1986 Yamaha 25hp Ignition or Fuel Problems

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wmk0002

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A friend bought a 1986 Yamaha 25hp (25SJ). Motor had 120 psi on both cylinders and was in good shape overall. I'll note that the head is a gold color while the rest of the motor looks more like a factory blue and the thermostat was broken and stuck in the open position.

We ran it as is at first and it started pretty easy and ran but very poorly both on muffs and in a barrel. It would cough very hard every so often. Let it run in a barrel for a while one day and it died and wouldn't restart. Compression was still good but it had an incredibly weak spark when using an air gap tester. Pulled the coil to find that one of the two leads from the CDI was melted through and exposing the wire where it contacted the powerhead. Also noticed that the starting lockout cable that runs near that area was melted to the metal sleeve. We checked the primary and secondary winding of the coil and both were in spec so I temporarily taped up the exposed wire and reinstalled. Still weak to no spark. We pulled the CDI and tried bench testing it but pretty much every combination showed an open circuit. Unsure if it was in fact dead, we ordered a fairly cheap used one that was said to have come off of a running motor that had an internal failure. Installed the used one and had very nice spark jumping 3/4" gap on both leads. So we are thinking that this motor overheated in the past and either damaged the CDI with heat, or by having the output wire to the coil shorting out. Also of note, the charge and pulse coils under the flywheel tested within spec.

While waiting on that part, we rebuilt the carb and checked the timing. Timing was good and the carb was not too dirty and after blowing out all orifices with solvent and compressed air seemed pretty spotless. After we have the CDI on we run it in a barrel and get it to start easy and run very well at neutral and forward idle. I do have to set the mixture screw to 2.5 turns out which is a lot but it runs pretty well there. Also set the forward idle to 750 rpms. We lake test that same afternoon...it starts first pull and appears to be running well. Slowly accelerate, where it does seem to have a slight miss or something off at maybe 1500 rpms, but it quickly clears out and accelerates strong and smooth up to around 3000 rpms where it then begins to sound like it is missing hard and is shaking and only climbs to maybe 3100 rpms.

From there did the typical stuff... checked tank vent - open, checked bulb - firm, tried squeezing bulb - no change in performance, inspected the fuel filter - full and free of air bubbles or noticeable debris at idle and WOT, checked the throat at WOT - good spray of fuel coming from the main nozzle, pull the choke at WOT - engine bogged down to an idle and cleared itself out after removing choke, checked engine temperature and water pump - both fine. Called it quits and drove back to the ramp. It never died and continued to run well and feel powerful in that lower half of the rpm range. After getting back, checked the fuel pump and it was in great shape including the diaphragm. Spark was still strong as well. Plugs also looked a little dark but not overly rich but the return trip to the ramp in the good performance window may have cleaned them off.

Now to present time, we are somewhat stumped. Granted one of the simplest things yet to do is change the plugs, which we will do asap, but aside from that not really sure what to try next. I feel like the on the water tests tell me that it isn't' a fuel starvation issue but more so another ignition issue. I guess its possible it could be getting too much fuel but I know the float height appears good and the only thing I may be missing is if someone did some home handywork on the main jet size. The ignition coil tests ok but I'm wondering if it was heat damaged as well or due to the bare wire arcing/shorting. Ive seen a number of bad powerpacks/CDIs fail due to heat but only one other coil I can think of. I think the coil is the next item to replace since it has the bad input wire already. I was thinking maybe cover it in heat shrink and call it good but IDK if that is an acceptable fix.

I'd appreciate some thoughts on what we may have missed or if anyone has any insight on the potential ignition system issues. I really want to help him get this one going as he got burned on the last one he bought when he didn't check compression prior to buying. But also since it isnt my motor its on him to decide what all he wants to spend on it to try to get it going. BTW, the boat is a 1542 Fisher mod-v which runs out great at 21-22 mph with two adults and gear so I don't think setup is at play here.
 

nccatfisher

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That slight miss @1500 may be the "spot" where it is transitioning over from low speed to high. All engines do it, Yamahas for some reason are more pronounced, most in the 25 HP range is right at 1200 RPM.

When you say it has a "miss" that is kind of generic. It would be nice to know if it was electrical. You could probably put it in a barrel and put a hose in there and you will get wet but put your air gap tester on it and run it at 3K RPMs in gear and tell if it was electrical. If not I would go back to the carb. Given you are running the mixture screw at 2.5 rounds out I would be suspect that the carb may still be the issue. I would say a thorough deep cleaning with an OEM kit installed may be a worthwhile endeavor.
 

turbotodd

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The one post about yamaha being notorious for the transition is not 100% true. When tuned properly, they are silky smooth in the transitions from idle to main and from no advance to full advance on the ignition side. Trick is, to get it all correct....and not many know how.

With regard to the OP. The cylinder head is golden colored because it has been overheated, severely. Perhaps something stuck in the cooling passage somewhere, or just run out of water due to failed water pump, whatever--but it has been hot. 120 psi means it's still making compression but that doesnt' give you the entire picture, for that you'll need to do a leak down test on both the top end of the engine and the bottom. I suspect several things. One, you may have a cdi going south, two you may have an intermittent ignition coil issue, and three you may also have a problem in the bottom of the engine, specifically the labyrinth seal that seals off the two crankcases from each other. Or four, perhaps an issue with the reed valve but that is very rare with Yamaha and I typically don't even mention it because of it's rarity.

Now that said, that engine is a twin cylinder single carburetor, right? Reason I ask, is that yamaha had about 6 different 25hp motors over the years ranging from twin cyl single carb to twin cyl twin carb to 3 cyl 3 carb and then 3 different 4 stroke motors. The old twin cyl single carb motors (which later became the C25 to differentiate it from the "regular" 25 which was twin carburetor) didn't idle all that great, didn't run real smooth either, and quite noisy. That, is in comparison to the twin carb version which is almost as quiet and smooth running as a new ETEC 25hp, and about 50 lbs lighter weight.In other words, the old style ain't ever gonna be perfectly smooth like the "newer" ones (up to 2006), but it shouldn't be doing what your describing either. Ignition timing is critical, and at ALL RPM ranges, meaning you may have to adjust the pickup timing, idle timing and wide open throttle timing, and there is some fiddling around to get all 3 of them working properly. You need a good service manual to get all 3 of them correct. Over-advancing the timing particularly at full throttle will hole the piston(s) just as easily and quickly as a lean running engine will. Not enough timing will result in sneezing at part throttle, etc. You get the idea. It is critical.
 

wmk0002

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nccatfisher said:
That slight miss @1500 may be the "spot" where it is transitioning over from low speed to high. All engines do it, Yamahas for some reason are more pronounced, most in the 25 HP range is right at 1200 RPM.

When you say it has a "miss" that is kind of generic. It would be nice to know if it was electrical. You could probably put it in a barrel and put a hose in there and you will get wet but put your air gap tester on it and run it at 3K RPMs in gear and tell if it was electrical. If not I would go back to the carb. Given you are running the mixture screw at 2.5 rounds out I would be suspect that the carb may still be the issue. I would say a thorough deep cleaning with an OEM kit installed may be a worthwhile endeavor.

Thanks for the reply. If I had some type of tester I could use while under power I would. I agree it would be nice to know it was electrical though.

I'll of course revisit the carb but I am pretty confident it is not the issue. The fact that it didn't respond positively to choking and I could see the fuel from the main jet just seems to me that it should at least be running out better in the upper rpm range. I will say, I bought the Sierra carb kit which seemed lower quality than Sierra kits I've gotten in the past.
 

wmk0002

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turbotodd said:
The one post about yamaha being notorious for the transition is not 100% true. When tuned properly, they are silky smooth in the transitions from idle to main and from no advance to full advance on the ignition side. Trick is, to get it all correct....and not many know how.

With regard to the OP. The cylinder head is golden colored because it has been overheated, severely. Perhaps something stuck in the cooling passage somewhere, or just run out of water due to failed water pump, whatever--but it has been hot. 120 psi means it's still making compression but that doesnt' give you the entire picture, for that you'll need to do a leak down test on both the top end of the engine and the bottom. I suspect several things. One, you may have a cdi going south, two you may have an intermittent ignition coil issue, and three you may also have a problem in the bottom of the engine, specifically the labyrinth seal that seals off the two crankcases from each other. Or four, perhaps an issue with the reed valve but that is very rare with Yamaha and I typically don't even mention it because of it's rarity.

Now that said, that engine is a twin cylinder single carburetor, right? Reason I ask, is that yamaha had about 6 different 25hp motors over the years ranging from twin cyl single carb to twin cyl twin carb to 3 cyl 3 carb and then 3 different 4 stroke motors. The old twin cyl single carb motors (which later became the C25 to differentiate it from the "regular" 25 which was twin carburetor) didn't idle all that great, didn't run real smooth either, and quite noisy. That, is in comparison to the twin carb version which is almost as quiet and smooth running as a new ETEC 25hp, and about 50 lbs lighter weight.In other words, the old style ain't ever gonna be perfectly smooth like the "newer" ones (up to 2006), but it shouldn't be doing what your describing either. Ignition timing is critical, and at ALL RPM ranges, meaning you may have to adjust the pickup timing, idle timing and wide open throttle timing, and there is some fiddling around to get all 3 of them working properly. You need a good service manual to get all 3 of them correct. Over-advancing the timing particularly at full throttle will hole the piston(s) just as easily and quickly as a lean running engine will. Not enough timing will result in sneezing at part throttle, etc. You get the idea. It is critical.

Thanks Todd. It indeed is a 2 cylinder single carb model. Also of note, it has a singe coil that fires both cylinders. Based on the inductive tach I put on it and my timing light readings I believe it utilizes a wasted spark.

I have never done a leak down test although its something I have been wanting to start doing. I'll look into what exactly I need to adapt my compressor to the plug hole and give that a try once I get set up. I also have not done a link and sync. I bought a 1 month pdf copy of the factory service manual from Yamaha so I should have what I need for that.

Back to the 3000 rpm issue - I have been thinking about how to describe it and it does seem similar to a coil misfire on a vehicle. Granted the only ones of those I have experienced are a single coil failure on a 4 cylinder car but nonetheless the loss of power paired with the shaking the engine does is quite similar. I would guess with this Yamahas singe coil and having a wasted spark means any failure will affect both cylinders equally since it seems to send out one fire sequence to each one simultaneously.

I'll post some pics of the head soon just for reference.
 

wmk0002

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A few pics of the head discoloration. Ignore the redneck engineered pull starter hold down - it was there when it was purchased :shock:

0GrdjiF.jpg


6O7T64a.jpg


MyYOpF0.jpg
 

RaisedByWolves

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Sound anything like this?. This was a bad temp sensor making it go into limp mode.
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https://youtu.be/UwMU9Q5_XP0


:40



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https://youtu.be/cAeJAnX-D4M
 

wmk0002

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RaisedByWolves said:
Sound anything like this?. This was a bad temp sensor making it go into limp mode.

Not quite. This was at a much higher frequency/rate. I think either my memory is fading of it already or I just have trouble describing it though.

My friend picked up new fuel lines, spark plugs, and an extra small inline filter today so we will put those on tomorrow hopefully and lake test once more and I will take a video of it. I'm also gonna start it cold at the ramp and just punch it just to see if it behaves any better before everything gets warm.
 

muddywaders

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I would suspect short.HT leads or bad plug.You may try choking or spraying premix directly into carb if rpms increase you can assume not electrical. m.w.
 

wmk0002

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muddywaders said:
I would suspect short.HT leads or bad plug.You may try choking or spraying premix directly into carb if rpms increase you can assume not electrical. m.w.

I tried choking and it bogged down instead of taking off like you'd expect if it was starving for fuel. Given the carb has been cleaned and a kit installed, fuel pump diaphragm looks good, and bulb is hard and pumping it has no effect I don't believe its a fuel issue. I ordered a non-oem ignition coil online yesterday. Hope to give it a try with fresh plugs this weekend and see how it does while its still cold. Then will install the new coil and hope for the best. If issues still present I'll start leaning to some type of internal issue.
 

turbotodd

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based on the picture of the cyl head, it's been really hot at some point in it's life. Same cyl head as some of the old 500cc wave runners I used to work on; typically when I saw one in that color, it was either seized or had parts hanging out of it, sometimes both. Of course that's a different application but yeah, that motor's been HOT.
 

Pappy

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Before you keep throwing money at parts replacement............... I will be referring to the video iMG 00371.
Take a digital temp gun with you and shoot the top of the cylinder head when the problem occurs. See what it is reading.
I will admit I am not as familiar with these engines as I am the OMC family of engines but what I saw was an engine that was going into limp home mode for lack of a better way to describe it.
On this series of Yamaha is there a temp sender on the cylinder head or anywhere? If there is you can take an ohmeter with you and pull the connection to it when it happens and check continuity between the sender lead and powerhead ground.
Your engine also sounds like there is quite a bit of cavitation when you nail the throttle. Is there anything at all in front of the gearcase on the bottom of the boat? A pick up for a live well or a transducer or anything?
When you get this to happen I see that it still happens as long as the engine is running once it starts. Try shutting the engine off and re-starting and see if it accelerates. If it does then we are on the right track here as the key cycle will reset the safety feature.
 

wmk0002

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Hey Pappy, that video above is not mine. That was Raised byWolves showing me an example.

To answer your question though, this motor does not have a temp sensor. I don't think it even has a limp mode...and may not have a built in rev limiter either. I do have an IR temp gun I will take with us on the next outing. Running it in a barrel at about 1750 rpms in forward gear it was reading about 120-130 deg on the top of the head if I remember right.

This coil came in the mail today so can now try it. Like I mentioned it isn't OEM and smells like Harbor Freight aka Chinesium so if it does happen to correct any issues not sure we will rely on it too long.
 

thill

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I haven't read all the comments, but I've seen similar issues and it ended up being a weak stator. Ran fine at lower RPM's but couldn't keep up at WOT. Pulled the flywheel and the stator was discolored from heat.

Hope you find it.
 

wmk0002

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I haven't read all the comments, but I've seen similar issues and it ended up being a weak stator. Ran fine at lower RPM's but couldn't keep up at WOT. Pulled the flywheel and the stator was discolored from heat.

Hope you find it.
Appreciate the reply but he sold that motor about a year ago I believe. We never figured it out. I was all in on tearing it apart and doing some more diagnostics but he doesn't have the same interest as me in working on motors so he was just wanting to get rid of it out of frustration.

The stator would have been a good thing to check though. I really think it had a bad lower crank seal and maybe a bad bearing too because it would puff smoke from that area once it began to warm up. Ran decent on first startup and then started puffing smoke and popping and sneezing once it warmed up some. I think its safe to say it was overheated so those are plausible and it may have had even more internal damage.

Personally I'll stick with the later 2 carbed models. Have 3 of them in my shop I need to get to work on if I can ever find the time.
 

thill

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Sometimes selling is the way to go. You only have so much time. If you don't like working on engines and can afford it, just get another motor and give someone else an interesting project to figure out.
 
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