

 

[lark2004]

Re: 1963 40hp Big Twin Low Compression

Could be a bit more than just rings.<br /><br />You would need to strip the engine, and measure the cylinder bore, the piston diameter, check to see if the bore is still round, ring gap etc....

 

[Bollie]

Re: 1963 40hp Big Twin Low Compression

Thank you for the reply I think I will strip the motor. Do you know where I can get this rings I could not find it at iboats parts listings

 

[WillyBWright]

Re: 1963 40hp Big Twin Low Compression

Try this first. You'll need a new head gasket and a brand new Scotchbrite. Remove the head to inspect. If it looks pretty good but glazed, roll each piston to bottom in turn and Scotchbrite some cross-hatch into the cylnder. Flush things out with WD-40, put it back together, run it for awhile. Then recheck your compression. It'd be a whole lot cheaper and faster than an overhaul if that's all it neeeded. I know you can't get any cross-hatch on the bottom of the cylinder this way. But with a 2-stroke, the important place to have a good ring seal is above the intake and exhaust ports.

 

[Bollie]

Re: 1963 40hp Big Twin Low Compression

Thank you for the input, will this help even if the rings are worn out and will the cylinder not get glazed over quickly again.

 

[Bollie]

Re: 1963 40hp Big Twin Low Compression

I think this motor did a lot of low speed trolling. It belonged to a guy that liked fishing and by the way the boat was set up with rod holders and windscreen removed.

 

[WillyBWright]

Re: 1963 40hp Big Twin Low Compression

It'll glaze-over quicker than if the bore were honed. The cross-hatch retains oil which makes the rings seal better. The rings may not be worn out. It's something cheap to try that has a good chance of positive results with a minimum amount of labor and downtime. If the cylinder walls are damaged, not just glazed, then it's not going to help. You basically need to take the head off anyhow to see what's up. That could be a cheap and easy fix that could help get some compression back for a few years.<br /><br />One possibility that shouldn't be discounted is a decarb treatment. It's described in the FAQs. It doesn't generally help, but it's easy to try and there is no disassembly. I have had little luck getting compression back this way, but it did work out well on a Chrysler that I figured had toasted cylinders. They were glazed. I did the Scotchbrite thing and got a little compression back, but the decarb made the biggest difference.

 

[Bollie]

Re: 1963 40hp Big Twin Low Compression

Thank you WillyBWright I have talked to a vintage outboard shop here in Denver the owner told me that he work on a lot of this old motors and that is not worth replacing the rings if the two cylinders psi is balanced he recon that 60 psi is not to bad according to him 85 psi was a new motor compression back in the day. I think I am going to do the Scots bright thing that will be easy. I would like to know how much scouring to do can it be overdone with a scotch bright

 

[itstippy]

Re: 1963 40hp Big Twin Low Compression

It is very possible that your 40HP is putting out 40HP but the boat is not set up to make best use of it. Especially if the prior owner was interested in slow trolling a lure, not in planing a tube. I'm no expert, but you want to start with the anti-cavitation plate about even with the bottom of the boat, then try shimming the motor up by small increments to get as little of the lower unit in the water as you can without cavitating. Less lower unit in the water means less drag. Makes a big difference. <br />The style of boat hull, pitch of the prop, and the angle of the motor are also important factors. Experts will be along to tell you what to look at. Lots of great advice on "setup" in the Boat Topics Non-Motor forum too. With the right hull and setup you should be able to pull a tube. As a skinny teenager I skied behind a 25HP!