1984 Mariner 25hp XD not idling

This is a start to finish post luckily. Hope this helps someone else with a similar issue.

THE ISSUE:

Problems started at the end of last year where my motor wouldn't hold idle & would die out if I didn't race the engine. If I could keep it going and get it into gear I could run it with a decent amount of throttle but could not slow down & crawl along without it dying.



First thing I ruled out was a spark issue. That is easy to do with an inline spark indicator, and well worth the $10 they cost to anyone looking to maintain their own motor. I pulled the plugs just to check them & they looked fine, not all coated over in carbon or corroded away etc. Without a spark issue I could for the moment rule out ignition coils, stator, trigger & the switch box - all of the costly possibilities.

PHEW...fuel issue...much cheaper fix (hopefully.)

So I checked the gas delivery to the carb first - primer bulb was solid and as I have clear fuel lines on the motor I'm able to see there was always plenty of fuel getting to the carb - no reason it shouldn't start there. Cleaned the fuel filter for good measure, nothing in there that would be obstructing fuel flow and no indication of water mixing in the gas. All good so far - next up is the carb.

Bit of history on the motor - I swapped the carbs on the motor to up the power from 20hp to 25hp (this is the only difference between the two power variants of these motors, I checked every part number on two motors of the same vintage) but I did not rebuild the 25hp carb at the time I made the switch. It hasn't given me any issues in several years of running it so I suspected the ethanol may finally have gotten to it. These carbs use a rubber diaphragm to use pulses of vacuum from the motor to pump fuel into the carb, so this was a likely suspect.

So I cleaned rebuilt the carb with new gaskets. I snaked all the lines and passages with thin gauge mono-filament and blew everything out on my air compressor to make sure all passages were open & free in both directions. There was no wear or deformation of the rubber tip on the inlet needle valve operated by the float, something that ethanol commonly wrecks. Other common areas are the fuel pump diaphragm and the diaphragm on the enrichment valve, both of which had noticeable cracking, so I thought I'd found my gremlin. Put the carb back together with the new gaskets and diaphragms and tried starting it up again, but the issue persisted.

Now I'm looking for a vacuum leak that would be robbing my carb of the vacuum it needs to function because clearly it has fuel to the carb and spark, and there is no obstruction in the carb itself or issue within the carb that should prevent it from operating. Inspected both vacuum lines, each was flexible and showed no sign of damage, so I dismissed them as possible problems. Next deeper component was the reed block assembly. I pulled this out and started to find more issues. The gasket between the reed block and the cover plate showed no signs of air intrusion, but it had what looked to be pollen deposits on the flat face of the gasket (the tears were when I removed the gasket and were not there prior to me pulling it all apart as best I could tell:



No biggie, I ordered gaskets prior to taking this all apart as I figured after 30 years it was probably not going to survive being pulled apart anyway. I took the reed block and held it up to see if the reeds were fully closed as they should be and found this on both sides of the block:



GREMLINS! There we go - there's my vacuum leak. Fortunately, I have the original style reed block which uses separate metal reeds instead of the newer ones where they're all one integrated assembly. Parts #30 in this diagram:



I took the reed stops off and inspected the reeds for metal fatigue - absolutely no sign of it what-so-ever. PHEW! Those are not available anymore and the replacement assembly off newer motors is $85. There was hardly any wear on the painted aluminum seat in the block where the reeds close against, confirming my belief that this motor, despite its age, had not seen much use over the years. First I tried flipping the reeds over, and while better, they still had some gaps so instead I proceeded to gently apply pressure to each reed tab being careful not to bend the thin sections of metal that tie all the reeds together until they all sat tight to the block with the stops reinstalled. This is the result:



No light showing through the individual reeds on either side as it should be. When you think about it, the vacuum lines running into the carb are not any bigger than 1/16th ID, so all these little gaps allowed a comparably tremendous amount of vacuum to be released without going through the carb. Needless to say once it had new gaskets on the reed block & plates and was all put back together it functioned perfectly. Started on the 1st pull & needed the typical carb adjustment common with any carb that had just been rebuilt.



I'm not entirely sure how it got to be this bad, I certainly don't use the motor much each trip - 95% of my time is on the trolling motor (separate diagnosis/repair thread coming on that one.) I'm thinking perhaps using the motor in the cold (sub freezing when starting in the morning warming to above freezing later in the day) might have lead to deformation of these parts, it seems newer reeds are made of a composite material rather than metal possibly for this reason.

There has been many motors given up on over that very issue or issues in that area. If a carb rebuild or replacement doesn't get it many times they find a new home. I agree that a big portion of the times that it does stop at the carb or even prior to the carb. But also many times you have to go a little deeper, and most folks are lost then.

I forgot to run a compression test when I had the plugs out, but if I were covering all my bases that should have been on the list at that point for simplicity.

Problems with engine internals or the gearbox are the stopping point for me. I'm sure I could do it, if I had the tooling, but that's the point where there is a line between could and should. You don't know what you don't know - the machining and assembly of the motor internals are where experience and training are required, and I know I lack both and can only make up for it with research but so much.

I had a 25 merc that would not idle too. I changed out all of the gaskets and rebuilt the carb. Still would not idle. It ended up being the seal between the two halves of the block. It has no gasket other than a type of silicone that never cures. It is a special type you have to use.

tomme boy said:

I had a 25 merc that would not idle too. I changed out all of the gaskets and rebuilt the carb. Still would not idle. It ended up being the seal between the two halves of the block. It has no gasket other than a type of silicone that never cures. It is a special type you have to use.

Click to expand...

Did you compression test that motor with the leaking seal? My gut tells me it wouldn't show on a compression test. How did you find it? Process of elimination or did you do something to confirm it?

I've heard about that issue with these motors from a few people. I consider myself lucky that it wasn't the problem.

Just look at the motor under the hood. It will have gas and oil mix all over it. Like it is leaking out of the carb. And NO a compression test will not show this as the block is in two pieces. Right at the crank. It is usually happens from over heating it. Mine was like this when I bought it. I got it cheap enought that I had it fixed.

This is a very common thing with Mercury motors that use this style of block

onthewater102 said:

I forgot to run a compression test when I had the plugs out, but if I were covering all my bases that should have been on the list at that point for simplicity.

Problems with engine internals or the gearbox are the stopping point for me. I'm sure I could do it, if I had the tooling, but that's the point where there is a line between could and should. You don't know what you don't know - the machining and assembly of the motor internals are where experience and training are required, and I know I lack both and can only make up for it with research but so much.

Click to expand...

I quit at splitting the block anymore. I have some bad arthritis type symptoms in my hands from undiagnosed lymes, or lymes that went undiagnosed for a long time to they caught it. My dexterity is pretty sorry. In fact some tedious work getting to some carbs and stuff aren't worth it either but sooner or later I can usually get that. But there is no way I would take a chance on having to go back and split a block because I made a bad seal.

I didn't need to get into splitting the block as he's described. Looking at it that makes sense that a failure in this seal would make a horrible mess under the engine cover - which thankfully is not the case with my motor.

Here's the full engine block diagram for anyone not familiar with these motors - the combustion chambers are completely separate of the seam Tomme Boy is speaking of. Its the surface between the two parts labeled "1" in this diagram that are joined and yet have no gasket between them:

I was just saying that was where I drew the line anymore as my abilities. I knew you could get the reeds out of that model without splitting.

What we are getting at is sometimes it is NOT the carb that makes a motor have idle issues. The blosk seal is just another item to look for in trying to diagnose a problem.

This style of motor is one reason you can not hop up the motor. One of the easiest ways to make HP was to increase the compression. The way you achieved that was to mill the head to decrease to chamber volume. Since this motor does not have a head to remove, the only way is to get different pistons that are made for a higher compression.

I will let you in on a little secret of these motors. If you take off the side inspection cover labeled as 7,8,9,10 in the photo, plug the compression relief ports. This will raise the PSI about 15-25PSI. You may have to rejet the carb to take advantage of the higher PSI.

I'm not looking to hop this thing up - I just wanted to get rid of the restrictive carb that was preventing it from making the HP it was designed to make.

Original vs. 25hp carb comparison:

The one of the left looks like the 18XD carb. I had one of those too. If that was on your motor then someone messed up.

Wouldn't surprise me - according to mercury's customer service the S/N for the motor indicates originally it was a 20hp, but when I got it the carb and all the linkages were in a child's sand castle pale all taken apart.

Also, it was one of the earliest motors produced in that line, so it wouldn't surprise me if things changed over time. The difference between an 18hp and a 20hp are negligible .

That new carb should make a big difference. Did you get the upgraded primer when you rebuilt the carb?

You are lucky that you have the WMC carb and not the BC style. Mine was acting funny today. I think I am going to have to replace the inlet needle. Or adjust the float. It wanted to flood out a little bit. It would spit out gas from the front of the carb and then would take off and run fine. Last time it did this the little rubber tip on that needle was not sealing and gas would spray out the front of the carb. Going to get one tomorrow just to be safe.

It made a noticeable difference -- with the hydrofoil on the motor the top speed went from ~20mph to ~23mph. I've since cupped the edge of the prop (was experiencing terrible slip) and removed the foil & it'll do 26mph with just me in the boat.

Once I get the 16' finished I'm going to circle back to this and install a jackplate I built to offset and raise the motor as right now the cavitation plate is a full 2" below the bottom line of the 1436's hull. I'm already experiencing chine walk on occasion, but I need to get the RPM's on the motor up and I'd rather play around this way and give myself adjustable trim than start playing 'which prop works for me'. I've got a Lenco trim tab actuator that I'm going to try to use to give myself trim adjustment - but as I said all that playing will have to wait until I have my 16' functional.