I just purchased a 40 HP Nissan.

[59merc]

It is low on compression on the number two cylinder. The indidvidual I purchased it from said he thought it had quit oiling on that cylinder. It is off the boat but I have turned it over manually and there dont seem to be any noises. I also checked the reed valves and they seem to be ok. What would be the next step for me to determine what is wrong? I would love to have the motor going. It also has power trim and tilt which I really like. Also is there a very good market for these parts if I have to scrap it out?<br /><br />TIA<br />Larry

 

[Johnshan1]

Re: I just purchased a 40 HP Nissan.

I would take the head off to have a look at the pistons and bore, its not hard to all with a 2 stroke.

 

[TOHATSU GURU]

Re: I just purchased a 40 HP Nissan.

Pull the head and see what is wrong with the # 2 cylinder.

 

[ziemann]

Re: I just purchased a 40 HP Nissan.

Yo see a few parts showing up now and then on ebay, but not alot. There are alot of 40's out there, so you would probably do fine if you scraped it out. But why?

The 40 is a nice simple design that is easy to work on. There is a chance that a top end rebuild will take are of your issue and you are back in business at a relatively low cost. Of course if its more serious than that....

 

[Silvertip]

Re: I just purchased a 40 HP Nissan.

If you decide to repair the engine, make sure you fix the oiling issue (or whatever caused the damage in the first place) so you don't end up damaging the rebuild. Lean carburetion can also damage an individual cylinder. People tend to blame the oiling system for individual cylinder damage but oiling systems don't pick one cylinder. If the oiling system fails, all cylinders will generally show damage. My take is that a lean carb caused the issue. They need to be removed, disassembled, soaked, cleaned, and reassembled with new kits.

 

[59merc]

Re: I just purchased a 40 HP Nissan.

Thanks for the replies. I pulled the head and side cover, but still don't see any obvious problems. Not sure where to go from here.

 

[TOHATSU GURU]

Re: I just purchased a 40 HP Nissan.

You are going to have to take the engine all the way apart.

 

[59merc]

Re: I just purchased a 40 HP Nissan.

what should I be looking for. The piston, rings and head gasket appear to be ok.

 

[TOHATSU GURU]

Re: I just purchased a 40 HP Nissan.

If an engine is low on compression it can be any of the following:

Head gasket
Cylinder head
Block
Basegasket
Sideplate
Lower main seal
Piston/Ring
Reeds
etc

 

[polychromeuganda]

Re: I just purchased a 40 HP Nissan.

... since this 40HP has power trim and oiling it must be a 3 cylinder 40D, not the 2 cylinder 40C, so the no. 2 cylinder is in the middle, and he says the reeds look good, so...

get a good compression tester. crank it to a stop with each cylinder near TDC (one at a time) and find out how many PSI you lose in 10 or 30 seconds. If they are all the same, its the head gasket, the cylinder, the piston, or the rings. Carbon build up can make the rings stick and tank the compression, so do a carbon remover treatment, use SeaFoam, unless you like waiting overnight better. If that didn't fix it, when you remove the head to look, you'll end up installing a new gasket, so you have that covered. If you see anything that gets your attention you can very lightly hone the cylinder and see if the rings will re-seat. You might have a cracked ring, which would be some work to replace.
If the rate of pressure loss was the nearly the same for all three cylinders, then the problem is in the crankcase. 2 stroke motors use the back side of the cylinder to draw in and pre-compress the air-fuel charge, so any leaks there can spoil the compression. Bubble solution is an effective diagnostic tool...

 

[59merc]

Re: I just purchased a 40 HP Nissan.

When I did the compression check #1 and #3 had about 110 lbs while #2 had 75. I am waiting on the title for the motor before I do to much more work on it. It is just strange you would think a problem could be seen on the number 2 cylinder but certainly not any thing obvious. It is currently apart so the seafoam is not an option.

 

[59merc]

Re: I just purchased a 40 HP Nissan.

[Well today I finally took a good hard look at the number 2 piston. It appears to have a few scratches on top and it appears there is a small indention on one side. A check of the cylinder wall in this area reveals a small scratch. So just a guess a ring broke. I haven't pulled the piston yet. Does this sound like a ring broke?

Thanks for all the replies I have had on this. I am going to check for parts and see if it is worth repairing. Any suggestions on best place to buy parts?

 

[polychromeuganda]

Re: I just purchased a 40 HP Nissan.

75 PSI is 5:1 compression, just loafing and wheezing, barely burning fuel. 112 is just under 7.5:1 compression, OK, but sound tired and old, 9:1 (about 135PSI) is more desirable. >10:1 may knock without premium fuel, 50:1 is a deisel.

Without knowing the length and depth of the scratches, its a bit hard to tell. Scratches on the top of a piston and a dent in the top of the piston would be something pulled into and through the motor. Remove the reed plate for No. 2 and look for a missing nut, then check the other 2 reed plates. Use metric stainless hardware with lockwashers and blue loctite during reassembly.

The dent in the piston is a little troubling, I assume this is more .010" deep, or you probably wouldn't see it or think much of it. and it probably hapened when the cylinder was at TDC with a foreign object in it. Are you sure the "scratches" aren't cracks? The piston sounds like it should be replaced. Measure the old piston and cylinder before ordering the replacement piston in case the engine has been rebored previously. The cylinder should be honed before installing the replacement piston, so any shallow scratches will disappear anyway during honing and as the new rings seat.

If the cylinder wall scratches are more profound, say more than .010" (2+ hairs) deep and over 1" long you may need to rebore the engine. If you tear it down motorcycle machine shops charge around $50 a hole to re-bore. You need to do all 3, and 3 new pistons. If this is a sleeved engine, you can just replace the sleeve in #2. If you don't know how, find a motorcycle machine shop. They won't have outboard parts, but if you bring the parts they do better major overhaul work than marine people at 1/10th the price.

 

[TOHATSU GURU]

Re: I just purchased a 40 HP Nissan.

Parts are available from any Nissan or Tohatsu dealer.

 

[59merc]

Re: I just purchased a 40 HP Nissan.

thanks for the indepth reply. Will post how it goes.