View Full Version : 1990 40hp Mariner weak lower cylinders

09-06-2019, 12:41 AM
Hi all, brand new to the forum here. Having a issue with a Mariner 40hp magnum Ser# 0D036677, I think its about a 1990. A little backstory on it, I got the boat this summer hadn't ran since last year, got it running and took it out ran okay, would get up on plain slowly but was no power house (on a 16' smokercraft). Second trip out made it about a mile up the river and lost power, was like only having half throttle. Got back in and checked it out, and had no spark on the lower 2 cylinders. Upon inspection the wires from the trigger coil were rubbing against the flywheel, fixed that problem and had to replace the CDI box as well to get spark back on the lower cylinders, had to replace the rectifier as well. So now have great spark on all the cylinders, the stator is charging after replacing the rectifier, the trigger coil and stator both ohm out in speck. But the lower cylinders #3 & 4 are still not firing, compression checked all the cylinders three times now, and they are all at 140 psi. Removed the lower carb and went through it completely , inspected the Reed valves while the carb was off, visually they looked fine, stuck my pinky finger in and felt them up couldn't feel anything wrong with them. After getting the carb back on the lower cylinders still won't pull themselves. Doing the drop test removing the spark plug wires while its running the lower two cylinders will seem to fire a little but if you remove the plug wires on the upper two cylinders the engine dies immediately . Now when its running we've tried spraying the lower carb with carb spray and the lower cylinders still don't pickup. We put spark checkers on all four cylinders while its running and it has great spark on four cylinders. We even removed the fuel pump and blocked off the port and ran it to eliminate a vacuum leak there and no change. Removed the side plate for the exhaust chamber to inspect the pistons and cylinders to see if the lower half of the cylinders were scored to where it couldn't build pressure in the crankcase, but the cylinders and pistons look fine, just like the upper two. Now when running when you put your hand in front of the top carburetor and it wants to suck your hand in, definite vacuum, the lower carb doesn't feel like it has hardly any vacuum. Did the paper test with putting a piece of paper in front of the carb while running to see if its spitting fuel back out like a bad reed valve, but that didn't yield a definite answer, didn't seem to have any fuel pushing back out of the carb. So what I'm too is possibly the lower crank seal could be blowed out and not letting it build crankcase pressure for the fuel to suck into the cylinders, would that affect both the lower cylinders?? Cracked block possibly, but I don't know how that would happen, and the motor is in great shape, all the paint on the engine is the same color doesn't appear to ever have gotten to hot. Now its basically the Classic 50 which doesn't have a head gasket at all. Or the reed valves could be bad but didn't visually appear to be, but what I've read this style of engine didn't have much reed valve trouble. So just looking for any suggestions before I pull the powerhead out and dig in deeper. Sorry for writing a darn novel, just want to be thorough, been pulling my hair out with this darn thing. I will add that I haven't done a leak down test yet on the lower cylinders just compression tests. Any help is greatly appreciated, Thanks!

09-07-2019, 06:12 PM
Have you verified that the check valves for the crankcase are working correctly? Just a suggestion..

09-07-2019, 08:00 PM
Please excuse my ignorance, the check valves would be different than the Reed valves? If so where are they located?

09-07-2019, 08:48 PM
When you say that you went thru the lower carb, what did you mean by that? Have you switched the top spark plugs with the bottom spark plugs? Is there any fuel on the bottom plugs? Is fuel making it into the float bowl of the lower carb (loosen the bowls nut a little to see if fuel comes out - do not remove the bowl completely as its gasket and baffle are a PITA to get back in) - just trying to eliminate the carbs. Have you checked the Ohms on the coils for the lower cylinders? Somewhere in this forum, I thought I read where you should not disconnect the plug wires while the engine is running - don't recall if that does damage.

09-07-2019, 09:04 PM
We tore the carb totally apart, soaked in seafoam overnight, blew it all out reassembled with a new gasket kit. Confirmed its getting fuel and it appears that it is spraying just like the top one is when revving up the engine. We even sprayed carb spray into it while running essentially bypassing the carb and the lower cylinders still didn't pickup. We've switched the spark plugs around a few times. The plugs appear to be getting some fuel on them. I haven't ohm'd the coils but there appears to be plenty of spark when checking with a spark checker.

09-08-2019, 12:18 AM
Check valves are located on the block that jumper to each cylinder, some of these older units have one and some have two. If it has good compression on all cylinders and the carburetor is not drawing a vacuum I'm thinking it is a check valve issue. Are the Reed's on the crankshaft or are they in the housing behind the carburetors?

09-08-2019, 10:12 AM
When looking at the breakdown on parts page here on MarineEngine it looks like to me they are on the crankshaft, the reed valves that is.

09-08-2019, 10:20 AM
When looking at the breakdown on the parts page here on MarineEngine it looks like to me they are on the crankshaft, the reed valves that is. I won't be able to work on it till next weekend, but it looks like the check valve is fairly inexpensive, think I'll just order one and replace it and report back. Thanks again for all the help and suggestions, much appreciated.

09-08-2019, 10:52 AM
1. Those Merc reeds rarely ever fail.

2. Do compression test and see what's happening there.


09-08-2019, 01:43 PM
You used neon or open gap spark tester???? The neon will lie to you...

09-08-2019, 10:28 PM
Used open gap spark testers

09-11-2019, 10:54 AM
Am not trying to hijack your thread, but regarding spark testers, I saw this open gap spark tester https://www.amazon.com/Lisle-20700-Coil-Spark-Tester/dp/B00IXMA1RY/ref=sr_1_16?crid=2MCR083G41VVW&keywords=spark+tester+small+engine&qid=1568213108&s=gateway&sprefix=spark+tester%2Caps%2C319&sr=8-16

I take it that testing spark wrong can damage electrical components - so, how does one do it wrong?

09-11-2019, 11:06 AM
Any evidence that this motor was apart before ?---------Perhaps remove intake port bypass covers to see if you can spot something there.-----Put tape over those cover openings and pull motor over to see what happens to the tape.

09-11-2019, 11:09 AM
That's essentially the spark tester I used. I dont see how it can hurt anything it pretty much does the same thing a sparkplug does, creates a open spark to ground.

09-11-2019, 01:15 PM
You've got good compression on all cylinders - my '92 40hp 4 cylinder 2 stroke is at around 140 on 3 and 127 on 1 but runs good. The clue is that you get no suction on the lower carb; that would be further confirmed if you try to adjust the idle screw on the lower carb with no effect on the idle. Maybe what racerone said above.

09-11-2019, 02:08 PM
Yeah after looking at parts breakdowns again I noticed the "Transfer port" cover, will check that out this weekend. Had no idea there was this much different stuff on 2 stroke engine. Not quite as simple as some people think, myself included lol!

09-11-2019, 04:50 PM
The whole concept of crankcase compression is mis-understood by many folks.-----Difficult to trouble shoot for some.

09-13-2019, 10:00 PM
Got a chance to mess with the motor a little tonight. Checked the crankcase check valve and it seems to be working properly. There is one check valve on this engine at the lower part of the crankcase, pulled the hose off where it goes into the top of the crankcase and checked it with my mouth, will flow one way but not the other, seems to be working from what I can tell. The equaliser tube seems to be fine not broke or anything. I removed the Transfer port/Intake port cover and the bolts were tight but it does look like someone had been in there before by looks of the Phillips headed bolts. Gasket was in fact. Put tape over the intake port openings and cranked over, they all look to be about the same, will post a video if I can figure it out. Going to leak down test the cylinders in the morning when I get my leak down tester here.

09-13-2019, 10:18 PM